P transporter OsPHT1;3 functions in P uptake, translocation, and remobilization of P in rice under extremely low P regimes.

Phosphorus (P) deficiency is one of the major nutrient stresses limiting plant growth. The uptake of P by plants is well considered to be mediated by a number of high-affinity phosphate (Pi) transporters belonging to the Pht1 family. Although the Pht1 genes have been extensively identified in several plant species, there is a lack of systematic analysis of the Pht1 gene family in any solanaceous species thus far.

The high-yielding Green Revolution varieties of cereal crops are characterized by a semi-dwarf architecture and lodging resistance. Plant height is tightly regulated by the availability of phosphate (Pi), yet the underlying mechanism remains obscure. Here, we report that rice (Oryza sativa) R2R3-type MYB transcription factor MYB110 is a Pi-dependent negative regulator of plant height. MYB110 is a direct target of PHOSPHATE STARVATION RESPONSE 2 (OsPHR2) and regulates OsPHR2-mediated inhibition of rice height. Inactivation of MYB110 increased culm diameter and bending resistance, leading to enhanced lodging resistance despite increased plant height. Strikingly, the grain yield of myb110 mutants was elevated under both high- and low-Pi regimes. Two divergent haplotypes based on single nucleotide polymorphisms in the putative promoter of MYB110 corresponded with its transcript levels and plant height in response to Pi availability. Thus, fine-tuning MYB110 expression may be a potent strategy for further increasing the yield of Green Revolution cereal crop varieties.

Summary: Plant Phosphate Transporter 1 (PHT1) proteins, probably the only influx transporters for phosphate (Pi) uptake, are partially degraded on sufficient Pi levels to prevent excessive Pi accumulation. Therefore, the basal/constitutive expression level of PHT1 genes is vital for maintaining Pi uptake under Pi‐replete conditions. Rice (Oryza sativa) OsPHT1;1 is a unique gene as it is highly expressed and not responsive to Pi, however the mechanism for maintaining its basal/constitutive expression remains unknown. Using biochemical and genetic approaches, we identified and functionally characterised the transcription factors maintaining the basal/constitutive expression of OsPHT1;1. OsWRKY21 and OsWRKY108 interact within the nucleus and both bind to the W‐box in the OsPHT1;1 promoter. Overexpression of OsWRKY21 or OsWRKY108 led to increased Pi accumulation, resulting from elevated expression of OsPHT1;1. By contrast, oswrky21 oswrky108 double mutants showed decreased Pi accumulation and OsPHT1;1 expression in a Pi‐dependent manner. Moreover, similar to ospht1;1 mutants, plants expressing the OsWRKY21–SRDX fusion protein (a chimeric dominant suppressor) were impaired in Pi accumulation in Pi‐replete roots, accompanied by downregulation of OsPHT1;1 expression. Our findings demonstrated that rice WRKY transcription factors function redundantly to promote Pi uptake by activating OsPHT1;1 expression under Pi‐replete conditions, and represent a novel pathway independent of the central Pi signalling system.

Inorganic phosphate (Pi) is the predominant form of phosphorus (P) readily accessible to plants, and Pi Transporter 1 (PHT1) genes are the major contributors to root Pi uptake. However, the mechanisms underlying the transport and recycling of Pi within plants, which are vital for optimizing P use efficiency, remain elusive. Here, we characterized a functionally unknown rice (Oryza sativa) PHT1 member barely expressed in roots, OsPHT1;7. Yeast complementation and Xenopus laevis oocyte assay demonstrated that OsPHT1;7 could mediate Pi transport. Reverse-transcription quantitative polymerase chain reaction and histochemical analyses showed that OsPHT1;7 was preferentially expressed in source leaves and nodes. A further fine-localization analysis by immunostaining showed that OsPHT1;7 expression was restricted in the vascular bundle (VB) sheath and phloem of source leaves as well as in the phloem of regular/diffuse- and enlarged-VBs of nodes. In accordance with this expression pattern, mutation of OsPHT1;7 led to increased and decreased P distribution in source (old leaves) and sink organs (new leaves/panicles), respectively, indicating that OsPHT1;7 is involved in P redistribution. Furthermore, OsPHT1;7 showed an overwhelmingly higher transcript abundance in anthers than other PHT1 members, and ospht1;7 mutants were impaired in P accumulation in anthers but not in pistils or husks. Moreover, the germination of pollen grains was significantly inhibited upon OsPHT1;7 mutation, leading to a >80% decrease in seed-setting rate and grain yield. Taken together, our results provide evidence that OsPHT1;7 is a crucial Pi transporter for Pi transport and recycling within rice plants, stimulating both vegetative and reproductive growth.