Rice shaker potassium channel OsAKT2 positively regulates salt tolerance and grain yield by mediating K⁺ redistribution

Abstract

Maintaining Na + /K + homeostasis is a critical feature for plant survival under salt stress, which depends on the operation of Na + and K + transporters. Although some K + transporters mediating root K + uptake have been reported to be essential to the maintenance of Na + /K + homeostasis, the effect of K + long-distance translocation via phloem on plant salt tolerance remains unclear. Here, we provide physiological and genetic evidence of the involvement of phloem-localized OsAKT2 in rice salt tolerance. OsAKT2 is a K + channel permeable to K + but not to Na + . Under salt stress, a T-DNA knock-out mutant, osakt2 and two CRISPR lines showed a more sensitive phenotype and higher Na + accumulation than wild type. They also contained more K + in shoots but less K + in roots, showing higher Na + /K + ratios. Disruption of OsAKT2 decreases K + concentration in phloem sap and inhibits shoot-to-root redistribution of K + . In addition, OsAKT2 also regulates the translocation of K + and sucrose from old leaves to young leaves, and affects grain shape and yield. These results indicate that OsAKT2-mediated K + redistribution from shoots to roots contributes to maintenance of Na + /K + homeostasis and inhibition of root Na + uptake, providing novel insights into the roles of K + transporters in plant salt tolerance.

Rice Na+-Permeable Transporter OsHAK12 Mediates Shoots Na+ Exclusion in Response to Salt Stress

“…The Nip and Oshak12 mutants plants showed no significant differences ( n = 30 for each data point) ( P > 0.05 by Student’s t -test). The methods for the shoot excision, collecting the xylem sap and phloem secretion, examining Na + and K + concentration by ICP-MS were described previously by Tian et al (2021) . The experiment was repeated four times with similar results.…”

“…Measurement of chlorophyll and ion content (Na + , K + ) analysis as previous methods ( Porra et al, 1989 ; Wang et al, 2021 ). The collected method, Na + and K + concentration in the xylem sap and phloem exudates were determined using inductively coupled plasma/optical emission spectrometry ICP-AES (Varian 715-ES) following the method reported by Tian et al (2021) . Briefly, 5-days-old rice seedlings were cultivated in the solutions for 14 days and then transferred to the hydroponic cultures containing 0 or 100 mM Na + for 2 days.…”

“…Cellular sodium ion (Na + ) toxicity is the dominant ion toxicity among salinity stress factors, leading to the inhibition of a series of physiological and biochemical processes such as photosynthesis, protein synthesis and K + absorption ( Tester and Davenport, 2003 ; Ismail and Horie, 2017 ; Zelm et al, 2020 ). To adapt to high-Na + environments, plants utilize various mechanisms to cope with Na + toxicity, including Na + efflux from roots to the rhizosphere, Na + sequestration in vacuoles, and Na + recycling in plants through vasculature ( Munns and Tester, 2008 ; Ismail and Horie, 2017 ; Yang and Guo, 2018a , b ; Tian et al, 2021 ). Understanding and harnessing the mechanisms responsed to salt stress will contribute to breeding salt-tolerant crops, thereby safeguarding global food security.…”

“…Thus, mechanisms for Na + exclusion from shoot are pivotal for the adaptation of plants in high-Na + environments. Previous studies showed that Na + retrieving from xylem sap in the root is an essential physiological strategy to achieve low shoots Na + concentrations during salt toxicity ( Ismail and Horie, 2017 ; Zelm et al, 2020 ; Tian et al, 2021 ). This process is mediated by a number of ion transporters ( Horie et al, 2009 ; Zelm et al, 2020 ).…”

Effects of improved sodium uptake ability on grain yields of rice plants under low potassium supply

“…Tian et al (2021) reported that the K + concentration in phloem exudate was higher in salt‐stressed rice plants than in control plants and that the phloem K + concentration was higher in wild‐type plants than in OsAKT2—a phloem localized K + channel—knock‐out mutant plants. They proposed that accumulated Na + in the shoot apoplast may have caused hyperpolarization of the phloem cell membrane and promoted OsAKT2‐mediated K + loading (Tian et al, 2021). In K + ‐deficient rice plants, although the absolute cation concentration is not as high as that in salt‐stressed plants, the ratio of Na + to K + in plants increases.…”