ZxNHX1 from a xerophyte outperforms AtNHX1 in sequestering Na+ into vacuoles to enhance plant stress resistance and yield

Abstract

Uncovering the mechanisms underlying stress-resistant traits in xerophytes thriving in harsh environments can aid the genetic improvement of crops. The xerophyte Zygophyllum xanthoxylum features high Na⁺ accumulation in leaves, mediated by the vacuolar antiporter ZxNHX1. Co-expression of ZxNHX1 and vacuolar H⁺-PPase gene ZxVP1-1 has been demonstrated to enhance the stress resistance and biomass of alfalfa. However, it remains unknown if ZxNHX1 outperforms its homologues from the Na⁺-excluding and stress-sensitive glycophytes such as Arabidopsis in enhancing plant stress resistance and yield. Here, we found that expression of ZxNHX1 conferred superior growth under salt stress in alfalfa, compared to the Arabidopsis homologue AtNHX1. When expressed in yeast, ZxNHX1 displays stronger Na⁺/H⁺ but weaker K⁺/H⁺ exchange activity than AtNHX1. Under both K⁺ sufficient and deficient conditions, an Arabidopsis atnhx1-1 mutant expressing ZxNHX1 accumulated higher Na⁺ and lower K⁺ concentrations, with more Na⁺ being sequestered into vacuoles and a larger proportion of K⁺ retained in the cytosol. This optimized cellular ion distribution ensures energy-conserving osmotic adjustment, leading to stronger stress resistance and higher biomass than plants expressing AtNHX1. Moreover, ZxNHX1 governed the root uptake and root-to-leaf transport of Na⁺ at the whole-plant level, whereas AtNHX1 acted mainly in K⁺ transport processes. We also identified a polar residue Thr265 in a membrane-spanning region of ZxNHX1 that influences its Na⁺ and K⁺ selectivity. These findings reveal a new energy-conserving, Na⁺-based osmotic adjustment mechanism that can enhance crop stress resistance without sacrificing yield, providing an effective way for utilizing saline soils to expand crop production into marginal lands.

Keywords: NHX1; Na+ and K+ selectivity; glycophyte; high yield; stress resistance; xerophyte.