Function of wheat phosphate transporter gene TaPHT2;1 in Pi translocation and plant growth regulation under replete and limited Pi supply conditions

Adaptation of potato cultivars to phosphorus variability and enhancement of phosphorus efficiency by Bacillus subtilis

BACKGROUND

Plants utilize a variety of mechanisms to adapt to fluctuations in phosphorus (P) availability. Potatoes, in comparison to other crops, often display reduced phosphorus use efficiency (PUE) due to their underdeveloped root systems; therefore, understanding the mechanisms underlying PUE is critical for improving it. This study aimed to evaluate the morphological and physiological responses of potatoes to different P levels, with a focus on root system alterations and PUE. Two potato cultivars, a table potato (cv. Milva) and a starch potato (cv. Lady Claire), were subjected to varying P levels (0.5, 2, 5, and 30 mg P L-1 supplied as KH2PO4) in a hydroponic system. Additionally, the plants grown under 0.5 and 2 mg P L-1 were treated with plant growth-promoting Bacillus subtilis (B. subtilis), compared to untreated controls, to investigate the effectiveness of B. subtilis in addressing P deficiency. B. subtilis inoculation was performed by adding a bacterial suspension weekly to the hydroponic nutrient solution.

RESULTS

The findings illustrated Milva's ability to efficiently allocate P and sugars to its roots under low P levels, thereby enhancing biomass and facilitating increased P uptake and PUE. Conversely, Lady Claire exhibited lower P assimilation efficiency under low P levels but demonstrated improved efficiency under high P availability. The concentration of P in the nutrient solution affected P uptake and several factors believed to be involved in P utilization, such as root morphology, sugar and indole-3-acetic acid concentration in the roots, and acid phosphatase activity. Gene expression analyses underscored the pivotal roles of StPHT1;1 and StPHT2;1 in P translocation to shoots, particularly in Lady Claire. Inoculation with B. subtilis improved P acquisition efficiency by 10% under low phosphorus levels (P0.5 and P2), particularly in Lady Claire, where shoot and root phosphorus contents increased by 13-25% and 4-13%, respectively. Additionally, B. subtilis displayed higher efficacy in mitigating P deficiency in Lady Claire compared to Milva, particularly under low P levels (P0.5 and P2).

CONCLUSION

Milva showed greater phosphorus efficiency than Lady Claire under low P conditions, attributed to higher P and sugar levels in roots, enhancing root growth, P uptake, and translocation to shoots, particularly to young leaves. However, Lady Claire demonstrated a notable increase in P uptake and enhanced responsiveness to B. subtilis inoculation, particularly under low P levels (P0.5 and P2). These findings provide valuable insights for optimizing P management strategies to improve PUE in potatoes, especially under low P levels.

Wheat Tae-MIR1118 Constitutes a Functional Module With Calmodulin TaCaM2-1 and MYB Member TaMYB44 to Modulate Plant Low-N Stress Response

Distinct target genes are modulated by microRNA members and affect various biological processes associated with abiotic stress responses in plants. In this study, we characterized a functional module comprising miRNA/target and a downstream MYB transcription factor partner, Tae-MIR1118/TaCaM2/TaMYB44, in Triticum aestivum to mediate the plant low-nitrogen (N) stress response. Dual luciferase (LUC) assay and expression analysis indicated that TaCaM2 is regulated by Tae-MIR1118 through a posttranscriptional cleavage mechanism. Reporter LUC activity in N. benthamiana leaves co-transformed with effector CaMV35S::Tae-MIR1118 and reporter TaCaM2::LUC was significantly reduced, and the transcripts of Tae-MIR1118 and TaCaM2 in tissues exhibited converse expression patterns under varying N levels. Specifically, the transcripts of Tae-MIR1118 decreased, whereas those of TaCaM2 increased under low-N stress in a temporal-dependent manner. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP) assays indicated that TaCaM2 interacted with the MYB transcription factor TaMYB44. Transgene analysis revealed the negative roles of Tae-MIR1118 and the positive functions of TaCaM2 and TaMYB44 in regulating plants for low-N stress adaptation by modulating glutamine synthetase activity, N uptake capacity, and root morphology. Yeast one-hybrid, transcriptional activation, and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-PCR) assays indicated that TaMYB44 could bind to the promoters of genes TaGS2.2, TaNRT2.1, and TaPIN4 and induce transcription of these stress-defensive genes. Knockdown of these three genes reduced GS activity, N accumulation, and root growth traits in plants subjected to N starvation. The yield in the wheat variety panel was highly correlated with the transcripts of Tae-MIR1118, TaCaM2, and TaMYB44 in plants cultured under N-deprived field conditions. A major haplotype of Tae-MIR1118, TaMIR1118-Hap1, enhanced the low-N stress tolerance of plants. Our findings indicate that the Tae-MIR1118/TaCaM2/TaMYB44 pathway primarily affects the low-N response of plants by modulating associated physiological processes.

Transcription factor gene TaWRKY76 confers plants improved drought and salt tolerance through modulating stress defensive-associated processes in Triticum aestivum L

WRKY transcription factor (TF) family acts as essential regulators in plant growth and abiotic stress responses. This study reported the function of TaWRKY76, a member of WRKY TF family in Triticum aestivum L., in regulating plant osmotic stress tolerance. TaWRKY76 transcripts were significantly upregulated upon drought and salt signaling, with dose extent- and stress temporal-dependent manners. Plant GUS activity assays suggested that stress responsive cis-acting elements, such as DRE and ABRE, exert essential roles in defining gene transcription under osmotic stress conditions. The TaWRKY76 protein targeted onto nucleus and possessed ability interacting with TaMYC2, a MYC TF member of wheat. TaWRKY76 and TaMYC2 positively regulated plant drought and salt adaptation by modulating osmotic stress-related physiological indices, including osmolyte contents, stomata movement, root morphology, and reactive oxygen species (ROS) homeostasis. Yeast one-hybrid assay indicated the binding ability of TaWRKY76 with promoters of TaDREB1;1, TaNCEB3, and TaCOR15;4. ChIP-PCR analysis confirmed that the osmotic stress genes are transcriptionally regulated by TaWRKY76. Moreover, the transgenic lines with knockdown of these stress-response genes displayed lowered plant biomass together with worsened root growth traits, decreased proline contents, and elevated ROS amounts. These results suggested that these stress defensive genes contributed to TaWRKY76-modulated osmotic stress tolerance. Highly positive correlations were observed between yield and the transcripts of TaWRKY76 in a wheat variety panel under field drought condition. A major haplotype TaWRKY76 Hap1 conferred improved drought tolerance. Our results suggested that TaWRKY76 is essential in plant drought and salt adaptation and a valuable target for molecular breeding stress-tolerant cultivars in Triticum aestivum L..

TaCDPK1-5A positively regulates drought response through modulating osmotic stress responsive-associated processes in wheat (Triticum aestivum)

KEY MESSAGE

Wheat TaCDPK1-5A plays critical roles in mediating drought tolerance through regulating osmotic stress-associated physiological processes. Calcium (Ca2+) acts as an essential second messenger in plant signaling pathways and impacts plant abiotic stress responses. This study reported the function of TaCDPK1-5A, a calcium-dependent protein kinase (CDPK) gene in T. aestivum, in mediating drought tolerance. TaCDPK1-5A sensitively responded to drought and exogenous abscisic acid (ABA) signaling, displaying induced transcripts in plants under drought and ABA treatments. Yeast two-hybrid and co-immunoprecipitation assays revealed that TaCDPK1-5A interacts with the mitogen-activated protein kinase TaMAPK4-7D whereas the latter with ABF transcription factor TaABF1-3A, suggesting that TaCDPK1-5A constitutes a signaling module with above partners to transduce signals initiated by drought/ABA stressors. Overexpression of TaCDPK1-5A, TaMAPK4-7D and TaABF1-3A enhanced plant drought adaptation by modulating the osmotic stress-related physiological indices, including increased osmolyte contents, enlarged root morphology, and promoted stomata closure. Yeast one-hybrid assays indicated the binding ability of TaABF1-3A with promoters of TaP5CS1-1B, TaPIN3-5A, and TaSLAC1-3-2A, the genes encoding P5CS enzyme, PIN-FORMED protein, and slow anion channel, respectively. ChIP-PCR and transcriptional activation assays confirmed that TaABF1-3A regulates these genes at transcriptional level. Moreover, transgene analysis indicated that these stress-responsive genes positively regulated proline biosynthesis (TaP5CS1-1B), root morphology (TaPIN3-5A), and stomata closing (TaSLAC1-3-2A) upon drought signaling. Positive correlations were observed between yield and the transcripts of TaCDPK1-5A signaling partners in wheat cultivars under drought condition, with haplotype TaCDPK1-5A-Hap1 contributing to improved drought tolerance. Our study concluded that TaCDPK1-5A positively regulates drought adaptation and is a valuable target for molecular breeding the drought-tolerant cultivars in T. aestivum.

Transcription factor TaNF-YB2 interacts with partners TaNF-YA7/YC7 and transcriptionally activates distinct stress-defensive genes to modulate drought tolerance in T. Aestivum

BACKGROUND

Drought stress limits significantly the crop productivity. However, plants have evolved various strategies to cope with the drought conditions by adopting complex molecular, biochemical, and physiological mechanisms. Members of the nuclear factor Y (NF-Y) transcription factor (TF) family constitute one of the largest TF classes and are involved in plant responses to abiotic stresses.

RESULTS

TaNF-YB2, a NY-YB subfamily gene in T. aestivum, was characterized in this study focusing on its role in mediating plant adaptation to drought stress. Yeast two-hybrid (Y-2 H), biomolecular fluoresence complementation (BiFC), and Co-immunoprecipitation (Co-IP) assays indicated that TaNF-YB2 interacts with the NF-YA member TaNF-YA7 and NF-YC family member TaNF-YC7, which constitutes a heterotrimer TaNF-YB2/TaNF-YA7/TaNF-YC7. The TaNF-YB2 transcripts are induced in roots and aerial tissues upon drought signaling; GUS histochemical staining analysis demonstrated the roles of cis-regulatory elements ABRE and MYB situated in TaNF-YB2 promoter to contribute to target gene response to drought. Transgene analysis on TaNF-YB2 confirmed its functions in regulating drought adaptation via modulating stomata movement, osmolyte biosynthesis, and reactive oxygen species (ROS) homeostasis. TaNF-YB2 possessed the abilities in transcriptionally activating TaP5CS2, the P5CS family gene involving proline biosynthesis and TaSOD1, TaCAT5, and TaPOD5, the genes encoding antioxidant enzymes. Positive correlations were found between yield and the TaNF-YB2 transcripts in a core panel constituting 45 wheat cultivars under drought condition, in which two types of major haplotypes including TaNF-YB2-Hap1 and -Hap2 were included, with the former conferring more TaNF-YB2 transcripts and stronger plant drought tolerance.

CONCLUSIONS

TaNF-YB2 is transcriptional response to drought stress. It is an essential regulator in mediating plant drought adaptation by modulating the physiological processes associated with stomatal movement, osmolyte biosynthesis, and reactive oxygen species (ROS) homeostasis, depending on its role in transcriptionally regulating stress response genes. Our research deepens the understanding of plant drought stress underlying NF-Y TF family and provides gene resource in efforts for molecular breeding the drought-tolerant cultivars in T. aestivum.

A genome-wide association study reveals novel loci and candidate genes associated with plant height variation in Medicago sativa

BACKGROUND

Plant height (PH) is an important agronomic trait influenced by a complex genetic network. However, the genetic basis for the variation in PH in Medicago sativa remains largely unknown. In this study, a comprehensive genome-wide association analysis was performed to identify genomic regions associated with PH using a diverse panel of 220 accessions of M. sativa worldwide.

RESULTS

Our study identified eight novel single nucleotide polymorphisms (SNPs) significantly associated with PH evaluated in five environments, explaining 8.59-12.27% of the phenotypic variance. Among these SNPs, the favorable genotype of chr6__31716285 had a low frequency of 16.4%. Msa0882400, located proximal to this SNP, was annotated as phosphate transporter 3;1, and its role in regulating alfalfa PH was supported by transcriptome and candidate gene association analysis. In addition, 21 candidate genes were annotated within the associated regions that are involved in various biological processes related to plant growth and development.

CONCLUSIONS

Our findings provide new molecular markers for marker-assisted selection in M. sativa breeding programs. Furthermore, this study enhances our understanding of the underlying genetic and molecular mechanisms governing PH variations in M. sativa.

Comparative transcriptomic and physiological analyses unravel wheat source root adaptation to phosphorous deficiency

Phosphorus (P) is a crucial macronutrient for plant growth and development. Basic metabolic processes regulate growth; however, the molecular detail of these pathways under low phosphorous (LP) in wheat is still unclear. This study aims to elucidate the varied regulatory pathways responses to LP stress in wheat genotypes. Phenotypic, physiological, and transcriptome analyses were conducted on Fielder (P efficient) and Ardito (P inefficient) wheat genotypes after four days of normal phosphorous (NP) and LP stress. In response to LP, Fielder outperformed Ardito, displaying higher chlorophyll content-SPAD values (13%), plant height (45%), stem diameter (12%), shoot dry weight (42%), and root biomass (75%). Root structure analysis revealed that Fielder had greater total root length (50%), surface area (56%), volume (15%), and diameter (4%) than Ardito under LP. These findings highlight Fielder's superior performance and adaptation to LP stress. Transcriptome analysis of wheat genotype roots identified 3029 differentially expressed genes (DEGs) in Fielder and 1430 in Ardito, highlighting LP-induced changes. Key DEGs include acid phosphatases (PAPs), phosphate transporters (PHT1 and PHO1), SPX, and transcription factors (MYB, bHLH, and WRKY). KEGG enrichment analysis revealed key pathways like plant hormones signal transduction, biosynthesis of secondary metabolites, and carbohydrate biosynthesis metabolism. This study unveils crucial genes and the intricate regulatory process in wheat's response to LP stress, offering genetic insights for enhancing plant P utilization efficiency.

Chloroplast phosphate transporter CrPHT4-7 regulates phosphate homeostasis and photosynthesis in Chlamydomonas

In eukaryotic cells, phosphorus is assimilated and utilized primarily as phosphate (Pi). Pi homeostasis is mediated by transporters that have not yet been adequately characterized in green algae. This study reports on PHOSPHATE TRANSPORTER 4-7 (CrPHT4-7) from Chlamydomonas reinhardtii, a member of the PHT4 transporter family, which exhibits remarkable similarity to AtPHT4;4 from Arabidopsis (Arabidopsis thaliana), a chloroplastic ascorbate transporter. Using fluorescent protein tagging, we show that CrPHT4-7 resides in the chloroplast envelope membrane. Crpht4-7 mutants, generated by the CRISPR/Cas12a-mediated single-strand templated repair, show retarded growth, especially in high light, reduced ATP level, strong ascorbate accumulation, and diminished non-photochemical quenching in high light. On the other hand, total cellular phosphorous content was unaffected, and the phenotype of the Crpht4-7 mutants could not be alleviated by ample Pi supply. CrPHT4-7-overexpressing lines exhibit enhanced biomass accumulation under high light conditions in comparison with the wild-type strain. Expressing CrPHT4-7 in a yeast (Saccharomyces cerevisiae) strain lacking Pi transporters substantially recovered its slow growth phenotype, demonstrating that CrPHT4-7 transports Pi. Even though CrPHT4-7 shows a high degree of similarity to AtPHT4;4, it does not display any substantial ascorbate transport activity in yeast or intact algal cells. Thus, the results demonstrate that CrPHT4-7 functions as a chloroplastic Pi transporter essential for maintaining Pi homeostasis and photosynthesis in C. reinhardtii.

Improved chloroplast Pi allocation helps sustain electron transfer to enhance photosynthetic low-phosphorus tolerance of wheat

Phosphorus (P) deficit limits high wheat (Triticum aestivum L.) yields. Breeding low-P-tolerant cultivars is vital for sustainable agriculture and food security, but the low-P adaptation mechanisms are largely not understood. Two wheat cultivars, ND2419 (low-P-tolerant) and ZM366 (low-P-sensitive) were used in this study. They were grown under hydroponic conditions with low-P (0.015 mM) or normal-P (1 mM). Low-P suppressed biomass accumulation and net photosynthetic rate (A) in both cultivars, whereas ND2419 was relatively less suppressed. Intercellular CO2 concentration did not decrease with the decline of stomatal conductance. Additionally, maximum electron transfer rate (Jmax) decreased sooner than maximum carboxylation rate (Vcmax). Results indicate that impeded electron transfer is directly responsible for decreased A. Under low-P, ND2419 exhibited greater PSII functionality (potential activity (Fv/Fo), maximum quantum efficiency (Fv/Fm), photochemical quenching (qL) and non-photochemical quenching (NPQ) required for electron transfer than ZM366, resulting more ATP for Rubisco activation. Furthermore, ND2419 maintained higher chloroplast Pi concentrations by enhancing chloroplast Pi allocation, compared with ZM366. Overall, the low-P-tolerant cultivar sustained electron transfer under low-P by enhancing chloroplast Pi allocation, allowing more ATP synthesis for Rubisco activation, ultimately presenting stronger photosynthesis capacities. The improved chloroplasts Pi allocation may provide new insights into improve low-P tolerance.

Root epidermis-specific expression of a phosphate transporter TaPT2 enhances the growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions

Although attempts to improve the phosphate (Pi) uptake and use efficiency by constitutively overexpressing phosphate transporters have resulted in enhanced Pi or total phosphorous contents, growth promotion by Pi acquisition was observed in only a few cases. This study examined the effect of the tissue-specific overexpression of phosphate transporter on Pi acquisition and plant growth. We cloned cDNA for a wheat phosphate transporter, TaPT2, using PCR and confirmed its Pi transport activity in Arabidopsis suspension cells. The overexpression of TaPT2 by the Arabidopsis Shaker family inward rectifying potassium channel 1 (AKT1) promoter, dominantly expressed in root epidermal cells, resulted in increased root and shoot growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions. However, their Pi and total P contents did not increase. The overexpression of TaPT2 by the constitutive promoter, actin8 (ACT8), increased shoot total P contents in transgenic plants, but did not promote their growth. These results suggested that enhanced Pi uptake in root epidermal cells is suitable as a driving force for Pi transport from roots to shoots, improving subsequent Pi use in shoots. Thus, the root epidermal cell-specific expression of TaPT2 may be a simple and promising strategy for enhancing plant Pi uptake and efficiency.

Roles of plastid-located phosphate transporters in carotenoid accumulation

Enhanced carotenoid accumulation in plants is crucial for the nutritional and health demands of the human body since these beneficial substances are acquired through dietary intake. Plastids are the major organelles to accumulate carotenoids in plants and it is reported that manipulation of a single plastid phosphate transporter gene enhances carotenoid accumulation. Amongst all phosphate transport proteins including phosphate transporters (PHTs), plastidial phosphate translocators (pPTs), PHOSPHATE1 (PHO1), vacuolar phosphate efflux transporter (VPE), and Sulfate transporter [SULTR]-like phosphorus distribution transporter (SPDT) in plants, plastidic PHTs (PHT2 & PHT4) are found as the only clade that is plastid located, and manipulation of which affects carotenoid accumulation. Manipulation of a single chromoplast PHT (PHT4;2) enhances carotenoid accumulation, whereas manipulation of a single chloroplast PHT has no impact on carotenoid accumulation. The underlying mechanism is mainly attributed to their different effects on plastid orthophosphate (Pi) concentration. PHT4;2 is the only chromoplast Pi efflux transporter, and manipulating this single chromoplast PHT significantly regulates chromoplast Pi concentration. This variation subsequently modulates the carotenoid accumulation by affecting the supply of glyceraldehyde 3-phosphate, a substrate for carotenoid biosynthesis, by modulating the transcript abundances of carotenoid biosynthesis limited enzyme genes, and by regulating chromoplast biogenesis (facilitating carotenoid storage). However, at least five orthophosphate influx PHTs are identified in the chloroplast, and manipulating one of the five does not substantially modulate the chloroplast Pi concentration in a long term due to their functional redundancy. This stable chloroplast Pi concentration upon one chloroplast PHT absence, therefore, is unable to modulate Pi-involved carotenoid accumulation processes and finally does affect carotenoid accumulation in photosynthetic tissues. Despite these advances, several cases including the precise location of plastid PHTs, the phosphate transport direction mediated by these plastid PHTs, the plastid PHTs participating in carotenoid accumulation signal pathway, the potential roles of these plastid PHTs in leaf carotenoid accumulation, and the roles of these plastid PHTs in other secondary metabolites are waiting for further research. The clarification of the above-mentioned cases is beneficial for breeding high-carotenoid accumulation plants (either in photosynthetic or non-photosynthetic edible parts of plants) through the gene engineering of these transporters.

Adaptive strategies of plants to conserve internal phosphorus under P deficient condition to improve P utilization efficiency

Phosphorus (P) is one of the limiting factors for plant growth and productivity due to its slow diffusion and immobilization in the soil which necessitates application of phosphatic fertilizers to meet the crop demand and obtain maximum yields. However, plants have evolved mechanisms to adapt to low P stress conditions either by increasing acquisition (alteration of belowground processes) or by internal inorganic P (Pi) utilization (cellular Pi homeostasis) or both. In this review, we have discussed the adaptive strategies that conserve the use of P and maintain cellular Pi homeostasis in the cytoplasm. These strategies involve modification in membrane lipid composition, flavanol/anthocyanin level, scavenging and reutilization of Pi adsorbed in cell wall pectin, remobilization of Pi during senescence by enzymes like RNases and purple acid phosphatases, alternative mitochondrial electron transport, and glycolytic pathways. The remobilization of Pi from senescing tissues and its internal redistribution to various cellular organelles is mediated by various Pi transporters. Although much efforts have been made to enhance P acquisition efficiency, an understanding of the physiological mechanisms conserving internal Pi and their manipulation would be useful for plants that can utilize P more efficiently to produce optimum growth per unit P uptake.

TaMPK2B, a member of the MAPK family in T. aestivum, enhances plant low-Pi stress tolerance through modulating physiological processes associated with phosphorus starvation defensiveness

The mitogen-activated protein kinase (MAPK) cascades are present in plant species and modulate plant growth and stress responses. This study characterizes TaMPK2B, a MAPK family gene in T. aestivum that regulates plant adaptation to low-Pi stress. TaMPK2B harbors the conserved domains involving protein phosphorylation and protein-protein interaction. A yeast two-hybrid assay reveals an interaction between TaMPK2B and TaMPKK2 and between the latter and TaMPKKK;A, suggesting that all comprise a MAPK signaling cascade TaMPKKK;A-TaMPKK2-TaMPK2B. TaMPK2B expression levels were elevated in roots and leaves under a Pi starvation (PS) condition. Additionally, the induced TaMPK2B transcripts under PS in tissues were gradually restored following the Pi normal recovery condition. TaMPK2B overexpression conferred on plants improved PS adaptation; the tobacco lines with TaMPK2B overexpression enhanced the plant's dry mass production, Pi uptake capacity, root system architecture (RSA) establishment, and ROS homeostasis relative to wild type under PS treatment. Moreover, the transcripts of genes in phosphate transporter (PT), PIN-FORMED, and antioxidant enzyme (AE) families, including NtPT3 and NtPT4, NtPIN9, and NtMnSOD1 and NtPOD1;7, were elevated in Pi-deprived lines overexpressing TaMPK2B. Transgene analyses validated their functions in regulating Pi uptake, RSA establishment, and AE activities of plants treated by PS. These results suggest that TaMPK2B-mediated plant PS adaptation is correlated with the modified transcription of distinct PT, PIN, and AE genes. Our investigation suggests that TaMPK2B is one of the crucial regulators in plant low-Pi adaptation by improving Pi uptake, RSA formation, and ROS homeostasis via transcriptionally regulating genes associated with the above physiological processes.

TaNF-YA7-5B, a gene encoding nuclear factor Y (NF-Y) subunit A in Triticum aestivum, confers plant tolerance to PEG-inducing dehydration simulating drought through modulating osmotic stress-associated physiological processes

Members of nuclear factor-Y (NF-Y) transcription factors play important roles in regulating physiological processes associated with abiotic stress responses. In this study, we characterized TaNF-YA7-5B, a gene encoding wheat NY-YA subunit, in mediating plant adaptation to PEG-inducing dehydration stress. TaNF-YA7-5B shares high similarities to its homologs across various plant species. The TaNF-YA7-5B protein is specified by its conserved domains as plant NF-YA members and targets onto nucleus after endoplasmic reticulum assortment. Yeast two-hybrid assays indicated that TaNF-YA7-5B interacts with TaNF-YB2 and TaNF-YC7, two members of NF-YB and NF-YC subfamilies, suggesting a heterotrimer constituted by TaNF-YA7-5B and above NF-YB and -YC partners. TaNF-YA7-5B displayed induced expression upon drought and whose PEG-inducing dehydration-elevated transcripts were restored under normal recovery condition, suggesting its involvement in plant PEG-inducing dehydration response through modifying transcription efficiency. Overexpressing TaNF-YA7-5B conferred plant improved growth under PEG-inducing dehydration, which was ascribed largely to the gene function in regulating stomata closing and leaf water retention, osmolyte biosynthesis, and cellular ROS homeostasis. The expression of P5CS gene TaP5CS2 and antioxidant enzyme (AE) genes, namely, TaSOD3, TaCAT1, and TaPOD4, was upregulated and downregulated in lines with overexpression and knockdown of TaNF-YA7-5B, respectively; transgene analysis on them validated their functions in positively regulating proline accumulation and ROS scavenging under PEG-inducing dehydration. RNA-seq analysis revealed modified transcription of numerous genes underlying TaNF-YA7-5B enriched by GO terms 'biological process', 'cellular components', and 'molecular function'. Therefore, TaNF-YA7-5B is a crucial regulator for plant drought adaptation through comprehensively integrating diverse physiological processes associated with drought acclimation.

TaPYL4, an ABA receptor gene of wheat, positively regulates plant drought adaptation through modulating the osmotic stress-associated processes

BACKGROUND

Abscisic acid receptors (ABR) involve transduction of the ABA signaling in plants, impacting largely on stress-defensive physiological processes and plant osmotic stress response. In this study, we characterized TaPYL4, a gene of ABR family in T. aestivum, in mediating plant drought tolerance given scarcity of functional characterization on wheat ABR members thus far.

RESULTS

TaPYL4 harbors nine conserved domains shared by its PYL counterparts, targeting onto plasma membrane and nucleus after endoplasmic reticulum assortment. TaPYL4 interacts with TaPP2C2 whereas the latter with TaSnRK2.1, which establish a core module of the ABA signaling pathway. TaPYL4 expression was upregulated in root and aerial tissues upon drought stress. Overexpressing TaPYL4 conferred plants improved growth traits whereas knockdown expression of target gene alleviated growth feature compared with wild type under drought treatment. The TaPYL4-enhanced drought adaptation associates gene function in positively regulating stomata movement, osmolyte biosynthesis, and root system architecture (RSA) establishment. Expression analysis on the P5CS family genes involving proline biosynthesis indicated that TaP5CS1 exerts critical roles in promoting osmolytes accumulation in drought-challenged TaPYL4 lines. TaPIN9, a PIN-FORMED gene modulating cellular auxin translocation, was validated to function as a crucial mediator in defining RSA establishment underlying TaPYL4 regulation. Transcriptome analysis revealed that TaPYL4 controls transcription of numerous genes, which impact on physiological processes associated with 'biological process', 'molecular component', and 'cellular process'. Moreover, the differentially expressed genes mediated by TaPYL4 were closely related to stress defensive pathways.

CONCLUSIONS

Our investigation suggested that TaPYL4 acts as a positive regulator in plant drought tolerance and a valuable target for engineering drought-tolerant cultivars in T. aestivum.

Transcriptome analysis revealed accumulation-assimilation of selenium and physio-biochemical changes in alfalfa (Medicago sativa L.) leaves

BACKGROUND

Selenium (Se) is an increasing concern for investigators predominantly because of its consumption in the human body mainly from crops. As the fourth largest plant crop globally, alfalfa is one of the most important forages. Alfalfa was fertilized with selenium(IV) (Se(IV)) under field conditions to study the accumulation and assimilation of Se(IV) and to assess the impact of Se fertilization.

RESULTS

It was analyzed that the physio-biochemistry, Se species, combined with transcriptome after spraying Se(IV) at different times (0, 12, and 48 h). 9402 and 12 607 differentially expressed genes (DEGs) were identified at 12 h (versus 0 h) and 48 h (versus 12 h). DEG functional enrichments proposed two time-specific biological processes: Se(IV) accumulation was the primary process at 0-12 h, and its assimilation mainly occurred during 12-48 h. This was further proved by the separation of various Se speciation at different times. It showed that Se-supplementation also affected the soluble protein, soluble sugar, pigment contents and antioxidant capacity. Selenium-biofortification could improve the stress resistance of alfalfa by enhancing antioxidant system to scavenge reactive oxygen species (e.g. hydrogen peroxide) and boosting carbohydrate metabolism.

CONCLUSION

By integrating physio-biochemistry, Se-related metabolites, and transcriptome under Se(IV) treatment, this study provides data to guide further work on Se-fortification in alfalfa. © 2022 Society of Chemical Industry.

Analysis of miRNAs responsive to long-term calcium deficiency in tef (Eragrostis tef (Zucc.) Trotter)

MicroRNAs (miRNAs) play an important role in growth, development, stress resilience, and epigenetic modifications of plants. However, the effect of calcium (Ca²⁺) deficiency on miRNA expression in the orphan crop tef (Eragrostis tef) remains unknown. In this study, we analyzed expression of miRNAs in roots and shoots of tef in response to Ca²⁺ treatment. miRNA-seq followed by bioinformatic analysis allowed us to identify a large number of small RNAs (sRNAs) ranging from 17 to 35 nt in length. A total of 1380 miRNAs were identified in tef experiencing long-term Ca²⁺ deficiency while 1495 miRNAs were detected in control plants. Among the miRNAs identified in this study, 161 miRNAs were similar with those previously characterized in other plant species and 348 miRNAs were novel, while the remaining miRNAs were uncharacterized. Putative target genes and their functions were predicted for all the known and novel miRNAs that we identified. Based on gene ontology (GO) analysis, the predicted target genes are known to have various biological and molecular functions including calcium uptake and transport. Pairwise comparison of differentially expressed miRNAs revealed that some miRNAs were specifically enriched in roots or shoots of low Ca²⁺-treated plants. Further characterization of the miRNAs and their targets identified in this study may help in understanding Ca²⁺ deficiency responses in tef and related orphan crops.

Cross-Talk between Transcriptome Analysis and Physiological Characterization Identifies the Genes in Response to the Low Phosphorus Stress in Malus mandshurica

Phosphorus (Pi) is a macronutrient essential for plant growth, development, and reproduction. However, there is not an efficient available amount of Pi that can be absorbed by plants in the soil. Previously, an elite line, MSDZ 109, selected from Malus mandshurica, was justified for its excellent tolerance to low phosphorus (low−Pi) stress. To date, however, the genes involved in low−Pi stress tolerance have not yet been unraveled in this species. Currently, the physiological responses of this line for different days to low−Pi stress were characterized, and their roots as well as leaves were used to carry out transcriptome analysis, so as to illuminate the potential molecular pathways and identify the genes involved in low−Pi stress−response. After exposure to low−Pi treatment (32 µmol/L KH₂PO₄) for 20 day after treatment (DAF) the biomass of shoots was significantly reduced in comparison with that of the stress−free (control), and root architecture diversely changed. For example, the root growth parameters e.g., length, surface area, and total volume somewhat increase in comparison with those of the control. The activity of acid phosphatase (ACP) increased with the low−Pi treatment, whereas the photosynthetic rate and biomass were declining. The activity of antioxidant enzymes, e.g., superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), were substantially elevated in response to low−Pi treatment. Many enzyme−related candidate genes e.g., MmCAT1, MmSOD1 and MmPOD21 were up−regulated to low−Pi treatment. Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that the processes of photosynthesis, plant hormone signal transduction, and MAPK signaling pathway were affected in the low−Pi response. In combination with the physiological characterization, several low−Pi−responsive genes, e.g., PHT, PHO, were identified, and the genes implicated in Pi uptake and transport, such as MmPHT1;5, MmPHO1, MmPAP1, etc., were also obtained since their expression status varied among the exposure times, which probably notifies the candidates involved in low−Pi−responsive tolerance in this line. Interestingly, low−Pi treatment activated the expression of transcription factors including the WRKY family, MYB family, etc. The available evidences will facilitate a better understanding of the roles of this line underlying the high tolerance to low−Pi stress. Additionally, the accessible data are helpful for the use of the apple rootstock M. mandshurica under low−Pi stress.

Insights to improve the plant nutrient transport by CRISPR/Cas system

We need to improve food production to feed the ever growing world population especially in a changing climate. Nutrient deficiency in soils is one of the primary bottlenecks affecting the crop production both in developed and developing countries. Farmers are forced to apply synthetic fertilizers to improve the crop production to meet the demand. Understanding the mechanism of nutrient transport is helpful to improve the nutrient-use efficiency of crops and promote the sustainable agriculture. Many transporters involved in the acquisition, export and redistribution of nutrients in plants are characterized. In these studies, heterologous systems like yeast and Xenopus were most frequently used to study the transport function of plant nutrient transporters. CRIPSR/Cas system introduced recently has taken central stage for efficient genome editing in diverse organisms including plants. In this review, we discuss the key nutrient transporters involved in the acquisition and redistribution of nutrients from soil. We draw insights on the possible application CRISPR/Cas system for improving the nutrient transport in plants by engineering key residues of nutrient transporters, transcriptional regulation of nutrient transport signals, engineering motifs in promoters and transcription factors. CRISPR-based engineering of plant nutrient transport not only helps to study the process in native plants with conserved regulatory system but also aid to develop non-transgenic crops with better nutrient use-efficiency. This will reduce the application of synthetic fertilizers and promote the sustainable agriculture strengthening the food and nutrient security.

Phosphorus homeostasis: acquisition, sensing, and long-distance signaling in plants

Phosphorus (P), an essential nutrient required by plants often becomes the limiting factor for plant growth and development. Plants employ various mechanisms to sense the continuously changing P content in the soil. Transcription factors, such as SHORT ROOT (SHR), AUXIN RESPONSE FACTOR19 (ARF19), and ETHYLENE-INSENSITIVE3 (EIN3) regulate the growth of primary roots, root hairs, and lateral roots under low P. Crop improvement strategies under low P depend either on improving P acquisition efficiency or increasing P utilization. The various phosphate transporters (PTs) are involved in the uptake and transport of P from the soil to various plant cellular organelles. A plethora of regulatory elements including transcription factors, microRNAs and several proteins play a critical role in the regulation of coordinated cellular P homeostasis. Among these, the well-established P starvation signaling pathway comprising of central transcriptional factor phosphate starvation response (PHR), microRNA399 (miR399) as a long-distance signal molecule, and PHOSPHATE 2 (PHO2), an E2 ubiquitin conjugase is crucial in the regulation of phosphorus starvation responsive genes. Under PHR control, several classes of PHTs, microRNAs, and proteins modulate root architecture, and metabolic processes to enable plants to adapt to low P. Even though sucrose and inositol phosphates are known to influence the phosphorus starvation response genes, the exact mechanism of regulation is still unclear. In this review, a basic understanding of P homeostasis under low P in plants and all the above aspects are discussed.

Effects of Low Temperature Stress on Source-Sink Organs in Wheat and Phosphorus Mitigation Strategies

The 21st century presents many challenges to mankind, including climate change, fast growing human population, and serious concerns over food security. Wheat is a leading cereal crop that largely fulfills the global food needs. Low temperature stress accompanied by nutrient-starved soils is badly disrupting the source-sink relationship of wheat, thus causing an acute decline in final yield and deteriorating the grain quality. This review paper aimed to understand how low temperature stress affects wheat source-sink organs (i.e., leaves, roots, and spikes) and how phosphorus application reliefs in alleviating its harmful consequences. Also, we discussed mitigation strategies to enhance wheat capacity to adapt to varying temperature extremes and made rational recommendations based on modern agronomic and breeding approaches. Therefore, this study is likely to establish a solid foundation for improving the tolerance to low temperature stress and to improve its phosphorus utilization efficiency in wheat.

Intracellular phosphate sensing and regulation of phosphate transport systems in plants

Recent research on the regulation of cellular phosphate (Pi) homeostasis in eukaryotes has collectively made substantial advances in elucidating inositol pyrophosphates (PP-InsP) as Pi signaling molecules that are perceived by the SPX (Syg1, Pho81, and Xpr1) domains residing in multiple proteins involved in Pi transport and signaling. The PP-InsP-SPX signaling module is evolutionarily conserved across eukaryotes and has been elaborately adopted in plant Pi transport and signaling systems. In this review, we have integrated these advances with prior established knowledge of Pi and PP-InsP metabolism, intracellular Pi sensing, and transcriptional responses according to the dynamics of cellular Pi status in plants. Anticipated challenges and pending questions as well as prospects are also discussed.

Genome-wide identification and characterization of phosphate transporter gene family members in tea plants (Camellia sinensis L. O. kuntze) under different selenite levels

Selenium (Se) is an essential element for human health and an important nutrient for plant growth. Selenite is the main form of Se available to plants in acidic soils. Previous studies have shown that phosphate transporters (PTHs) participate in selenite uptake in plants. Research on the PHT gene family is therefore vital for production of Se-rich products. Here, 23 CsPHT genes were identified in the tea (Camellia sinensis) genome and renamed based on homology with AtPHT genes in Arabidopsis thaliana. The CsPHT genes were divided into four subfamilies: PHT1, PHT3, PHT4, and PHO, containing nine, three, six, and five genes, respectively. Phylogenetic analysis indicated that fewer duplication events occurred in tea plants than in A. thaliana, rice, apple, and poplar. Genes in the same subfamily tended to share similar gene structures, conserved motifs, and potential functions. CsPHT genes were differentially expressed in various tissues and in roots under different Se levels, suggesting key roles in selenite uptake, translocation, and homeostasis. The results illuminate the contributions of CsPHT genes to selenite supply in tea plants, and lay a foundation for follow-up studies on their potential functions in this plant species.

Knox homologs shoot meristemless (STM) and KNAT6 are epistatic to CLAVATA3 (CLV3) during shoot meristem development in Arabidopsis thaliana

BACKGROUND

In Arabidopsis, the genes SHOOT MERISTEMLESS (STM) and CLAVATA3 (CLV3) antagonistically regulate shoot meristem development. STM is essential for both development and maintenance of the meristem, as stm mutants fail to develop a shoot meristem. CLV3, on the other hand, negatively regulates meristem proliferation, and clv3 mutants possess an enlarged shoot meristem. Genetic interaction studies revealed that stm and clv3 dominantly suppress each other's phenotypes. STM works in conjunction with its closely related homologue KNOTTED1-LIKE HOMEOBOX GENE 6 (KNAT6) to promote meristem development and organ separation, as stm knat6 double mutants fail to form shoot meristem and produce a fused cotyledon.

RESULTS

In this study, we show that clv3 fails to promote shoot meristem formation in stm-1 background if we also remove KNAT6. stm-1 knat6 clv3 triple mutants result in shoot meristem termination and produce fused cotyledons similar to stm knat6 double mutant. Notably, the stm-1 knat6 and stm-1 knat6 clv3 alleles lack tissue in the presumed region of SAM that is a novel phenotype reported in Arabidopsis mutants. stm-1 knat6 clv3 also showed reduced inflorescence size as compared to clv3 single or stm clv3 double mutants.

CONCLUSION

In contrast to previously published data, these data suggest that STM and KNAT6 are redundantly required for the vegetative SAM, but insufficient for the inflorescence meristem.

The N uptake-associated physiological processes at late growth stage in wheat (Triticum aestivum) under N deprivation combined with deficit irrigation condition

Elucidating physiological mechanisms underlying the plant N uptake benefits breeding of high N use efficiency (NUE) crop cultivars. In this study, we investigated the growth and N uptake-associated processes in wheat under N deprivation and deficit irrigation, using two contrasting NUE cultivars. Compared with sufficient-N (SN), deficient-N (DN) treatment reduced plant biomass, N accumulation, and yields in two cultivars (high NUE Shinong 086 and N deprivation-sensitive Jimai 585), suggesting that N deprivation negatively regulates plant growth and N uptake. Shinong 086 was better on growth and N uptake-associated traits than Jimai 585 due to the improved root biomass across soil profile, which was consistent with the decrease of available N contents in soil layers. These results suggested that the improved root system architecture (RAS) enhances plant acquirement for soil N under N- and water-deprivation condition, contributing to the plant N uptake and yield formation capacities. Transcriptome investigation revealed that numerous genes were differentially expressed (DE) in the N-deprived Shinong 086 plants, which involve the regulation of complicate biochemical pathways. These results suggested that the modified RAS and N uptake in high NUE plants are accomplished underlying the regulation of numerous DE genes. TaWRKY20, a gene in ZFP transcription factor family, was functionally characterized for the role in mediating plant N uptake. Overexpression of it conferred plants improved growth and N uptake under DN due to its regulation on TaNRT2.1 and TaNRT2.2, two nitrate transporter genes. Our investigation provides insights in high NUE mechanisms in wheat under N deprivation.

Expressing Phosphate Transporter PvPht2;1 Enhances P Transport to the Chloroplasts and Increases Arsenic Tolerance in Arabidopsis thaliana

Arsenic (As) contamination in soils is of great concerns due to its toxicity to plants. As an analogue, phosphorus plays an important role in protecting plants from As toxicity. In this study, we identified a new phosphate transporter 2 (PHT2), PvPht2;1, from As-hyperaccumulator Pteris vittata and analyzed its functions in As and P transport in a yeast mutant, and model plant Arabidopsis thalian. PvPht2;1 contained 12 transmembrane domains, sharing high identity with PHT2 genes in diverse plants. Further, independent of external P or As levels, PvPht2;1 was mainly expressed in P. vittata fronds with the expression being 3-4 folds higher than that in the roots and rhizomes. Localized to the chloroplasts based on GFP-fused PvPht2;1 in model plant tobacco, PvPht2;1 functioned as a low-affinity P transporter. Under As exposure, PvPht2;1 yeast transformants showed comparable growth with the control while high-affinity P transporter PvPht1;3 transformants showed better growth, suggesting that PvPht2;1 transported P but slower than PvPht1;3 transporter. Expressing PvPht2;1 in A. thaliana increased its shoot P concentration without influencing its As accumulation. Further, the chloroplasts' P content in transgenic A. thaliana increased by 37-59% than wild-type (WT) plants. Under As exposure, the photosynthesis of PvPht2;1-expressing A. thaliana remained stable but that of WT plants decreased. The data indicate that, under As stress, expressing PvPht2;1 in A. thaliana enhanced its P transport to the chloroplasts and protected its photosynthesis. In short, highly expressed in the fronds and not impacted by As exposure, chloroplast-located PvPht2;1 may have protected As-hyperaccumulator P. vittata from As toxicity by efficiently transporting only P to its chloroplasts.

Potassium and phosphorus transport and signaling in plants

Nitrogen (N), potassium (K), and phosphorus (P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice (Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

Characterization on TaMPK14, an MAPK family gene of wheat, in modulating N-starvation response through regulating N uptake and ROS homeostasis

Wheat MAPK gene TaMPK14 is N starvation response and is crucial in modulating plant low-N stress tolerance. Improving plant N use efficiency (NUE) contributes largely to the sustainable crop production worldwide. In this study, TaMPK14, a mitogen-activated protein kinase (MAPK) family gene in T. aestivum, was characterized for the role in mediating N starvation response. TaMPK14 harbors conserved domain/motifs specified by the plant MAPK proteins. In vitro assay for kinase activity of TaMPK14 validated its phosphorylation nature. TaMPK14 transcripts were upregulated in both roots and leaves under low-N treatment; moreover, the expression levels induced by N starvation were gradually restored following the N recovery progression. These results suggested transcriptional response of TaMPK14 upon the low-N stress. Compared with wild type (WT), the TaMPK14 overexpressing lines in N. tabacum displayed improved growth and N accumulation traits under deficient-N treatment, which indicated the crucial roles of the MAPK gene in mediating N starvation response. Additionally, the lines treated by N starvation were shown to be improved on cellular ROS homeostasis, displaying higher antioxidant enzymes (AE) activities and less ROS accumulative amount than WT. The transcripts of nitrate transporter gene NtNRT2.1 and those of AE genes NtSOD1, NtCAT1;2, and NtPOD4 were significantly upregulated in N-deprived TaMPK14 lines; overexpression of them conferred plants enhanced N uptake capacity and AE activities, respectively. Moreover, RNA-seq datasets generated from N-deprived transgenic lines contained numerous differential genes involving modulating various biological process, cellular component, and molecular function. Together, our investigation suggested that TaMPK14 improves plant N starvation response through transcriptional regulation of distinct NRT and AE genes as well as modulation of associated biological processes.

Mutation of the chloroplast-localized phosphate transporter OsPHT2;1 reduces flavonoid accumulation and UV tolerance in rice

Phosphorus (P) is an essential macronutrient required for plant development and production. The mechanisms regulating phosphate (Pi) uptake are well established, but the function of chloroplast Pi homeostasis is poorly understood in Oryza sativa (rice). PHT2;1 is one of the transporters/translocators mediating Pi import into chloroplasts. In this study, to gain insight into the role of OsPHT2;1-mediated stroma Pi, we analyzed OsPHT2;1 function in Pi utilization and photoprotection. Our results showed that OsPHT2;1 was induced by Pi starvation and light exposure. Cell-based assays showed that OsPHT2;1 localized to the chloroplast envelope and functioned as a low-affinity Pi transporter. The ospht2;1 had reduced Pi accumulation, plant growth and photosynthetic rates. Metabolite profiling revealed that 52.6% of the decreased metabolites in ospht2;1 plants were flavonoids, which was further confirmed by 40% lower content of total flavonoids compared with the wild type. As a consequence, ospht2;1 plants were more sensitive to UV-B irradiation. Moreover, the content of phenylalanine, the precursor of flavonoids, was also reduced, and was largely associated with the repressed expression of ADT1/MTR1. Furthermore, the ospht2;1 plants showed decreased grain yields at relatively high levels of UV-B irradiance. In summary, OsPHT2;1 functions as a chloroplast-localized low-affinity Pi transporter that mediates UV tolerance and rice yields at different latitudes.

Global Survey and Expressions of the Phosphate Transporter Gene Families in Brassica napus and Their Roles in Phosphorus Response

Phosphate (Pi) transporters play critical roles in Pi acquisition and homeostasis. However, currently little is known about these genes in oil crops. In this study, we aimed to characterize the five Pi transporter gene families (PHT1-5) in allotetraploid Brassica napus. We identified and characterized 81 putative PHT genes in B. napus (BnaPHTs), including 45 genes in PHT1 family (BnaPHT1s), four BnaPHT2s, 10 BnaPHT3s, 13 BnaPHT4s and nine BnaPHT5s. Phylogenetic analyses showed that the largest PHT1 family could be divided into two groups (Group I and II), while PHT4 may be classified into five, Groups I-V. Gene structure analysis revealed that the exon-intron pattern was conservative within the same family or group. The sequence characteristics of these five families were quite different, which may contribute to their functional divergence. Transcription factor (TF) binding network analyses identified many potential TF binding sites in the promoter regions of candidates, implying their possible regulating patterns. Collinearity analysis demonstrated that most BnaPHTs were derived from an allopolyploidization event (~40.7%) between Brassica rapa and Brassica oleracea ancestors, and small-scale segmental duplication events (~39.5%) in the descendant. RNA-Seq analyses proved that many BnaPHTs were preferentially expressed in leaf and flower tissues. The expression profiles of most colinearity-pairs in B. napus are highly correlated, implying functional redundancy, while a few pairs may have undergone neo-functionalization or sub-functionalization during evolution. The expression levels of many BnaPHTs tend to be up-regulated by different hormones inductions, especially for IAA, ABA and 6-BA treatments. qRT-PCR assay demonstrated that six BnaPHT1s (BnaPHT1.11, BnaPHT1.14, BnaPHT1.20, BnaPHT1.35, BnaPHT1.41, BnaPHT1.44) were significantly up-regulated under low- and/or rich- Pi conditions in B. napus roots. This work analyzes the evolution and expression of the PHT family in Brassica napus, which will help further research on their role in Pi transport.

Intracellular phosphate homeostasis - A short way from metabolism to signaling

Phosphorus in plant cells occurs in inorganic form as both ortho- and pyrophosphate or bound to organic compounds, like e.g., nucleotides, phosphorylated metabolites, phospholipids, phosphorylated proteins, or phytate as P storage in the vacuoles of seeds. Individual compartments of the cell are surrounded by membranes that are selective barriers to avoid uncontrolled solute exchange. A controlled exchange of phosphate or phosphorylated metabolites is accomplished by specific phosphate transporters (PHTs) and the plastidial phosphate translocator family (PTs) of the inner envelope membrane. Plastids, in particular chloroplasts, are the site of various anabolic sequences of enzyme-catalyzed reactions. Apart from their role in metabolism PHTs and PTs are presumed to be also involved in communication between organelles and plant organs. Here we will focus on the integration of phosphate transport and homeostasis in signaling processes. Recent developments in this field will be critically assessed and potential future developments discussed. In particular, the occurrence of various plastid types in one organ (i.e. the leaf) with different functions with respect to metabolism or sensing, as has been documented recently following a tissue-specific proteomics approach (Beltran et al., 2018), will shed new light on functional aspects of phosphate homeostasis.

TaZFP1, a C2H2 type-ZFP gene of T. aestivum, mediates salt stress tolerance of plants by modulating diverse stress-defensive physiological processes

Salt stress suppresses plant growth, development, and crop productivity. In this study, we characterized the role of TaZFP1, a C2H2 type-zinc finger protein family member of T. aestivum, in salt stress tolerance. TaZFP1 possesses a conserved C2H2 motif (CX2-4CX12HX3-5H) shared by plant ZFP proteins, translocates to the nucleus after endoplasmic reticulum (ER) assortment, and displays a ZF 3-D structure similar to its eukaryote homologs. The transcripts of TaZFP1 were upregulated during salt stress condition and this effect was restored under normal conditions. Compared to wild type (WT), the transgenic lines of TaZFP1 overexpression or knockdown displayed improved phenotypes, biomass, photosynthesis parameters (Pn, ΨPSII, and NPQ), osmolytes contents (i.e. proline and soluble sugar), and enhanced antioxidant enzyme (AE) activity following salt stress treatment. A set of genes associated with proline synthesis (i.e., NtP5CS1 and NtP5CS2) and encoding AEs (i.e., NtSOD2, NtCAT1, and NtPOD4) were upregulated in the salt-challenged transgenic lines of TaZFP1 expression. Additionally, the transgenic lines exhibited similar stomata movement patterns and leaf water retention properties under salinity conditions compared to those induced by exogenous abscisic acid (ABA) treatment, suggesting that the TaZFP1-mediated salt response is dependent on the ABA signaling. High throughput RNAseq analysis revealed significant alteration of gene transcription in transgenic lines upon salt stress. Among them, the differentially expressed genes (DEGs) represented by the gene ontology (GO) terms were associated with organic acid, carboxylic acid, carbohydrate, and coenzyme as well as organonitrogen compounds, translation, peptide metabolism, and peptide biosynthesis. A set of upregulated DEGs were found to be thylakoid- and photosystem-associated, which is consistent with the TaZFP1-mediated improvement in photosynthesis in salt-stressed transgenic lines. Our investigation indicated that the TaZFP1-mediated salt tolerance is ascribed to the regulation of gene functions related to photosynthesis, osmolytes metabolism and ROS homeostasis mediated by ABA signaling.

TaMIR1139: a wheat miRNA responsive to Pi-starvation, acts a critical mediator in modulating plant tolerance to Pi deprivation

Wheat miRNA member TaMIR1139 targets genes functional in various families and plays crucial roles in regulating plant Pi starvation tolerance. Through regulating target genes at posttranscriptional or translational level, plant miRNAs are involved in mediating diverse biological processes associated with growth, development, and responses to adverse stresses. In this study, we characterized the expression pattern and function of TaMIR1139, a miRNA member of wheat (T. aestivum) under Pi deprivation. TaMIR1139 precursor is also present in N. tabucum, suggesting the conserved nature of miR1139 across monocots and eudicots. TaMIR1139 targets seven genes within different families. The transcripts abundance of TaMIR1139 was induced upon Pi deprivation and the upregulated expression under Pi starvation was downregulated by the Pi recovery treatment, In contrast, the genes targeted by TaMIR1139 exhibited reduced transcripts upon Pi starvation and their downregulated expression was recovered by Pi-recovery condition, suggesting the regulation of them under TaMIR1139 through a cleavage mechanism. TaMIR1139 overexpression conferred the Pi-deprived plants improved phenotype, biomass, photosynthesis, and Pi acquisition. Transcriptome analysis identified numerous genes involving biological process, cellular components, and molecular function were differentially expressed in the TaMIR1139 overexpression lines, which suggests the TaMIR1139-mediated plant Pi starvation tolerance to be associated with the role of miRNA in extensively modulating the transcript profiling. A phosphate transporter (PT) gene NtPT showed significantly upregulated expression in TaMIR1139 overexpression lines; overexpression of it conferred plants improved Pi acquisition upon Pi starvation, suggesting its contribution to the TaMIR1139-mediated plant low-Pi stress resistance. Our investigation indicates that TaMIR1139 is critical in plant Pi starvation tolerance through transcriptionally regulating the target genes and modulating the Pi stress-defensiveness processes.

TaNBP1, a guanine nucleotide-binding subunit gene of wheat, is essential in the regulation of N starvation adaptation via modulating N acquisition and ROS homeostasis

BACKGROUND

Nitrate (NO3-) is the major source of nitrogen (N) for higher plants aside from its function in transducing the N signaling. Improving N use efficiency of crops has been an effective strategy for promotion of the sustainable agriculture worldwide. The regulatory pathways associating with N uptake and the corresponding biochemical processes impact largely on plant N starvation tolerance. Thus, exploration of the molecular mechanism underlying nitrogen use efficiency (NUE) and the gene wealth will pave a way for molecular breeding of N starvation-tolerant crop cultivars.

RESULTS

In the current study, we characterized the function of TaNBP1, a guanine nucleotide-binding protein subunit beta gene of wheat (T. aestivum), in mediating the plant N starvation response. TaNBP1 protein harbors a conserved W40 domain and the TaNBP1-GFP (green fluorescence protein) signals concentrate at positions of cytoplasm membrane and cytosol. TaNBP1 transcripts are induced in roots and leaves upon N starvation stress and that this upregulated expression is recovered by N recovery treatment. TaNBP1 overexpression confers improved phenotype, enlarged root system architecture (RSA), and increased biomass for plants upon N deprivation relative to the wild type, associating with its role in enhancing N accumulation and improving reactive oxygen species (ROS) homeostasis. Nitrate transporter (NRT) gene NtNRT2.2 and antioxidant enzyme genes NtSOD1, NtSOD2, and NtCAT1 are transcriptionally regulated under TaNBP1 and contribute to the improved N acquisition and the increased AE activities of plants.

CONCLUSIONS

Altogether, TaNBP1 is transcriptional response to N starvation stress. Overexpression of this gene enhances plant N starvation adaptation via improvement of N uptake and cellular ROS homeostasis by modifying transcription of NRT gene NtNRT2.2 and antioxidant enzyme genes NtSOD1, NtSOD2, and NtCAT1, respectively. Our research helps to understand the mechanism underlying plant N starvation response and benefits to genetically engineer crop cultivars with improved NUE under the N-saving cultivation conditions.

Identification and expression profiling of Pht1 phosphate transporters in wheat in controlled environments and in the field

Phosphorus (P) is an important macronutrient with critical functions in plants. Phosphate (Pi) transporters, which mediate Pi acquisition and Pi translocation within the plant, are key factors in Pi deficiency responses. However, their relevance for adaptation to long-term Pi limitation under agronomic conditions, particularly in wheat, remains unknown. Here, we describe the identification of the complete Pi transporter gene family (Pht1) in wheat (Triticum aestivum). Gene expression profiles were compared for hydroponic and field-grown plant tissues of wheat at multiple development stages. Cis-element analysis of selected Pht1 promoter regions was performed. A broad range of expression patterns of individual TaPht1 genes was observed in relation to tissue specificity and the nutrient supply in the soil or in liquid culture, as well as an influence of the experimental system. The expression patterns indicate the involvement of specific transporters in Pi uptake, and in Pi transport and remobilisation within the plant, at different growth developmental stages. Specifically, the influence of Pi nutrition indicates a complex regulatory pattern of TaPht1 gene transcript abundances as a response to low Pi availability in different culture systems, correlating with the existence of different cis-acting promoter elements.

Wheat miRNA TaemiR408 Acts as an Essential Mediator in Plant Tolerance to Pi Deprivation and Salt Stress via Modulating Stress-Associated Physiological Processes

MicroRNAs (miRNA) families act as critical regulators for plant growth, development, and responses to abiotic stresses. In this study, we characterized TaemiR408, a miRNA family member of wheat (Triticum aestivum), for the role in mediating plant responses to Pi starvation and salt stress. TaemiR408 targets six genes that encode proteins involving biochemical metabolism, microtubule organization, and signaling transduction. 5'- and 3'-RACE analyses confirmed the mRNA cleavage of target genes mediated by this wheat miRNA. TaemiR408 showed induced expression patterns upon Pi starvation and salt stress and whose upregulated expression was gradually repressed by the normal recovery treatments. The target genes of TaemiR408 exhibited reverse expression patterns to this miRNA, whose transcripts were downregulated under Pi starvation and salt stress and the reduced expression was recovered by the followed normal condition. These results suggest the regulation of the target genes under TaemiR408 through a cleavage mechanism. Tobacco lines with TaemiR408 overexpression exhibited enhanced stress tolerance, showing improved phenotype, biomass, and photosynthesis behavior compared with wild type under both Pi starvation and salt treatments, which closely associate increased P accumulation upon Pi deprivation and elevated osmolytes under salt stress, respectively. Phosphate transporter (PT) gene NtPT2 displays upregulated transcripts in the Pi-deprived TaemiR408 overexpressors; knockdown of this PT gene reduces Pi acquisition under low-Pi stress, confirming its role in improving plant Pi taken up. Likewise, NtPYL2 and NtSAPK3, genes encoding abscisic acid (ABA) receptor and SnRK2 protein, respectively, exhibited upregulated transcripts in salt-challenged TaemiR408 overexpressors; knockdown of them caused deteriorated growth and lowered osmolytes amounts of plants upon salt treatment. Thus, TaemiR408 is crucial for plant adaptations to Pi starvation and salt stress through regulating Pi acquisition under low-Pi stress and remodel ABA signaling pathway and osmoprotects biosynthesis under salt stress.

Intracellular transport and compartmentation of phosphate in plants

Phosphate (Pi) is an essential macronutrient with structural and metabolic roles within every compartment of the plant cell. Intracellular Pi transporters direct Pi to each organelle and also control its exchange between subcellular compartments thereby providing the means to coordinate compartmented metabolic processes, including glycolysis, photosynthesis, and respiration. In this review we summarize recent advances in the identification and functional analysis of Pi transporters that localize to vacuoles, chloroplasts, non-photosynthetic plastids, mitochondria, and the Golgi apparatus. Electrical potentials across intracellular membranes and the pH of subcellular environments will also be highlighted as key factors influencing the energetics of Pi transport, and therefore pose limits for Pi compartmentation.

Identification of major quantitative trait loci for root diameter in synthetic hexaploid wheat under phosphorus-deficient conditions

Synthetic hexaploid wheat (SHW) possesses numerous genes for resistance to stress, including phosphorus (P) deficiency. Root diameter (RDM) plays an important role in P-deficiency tolerance, but information related to SHW is still limited. Thus, the objective of this study was to investigate the genetic architecture of RDM in SHW under P-deficient conditions. To this end, we measured the RDM of 138 F₉ recombinant inbred lines derived from an F₂ population of a synthetic hexaploid wheat line (SHW-L1) and a common wheat line (Chuanmai32) under two P conditions, P sufficiency (PS) and P deficiency (PD), and mapped quantitative trait loci (QTL) for RDM using an enriched high-density genetic map, containing 120,370 single nucleotide polymorphisms, 733 diversity arrays technology markers, and 119 simple sequence repeats. We identified seven RDM QTL for P-deficiency tolerance that individually explained 11-14.7% of the phenotypic variation. Five putative candidate genes involved in root composition, energy supply, and defense response were predicted. Overall, our results provided essential information for cloning genes related to P-deficiency tolerance in common wheat that might help in breeding P-deficiency-tolerant wheat cultivars.

Roles, Regulation, and Agricultural Application of Plant Phosphate Transporters

Phosphorus (P) is an essential mineral nutrient for plant growth and development. Low availability of inorganic phosphate (orthophosphate; Pi) in soil seriously restricts the crop production, while excessive fertilization has caused environmental pollution. Pi acquisition and homeostasis depend on transport processes controlled Pi transporters, which are grouped into five families so far: PHT1, PHT2, PHT3, PHT4, and PHT5. This review summarizes the current understanding on plant PHT families, including phylogenetic analysis, function, and regulation. The potential application of Pi transporters and the related regulatory factors for developing genetically modified crops with high phosphorus use efficiency (PUE) are also discussed in this review. At last, we provide some potential strategies for developing high PUE crops under salt or drought stress conditions, which can be valuable for improving crop yields challenged by global scarcity of water resources and increasing soil salinization.

Phosphate, phytate and phytases in plants: from fundamental knowledge gained in Arabidopsis to potential biotechnological applications in wheat

Phosphorus (P) is an essential macronutrient for all living organisms. In plants, P is taken up from the rhizosphere by the roots mainly as inorganic phosphate (Pi), which is required in large and sufficient quantities to maximize crop yields. In today's agricultural society, crop yield is mostly ensured by the excessive use of Pi fertilizers, a costly practice neither eco-friendly or sustainable. Therefore, generating plants with improved P use efficiency (PUE) is of major interest. Among the various strategies employed to date, attempts to engineer genetically modified crops with improved capacity to utilize phytate (PA), the largest soil P form and unfortunately not taken up by plants, remains a key challenge. To meet these challenges, we need a better understanding of the mechanisms regulating Pi sensing, signaling, transport and storage in plants. In this review, we summarize the current knowledge on these aspects, which are mainly gained from investigations conducted in Arabidopsis thaliana, and we extended it to those available on an economically important crop, wheat. Strategies to enhance the PA use, through the use of bacterial or fungal phytases and other attempts of reducing seed PA levels, are also discussed. We critically review these data in terms of their potential for use as a technology for genetic manipulation of PUE in wheat, which would be both economically and environmentally beneficial.

Tissue specific transcript profiling of wheat phosphate transporter genes and its association with phosphate allocation in grains

Approaches enabling efficient phosphorus utilization in crops are of great importance. In cereal crop like wheat, utilization of inorganic phosphate (Pi) is high and mature grains are the major sink for Pi utilization and storage. Research that addresses the importance of the Pi homeostasis in developing grains is limited. In an attempt to understand the Pi homeostasis in developing wheat grains, we identified twelve new phosphate transporters (PHT), these are phyologentically well distributed along with the members reported from Arabidopsis and rice. Enhanced expression of PHT1-subfamily genes was observed in roots subjected to the Pi starvation suggesting their active role in Pi homeostasis. Differential expression patterns of all the PHT genes during grain filling stages suggested their importance in the filial tissues. Additionally, high accumulation of Pi and total P in aleurone correlates well with the expression of TaPHTs and other phosphate starvation related genes. Tissue specific transcript accumulation of TaPHT1.1, TaPHT1.2, TaPHT1.4 in aleurone; TaPHT3.1 in embryo and TaPHT4.2 in the endosperm was observed. Furthermore, their transcript abundance was affected in low phytate wheat grains. Altogether, this study helps in expanding the knowledge and prioritize the candidate wheat Pi-transporters to modulate the Pi homeostasis in cereal grains.

Wheat bHLH-type transcription factor gene TabHLH1 is crucial in mediating osmotic stresses tolerance through modulating largely the ABA-associated pathway

KEY MESSAGE

Wheat bHLH family gene TabHLH1 is responsive to drought and salt stresses, and it acts as one crucial regulator in mediating tolerance to aforementioned stresses largely through an ABA-associated pathway. Osmotic stresses are adverse factors for plant growth and crop productivity. In this study, we characterized TabHLH1, a gene encoding wheat bHLH-type transcription factor (TF) protein, in mediating plant adaptation to osmotic stresses. TabHLH1 protein contains a conserved basic-helix-loop-helix (bHLH) domain shared by its plant counterparts. Upon PEG-simulated drought stress, salt stress, and exogenous abscisic acid (ABA), the TabHLH1 transcripts in roots and leaves were induced. Under PEG-simulated drought stress and salt stress treatments, the tobacco seedlings with TabHLH1 overexpression exhibited improved growth and osmotic stress-associated traits, showing increased biomass and reduced leaf water loss rate (WLR) relative to wild type (WT). The transgenic lines also possessed promoted stomata closure under drought stress, salt stress, and exogenous ABA and increased proline and soluble sugar contents and reduced hydrogen peroxide (H2O2) amount under osmotic stress conditions, indicating that TabHLH1-mediated osmolyte accumulation and cellular ROS homeostasis contributed to the drought stress and salt stress tolerance. NtPYL12 and NtSAPK2;1, the genes encoding ABA receptor and SnRK2 family kinase, respectively, showed up-regulated expression in lines overexpressing TabHLH1 under osmotic stress and exogenous ABA conditions; overexpression of them conferred plants modified stomata movement, leaf WLR, and growth feature under drought and high salinity, suggesting that these ABA-signaling genes are mediated by wheat TabHLH1 gene and involved in regulating plant responses to simulated drought and salt stresses. Our investigation indicates that the TabHLH1 gene plays critical roles in plant tolerance to osmotic stresses largely through an ABA-dependent pathway.

TaZAT8, a C2H2-ZFP type transcription factor gene in wheat, plays critical roles in mediating tolerance to Pi deprivation through regulating P acquisition, ROS homeostasis and root system establishment

Transcription factors (TFs) play critical roles in mediating defense of plants to abiotic stresses through regulating downstream defensive genes. In this study, a wheat C2H2-ZFP (zinc finger protein) type TF gene designated as TaZAT8 was functionally characterized in mediating tolerance to the inorganic phosphate (Pi)-starvation stress. TaZAT8 bears conserved motifs harboring in the C2H2-ZFP type counterparts across vascular plant species. The expression of TaZAT8 was shown to be induced in roots upon Pi deprivation, with a Pi concentration- and temporal-dependent manner. Overexpression of TaZAT8 in tobacco conferred plants improved tolerance to Pi deprivation; the transgenic lines exhibited enlarged phenotype and elevated biomass and phosphorus (P) accumulation relative to wild-type (WT) after Pi-starvation treatment. NtPT1 and NtPT2, the tobacco phosphate transporter (PT) genes, showed increased transcripts in the Pi-deprived transgenic lines, indicative of their transcriptional regulation by TaZAT8. Overexpression analysis of these PT genes validated their function in mediating Pi acquisition under the Pi deprivation conditions. Additionally, the TaZAT8-overexpressing lines also behaved enhanced antioxidant enzyme (AE) activities and enlarged root system architecture (RSA) with respect to WT. Evaluation of the transcript abundance of tobacco genes encoding AE and PIN proteins, including NtMnSOD1, NtSOD1, NtPOD1;2, NtPOD1;5, NtPOD1;6, and NtPOD1;9, and NtPIN1 and NtPIN4 are upregulated in the TaZAT8-overexpressing lines. Overexpression of NtPIN1 and NtPIN4 conferred plants to enlarged RSA and elevated biomass under the Pi-starvation stress conditions. Our investigation provides insights into plant adaptation to the Pi-starvation stress mediated by distinct ZFP TFs through modulation of Pi acquisition and cellular ROS detoxicity.

Phosphate Uptake and Allocation - A Closer Look at Arabidopsis thaliana L. and Oryza sativa L

This year marks the 20th anniversary of the discovery and characterization of the two Arabidopsis PHT1 genes encoding the phosphate transporter in Arabidopsis thaliana. So far, multiple inorganic phosphate (Pi) transporters have been described, and the molecular basis of Pi acquisition by plants has been well-characterized. These genes are involved in Pi acquisition, allocation, and/or signal transduction. This review summarizes how Pi is taken up by the roots and further distributed within two plants: A. thaliana and Oryza sativa L. by plasma membrane phosphate transporters PHT1 and PHO1 as well as by intracellular transporters: PHO1, PHT2, PHT3, PHT4, PHT5 (VPT1), SPX-MFS and phosphate translocators family. We also describe the role of the PHT1 transporters in mycorrhizal roots of rice as an adaptive strategy to cope with limited phosphate availability in soil.

Acclimation of the crucifer Eutrema salsugineum to phosphate limitation is associated with constitutively high expression of phosphate-starvation genes

Eutrema salsugineum, a halophytic relative of Arabidopsis thaliana, was subjected to varying phosphate (Pi) treatments. Arabidopsis seedlings grown on 0.05 mm Pi displayed shortened primary roots, higher lateral root density and reduced shoot biomass allocation relative to those on 0.5 mm Pi, whereas Eutrema seedlings showed no difference in lateral root density and shoot biomass allocation. While a low Fe concentration mitigated the Pi deficiency response for Arabidopsis, Eutrema root architecture was unaltered, but adding NaCl increased Eutrema lateral root density almost 2-fold. Eutrema and Arabidopsis plants grown on soil without added Pi for 4 weeks had low shoot and root Pi content. Pi-deprived, soil-grown Arabidopsis plants were stunted with senescing older leaves, whereas Eutrema plants were visually indistinguishable from 2.5 mm Pi-supplemented plants. Genes associated with Pi starvation were analysed by RT-qPCR. EsIPS2, EsPHT1;4 and EsPAP17 showed up-regulated expression in Pi-deprived Eutrema plants, while EsPHR1, EsWRKY75 and EsRNS1 showed no induction. Absolute quantification of transcripts indicated that PHR1, WRKY75 and RNS1 were expressed at higher levels in Eutrema plants relative to those in Arabidopsis regardless of external Pi. The low phenotypic plasticity Eutrema displays to Pi supply is consistent with adaptation to chronic Pi deprivation in its extreme natural habitat.

TabHLH1, a bHLH-type transcription factor gene in wheat, improves plant tolerance to Pi and N deprivation via regulation of nutrient transporter gene transcription and ROS homeostasis

Basic helix-loop-helix (bHLH) transcription factors (TFs) comprise a large TF family and act as crucial regulators in various biological processes in plants. Here, we report the functional characterization of TabHLH1, a bHLH TF member in wheat (Triticum aestivum). TabHLH1 shares conserved bHLH domain and targets to nucleus with transactivation activity. Upon Pi and N deprivation, the expression of TabHLH1 was up-regulated in roots and leaves, showing a pattern to be gradually increased within 23-h treatment regimes. The lines with overexpression of TabHLH1 exhibited drastically improved tolerance to Pi and N deprivation, showing larger plant phenotype, more biomass, higher concentration and more accumulation of P and N than wild type (WT) upon the Pi- and N-starvation stresses. NtPT1 and NtNRT2.2, the genes encoding phosphate transporter (PT) and nitrate transporter (NRT) in tobacco, respectively, showed up-regulated expression in TabHLH1-overexpressing plants; knockdown expression of them led to deteriorated growth feature, lowered biomass, and decreased nutrient accumulation of plants under Pi- and N-deficient conditions. Compared with WT, the TabHLH1-overexpressing plants also showed lowered reactive oxygen species (ROS) accumulation and improved antioxidant enzyme (AE) activities, such as those of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). NtSOD1, NtCAT1, and NtPOD1;6 that encode SOD, CAT, and POD, respectively, were up-regulated in TabHLH1-overexpressing plants. Further knockdown of these AE gene expression caused reduced antioxidant enzymatic activities, indicative of their crucial roles in mediating cellular ROS homeostasis in Pi- and N-starvation conditions. Together, TabHLH1 plays an important role in mediating adaptation to the Pi- and N-starvation stresses through transcriptional regulation of a set of genes encoding PT, NRT and AEs that mediate the taken up of Pi and N and the cellular homeostasis of ROS initiated by the nutrient stresses.

Molecular Mechanisms of Phosphorus Metabolism and Transport during Leaf Senescence

Leaf senescence, being the final developmental stage of the leaf, signifies the transition from a mature, photosynthetically active organ to the attenuation of said function and eventual death of the leaf. During senescence, essential nutrients sequestered in the leaf, such as phosphorus (P), are mobilized and transported to sink tissues, particularly expanding leaves and developing seeds. Phosphorus recycling is crucial, as it helps to ensure that previously acquired P is not lost to the environment, particularly under the naturally occurring condition where most unfertilized soils contain low levels of soluble orthophosphate (Pi), the only form of P that roots can directly assimilate from the soil. Piecing together the molecular mechanisms that underpin the highly variable efficiencies of P remobilization from senescing leaves by different plant species may be critical for devising effective strategies for improving overall crop P-use efficiency. Maximizing Pi remobilization from senescing leaves using selective breeding and/or biotechnological strategies will help to generate P-efficient crops that would minimize the use of unsustainable and polluting Pi-containing fertilizers in agriculture. This review focuses on the molecular mechanisms whereby P is remobilized from senescing leaves and transported to sink tissues, which encompasses the action of hormones, transcription factors, Pi-scavenging enzymes, and Pi transporters.

Wheat mitogen-activated protein kinase gene TaMPK4 improves plant tolerance to multiple stresses through modifying root growth, ROS metabolism, and nutrient acquisitions

KEY MESSAGE

Wheat MAPK member TaMPK4 responds to abiotic stresses of Pi and N deprivations and high salinity and is crucial in regulating plant tolerance to aforementioned stresses. Mitogen-activated protein kinase (MAPK) cascades are important signal transduction modules in regulating plant responses to various environmental stresses. In this study, a wheat MAPK member referred to TaMPK4 was characterized for its roles in mediating plant tolerance to diverse stresses. TaMPK4 shares conserved domains generally identified in plant MAPKs and possesses in vitro kinase activity. Under stresses of Pi and N deprivations and high salinity, TaMPK4 was strongly upregulated and its expressions were restored upon recovery treatments from above stresses. Sense- and antisense-expressing TaMPK4 in tobacco significantly modified plant growth under the stress conditions and dramatically modified the root architecture through transcriptional regulation of the auxin transport-associated genes NtPIN3 and NtPIN9, whose downregulated expressions dramatically reduced the root growth. Compared with wild type (WT), the antioxidant enzymatic activities under the stress conditions, P accumulation under P deprivation, and N amount under N deficiency were altered dramatically in the transgenic plants, showing higher in the TaMPK4-overexpressing and lower in the TaMPK4-knockout plants, which were in concordance with the modified expressions of a set of antioxidant enzyme genes (NtPOD2;1, NtPOD9, NtSOD2, NtFeSOD, and NtCAT), two phosphate transporter genes (NtPT and NtPT2), and two nitrate transporter genes (NtNRT1.1-s and NtNRT1.1-t), respectively. Downregulated expression of above genes in tobacco largely reduced the plant growth, and Pi and N acquisitions under the stress conditions. TaMPK4 also involved regulations of plant K(+) and osmolyte contents under high salinity. Thus, TaMPK4 is functional in regulating plant tolerance to diverse stresses through modifying various biological processes.

Genome-wide association study of phosphorus-deficiency-tolerance traits in Aegilops tauschii

Using GWAS, 13 significant SNPs distributed on six of the seven Aegilops tauschii chromosomes (all but 5D) were identified, and several candidate P-deficiency-responsive genes were proposed from searches of public databases. Aegilops tauschii, the wheat (Triticum aestivum) D-genome progenitor, possesses numerous genes for stress resistance, including genes for tolerance of phosphorus (P) deficiency. Investigation of the genetic architecture of A. tauschii will help in developing P-deficiency-tolerant varieties of wheat. We evaluated nine traits in a population of 380 A. tauschii specimens under conditions with and without P application, and we performed genome-wide association studies for these traits using single nucleotide polymorphism (SNP) chips containing 7185 markers. Using a general linear model, we identified 119 SNPs that were significantly associated with all nine traits, and a mixed linear model revealed 18 SNPs associated with all traits. Both models detected 13 significant markers distributed on six of the seven A. tauschii chromosomes (all but 5D). Searches of public databases revealed several candidate/flanking genes related to P-deficiency tolerance. These genes were grouped in five categories by the types of proteins they encoded: defense response proteins, enzymes, promoters and transcription factors, storage proteins, or proteins triggered by P deficiency. The identified SNPs and genes contain essential information for cloning genes related to P-deficiency tolerance in A. tauschii and wheat, and they provide a foundation for breeding P-deficiency tolerant wheat cultivars.

Comparative proteomics exploring the molecular mechanism of eutrophic water purification using water hyacinth (Eichhornia crassipes)

Eutrophication is a serious threat to ecosystem stability and use of water resources worldwide. Accordingly, physical, chemical, and biological technologies have been developed to treat eutrophic water. Phytoremediation has attracted a great deal of attention, and water hyacinth (Eichhornia crassipes) is regarded as one of the best plants for purification of eutrophic water. Previous studies have shown that water hyacinths remove nitrogen (N) and phosphorus (P) via diverse processes and that they can inhibit the growth of algae. However, the molecular mechanisms responsible for these processes, especially the role of proteins, are unknown. In this study, we applied a proteomics approach to investigate the protein dynamics of water hyacinth under three eutrophication levels. The results suggested that proteins with various functions, including response to stress, N and P metabolic pathways, synthesis and secretion, photosynthesis, biosynthesis, and energy metabolism, were involved in regulating water hyacinth to endure the excess-nutrient environment, remove N and P, and inhibit algal growth. The results help us understand the mechanism of purification of eutrophic water by water hyacinth and supply a theoretical basis for improving techniques for phytoremediation of polluted water.