A Short Shaker Channel Mediates K+ Loading to Root Conducting Tissues and Contributes to Rice Grain Yield under Field Conditions

Abstract K + uptake to root conducting cells is a prerequisite for its release to the xylem vessels and long-distance delivery to the aerial parts, though the molecular mechanism underpinning such process remains insufficiently understood. Here we report the discovery in rice root of an essential component involved in loading of K + to the stelar tissues encoded by an unusual short Shaker K + channel OsK2.2. OsK2.2 represents a novel type of short Shaker channels lacking the sequences immediately downstream of the putative cyclic nucleotide binding domain that present to the C-termini of all plant Shaker channels reported to date. In silico analysis suggests that such short Shaker channels are monocot-specific and especially found in the Poaceae species. Functional assessment in Xenopus oocytes reveals that OsK2.2 uses such innate C-terminal shortness as an intrinsic strategy for maintaining large capacity of K + uptake to the cells. OsK2.2 predominantly localizes to the phloem and xylem peripheral cells of rice root vasculature. Disruption of OsK2.2 function causes approximately 30% lower K + in both phloem and xylem sap and reduces K + accumulation to the shoots. OsK2.2 strongly contributes to the grain yield of rice under field conditions. The essential contribution of such particular type of inward Shaker channel, operating in stelar tissues, to K + transport between roots and shoots and to grain yield, here evidenced in rice, may be a common trait amongst Poaceae species.