The Roles Of Genes Encoding Na~+ Transport Proteins In Response To Salt And Drought Conditions In The Desert Plant Zygophyllum Xanthoxylum

Drought affects around one-third of land surface on the earth and is one of the major environmental factors limiting growth and productivity of crop plants worldwide. Most of crop plants have grown in rich environment for a long period, thus their genetic potential of drought tolerance is very limited. This is also an important reason why agricultural and pastoral production is reduced by drought stress. Desert xerophytes which grow in arid regions, however, have developed various mechanisms to adapt to stressful environments in the process of long-term evolution. This kind of plant has been identified as salt diluting species that absorbed much Na+ by roots, and the Na+ was transported to the succulent leaves and photosynthesizing branches. Na+ accumulation into the vacuole may be one of the most effective strategies for adaptation of desert xerophytes to arid environment. However, very little is known about Na+ transporter involved in Na+ transport and compartmentation in the xerophyte, especially, their roles in response of xerophyte to drought. The aims of this Ph.D project, therefore, are to clone and characterize K+ transporter gene ZxKUP, K+ channel gene ZxKAT1 and tonoplast Na+/H+ antiporter gene ZxNHX from the xerophyte Zygophyllum xanthoxylum, and to reveal their functions in response to salt and drought conditions. The main foundings and conclusions are as follows:1. K+ transporter gene ZxKUP, K+ channel gene ZxKAT1 and tonoplast Na+/H+ antiporter gene ZxNHX were firstly cloned and characterized from the desert xerophyte Z. xanthoxylum. The data showed that all these genes have over 65% similarities with the corresponding genes previously characterised in other plants.2. We found that either under normal condition or under NaCl condition, the mRNA level of ZxNHX was significantly higher in the leaf than in stem or root. Under 5,50 and 150 mM NaCl, the transcript levels of ZxNHX were remarkably induced and up-regulated. Compared with those under 5 or 150 mM NaCl, its expression level under 50 mM NaCl maintained relatively higher and stabler trend over a period. This is a molecular basis that 50 mM NaCl can stimulate growth of Z. xanthoxylum seedlings. Our further research showed that there is a positive correlation between up-regulation of ZxNHX and accumulation of Na+ in Z. xanthoxylum exposed to salt.3. At the whole plant level, we firstly investigated the effects of 0.5 mM amiloride, an inhibitor of NHX, on Na+ accumulation and transport in Z. xanthoxylum exposed to salt (25-100 mM NaCl). The results showed that amiloride disrupted Na+ compartmentation, reduced the amounts of Na+ in leaf and increased the proportions in stem, and down-regulated the expression level of ZxNHX, thus inhibited growth of Z. xanthoxylum. These results further confirmed that ZxNHX plays important roles in Na+ compartmentation and homeostasis of Z. xanthoxylum.4. Under different drought stress levels (50%-15% FWC), Na+ concentrations in Z. xanthoxylum plants remarkably increased, and K+ concentrations in both leaf and root maintained relatively stable. It is also showed that the expression of ZxNHX was positively correlated to Na+ accumulation in leaf.5. Under moderate drought stress (30% FWC) and drought plus salt treatment (30%FWC+50 mM NaCl), ZxSOS1, ZxKAT1 and ZxKUP were preferentially expressed in the stem of Z. xanthoxylum, while ZxNHX and ZxVP1 were preferentially expressed in the leaf tissue. These data suggested that ZxSOS1, ZxKAT1 and ZxKUP in the stem could tansport Na+ from root to leaf, and Na+ was accumulated into the vacuole of leaf by ZxNHX and ZxVP1, to reduce osmotic potential of cells and increase water uptake capacity.In summary, our results suggest that the genes involved in Na+ transport and compartmentation play important roles in response of the desert xerophyte to salt and drought conditions.