OsHKT1;5 mediates Na⁺ exclusion in the vasculature to protect leaf blades and reproductive tissues from salt toxicity in rice

Abstract

Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC1 locus, which is involved in a higher K /Na ratio in shoots, corresponds to the OsHKT1;5 gene encoding a Na -selective transporter. However, physiological roles of OsHKT1;5 in rice exposed to salt stress remain elusive, and no OsHKT1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T-DNA insertional OsHKT1;5 mutants. Measurements of ion contents in tissues and Na tracer imaging experiments showed that loss-of-function of OsHKT1;5 in salt-stressed rice roots triggers massive Na accumulation in shoots. Salt stress-induced increases in the OsHKT1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno-staining using an anti-OsHKT1;5 peptide antibody indicated that OsHKT1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of Na tracer to leaf sheaths also demonstrated the involvement of OsHKT1;5 in xylem Na unloading in leaf sheaths. Furthermore, OsHKT1;5 was indicated to be present in the plasma membrane and found to localize also in the phloem of diffuse vascular bundles in basal nodes. Together with the characteristic Na allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of OsHKT1;5 in mediating Na exclusion in the phloem to prevent Na transfer to young leaf blades. Our findings further demonstrate that the function of OsHKT1;5 is crucial over growth stages of rice, including the protection of the next generation seeds as well as of vital leaf blades under salt stress.

A Grain of Salt

“…After growing representatives of each haplotype under saline conditions, our observations align with previous findings in barley 19, 20 , and in rice 25 in response to short-term NaCl stress. However, in the latter, longer term stress (21 days of 40mM or 80 mM NaCl) led to a 72% decrease in biomass in an OsHKT1;5 expression mutant compared to wild type.…”

“…The highest overall expression was observed in the maturation zone of the roots in cv. Viivi (Na + HAP2 ) with in situ hybridisations using sections from this region showing that HvHKT1;5 was predominantly expressed in the xylem parenchyma and endodermal cells adjacent to the xylem vessels ( Figure 2c ) 24, 25, 28 . These results are consistent with HvHKT1;5 transcript abundance in root and shoot tissues reflecting the observed high and low grain Na + haplotypes.…”

Expression and Ion Transport Activity of Rice OsHKT1;1 Variants

“…The SKC1-dependent phenotype in the accumulation of K + and Na + was similar to the mechanism found in the athkt1;1 mutant of Arabidopsis [18,19], which supports a hypothetical model that HKT1-mediated Na + unloading at the xylem indirectly couples with K + loading, which eventually accelerates lower Na + but higher K + accumulation in shoots [5]. More recently, OsHKT1;5 was demonstrated to have a critical role in reducing Na + contents of young leaves of rice under salt stress via Na + unloading at the xylem of roots and leaf sheaths and the phloem of the basal node [20].…”

ANALYSIS OF NATURAL VARIATION IN OsHKT1;1 GENE SEQUENCE AND GENE EXPRESSION IN RELATION TO SALINITY IN RICE (Oryza sativa L.)

“…High affinity K + transporters (HKTs) are an essential determinant for salt tolerance in plants. Members of HKTs are proposed to protect the leaves from overaccumulation of Na + by mediating xylem Na + unloading in the roots, phloem Na + loading in the shoots (Berthomieu et al, 2003;Sunarpi et al, 2005;Ren et al, 2005;Byrt et al, 2007;Davenport et al, 2007;Almeida et al, 2017), and by enhancing exclusion of Na + ion from leaf blades during salt stress (Wang et al, 2015;Kobayashi et al, 2017). Overexpression of AtHKT1;1 enhances salt tolerance in rice, and GmHKT1;4overexpressed transgenic tobacco showed improvement of salinity tolerance (Plett et al, 2010;Chen et al, 2014).…”

“…The rice HKT genes are found to diversely express (Almeida et al, 2013 andAlmeida et al, 2017 andreference herein). Some members of the rice HKT transporter family are involved in the salt tolerance mechanism, such as OsHKT1;1, OsHKT1;4, OsHKT1;5 (Ren et al, 2005;Cotsaftis et al, 2012;Wang et al, 2015;Suzuki et al, 2016;Campbell et al, 2017;Kobayashi et al, 2017). OsHKT1;1 belongs to class 1 of rice HKT family.…”