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New insights into the role of outward Shaker channels in xylem sap K+ loading through shoot-to-root K+ circulation in rice plants
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International audience. Background and aim: Potassium (K+) is among essential nutrients and required in large amount for plant growth. Inplants, Shaker channels dominate the membrane K+ conductance in most cell types. In Arabidopsis, the SKOR Shakerchannel, essentially expressed in the root pericycle, plays key role in K+ secretion to the xylem sap. In my project, Ifocused on OsK5.2, a SKOR homologue from rice, expressed in both root vasculature and guard cells and I analyzedits role in K+ translocation to shoots, via the combined involvement in xylem sap loading and transpiration control.Methods: In this study, I developed a molecular and integrative physiology approach. I used insertional and CRISPR-Cas9-induced loss-of-function technologies to produce Knock-out (KO) mutant plants on which I analyzed the sapcirculation (collection and analysis of xylem and phloem sap K+ concentration) and I determined the transpiration rateand stomatal conductance parameters under different conditions (control, K+ shortage, different relative humidity).These physiological analyses are coupled with an expression analysis (GUS reporter lines, qRT-PCR) and a functionalanalysis of the OsK5.2 transport system in the Xenopus oocyte by electrophysiology.Results: Electrophysiological characterization of chimeric constructions allowed to identify the crucial domain (in theC-terminus) for OsK5.2 channel function in Xenopus oocyte, and showed that this channel is a highly selectiveoutwardly rectifying K+ channel. OsK5.2, mostly expressed in the root xylem parenchyma and phloem, stronglycontributes to the circulation of K+ between roots and shoots. My results show that OsK5.2 is involved in xylem sapK+ loading via two distinct pathways: (i) a direct secretion from external flow and (ii) a recirculation from the phloem.The contribution of each pathway to the xylem sap K+ loading depends on the intensity of leaf transpiration, which wasvaried according to humidity conditions.Conclusion: Our results showed that OsK5.2 contributes to the K+-xylem sap loading via a broader mechanism thanAtSKOR. Its role in long-distance K+ transport varies according to external K+ conditions and hygrometry, and iscritical to biomass production in rice plants.
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