Search

The succulent xerophyte Zygophyllum xanthoxylum (Bunge) Engl. can absorb Na+ from the soil as an osmoticum in order to resist osmotic stress. The tonoplast Na+/H+ antiporter ZxNHX1 is essential for maintaining the salt-accumulation characteristics of Z. xanthoxylum by compartmentalizing Na+ into vacuoles. Previous results revealed that the silencing of ZxNHX1 greatly decreased Na+ accumulation in Z. xanthoxylum under 50 mM NaCl due to the weakened compartmentalisation; in addition, K+ concentration also significantly reduced in ZxNHX1-RNAi lines. Yet, whether the reduction of K+ concentration was directly triggered by the silencing of ZxNHX1 remains unclear. In this study, the growth parameters and expression levels of ZxSOS1, ZxHKT1;1, ZxAKT1 and ZxSKOR were measured in wild-type and ZxNHX1-RNAi lines under control or -0.5 MPa osmotic stress. The results showed that the silencing of ZxNHX1 inhibited the plant growth, decreased Na+ concentration in leaves, reduced the transcript abundance of ZxSOS1 and dramatically increased that of ZxHKT1;1 in roots of Z. xanthoxylum under osmotic stress; whereas tissue K+ concentrations and the expression level of ZxSKOR displayed no significant variations, and the expression of ZxAKT1 were significantly reduced in ZxNHX1-RNAi lines under osmotic stress, compared with the wild type. These results suggest that in Z. xanthoxylum, ZxNHX1 can maintain the normal growth by compartmentalizing Na+ into vacuoles, and regulate the spatial distribution of Na+ indirectly by affecting the expressions of ZxSOS1 and ZxHKT1;1. Moreover, the silencing of ZxNHX1 is not the main reason that led to the reduction of K+ concentration in ZxNHX1-RNAi lines under 50 mM NaCl, and ZxNHX1 might be indirectly involved in regulating K+ homeostasis.

Zygophyllum xanthoxylum, a succulent xerophyte, possesses excellent salt tolerance which is closely associated with vacuolar Na+ compartmentation via ZxNHX1. RNA interference (RNAi)-mediated ZxNHX1 silencing impaired the characteristics of Na+ accumulation and thus inhibited normal growth of Z. xanthoxylum. To explore the molecular mechanisms underlying the changed phenotype, here we compared the different expression of salt-responsive genes between ZxNHX1-RNAi line and wild type at transcriptome level. The result showed that vast genes were differently expressed in leaves or roots between ZxNHX1-RNAi line and wild type under control condition or salt treatment. Among them, 142 unigenes were differently expressed in both roots and leaves under 50 mM NaCl for 6 h and 24 h between ZxNHX1-RNAi line and WT. These differentially expressed genes (DEGs) were related to transmembrane transportation based on Gene Ontology (GO) annotations. In terms of ion transport, 49 DEGs were identified, in which some genes associated with Na+ and K+ homeostasis were down-regulated; meanwhile, silencing of ZxNHX1 triggered the differential expression of several genes involved in the transport of important nutrient elements including N, P, Ca, and Mg. Besides, silencing of ZxNHX1 affected the expression level of 18 and 26 genes related to photosynthesis and ROS scavenging, respectively. This study provided strong evidences to indicate the dominant role of ZxNHX1 in maintaining the characteristics of salt accumulation and regulating the salt tolerance of Z. xanthoxylum. [ABSTRACT FROM AUTHOR]