Mian Gu

Plant phosphate transporters (PTs) are active in the uptake of inorganic phosphate (Pi) from the soil and its translocation within the plant. Here, we report on the biological properties and physiological roles of OsPht1;8 (OsPT8), one of the PTs belonging to the Pht1 family in rice (Oryza sativa). Expression of a β-glucuronidase and green fluorescent protein reporter gene driven by the OsPT8 promoter showed that OsPT8 is expressed in various tissue organs from roots to seeds independent of Pi supply. OsPT8 was able to complement a yeast Pi-uptake mutant and increase Pi accumulation of Xenopus laevis oocytes when supplied with micromolar (33)Pi concentrations at their external solution, indicating that it has a high affinity for Pi transport. Overexpression of OsPT8 resulted in excessive Pi in both roots and shoots and Pi toxic symptoms under the high-Pi supply condition. In contrast, knockdown of OsPT8 by RNA interference decreased Pi uptake and plant growth under both high- and low-Pi conditions. Moreover, OsPT8 suppression resulted in an increase of phosphorus content in the panicle axis and in a decrease of phosphorus content in unfilled grain hulls, accompanied by lower seed-setting rate. Altogether, our data suggest that OsPT8 is involved in Pi homeostasis in rice and is critical for plant growth and development.

Abstract A number of phosphate (Pi) starvation- or mycorrhiza-regulated Pi transporters belonging to the Pht1 family have been functionally characterized in several plant species, whereas functions of the Pi transporters that are not regulated by changes in Pi supply are lacking. In this study, we show that rice (Oryza sativa) Pht1;1 (OsPT1), one of the 13 Pht1 Pi transporters in rice, was expressed abundantly and constitutively in various cell types of both roots and shoots. OsPT1 was able to complement the proton-coupled Pi transporter activities in a yeast mutant defective in Pi uptake. Transgenic plants of OsPT1 overexpression lines and RNA interference knockdown lines contained significantly higher and lower phosphorus concentrations, respectively, compared with the wild-type control in Pi-sufficient shoots. These responses of the transgenic plants to Pi supply were further confirmed by the changes in depolarization of root cell membrane potential, root hair occurrence, 33P uptake rate and transportation, as well as phosphorus accumulation in young leaves at Pi-sufficient levels. Furthermore, OsPT1 expression was strongly enhanced by the mutation of Phosphate Overaccumulator2 (OsPHO2) but not by Phosphate Starvation Response2, indicating that OsPT1 is involved in the OsPHO2-regulated Pi pathway. These results indicate that OsPT1 is a key member of the Pht1 family involved in Pi uptake and translocation in rice under Pi-replete conditions.

Legume biological nitrogen (N) fixation is the most important N source in agroecosystems, but it is also a process requiring a considerable amount of phosphorus (P). Therefore, developing legume varieties with effective N(2) fixation under P-limited conditions could have profound significance for improving agricultural sustainability. We show here that inoculation with effective rhizobial strains enhanced soybean (Glycine max) N(2) fixation and P nutrition in the field as well as in hydroponics. Furthermore, we identified and characterized a nodule high-affinity phosphate (Pi) transporter gene, GmPT5, whose expression was elevated in response to low P. Yeast heterologous expression verified that GmPT5 was indeed a high-affinity Pi transporter. Localization of GmPT5 expression based on β-glucuronidase staining in soybean composite plants with transgenic roots and nodules showed that GmPT5 expression occurred principally in the junction area between roots and young nodules and in the nodule vascular bundles for juvenile and mature nodules, implying that GmPT5 might function in transporting Pi from the root vascular system into nodules. Overexpression or knockdown of GmPT5 in transgenic composite soybean plants altered nodulation and plant growth performance, which was partially dependent on P supply. Through both in situ and in vitro (33)P uptake assays using transgenic soybean roots and nodules, we demonstrated that GmPT5 mainly functions in transporting Pi from roots to nodules, especially under P-limited conditions. We conclude that the high-affinity Pi transporter, GmPT5, controls Pi entry from roots to nodules, is critical for maintaining Pi homeostasis in nodules, and subsequently regulates soybean nodulation and growth performance.

Plant roots rely on inorganic orthophosphate (Pi) transporters to acquire soluble Pi from soil solutions that exists at micromolar levels in natural ecosystems. Here, we functionally characterized a rice (Oryza sativa) Pi transporter, Os Phosphate Transporter-1;3 (OsPHT1;3), that mediates Pi uptake, translocation, and remobilization. OsPHT1;3 was directly regulated by Os Phosphate Starvation Response-2 and, in response to Pi starvation, showed enhanced expression in young leaf blades and shoot basal regions and even more so in roots and old leaf blades. OsPHT1;3 was able to complement a yeast mutant strain defective in five Pi transporters and mediate Pi influx in Xenopus laevis oocytes. Overexpression of OsPHT1;3 led to increased Pi concentration both in roots and shoots. However, unlike that reported for other known OsPHT1 members that facilitate Pi uptake at relatively higher Pi levels, mutation of OsPHT1;3 impaired Pi uptake and root-to-shoot Pi translocation only when external Pi concentration was below 5 μm. Moreover, in basal nodes, the expression of OsPHT1;3 was restricted to the phloem of regular vascular bundles and enlarged vascular bundles. An isotope labeling experiment with 32P showed that ospht1;3 mutant lines were impaired in remobilization of Pi from source to sink leaves. Furthermore, overexpression and mutation of OsPHT1;3 led to reciprocal alteration in the expression of OsPHT1;2 and several other OsPHT1 genes. Yeast-two-hybrid, bimolecular fluorescence complementation, and coimmunoprecipitation assays all demonstrated a physical interaction between OsPHT1;3 and OsPHT1;2. Taken together, our results indicate that OsPHT1;3 acts as a crucial factor for Pi acquisition, root-to-shoot Pi translocation, and redistribution of phosphorus in plants growing in environments with extremely low Pi levels.