Overexpression of Populus euphratica xyloglucan endotransglucosylase/hydrolase gene confers enhanced cadmium tolerance by the restriction of root cadmium uptake in transgenic tobacco

摘要

Cadmium (Cd2+) is a toxic heavy metal impairing plant growth and development. Xyloglucan endotransglucosylase/hydrolase gene (XTH) is involved in the plant response to heavy metal toxicity, in addition to controlling cell wall extensibility. However, the link between XTH and Cd2+ stress has not yet been established in higher plants. PeXTH expression was up-regulated by 1.2-2.1-fold in Populus euphratica roots and leaves upon Cd2+ exposure (40-80 mu M CdCl2). Cellular Cd2+ analysis and flux data showed that the cadmium-elicited expression of PeXTH markedly restricted Cd2+ uptake and accumulation in P. euphratica roots. Moreover, tobacco plants overexpressing PeXTH were more tolerant to Cd2+ stress (80 mu M CdCl2) than wild-type tobacco in terms of root and shoot growth. Transgenic lines accumulated 49-58% less Cd2+ in root apical and mature regions, as compared to the wild type. The less buildup of Cd2+ in roots of transgenic lines was the result of lower influx of Cd2+ under Cd2+ stress. It is noting that transgenic plants displayed 56-87% higher xyloglucan degradation activity (XDA) than the wild type, leading to a 25-27% decline of xyloglucan content in the root cell walls. Therefore, overexpression of PeXTH increased the activity of XDA in transgenic plants, which enhanced the degradation of xyloglucan in the wall. The down-regulated amount of xyloglucan led to less binding sites for Cd2+ and thus reduced the root Cd2+ uptake and buildup in transgenic plants. Consequently, the Cd2+ toxicity was eventually alleviated in transgenic tobacco