A study of abiotic stress protecting xyloglucan endotransglycosylase (XTH) genes in plants

Xyloglucan endotransglycosylase/hydrolase (XTHs) enzymes which are found in the cell wall of all land plants, are recognized as cell-wall modifying enzymes having diverse roles during cell growth, maturation and response to external stresses. In many experiments, it was demonstrated that some of the XTH genes are upregulated during various abiotic stresses. In a recent work, it was shown that the heterologous over-expression of a hot pepper XTH gene increased the resistance to salt and drought stresses in Arabidopsis and tomato plants. This project aimed to examine a capsicum XTH enzyme (CaXTH2) at the genomic, transcriptomic and protein levels to get better insights about how those enzymes function and their roles under salt stress. Substrate specificity and enzyme characterization analyses show that unsubstituted xyloglucan was highly preferred by the CaXTH enzymes. CaXTH2 enzyme has shown the highest activity when tamarind seed xyloglucan (TXG) polysaccharide and XXXG (X7) oligosaccharide were used as donor-acceptor substrate couple. The next highest activity was shown when hydroxyethyl cellulose (HEC) was used as donor substrate with xyloglucan acceptor oligosaccharides. CaXTH2 enzyme gave activity with other oligosaccharides such as XT, CT, BB etc. with almost 0,1 percent of the TXG-XGO activity level, and this also supported that it had only low amounts of heterotransgylcosylase activity. Different pepper varieties had been phenotypically observed and they showed susceptibility or tolerance under salt stress conditions. Genomic and transcriptomic analyses have demonstrated that CaXTH2 gene was upregulated under salt stress conditions. However, CaXTH2 does not seem to be directly responsible for the stress resistance. Nevertheless, this does not prove that XTH genes have no effect in stress tolerance under native promoters, but that they are likely to be part of a larger, more complex mechanism.