Expression profiles and hormonal regulation of tobacco NtEXGT gene and its involvement in abiotic stress response.

Despite the intensive study of xyloglucan endotransglucosylases/hydrolases, their multifaceted role in plant growth regulation in changing environmental conditions is not yet clarified. The functional role of the large number of genes encoding this group of enzymes is also still unclear. NtEXGT gene encodes one of xyloglucan endotransglucosylases/hydrolases (XTHs) of Nicotiana tabacum L. The highest level of NtEXGT gene expression was detected in young flowers and leaves near the shoot apex. Expression of the NtEXGT gene in leaves was induced by cytokinins, auxins, brassinosteroids and gibberellins. NtEXGT gene was also up-regulated by salinity, drought, cold, cadmium and 10 µM abscisic acid treatments and down-regulated in response to 0 °C and 100 µM abscisic acid. Pretreatment of leaves with fluridone contributed to smaller increase in the level of NtEXGT transcripts in response to drought stress. These data suggest that NtEXGT gene is ABA-regulated and probably implicated in ABA-dependent signaling in response to stress factors. 35S::NtEXGT plants of tobacco showed higher rate of root growth under salt-stress conditions, greater frost and heat tolerance as compared with the wild type tobacco plants.

Expression profiles and hormonal regulation of tobacco expansin genes and their involvement in abiotic stress response.

Changes in the expression levels of tobacco expansin genes NtEXPA1, NtEXPA4, NtEXPA5, and NtEXPA6 were studied in different organs of tobacco (Nicotiana tabacum L.) as well as in response to phytohormone and stress treatments. It was shown that NtEXPA1, NtEXPA4 and NtEXPA5 transcripts were predominantly expressed in the shoot apices and young leaves, but almost absent in mature leaves and roots. The NtEXPA6 mRNA was found at high levels in calluses containing a large number of undifferentiated cells, but hardly detectable in the leaves of different ages and roots. In young leaves, expression levels of NtEXPA1, NtEXPA4 and NtEXPA5 genes were induced by cytokinins, auxins and gibberellins. Cytokinins and auxins were also found to increase NtEXPA6 transcripts in young leaves but to the much lower levels than the other expansin mRNAs. Expression analysis demonstrated that brassinosteroid phytohormones were able either to up-regulate or to down-regulate expression of different expansins in leaves of different ages. Furthermore, transcript levels of NtEXPA1, NtEXPA4, and NtEXPA5 genes were increased in response to NaCl, drought, cold, heat, and 10µM abscisic acid (ABA) treatments but reduced in response to more severe stresses, i.e. cadmium, freezing, and 100µM ABA. In contrast, no substantial changes were found in NtEXPA6 transcript level after all stress treatments. In addition, we examined the involvement of tobacco expansins in the regulation of abiotic stress tolerance by transgenic approaches. Transgenic tobacco plants with constitutive expression of NtEXPA1 and NtEXPA5 exhibited improved tolerance to salt stress: these plants showed higher growth indices after NaCl treatment and minimized water loss by reducing stomatal density. In contrast, NtEXPA4-silenced plants were characterized by a considerable growth reduction under salinity and enhanced water loss. Our findings indicate that expression levels of all studied tobacco expansins genes are modulated by plant hormones whereas NtEXPA1, NtEXPA4, and NtEXPA5 expansins may be involved in the regulation of stress tolerance in tobacco plants.