A role for the OsHKT 2;1 sodium transporter in potassium use efficiency in rice

Abstract

Increasing the potassium use efficiency (KUE) of crops is important for agricultural sustainability. However, a greater understanding of this complex trait is required to develop new, high-KUE cultivars. To this end, a genome-wide association study (GWAS) was applied to diverse rice (Oryza sativa L.) genotypes grown under potassium-stressed and -replete conditions. Using high-stringency criteria, the genetic architecture of KUE was uncovered, together with the breadth of physiological responses to low-potassium stress. Specifically, three quantitative trait loci (QTLs) were identified, which contained >90 candidate genes. Of these, the sodium transporter gene OsHKT2;1 emerged as a key factor that impacts on KUE based on (i) the correlation between shoot Na+ and KUE, and (ii) higher levels of HKT2;1 expression in high-KUE lines.

Keywords: Fertilizer use; GWAS; HKT2;1; potassium; potassium use efficiency; rice; sodium; sustainable agriculture.

Fig. 1.

Responses of rice genotypes to potassium stress. (a) Mean plant dry mass of cultivars when grown in the presence of 0.1 mM (LK) and 1 (HK) mM potassium. (b) Relative plant dry mass (dry mass LK/dry mass HK). (c) Reduction in relative growth rate (RGR) in LK compared with HK conditions.

Fig. 2.

Distribution of root (top two panels) and shoot (bottom two panels) K⁺ concentration across the diversity panel for plants grown on LK (0.1 mM) and HK (1 mM) K⁺ medium.

Fig. 3.

Significant (P<0.05) correlation between RGR reduction and shoot tissue Na⁺ concentration of plants grown on LK medium.

Fig. 4.

Manhattan plots for traits (RGR at LK, KUE-K, root [Na⁺] at LK, and shoot [Na⁺] at LK) that generated significant association signals (arrows) using criteria as explained in the Materials and methods. Note that ‘shoot Na’ and ‘root Na’ trait data associate with the same locus on chromosome 6.

Fig. 5.

Co-incidence of previously described QTLs and loci identified in this study related to low K⁺ growth in the rice genome. Each bar represents a chromosome, and previously reported QTLs are marked in white (Wu et al., 1998), yellow (Miyamoto et al., 2012), or red (Fang et al., 2015). Triangles indicate the position of QTLs derived from this study.

Fig. 6.

Reducing levels of medium K⁺ drastically increases Na⁺ concentrations in both roots and shoots of rice cultivar IR64. Plants were grown hydroponically for 7 weeks in the presence of varying K⁺ levels and 3 mM NaCl. Error bars show the SD of three biological replicates.

Fig. 7.

qPCR analysis of HKT2;1 expression in roots of five high-KUE cultivars (GSOR 54, 109, 133, 357, and 366; see Supplementary Table S1) and five low-KUE rice cultivars (GSOR 42, 115, 276, 377, and 401). Plants were grown for 4 weeks in medium containing 0.01 mM K⁺ supplemented with 0 mM or 1 mM NaCl. Data are means for three biological replicates, with error bars denoting the SD.