ZxNHX controls Na+ and K+ homeostasis at the whole-plant level In Zygophyllum xanthoxylum through feedback regulation of the expression of genes involved In their transport

Abstract

Background and Aims: In order to cope with arid environments, the xerohalophyte Zygophyllum xanthoxylum efficiently compartmentalizes Na+ into vacuoles, mediated by ZxNHX, and maintains stability of K+ in its leaves. However, the function of ZxNHX in controlling Na+ and K+ homeostasis at the whole-plant level remains unclear. In this study, the role of ZxNHX in regulating the expression of genes involved in Na+ and K+ transport and spatial distribution was investigated. Methods: The role of ZxNHX in maintaining Na+ and K+ homeostasis in Z. xanthoxylum was studied using posttranscriptional gene silencing via Agrobacterium-mediated transformation. Transformed plants were grown with or without 50 mM NaCl, and expression levels and physiological parameters were measured. Key Results: It was found that 50 mM NaCl induced a 620% increase in transcripts of ZxSOS1 but only an 80% increase in transcripts of ZxHKT1;1 in roots of wild-type (WT) plants. Consequently, the ability of ZxSOS1 to transport Na+ exceeded that of ZxHKT1;1, and Na+ was loaded into the xylem by ZxSOS1 and delivered to the shoots. However, in a ZxNHX-silenced line (L7), the capacity to sequester Na+ into vacuoles of leaveswasweakened, which in turn regulated long-distance Na+ transport from roots to shoots. In roots of L7, NaCl (50 mM) increased transcripts of ZxSOS1 by only 10 %, whereas transcripts of ZxHKT1;1 increased by 53 %. Thus, in L7, the transport ability of ZxHKT1;1 for Na+ outweighed that of ZxSOS1. Na+ was unloaded from the xylem stream, consequently reducing Na+ accumulation and relative distribution in leaves, but increasing the relative distribution of Na+ in roots and the net selective transport capacity forK+ overNa+ fromroots to shoots compared with the WT. Silencing of ZxNHXalso triggered a downregulation of ZxAKT1and ZxSKORin roots, resulting in a significant decrease in K+ accumulation in all the tissues in plants grownin 50 mM NaCl. These changes led to a significant reduction in osmotic adjustment, and thus an inhibition of growth in ZxNHX-silenced lines. Conclusions: The results suggest that ZxNHX is essential for controlling Na+, K+ uptake, long-distance transport and their homeostasis at whole-plant level via feedback regulation of the expression of genes involved in Na+, K+ transport. The net result is the maintenance of the characteristic salt accumulation observed in Z. xanthoxylum and the regulation of its normal growth. A model is proposed for the role of ZxNHX in regulating the Na+ transport system in Z. xanthoxylum under saline conditions.