Genome Wide Transcriptome Analysis Reveals Complex Regulatory Mechanisms Underlying Phosphate Homeostasis in Soybean Nodules.

Phosphorus (P) deficiency is a major limitation for legume crop production. Although overall adaptations of plant roots to P deficiency have been extensively studied, only fragmentary information is available in regard to root nodule responses to P deficiency. In this study, genome wide transcriptome analysis was conducted using RNA-seq analysis in soybean nodules grown under P-sufficient (500 µM KH2PO4) and P-deficient (25 µM KH2PO4) conditions to investigate molecular mechanisms underlying soybean (Glycine max) nodule adaptation to phosphate (Pi) starvation. Phosphorus deficiency significantly decreased soybean nodule growth and nitrogenase activity. Nodule Pi concentrations declined by 49% in response to P deficiency, but this was well below the 87% and 88% decreases observed in shoots and roots, respectively. Nodule transcript profiling revealed that a total of 2055 genes exhibited differential expression patterns between Pi sufficient and deficient conditions. A set of (differentially expressed genes) DEGs appeared to be involved in maintaining Pi homeostasis in soybean nodules, including eight Pi transporters (PTs), eight genes coding proteins containing the SYG1/PHO81/XPR1 domain (SPXs), and 16 purple acid phosphatases (PAPs). The results suggest that a complex transcriptional regulatory network participates in soybean nodule adaption to Pi starvation, most notable a Pi signaling pathway, are involved in maintaining Pi homeostasis in nodules.

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

As an essential nutrient element, phosphorus (P) plays an important role in plant growth and development. Low P availability is a limiting factor for crop production, especially for legume crops (e.g. soybean), which require additional P to sustain nitrogen fixation through symbiotic associations with rhizobia. Although PHOSPHATE STARVATION RESPONSE 1 (PHR1) or PHR1-like is considered as a central regulator of phosphate (Pi) homeostasis in several plant species, it remains undefined in soybean. In this study, 35 GmPHR members were cloned from the soybean genome and expression patterns in soybean were assayed under nitrogen (N) and P deficiency conditions. GmPHR25, which is up-regulated in response to Pi starvation, was then overexpressed in soybean hairy roots in vitro and in vivo to investigate its functions. The results showed that overexpressing GmPHR25 increased Pi concentration in transgenic soybean hairy roots under normal conditions, accompanied with a significant decrease in hairy root growth. Furthermore, transcripts of 11 out of 14 high-affinity Pi transporter (GmPT) members as well as five other Pi starvation-responsive genes were significantly increased in soybean hairy roots with GmPHR25 overexpression. Taken together, this study suggests that GmPHR25 is a vital regulator in the P signaling network, and controls Pi homeostasis in soybean.

SPX1 is an important component in the phosphorus signalling network of common bean regulating root growth and phosphorus homeostasis.

Proteins containing the SPX domain are believed to play vital roles in the phosphorus (P) signalling network in plants. However, the functions of SPX proteins in legumes remain largely unknown. In this study, three SPX members, PvSPX1-PvSPX3 were cloned from common bean (Phaseolus vulgaris L.). It was found that the transcripts of all three PvSPX members were significantly enhanced in both bean leaves and roots by phosphate (Pi) starvation. Among them, the expression of nuclear localized PvSPX1 showed more sensitive and rapid responses to Pi starvation. Consistently, only overexpression of PvSPX1 resulted in increased root P concentration and modified morphology of transgenic bean hairy roots, such as inhibited root growth and an enlarged root hair zone. It was further demonstrated that PvSPX1 transcripts were up-regulated by overexpressing PvPHR1, and overexpressing PvSPX1 led to increased transcripts of 10 Pi starvation-responsive genes in transgenic bean hairy roots. Taken together, it is suggested that PvSPX1 is a positive regulator in the P signalling network of common bean, and is downstream of PvPHR1.