GmPHR25, a GmPHR member up-regulated by phosphate starvation, controls phosphate homeostasis in soybean

Characterization of Purple Acid Phosphatase Family and Functional Analysis of GmPAP7a/7b Involved in Extracellular ATP Utilization in Soybean

Low phosphate (Pi) availability limits crop growth and yield in acid soils. Although root-associated acid phosphatases (APases) play an important role in extracellular organic phosphorus (P) utilization, they remain poorly studied in soybean (Glycine max), an important legume crop. In this study, dynamic changes in intracellular (leaf and root) and root-associated APase activities were investigated under both Pi-sufficient and Pi-deficient conditions. Moreover, genome-wide identification of members of the purple acid phosphatase (PAP) family and their expression patterns in response to Pi starvation were analyzed in soybean. The functions of both GmPAP7a and GmPAP7b, whose expression is up regulated by Pi starvation, were subsequently characterized. Phosphate starvation resulted in significant increases in intracellular APase activities in the leaves after 4 days, and in root intracellular and associated APase activities after 1 day, but constant increases were observed only for root intracellular and associated APase activities during day 5-16 of P deficiency in soybean. Moreover, a total of 38 GmPAP members were identified in the soybean genome. The transcripts of 19 GmPAP members in the leaves and 17 in the roots were upregulated at 16 days of P deficiency despite the lack of a response for any GmPAP members to Pi starvation at 2 days. Pi starvation upregulated GmPAP7a and GmPAP7b, and they were subsequently selected for further analysis. Both GmPAP7a and GmPAP7b exhibited relatively high activities against adenosine triphosphate (ATP) in vitro. Furthermore, overexpressing GmPAP7a and GmPAP7b in soybean hairy roots significantly increased root-associated APase activities and thus facilitated extracellular ATP utilization. Taken together, these results suggest that GmPAP7a and GmPAP7b might contribute to root-associated APase activities, thus having a function in extracellular ATP utilization in soybean.

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

Since 1999, various forward- and reverse-genetic approaches have uncovered nearly 200 genes required for symbiotic nitrogen fixation (SNF) in legumes. These discoveries advanced our understanding of the evolution of SNF in plants and its relationship to other beneficial endosymbioses, signaling between plants and microbes, the control of microbial infection of plant cells, the control of plant cell division leading to nodule development, autoregulation of nodulation, intracellular accommodation of bacteria, nodule oxygen homeostasis, the control of bacteroid differentiation, metabolism and transport supporting symbiosis, and the control of nodule senescence. This review catalogs and contextualizes all of the plant genes currently known to be required for SNF in two model legume species, Medicago truncatula and Lotus japonicus, and two crop species, Glycine max (soybean) and Phaseolus vulgaris (common bean). We also briefly consider the future of SNF genetics in the era of pan-genomics and genome editing.

Integration of metabolome and transcriptome analyses highlights soybean roots responding to phosphorus deficiency by modulating phosphorylated metabolite processes

Phosphorus (P) is a major constituent of biomolecules in plant cells, and is an essential plant macronutrient. Low phosphate (Pi) availability in soils is a major constraint on plant growth. Although a complex variety of plant responses to Pi starvation has been well documented, few studies have integrated both global transcriptome and metabolome analyses to shed light on molecular mechanisms underlying metabolic responses to P deficiency. This study is the first time to investigate global profiles of metabolites and transcripts in soybean (Glycine max) roots subjected to Pi starvation through targeted liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS/MS) and RNA-sequencing analyses. This integrated analysis allows for assessing coordinated transcriptomic and metabolic responses in terms of both pathway enzyme expression and regulatory levels. Between two Pi availability treatments, a total of 155 metabolites differentially accumulated in soybean roots, of which were phosphorylated metabolites, flavonoids and amino acids. Meanwhile, a total of 1644 differentially expressed genes (DEGs) were identified in soybean roots, including 1199 up-regulated and 445 down-regulated genes. Integration of metabolome and transcriptome analyses revealed Pi starvation responsive connection between specific metabolic processes in soybean roots, especially metabolic processes involving phosphorylated metabolites (e.g., phosphorylated lipids and nucleic acids). Taken together, this study suggests that complex molecular responses scavenging internal Pi from phosphorylated metabolites are typical adaptive strategies soybean roots employ as responses to Pi starvation. Identified DEGs will provide potential target region for future efforts to develop P-efficient soybean cultivars.

MYB-CC transcription factor, TaMYBsm3, cloned from wheat is involved in drought tolerance

BACKGROUND

MYB-CC transcription factors (TFs) genes have been demonstrated to be involved in the response to inorganic phosphate (Pi) starvation and regulate some Pi-starvation-inducible genes. However, their role in drought stress has not been investigated in bread wheat. In this study, the TaMYBsm3 genes, including TaMYBsm3-A, TaMYBsm3-B, and TaMYBsm3-D, encoding MYB-CC TF proteins in bread wheat, were isolated to investigate the possible molecular mechanisms related to drought-tolerance in plants.

RESULTS

TaMYBsm3-A, TaMYBsm3-B, and TaMYBsm3-D were mapped on chromosomes 6A, 6B, and 6D in wheat, respectively. TaMYBsm3 genes belonged to MYB-CC TFs, containing a conserved MYB DNA-binding domain and a conserved coiled-coil domain. TaMYBsm3-D was localized in the nucleus, and the N-terminal region was a transcriptional activation domain. TaMYBsm3 genes were ubiquitously expressed in different tissues of wheat, and especially highly expressed in the stamen and pistil. Under drought stress, transgenic plants exhibited milder wilting symptoms, higher germination rates, higher proline content, and lower MDA content comparing with the wild type plants. P5CS1, DREB2A, and RD29A had significantly higher expression in transgenic plants than in wild type plants.

CONCLUSION

TaMYBsm3-A, TaMYBsm3-B, and TaMYBsm3-D were associated with enhanced drought tolerance in bread wheat. Overexpression of TaMYBsm3-D increases the drought tolerance of transgenic Arabidopsis through up-regulating P5CS1, DREB2A, and RD29A.

Can Aluminum Tolerant Wheat Cultivar Perform Better under Phosphate Deficient Conditions?

Low availability of inorganic phosphate (Pi), together with aluminum (Al), is a major constraint for plant growth and development in acidic soils. To investigate whether or not Al-resistant cultivars can perform better under Pi deficiency, we chose two wheat cultivars with different Al-responses-Atlas 66, being Al-tolerant, and Scout 66, which is Al-sensitive-and analyzed their responses to Pi deficiency. Results showed that, unexpectedly, the Al-sensitive cultivar Scout 66 contained comparatively higher amount of soluble phosphate (Pi) and total phosphorus (P) both in the roots and in the shoots than Atlas 66 under P deficiency. In addition, Scout 66 exhibited higher root biomass, root volume, and root tip numbers, compared with Atlas 66. The expression of Pi-responsive marker genes, TaIPS1, TaSPX3, and TaSQD2 was strongly induced in both cultivars, but the extents of induction were higher in Scout 66 than in Atlas 66 under long-term Pi starvation. Taken together, our results suggest that the Al-sensitive cultivar Scout 66 performed much better under sole Pi starvation, which sets the following experimental stage to uncover the underlying mechanisms of why Scout 66 can display better under Pi deficiency. Our study also raises an open question whether Al-resistant plants are more sensitive to Pi deficiency.

Functional characterization of the gene promoter for an Elaeis guineensis phosphate starvation-inducible, high affinity phosphate transporter in both homologous and heterologous model systems

Oil palm is grown in tropical soils with low bioavailability of Pi. A cDNA clone specifically expressed under phosphate-starvation condition in oil palm roots was identified as a high-affinity phosphate transporter (EgPHT1). The deduced amino acid sequence has 6 transmembrane domains each at the N- and C-termini separated by a hydrophilic linker. Comparison of promoter motifs within 1500 bp upstream of ATG of 10 promoters from high- and low-affinity phosphate transporter from both dicots and monocots including EgPHT1 was performed. The EgPHT1 promoter was fused to β-glucuronidase (GUS) reporter gene and its activity was analysed by histochemical and fluorometric GUS assays in transiently transformed oil palm tissues and T₃ homozygous transgenic Arabidopsis plants. In response to Pi-starvation, no GUS activity was detected in oil palm leaves, but a strong inducible activity was observed in the roots (1.4 times higher than the CaMV35S promoter). GUS was specifically expressed in transgenic Arabidopsis roots under Pi deficiency and starvation of the other macronutrients (N and K) did not induce GUS activity. Eight motifs including ABRERATCAL (abscisic-acid responsive), RHERPATEXPA7 (root hair-specific), SURECOREATSULTR11 (sulfur-deficiency response), LTRECOREATCOR15 (temperature-stress response), MYB2CONSENSUSAT and ACGTATERD1 (water-stress response) as well as two novel motifs, 3 (TAAAAAAA) and 26 (TTTTATGT) identified through pattern discovery, occur at significantly higher frequency (p < 0.05) in the high-than the low-affinity phosphate transporter promoters. The Pi deficiency-responsive elements in EgPHT1 includes the P1BS, W-box, E-box and the G-box. Thus, EgPHT1 is important for improving Pi uptake in oil palm with potential for engineering efficient Pi acquisition.

Genome-wide identification and comparative analysis of phosphate starvation-responsive transcription factors in maize and three other gramineous plants

The present study identified several important candidate Pi regulation genes of maize and provides a better understanding on the generation of PHR genes in gramineous plants. Plants have evolved adaptive responses to cope with low phosphate (Pi) soils. The previous studies have indicated that phosphate starvation response (PHR) genes play central roles in regulating plant Pi starvation responses. However, the investigation of PHR family in gramineous plants is limited. In this study, we identified 64 PHR genes in four gramineous plants, including maize, rice, sorghum, and brachypodium, and conducted systematical analyses on phylogenetic, structure, collinearity, and expression pattern of these PHR genes. Genome synteny analysis revealed that a number of PHR genes were present in the corresponding syntenic blocks of maize, rice, sorghum, and brachypodium, indicating that large-scale duplication events contributed significantly to the expansion and evolution of PHR genes in these gramineous plants. Gene expression analysis showed that many PHR genes were expressed in various tissues, suggesting that these genes are involved in Pi redistribution and allocation. In addition, the expression levels of PHR genes from maize and rice under low Pi stress conditions revealed that some PHRs may play an important role in Pi starvation response. Our results provided a better understanding on the generation of PHR genes in gramineous plants and identified several important candidate Pi regulation genes of maize.

Functional Characterization of Arabidopsis PHL4 in Plant Response to Phosphate Starvation

Plants have evolved an array of adaptive responses to cope with phosphate (Pi) starvation. These responses are mainly controlled at the transcriptional level. In Arabidopsis, PHR1, a member of the MYB-CC transcription factor family, is a key component of the central regulatory system controlling plant transcriptional responses to Pi starvation. Its homologs in the MYB-CC family, PHL1 (PHR1-LIKE 1), PHL2, and perhaps also PHL3, act redundantly with PHR1 to regulate plant Pi starvation responses. The functions of PHR1's closest homolog in this family, PHL4, however, have not been characterized due to the lack of its null mutant. In this work, we generated two phl4 null mutants using the CRISPR/Cas9 technique and investigated the functions of PHL4 in plant responses to Pi starvation. The results indicated that the major developmental, physiological, and molecular responses of the phl4 mutants to Pi starvation did not significantly differ from those of the wild type. By comparing the phenotypes of the phr1 single mutant and phr1phl1 and phr1phl4 double mutants, we found that PHL4 also acts redundantly with PHR1 to regulate plant Pi responses, but that its effects are weaker than those of PHL1. We also found that the overexpression of PHL4 suppresses plant development under both Pi-sufficient and -deficient conditions. Taken together, the results indicate that PHL4 has only a minor role in the regulation of plant responses to Pi starvation and is a negative regulator of plant development.

Interaction and Regulation of Carbon, Nitrogen, and Phosphorus Metabolisms in Root Nodules of Legumes

Members of the plant family Leguminosae (Fabaceae) are unique in that they have evolved a symbiotic relationship with rhizobia (a group of soil bacteria that can fix atmospheric nitrogen). Rhizobia infect and form root nodules on their specific host plants before differentiating into bacteroids, the symbiotic form of rhizobia. This complex relationship involves the supply of C₄-dicarboxylate and phosphate by the host plants to the microsymbionts that utilize them in the energy-intensive process of fixing atmospheric nitrogen into ammonium, which is in turn made available to the host plants as a source of nitrogen, a macronutrient for growth. Although nitrogen-fixing bacteroids are no longer growing, they are metabolically active. The symbiotic process is complex and tightly regulated by both the host plants and the bacteroids. The metabolic pathways of carbon, nitrogen, and phosphate are heavily regulated in the host plants, as they need to strike a fine balance between satisfying their own needs as well as those of the microsymbionts. A network of transporters for the various metabolites are responsible for the trafficking of these essential molecules between the two partners through the symbiosome membrane (plant-derived membrane surrounding the bacteroid), and these are in turn regulated by various transcription factors that control their expressions under different environmental conditions. Understanding this complex process of symbiotic nitrogen fixation is vital in promoting sustainable agriculture and enhancing soil fertility.