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

摘要

>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 post-transcriptional 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 leaves was weakened, 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 for K⁺ over Na⁺ from roots to shoots compared with the WT. Silencing of ZxNHX also triggered a downregulation of  ZxAKT1 and ZxSKOR in roots, resulting in a significant decrease in K⁺ accumulation in all the tissues in plants grown in 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.