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Running head: Rice Phosphate Transporter ... - Plant Physiology

Running head: Rice Phosphate Transporter ... - Plant Physiology

283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314family in rice (Zhou et al., 2008). That the transcriptional expression of OsPT1 wasnot regulated by external Pi-supply level could be further supported by non-alteredexpression of OsPT1 in osphr2 mutant (Fig. 9). This suggests that OsPT1 is notdirectly regulated by OsPHR2 in rice.OsPT1 functions in Pi acquisition and distribution, Pi-mediated root hairdevelopment in Pi-replete riceIt is generally accepted that Pi uptake of roots from low Pi soil solution is catalysedby high-affinity PiTs (Raghothama, 1999). It has been shown that AtPht1;1 andAtPht1;4, which were most highly expressed in the epidermis and root hair cells,contributed to Pi transport into roots during growth in a wide range of external Piconcentrations (Shin et al., 2004), while AtPht1;5 which showed Pideficiency-induced expression specifically in the phloem cells of older leaves,cotyledons, and flowers (Mudge et al., 2002) functions in mobilizing Pi betweensource and sink organs (Nagarajan et al., 2011). The OsPT1 promoter directed itsconstitutive expression in epidermis, cortex and stele cells of various rice tissues (Fig.1) suggests that OsPT1 is likely to be involved both in Pi acquisition and translocation.However, either over-expression or knockdown of OsPT1 did not significantly affectshoot biomass, Pi concentration in both roots and shoots, and Pi distribution atdeficient-Pi supplied rice (Fig. 3, 4). There were only slight differences of the rootresponses to Pi-starvation in the transgenic rice in comparison to WT (Fig. 8). It hasbeen shown that Pi-starvation in rice strongly enhanced expression of high affinityPi-transporter gene OsPT6 which was expressed in multiple cell types from epidermis,cortex to stele, and low affinity Pi-transporter gene OsPT2 which was exclusivelyexpressed in stele cells of various rice tissues (Ai et al., 2009). Therefore, it could beassumed that OsPT1 has the functional redundancy with other Pi-starvationup-regulated Pht1 members for acquisition and transportation of Pi. Enhancedexpression of OsPT2 and OsPT6, possibly together with other non-characterized Pht1members, could compensate the altered expression of OsPT1 in the Pi-starved riceplants.Maintaining sufficient Pi in plant above ground parts depends not only on root Piacquisition from external solution, but also on transfer of Pi from roots to shoots viaxylem and re-distribution inside the plant via phloem. PHO1 protein primarily12

315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345expressed in root vascular cylinder is known in mediating Pi efflux for loading Pi toroot xylem in Arabidopsis (Hamburger et al., 2002; Stefanovic 2011). In rice, it hasbeen shown that OsPHO1;2 plays an important role in transferring Pi from roots toshoots (Secco et al., 2010). However, it is not clear if Pht1 members are directlyresponsible for Pi loading or unloading or retrieval in the vascular tissue, particularlyunder Pi-sufficient condition. Gómez-Ariza et al. (2009) analyzed the expression offive tomato Pi transporter genes in the different root tissues using the lasermicrodissection technology. The results indicated that no expression of the detected Pitransporter genes was observed in the central cylinder, irrespective of the presence ofthe arbuscular symbiosis. Previously, we showed that knockdown of OsPT2exclusively expressed in Pi-starved root stele cells decreased Pi transport from rootsto shoots in rice (Ai et al., 2009). In the present work, the increase of Pi-supplygradually enlarged the difference of shoot P concentration between the transgenic riceand WT, and thus resulted in significantly higher P in OsPT1-Ox and lower P inOsPT1-RNAi mutants (Fig. 4). OsPT1-Ox had significantly higher root cellmembrane potential depolarization responses to external Pi and higher Piconcentration in xylem sap than WT under P-replete condition (Fig. 5, 7). It is alsointeresting that over-expression of OsPT1 resulted in much higher Pi in young leavesof mature plants (Fig. 6). All these data demonstrated that OsPT1 is involved in rootPi uptake and allocation in Pi-replete rice.In Arabidopsis, over-expression of AtPht1;5 lead to increases of root hair numbersand length both under Pi-sufficient and -deficient conditions, while silencing of thegene did not have significant effect on the root hair development (Nagarajan et al.,2011). It was remarkable that the OsPT1-Ri lines, particularly OsPT1-Ox lines,developed much longer and dense root hairs only under P-replete condition (Fig. 8).Since there was no significant difference of Pi concentration in their roots underPi-sufficient condition (Fig. 3C), the enhanced growth of root hair length and numberof OsPT1-Ox in Pi-sufficient medium (Fig. 8) could not be simply explained byinternal Pi status induced changes of root growth. This could be due to differentialdistribution of P between different tissues and organs in the whole plants by OsPT1leading to altered root hair development.34613

Plant Physiology - Stony Brook University
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