OsHKT1;4-mediated Na+ transport in stems contributes to Na+ exclusion from leaf blades of rice at the reproductive growth stage upon salt stress

Abstract

BackgroundNa+ exclusion from leaf blades is one of the key mechanisms for glycophytes to cope with salinity stress. Certain class I transporters of the high-affinity K+ transporter (HKT) family have been demonstrated to mediate leaf blade-Na+ exclusion upon salinity stress via Na+-selective transport. Multiple HKT1 transporters are known to function in rice (Oryza sativa). However, the ion transport function of OsHKT1;4 and its contribution to the Na+ exclusion mechanism in rice remain to be elucidated.ResultsHere, we report results of the functional characterization of the OsHKT1;4 transporter in rice. OsHKT1;4 mediated robust Na+ transport in Saccharomyces cerevisiae and Xenopus laevis oocytes. Electrophysiological experiments demonstrated that OsHKT1;4 shows strong Na+ selectivity among cations tested, including Li+, Na+, K+, Rb+, Cs+, and NH4+, in oocytes. A chimeric protein, EGFP-OsHKT1;4, was found to be functional in oocytes and targeted to the plasma membrane of rice protoplasts. The level of OsHKT1;4 transcripts was prominent in leaf sheaths throughout the growth stages. Unexpectedly however, we demonstrate here accumulation of OsHKT1;4 transcripts in the stem including internode II and peduncle in the reproductive growth stage. Moreover, phenotypic analysis of OsHKT1;4 RNAi plants in the vegetative growth stage revealed no profound influence on the growth and ion accumulation in comparison with WT plants upon salinity stress. However, imposition of salinity stress on the RNAi plants in the reproductive growth stage caused significant Na+ overaccumulation in aerial organs, in particular, leaf blades and sheaths. In addition, 22Na+ tracer experiments using peduncles of RNAi and WT plants suggested xylem Na+ unloading by OsHKT1;4.ConclusionsTaken together, our results indicate a newly recognized function of OsHKT1;4 in Na+ exclusion in stems together with leaf sheaths, thus excluding Na+ from leaf blades of a japonica rice cultivar in the reproductive growth stage, but the contribution is low when the plants are in the vegetative growth stage.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0709-4) contains supplementary material, which is available to authorized users.

A Magnesium Transporter OsMGT1 Plays a Critical Role in Salt Tolerance in Rice

“…6, H-K). This tissue specificity of expression of OsMGT1 is similar to OsHKT1;1 and OsHKT1;4 (Wang et al, 2015;Suzuki et al, 2016). Therefore, there is a possibility that Mg 2+ transported by OsMGT1 in these tissues also plays a role in enhancing the transport activity of OsHKT1;1 and OsHKT1;5, although further work remains to be done.…”

“…2A), it is unlikely that OsSOS1 activity is associated with OsMGT1. On the other hand, OsHKT1;1 was mainly expressed in the shoots (Wang et al, 2015), and the contribution of OsHKT1;4 to the Na exclusion from shoots was found to be low at the vegetative growth stage (Suzuki et al, 2016). Therefore, the possibility of these transporters in restricting Na accumulation through OsMGT1 is also excluded at the young seedling stage.…”

“…Several salt tolerance genes in rice have been reported including OsSOS1, OsHKT1;1, OsHKT1;4, and OsHKT1;5, which function as Na + transporters (Martínez-Atienza et al, 2007;Wang et al, 2015;Suzuki et al, 2016;Ren et al, 2005 expression of OsSOS1 in the roots was not induced by NaCl within 24 h, and there was also no difference in the expression level of OsSOS1 between wild type and two osmgt1 mutants ( Fig. 5G; Supplemental Fig.…”

“…It is expressed in the vascular tissues of roots and leaves (Jabnoune et al, 2009), but its exact role in salt tolerance is unknown. OsHKT1;4 is mainly expressed in the leaf sheath and encodes a plasma membrane-localized protein (Suzuki et al, 2016). Recent functional analysis showed that OsHKT1;4 does not contribute to salt tolerance at the vegetative growth stage (Suzuki et al, 2016); however, at the reproductive stage, knockdown of OsHKT1;4 resulted in increased Na accumulation in the leaf sheath and leaf blade under salt stress, implicating that this gene may be involved in the Na + exclusion in the leaf sheath (Suzuki et al, 2016).…”

“…OsHKT1;4 is mainly expressed in the leaf sheath and encodes a plasma membrane-localized protein (Suzuki et al, 2016). Recent functional analysis showed that OsHKT1;4 does not contribute to salt tolerance at the vegetative growth stage (Suzuki et al, 2016); however, at the reproductive stage, knockdown of OsHKT1;4 resulted in increased Na accumulation in the leaf sheath and leaf blade under salt stress, implicating that this gene may be involved in the Na + exclusion in the leaf sheath (Suzuki et al, 2016). On the other hand, SKC1/OsHKT1;5 was suggested to be a quantitative trait locus controlling a higher K + /Na + ratio in the shoots (Ren et al, 2005).…”