Characterization of the phosphate starvation-induced glycerol-3-phosphate permease gene family in Arabidopsis

Peribacillus frigoritolerans T7-IITJ, a potential biofertilizer, induces plant growth-promoting genes of Arabidopsis thaliana

AIMS

This study aimed to isolate plant growth and drought tolerance-promoting bacteria from the nutrient-poor rhizosphere soil of Thar desert plants and unravel their molecular mechanisms of plant growth promotion.

METHODS AND RESULTS

Among our rhizobacterial isolates, Enterobacter cloacae C1P-IITJ, Kalamiella piersonii J4-IITJ, and Peribacillus frigoritolerans T7-IITJ, significantly enhanced root and shoot growth (4-5-fold) in Arabidopsis thaliana under PEG-induced drought stress. Whole genome sequencing and biochemical analyses of the non-pathogenic bacterium T7-IITJ revealed its plant growth-promoting traits, viz., solubilization of phosphate (40-73 µg/ml), iron (24 ± 0.58 mm halo on chrome azurol S media), and nitrate (1.58 ± 0.01 µg/ml nitrite), along with production of exopolysaccharides (125 ± 20 µg/ml) and auxin-like compounds (42.6 ± 0.05 µg/ml). Transcriptome analysis of A. thaliana inoculated with T7-IITJ and exposure to drought revealed the induction of 445 plant genes (log2fold-change > 1, FDR < 0.05) for photosynthesis, auxin and jasmonate signalling, nutrient uptake, redox homeostasis, and secondary metabolite biosynthesis pathways related to beneficial bacteria-plant interaction, but repression of 503 genes (log2fold-change < -1) including many stress-responsive genes. T7-IITJ enhanced proline 2.5-fold, chlorophyll 2.5-2.8-fold, iron 2-fold, phosphate 1.6-fold, and nitrogen 4-fold, and reduced reactive oxygen species 2-4.7-fold in plant tissues under drought. T7-IITJ also improved the germination and seedling growth of Tephrosia purpurea, Triticum aestivum, and Setaria italica under drought and inhibited the growth of two plant pathogenic fungi, Fusarium oxysporum, and Rhizoctonia solani.

CONCLUSIONS

P. frigoritolerans T7-IITJ is a potent biofertilizer that regulates plant genes to promote growth and drought tolerance.

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is "Pi recycling" in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.

Plant growth-promoting properties of the phosphate-solubilizing red yeast Rhodosporidium paludigenum

Phosphorus (P) is one of the essential elements that are necessary for plant development and growth. However, the availability of soluble forms of P for plants in the soils is limited, because a large proportion of it is bound to soil constituents. Thus, the concentration of P available to plants at any time is very low and, moreover, its availability depends on the soil pH. As a solution, phosphate-solubilizing microorganisms (PSMs) are employed that render inorganic P available to plants in soluble form. Thus far, research into PSMs has been insufficient, and only few such organisms have been considered for exploitation as microbial fertilizer strains. The characteristics of plant growth promotion with the plant-PSMs coculture system remain to be elucidated. In the current study, we report on the isolate Rhodosporidium paludigenum JYC100 that exhibits good performance for solubilizing calcium phosphate. We found that it can be regulated by the amount of soluble phosphate. Furthermore, R. paludigenum JYC100 promotes plant growth under specific conditions (P deficiency, but with insoluble phosphate) in different media and soil pots. In contrast, the yeast Aureobasidium pullulans JYC104 exhibited weak phosphate-solubilizing capacities and no plant growth-promoting ability. Compared to control plants, the biomass, shoot height, and cellular inorganic P content of plants increased in plants cocultivated with R. paludigenum JYC100. In addition, histochemical GUS and qRT-PCR assays of phosphate starvation-induced (PSI) genes showed that the transcript levels of these PSI genes are decreased in the plants cocultured with R. paludigenum JYC100. These findings reflect the unique ability of R. paludigenum JYC100 to convert insoluble P compounds to plant-available P, thereby leading to growth promotion. Our study results highlight the use of yeasts as potential substitutes for inorganic phosphate fertilizers to meet the P demands of plants, which may eventually improve yields in sustainable agricultures.

ATL8, a RING E3 ligase, modulates root growth and phosphate homeostasis in Arabidopsis

Ubiquitination-mediated post-translational modification of proteins is a pivotal regulatory mechanism involved in the growth and development of the plant. The Arabidopsis Tóxicos en Levadura (ATL) family is a group of RING-type ubiquitin ligases (E3) and ATL8 is a membrane-localized protein. Here, a reverse genetics approach was used to elucidate the role of ATL8 in phosphate (Pi) homeostasis. Deficiencies of Pi and sucrose (Suc) enhanced the relative expression level of ATL8 in different tissues of the wild-type (Wt). The relative expression level of ATL8 was attenuated and augmented in the mutant (atl8) and overexpression lines (Oe1 and Oe2), respectively. There were significant reductions in different root traits, root hairs, root to shoot ratio, and total Pi content in atl8 compared with the Wt under different Pi regimes. On the contrary, Oe1 and Oe2 lines exhibited enhancement in some of these traits. Noticeably, anthocyanin content was significantly reduced in Oe1 and Oe2 compared with the Wt and atl8 under P- condition. Abscisic acid (ABA) treatment led to an increase in the primary root length of atl8 compared with the Wt, suggesting a cross-talk between ABA and ATL8 on root growth. Furthermore, the relative expression levels of the genes involved in the maintenance of Pi homeostasis (WRKY75, RNS1, E3L, and ACP5) were differentially modulated in atl8, Oe1, and Oe2 compared with the Wt under different Pi regimes. The results revealed the pivotal role of ATL8 in mediating morphophysiological and molecular adaptive responses to Pi deficiency.

Phosphate starvation-inducible GLYCEROPHOSPHODIESTER PHOSPHODIESTERASE6 is involved in Arabidopsis root growth

Plants that are starved of phosphate trigger membrane lipid remodeling, which hydrolyses phospholipids and presumably allows their phosphate to be utilized, whilst replacing them with galactolipids to maintain the integrity of the membrane system. In addition to the two concurrent pathways of phospholipid hydrolysis by phospholipases C and D that have already been established, an emerging third pathway has been proposed that includes a reaction step catalysed by glycerophosphodiester phosphodiesterases (GDPDs). However, its functional involvement in phosphate-starved plants remains elusive. Here, we show that Arabidopsis GDPD6 is a functional isoform responsible for glycerophosphocholine hydrolysis in vivo. Overexpression of GDPD6 promoted root growth whilst gdpd6 mutants showed impaired root growth under phosphate starvation, and this defect was rescued by supplementing with the reaction product glycerol 3-phosphate but not with choline. Since GDPD6 is induced by phosphate starvation, we suggest that GDPD6 might be involved in root growth via the production of glycerol 3-phosphate in phosphate-starved plants.

Potential Networks of Nitrogen-Phosphorus-Potassium Channels and Transporters in Arabidopsis Roots at a Single Cell Resolution

Nitrogen (N), phosphorus (P), and potassium (K) are three major macronutrients essential for plant life. These nutrients are acquired and transported by several large families of transporters expressed in plant roots. However, it remains largely unknown how these transporters are distributed in different cell-types that work together to transfer the nutrients from the soil to different layers of root cells and eventually reach vasculature for massive flow. Using the single cell transcriptomics data from Arabidopsis roots, we profiled the transcriptional patterns of putative nutrient transporters in different root cell-types. Such analyses identified a number of uncharacterized NPK transporters expressed in the root epidermis to mediate NPK uptake and distribution to the adjacent cells. Some transport genes showed cortex- and endodermis-specific expression to direct the nutrient flow toward the vasculature. For long-distance transport, a variety of transporters were shown to express and potentially function in the xylem and phloem. In the context of subcellular distribution of mineral nutrients, the NPK transporters at subcellular compartments were often found to show ubiquitous expression patterns, which suggests function in house-keeping processes. Overall, these single cell transcriptomic analyses provide working models of nutrient transport from the epidermis across the cortex to the vasculature, which can be further tested experimentally in the future.

Transcriptome and GWAS analyses reveal candidate gene for seminal root length of maize seedlings under drought stress

Water deficits are a major constraint on maize growth and yield, and deep roots are one of the major mechanisms of drought tolerance. In this study, four root and shoot traits were evaluated within an association panel consisting of 209 diverse maize accessions under well-watered (WW) and water-stressed (WS) conditions. A significant positive correlation was observed between seminal root length (SRL) under WS treatment and the drought tolerance index (DI) of maize seedlings. The transcriptome profiles of maize seminal roots were compared between four drought-tolerant lines and four drought-sensitive lines under both water conditions to identify genes associated with the drought stress response. After drought stress, 343 and 177 common differentially expressed genes (DEGs) were identified in the drought-tolerant group and drought-sensitive group, respectively. In parallel, a coexpression network underlying SRL was constructed on the basis of transcriptome data, and 10 hub genes involved in two significant associated modules were identified. Additionally, a genome-wide association study (GWAS) of the SRL revealed 62 loci for the two water treatments. By integrating the results of the GWAS, the common DEGs and the coexpression network analysis, 7 promising candidate genes were prioritized for further research. Together, our results provide a foundation for the enhanced understanding of seminal root changes in response to drought stress in maize.

Early Responses of Brassica oleracea Roots to Zinc Supply Under Sufficient and Sub-Optimal Phosphorus Supply

Shoot zinc (Zn) concentration in Brassica oleracea is affected by soil Zn and phosphorus (P) supply. Most problematic is the negative impact of P fertilizers on Zn concentrations in crops, which makes balancing yield and mineral quality challenging. To evaluate early molecular mechanisms involved in the accumulation of large shoot Zn concentrations regardless of the P supply, two B. oleracea accessions differing in root architecture and root exudates were grown hydroponically for two weeks with different combinations of P and Zn supply. Ionome profiling and deep RNA sequencing of roots revealed interactions of P and Zn in planta, without apparent phenotypic effects. In addition, increasing P supply did not reduce tissue Zn concentration. Substantial changes in gene expression in response to different P and/or Zn supplies in roots of both accessions ensured nutritionally sufficient P and Zn uptake. Numerous genes were differentially expressed after changing Zn or P supply and most of them were unique to only one accession, highlighting their different strategies in achieving nutrient sufficiency. Thus, different gene networks responded to the changing P and Zn supply in the two accessions. Additionally, enrichment analysis of gene ontology classes revealed that genes involved in lipid metabolism, response to starvation, and anion transport mechanisms were most responsive to differences in P and Zn supply in both accessions. The results agreed with previously studies demonstrating alterations in P and Zn transport and phospholipid metabolism in response to reduced P and Zn supply. It is anticipated that improved knowledge of genes responsive to P or Zn supply will help illuminate the roles in uptake and accumulation of P and Zn and might identify candidate genes for breeding high-yield-high-Zn brassicas.

Functional identification of apple MdMYB2 gene in phosphate-starvation response

Inorganic phosphate (Pi) starvation severely affects the normal growth and development of plants. Here, a Pi-responsive gene, named MdMYB2 (MDP0000823458), was cloned and functionally identified in apple. Overexpression of MdMYB2 regulated the expression of Pi starvation-induced (PSI) genes and then promoted phosphate assimilation and utilization. The ectopic expression of MdMYB2 in Arabidopsis influenced plant growth and flowering, which was partially rescued by application of exogenous gibberellin (GA). These results indicated that MdMYB2 may be an essential regulator in phosphate utilization and GA-regulated plant growth and development.

More Transporters, More Substrates: The Arabidopsis Major Facilitator Superfamily Revisited

The Major Facilitator Superfamily (MFS) is ubiquitous in living organisms and represents the largest group of secondary active membrane transporters. In plants, significant research efforts have focused on the role of specific families within the MFS, particularly those transporting macronutrients (C, N, and P) that constitute the vast majority of the members of this superfamily. Other MFS families remain less explored, although a plethora of additional substrates and physiological functions have been uncovered. Nevertheless, the lack of a systematic approach to analyzing the MFS as a whole has obscured the high diversity and versatility of these transporters. Here, we present a phylogenetic analysis of all annotated MFS domain-containing proteins encoded in the Arabidopsis thaliana genome and propose that this superfamily of transporters consists of 218 members, clustered in 22 families. In reviewing the available information regarding the diversity in biological functions and substrates of Arabidopsis MFS members, we provide arguments for intensified research on these membrane transporters to unveil the breadth of their physiological relevance, disclose the molecular mechanisms underlying their mode of action, and explore their biotechnological potential.

Identification of vacuolar phosphate efflux transporters in land plants

Inorganic phosphate (Pi) is an essential component of all life forms. Land plants acquire Pi from the soil through roots and associated symbioses, and it is then transported throughout the plant. When sufficient, excess Pi is stored in vacuoles for remobilization following Pi deficiency. Although Pi release from the vacuoles to the cytoplasm serves as a critical mechanism for plants to adapt to low-Pi stress, the transporters responsible for vacuolar Pi efflux have not been identified. Here, we identified a pair of Oryza sativa vacuolar Pi efflux transporters (OsVPE1 and OsVPE2) that were more abundant in plants grown under Pi-deficient conditions. These OsVPE proteins can transport Pi into yeast cells and Xenopus laevis oocytes. Vacuolar Pi content was higher in the loss-of-function Osvpe1 Osvpe2 double mutant than in wild type, particularly under low-Pi stress. Overexpression of either OsVPE1 or OsVPE2 in transgenic plants reduced vacuolar Pi content, consistent with a role in vacuolar Pi efflux. We demonstrate that these VPE proteins evolved from an ancient plasma membrane glycerol-3-phosphate transporter protein. Together, these data indicate that this transporter was recruited to the vacuolar membrane to catalyse Pi efflux during the course of land plant evolution.

Early Transcriptomic Response to Phosphate Deprivation in Soybean Leaves as Revealed by RNA-Sequencing

Low phosphate (Pi) availability is an important limiting factor affecting soybean production. However, the underlying molecular mechanisms responsible for low Pi stress response and tolerance remain largely unknown, especially for the early signaling events under low Pi stress. Here, a genome-wide transcriptomic analysis in soybean leaves treated with a short-term Pi-deprivation (24 h) was performed through high-throughput RNA sequencing (RNA-seq) technology. A total of 533 loci were found to be differentially expressed in response to Pi deprivation, including 36 mis-annotated loci and 32 novel loci. Among the differentially expressed genes (DEGs), 303 were induced and 230 were repressed by Pi deprivation. To validate the reliability of the RNA-seq data, 18 DEGs were randomly selected and analyzed by quantitative RT-PCR (reverse transcription polymerase chain reaction), which exhibited similar fold changes with RNA-seq. Enrichment analyses showed that 29 GO (Gene Ontology) terms and 8 KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways were significantly enriched in the up-regulated DEGs and 25 GO terms and 16 KEGG pathways were significantly enriched in the down-regulated DEGs. Some DEGs potentially involved in Pi sensing and signaling were up-regulated by short-term Pi deprivation, including five SPX-containing genes. Some DEGs possibly associated with water and nutrient uptake, hormonal and calcium signaling, protein phosphorylation and dephosphorylation and cell wall modification were affected at the early stage of Pi deprivation. The cis-elements of PHO (phosphatase) element, PHO-like element and P responsive element were present more frequently in promoter regions of up-regulated DEGs compared to that of randomly-selected genes in the soybean genome. Our transcriptomic data showed an intricate network containing transporters, transcription factors, kinases and phosphatases, hormone and calcium signaling components is involved in plant responses to early Pi deprivation.

iTRAQ-based analysis of the Arabidopsis proteome reveals insights into the potential mechanisms of anthocyanin accumulation regulation in response to phosphate deficiency

Phosphate (Pi) deficiency significantly limits plant growth in natural and agricultural systems. Accumulation of anthocyanins in shoots is a common response of Arabidopsis thaliana to Pi deficiency. To elucidate the mechanisms underlying Pi deficiency-induced anthocyanin accumulation, we employed a proteomic approach based on isobaric tags for relative and absolute quantification (iTRAQ) to investigate protein expression profiles of Arabidopsis thaliana seedlings subjected to Pi deficiency for 7 days. In total, 5,106 proteins were identified, of which 156 displayed significant changes in abundance upon Pi deficiency. Bioinformatics analysis indicated that flavonoid biosynthesis was the most significantly elevated metabolic process under Pi deficiency. We further examined the potential role of the flavonoid biosynthetic pathway using a dihydroflavonol 4-reductase (DFR) mutant (tt3) and quantitative RT-PCR (qRT-PCR) analysis, and found that the tt3 mutant was deprived of transcriptional up-regulation of three genes related to anthocyanin biosynthesis, modification and transport under Pi deficiency. These results showed that Pi deficiency probably enhances the anthocyanin accumulation by promoting the flavonoid biosynthesis. The exact functions of these proteins remain to be examined. Nevertheless, our study increases the understanding of the mechanisms implicated in the anthocyanin accumulation induced by Pi deficiency and adaptive responses of plants to Pi starvation.

SIZ1-mediated SUMOylation during phosphate homeostasis in plants: Looking beyond the tip of the iceberg

Availability of phosphate (Pi) is often limited in rhizospheres in different agroclimatic zones and adversely affects growth and development of plants. To circumvent this impasse, there is an urgent need and global consensus to develop Pi use efficient crops. To achieve this goal, it is essential to identify the molecular entities that exert regulatory influences on the sensing and signaling cascade governing Pi homeostasis. SIZ1 encodes a small ubiquitin-like modifier (SUMO E3) ligase, and plays a pivotal role in the post-translational SUMOylation of proteins. In this review, we discuss the reverse genetics approach conventionally used for providing circumstantial evidence towards the regulatory influences of SIZ1 on several morphophysiological and molecular traits that govern Pi homeostasis in taxonomically diverse Arabidopsis thaliana (Arabidopsis) and Oryza sativa (rice) model species. However, the efforts have been rather modest in identifying SUMO protein targets that play key roles in the maintenance of Pi homeostasis in these model plants contrary to the plethora of them now known in lower organisms and animals. Therefore, to predict the SIZ1-mediated SUMOylome involved in Pi homeostasis, the state-of-the-art high-throughput technologies often used for animals thus provide an attractive paradigm towards achieving the long-term goal of developing Pi use efficient crops.

Two Glycerol-3-Phosphate Dehydrogenases from Chlamydomonas Have Distinct Roles in Lipid Metabolism

The metabolism of glycerol-3-phosphate (G3P) is important for environmental stress responses by eukaryotic microalgae. G3P is an essential precursor for glycerolipid synthesis and the accumulation of triacylglycerol (TAG) in response to nutrient starvation. G3P dehydrogenase (GPDH) mediates G3P synthesis, but the roles of specific GPDH isoforms are currently poorly understood. Of the five GPDH enzymes in the model alga Chlamydomonas reinhardtii, GPD2 and GPD3 were shown to be induced by nutrient starvation and/or salt stress. Heterologous expression of GPD2, a putative chloroplastic GPDH, and GPD3, a putative cytosolic GPDH, in a yeast gpd1Δ mutant demonstrated the functionality of both enzymes. C. reinhardtii knockdown mutants for GPD2 and GPD3 showed no difference in growth but displayed significant reduction in TAG concentration compared with the wild type in response to phosphorus or nitrogen starvation. Overexpression of GPD2 and GPD3 in C. reinhardtii gave distinct phenotypes. GPD2 overexpression lines showed only subtle metabolic phenotypes and no significant alteration in growth. In contrast, GPD3 overexpression lines displayed significantly inhibited growth and chlorophyll concentration, reduced glycerol concentration, and changes to lipid composition compared with the wild type, including increased abundance of phosphatidic acids but reduced abundance of diglycerides, triglycerides, and phosphatidylglycerol lipids. This may indicate a block in the downstream glycerolipid metabolism pathway in GPD3 overexpression lines. Thus, lipid engineering by GPDH modification may depend on the activities of other downstream enzyme steps. These results also suggest that GPD2 and GPD3 GPDH isoforms are important for nutrient starvation-induced TAG accumulation but have distinct metabolic functions.

Complex Regulation of Plant Phosphate Transporters and the Gap between Molecular Mechanisms and Practical Application: What Is Missing?

It has been almost 25 years since the first report of the gene encoding a high-affinity phosphate transporter (PT), PHO84, in yeast. Since then, an increasing number of yeast PHO84 homologs as well as other genes encoding proteins with phosphate (Pi) transport activities have been identified and functionally characterized in diverse plant species. Great progress has been made also in deciphering the molecular mechanism underlying the regulation of the abundance and/or activity of these genes and their products. The regulatory genes affect plant Pi homeostasis commonly through direct or indirect regulation of the abundance of PTs at different levels. However, little has been achieved in the use of PTs for developing genetically modified crops with high phosphorus use efficiency (PUE). This might be a consequence of overemphasizing Pi uptake from the rhizosphere and lack of knowledge about the roles of PTs in Pi transport and recycling within the plant that are required to optimize PUE. Here, we mainly focused on the genes encoding proteins with Pi transport activities and the emerging understanding of their regulation at the transcriptional, post-transcriptional, translational, and post-translational levels. In addition, we propose potential strategies for effective use of PTs in improving plant growth and development.

A new insight into root responses to external cues: Paradigm shift in nutrient sensing

Higher plants are sessile and their growth relies on nutrients present in the soil. The acquisition of nutrients is challenging for plants. Phosphate and nitrate sensing and signaling cascades play significant role during adverse conditions of nutrient unavailability. Therefore, it is important to dissect the mechanism by which plant roots acquire nutrients from the soil. Root system architecture (RSA) exhibits extensive developmental flexibility and changes during nutrient stress conditions. Growth of root system in response to external concentration of nutrients is a joint operation of sensor or receptor proteins along with several other cytoplasmic accessory proteins. After nutrient sensing, sensor proteins start the cellular relay involving transcription factors, kinases, ubiquitin ligases and miRNA. The complexity of nutrient sensing is still nebulous and many new players need to be better studied. This review presents a survey of recent paradigm shift in the advancements in nutrient sensing in relation to plant roots.

Ethylene Response Factor070 regulates root development and phosphate starvation-mediated responses

Inorganic phosphate (Pi) availability is a major factor determining growth and consequently the productivity of crops. However, it is one of the least available macronutrients due to its high fixation in the rhizospheres. To overcome this constraint, plants have developed adaptive responses to better acquire, utilize, and recycle Pi. Molecular determinants of these adaptive mechanisms include transcription factors (TFs) that play a major role in transcriptional control, thereby regulating genome-scale networks. In this study, we have characterized the biological role of Arabidopsis thaliana Ethylene Response Factor070 (AtERF070), a Pi starvation-induced TF belonging to the Apetala2/Ethylene Response Factor family of TFs in Arabidopsis (Arabidopsis thaliana). It is localized to the nucleus and induced specifically in Pi-deprived roots and shoots. RNA interference-mediated suppression of AtERF070 led to augmented lateral root development resulting in higher Pi accumulation, whereas there were reductions in both primary root length and lateral root number in 12-d-old transgenic seedlings overexpressing AtERF070. When the overexpressing lines were grown to maturity under greenhouse conditions, they revealed a stunted bushy appearance that could be rescued by gibberellic acid application. Furthermore, a number of Pi starvation-responsive genes were modulated in AtERF070-overexpressing and RNA interference lines, thereby suggesting a potential role for this TF in maintaining Pi homeostasis.

Glycerol affects root development through regulation of multiple pathways in Arabidopsis

Glycerol metabolism has been well studied biochemically. However, the means by which glycerol functions in plant development is not well understood. This study aimed to investigate the mechanism underlying the effects of glycerol on root development in Arabidopsis thaliana. Exogenous glycerol inhibited primary root growth and altered lateral root development in wild-type plants. These phenotypes appeared concurrently with increased endogenous glycerol-3-phosphate (G3P) and H₂O₂ contents in seedlings, and decreased phosphate levels in roots. Upon glycerol treatment, G3P level and root development did not change in glycerol kinase mutant gli1, but G3P level increased in gpdhc1 and fad-gpdh mutants, which resulted in more severely impaired root development. Overexpression of the FAD-GPDH gene attenuated the alterations in G3P, phosphate and H₂O₂ levels, leading to increased tolerance to exogenous glycerol, which suggested that FAD-GPDH plays an important role in modulating this response. Free indole-3-acetic acid (IAA) content increased by 46%, and DR5pro::GUS staining increased in the stele cells of the root meristem under glycerol treatment, suggesting that glycerol likely alters normal auxin distribution. Decreases in PIN1 and PIN7 expression, β-glucuronidase (GUS) staining in plants expressing PIN7pro::GUS and green fluorescent protein (GFP) fluorescence in plants expressing PIN7pro::PIN7-GFP were observed, indicating that polar auxin transport in the root was downregulated under glycerol treatment. Analyses with auxin-related mutants showed that TIR1 and ARF7 were involved in regulating root growth under glycerol treatment. Glycerol-treated plants showed significant reductions in root meristem size and cell number as revealed by CYCB1;1pro::GUS staining. Furthermore, the expression of CDKA and CYCB1 decreased significantly in treated plants compared with control plants, implying possible alterations in cell cycle progression. Our data demonstrated that glycerol treatment altered endogenous levels of G3P, phosphate and ROS, affected auxin distribution and cell division in the root meristem, and eventually resulted in modifications of root development.

Plant phosphoinositide-dependent phospholipases C: variations around a canonical theme

Phosphoinositide-specific phospholipase C (PI-PLC) cleaves, in a Ca(2+)-dependent manner, phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) into diacylglycerol (DAG) and inositol triphosphate (IP3). PI-PLCs are multidomain proteins that are structurally related to the PI-PLCζs, the simplest animal PI-PLCs. Like these animal counterparts, they are only composed of EF-hand, X/Y and C2 domains. However, plant PI-PLCs do not have a conventional EF-hand domain since they are often truncated, while some PI-PLCs have no EF-hand domain at all. Despite this simple structure, plant PI-PLCs are involved in many essential plant processes, either associated with development or in response to environmental stresses. The action of PI-PLCs relies on the mediators they produce. In plants, IP3 does not seem to be the sole active soluble molecule. Inositol pentakisphosphate (IP5) and inositol hexakisphosphate (IP6) also transmit signals, thus highlighting the importance of coupling PI-PLC action with inositol-phosphate kinases and phosphatases. PI-PLCs also produce a lipid molecule, but plant PI-PLC pathways show a peculiarity in that the active lipid does not appear to be DAG but its phosphorylated form, phosphatidic acid (PA). Besides, PI-PLCs can also act by altering their substrate levels. Taken together, plant PI-PLCs show functional differences when compared to their animal counterparts. However, they act on similar general signalling pathways including calcium homeostasis and cell phosphoproteome. Several important questions remain unanswered. The cross-talk between the soluble and lipid mediators generated by plant PI-PLCs is not understood and how the coupling between PI-PLCs and inositol-kinases or DAG-kinases is carried out remains to be established.

The role of strigolactones in nutrient-stress responses in plants

Strigolactones (SLs) are a new group of plant hormones, which have been intensively investigated during the last few years. The wide spectrum of SLs actions, including the regulation of shoot/root architecture, and the stimulation of the interactions between roots and fungi or bacteria, as well as the stimulation of germination of parasitic plants, indicates that this group of hormones may play an important role in the mechanisms that control soil exploration, and the root-mediated uptake of nutrients. Current studies have shown that SLs might be factors that have an influence on the plant response to a deficiency of macronutrients. Experimental data from the last four years have confirmed that the biosynthesis and exudation of SLs are increased under phosphorus and nitrogen deficiency. All these data suggest that SLs may regulate the complex response to nutrient stress, which include not only the modification of the plant developmental process, but also the cooperation with other organisms in order to minimize the effects of threats. In this paper the results of studies that indicate that SLs play an important role in the response to nutrient stress are reviewed and the consequences of the higher biosynthesis and exudation of SLs in response to phosphorus and nitrogen deficiency are discussed.

Understanding plant responses to phosphorus starvation for improvement of plant tolerance to phosphorus deficiency by biotechnological approaches

In both prokaryotes and eukaryotes, including plants, phosphorus (P) is an essential nutrient that is involved in various biochemical processes, such as lipid metabolism and the biosynthesis of nucleic acids and cell membranes. P also contributes to cellular signaling cascades by function as mediators of signal transduction and it also serves as a vital energy source for a wide range of biological functions. Due to its intensive use in agriculture, P resources have become limited. Therefore, it is critically important in the future to develop scientific strategies that aim to increase P use efficiency and P recycling. In addition, the biologically available soluble form of P for uptake (phosphate; Pi) is readily washed out of topsoil layers, resulting in serious environmental pollution. In addition to this environmental concern, the wash out of Pi from topsoil necessitates a continuous Pi supply to maintain adequate levels of fertilization, making the situation worse. As a coping mechanism to P stress, plants are known to undergo drastic cellular changes in metabolism, physiology, hormonal balance and gene expression. Understanding these molecular, physiological and biochemical responses developed by plants will play a vital role in improving agronomic practices, resource conservation and environmental protection as well as serving as a foundation for the development of biotechnological strategies, which aim to improve P use efficiency in crops. In this review, we will discuss a variety of plant responses to low P conditions and various molecular mechanisms that regulate these responses. In addition, we also discuss the implication of this knowledge for the development of plant biotechnological applications.

Regulation of miR399f transcription by AtMYB2 affects phosphate starvation responses in Arabidopsis

Although a role for microRNA399 (miR399) in plant responses to phosphate (Pi) starvation has been indicated, the regulatory mechanism underlying miR399 gene expression is not clear. Here, we report that AtMYB2 functions as a direct transcriptional activator for miR399 in Arabidopsis (Arabidopsis thaliana) Pi starvation signaling. Compared with untransformed control plants, transgenic plants constitutively overexpressing AtMYB2 showed increased miR399f expression and tissue Pi contents under high Pi growth and exhibited elevated expression of a subset of Pi starvation-induced genes. Pi starvation-induced root architectural changes were more exaggerated in AtMYB2-overexpressing transgenic plants compared with the wild type. AtMYB2 directly binds to a MYB-binding site in the miR399f promoter in vitro, as well as in vivo, and stimulates miR399f promoter activity in Arabidopsis protoplasts. Transcription of AtMYB2 itself is induced in response to Pi deficiency, and the tissue expression patterns of miR399f and AtMYB2 are similar. Both genes are expressed mainly in vascular tissues of cotyledons and in roots. Our results suggest that AtMYB2 regulates plant responses to Pi starvation by regulating the expression of the miR399 gene.

Role of metabolite transporters in source-sink carbon allocation

Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g., roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types developed according to their respective function, and cells of either tissue type are highly compartmentalized. Photoassimilates are allocated to distinct compartments of these tissues in all organs, requiring a set of metabolite transporters mediating this intercompartmental transfer. The general route of photoassimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triose phosphates either enter the cytosol for mainly sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil in oil bodies, or - in combination with nitrogen - as protein in protein storage vacuoles and protein bodies. Here, we focus on transport proteins known for either of these steps, and discuss the implications for yield increase in plants upon genetic engineering of respective transporters.

Enhanced organic phosphorus assimilation promoting biomass and shoot P hyperaccumulations in Lolium multiflorum grown under sterile conditions

Search for plant species - prodigious in P use - is important for both P-sufficient and -deficient conditions. Gulf and Marshall ryegrass (Lolium multiflorum), grown in sterile media containing different organic P substrates (AMP, ATP, GMP, and IHP), exhibited high rates of growth and shoot P concentrations. Growth increase in Gulf was significantly greater on IHP relative to other sources of organic P substrates. Growth was also dependent on an increasing concentration of IHP (0-20 mM) in this cultivar. P accumulations in Gulf exceeded 1% shoot dry weight from IHP, AMP, and ATP-equivalent to the P accrual from equimolar Pi source. Plants supplied with IHP had phytase activity in root extracts comparable to that in Pi-fed plants or control (no P). The extracellular phytase, however, increased by about 100% relative to that observed in root extracts- for both ryegrass cultivars, but there were no significant differences (P < 0.05) between plant groups grown on different substrates (IHP, Pi or control). No significant differences in phosphomonoesterase activities were evident between plant groups supplied with organic P (IHP, G1P) and inorganic source or control. This study establishes the high P-use efficiency in ryegrass, irrespective of P source.