Effect Of Glucose And Xyloglucan On The Resistance To Cadmium And Aluminum Toxicity Of Arabidopsis

When plants roots are growing in an environment with high concentration of metal ions, such as the cadmium (Cd) and aluminium (Al), plants will produce some special mechanisms to alleviate their toxicity. Among them, the most typical mechanisms are to reduce the entering of metal ions into the cell by fixation in the apoplast, or to sequester those metal ions as the metal ion-organic acid complex into the organ, such as vacuoles, to prevent them from taking part in metabolism. In this paper, we used wild type and some cell wall related mutants of Arabidopsis thaliana to investigate the contribution of glucose and xyloglucan in alleviateing Cd and Al toxicity, and possible destination of Cd or Al within cell; furthermore, we compared the enzymetic characteristics of two xyloglucan endotransglucosy -lase/hydrolases (XTHs), (XTH15 and XTH31), which play an important role in the formation of the cell wall by cracking and polymerizing of the β(1-4)- xyloglucan, the major component of hemicellulose. The major results are as follows:1. Exogenous glucose alleviates cadmium toxicity by increasing cadmium fixation in root cell wall and sequestration into vacuole in Arabidopsis thalianaGlucose is involved in not only plant physiological and developmental events but also plant responses to abiotic stresses. Here we found that exogenous glucose improved root and shoot growth, reduced shoot cadmium (Cd) concentration, and rescued Cd-induced chlorosis in Arabidopsis thaliana under Cd stressed condition. Glucose increased Cd retained in the roots, thus reduced its translocation from root to shoot significantly. When root Cd was fractionized into different cell wall components, the most Cd was found in the hemicellulose 1. Glucose combined with Cd (Glu+Cd) treatment did not affect the content of pectin and its binding capacity of Cd while it increased the content of hemicelluloses 1 and the amount of Cd retained in it significantly. Furthermore, LeadmiumTM Green staining indicated that more Cd was compartmented in the vacuole in Glu+Cd treatment compared with Cd treatment alone, which was in accordance with the significant upregulation of the expression of tonoplast-localized metal transporters, suggesting that compartmentation of Cd into vacuoles also contributes to the glucose-alleviated Cd toxicity. Taken together, we demonstrated that glucose-alleviated Cd toxicity is mediated through increasing Cd fixation in the root cell wall and sequestration into the vacuoles.2. XTH15 controls Al sensitivity by modification cell wall properties and up-regulating the expression of AIS3 in ArabidopsisThe functional characterization of AtXTH15 in its relationship with Al sensitivity was investigated. We found that xth15 mutant is more Al resistance, although the Al accumulated in the root was the same as Col-0. Subcellular localization and histochemical analysis showed that XTH15 was at the plasma membrane and expressed in the elongation zone of the root. xth15 showed the same XET activity as Col-0 and accumulated significant less Al in the cell wall. We demonstrated that the enhanced Al resistance in the mutant is a result of Al internal detoxifying mechanism through up-regulating the expression of ALS3, which involves in the redistribution of Al into sites with less metabolic activities.3. The difference of key catalytic properties of two very different Arabidopsis XTHs (XTH15 and XTH31, heterologously produced in Pichia), both of which are aluminium-repressedXyloglucan plays an important structural role in primary cell walls, possibly tethering adjacent microfibrils and restraining cell expansion. There is therefore considerable interest in understanding the role of xyloglucan endotransglucosylase /hydrolases (XTHs), which are encoded in Arabidopsis by a 33-member gene family. We compared the enzymological properties of two very different Arabidopsis XTHs, both of which are aluminium-repressed, heterologously produced in Pichia. XTH15, a classical group-Ⅰ/Ⅱ XTH, had high XET and undetectable XEH activity in vitro; its XET Km values were 31 μM XXXGol (acceptor substrate) and 2.9 mg/ml xyloglucan (donor substrate). In contrast, XTH31, a group-HI-A XTH, showed predominant XEH activity and only slight XET activity in vitro; its XET Km was 86μM XXXGol (acceptor), indicating a low affinity of this predominantly hydrolytic protein for a transglycosylation acceptor substrate. The Km of XTH31’s XEH activity was 1.6 mg/ml xyloglucan. For both proteins, the preferred XET acceptor substrate, among five cellotetraitol-based oligosaccharides tested, was XXXGol. XTH31’s XET activity was strongly compromised when the second Xyl residue was galactosylated. XTH15’s XET activity, in contrast, tolerated substitution at the second Xyl residue. The two enzymes also showed different pH preferences, XTH31 exhibiting an unusually low pH optimum and XTH15 an unusually broad optimum. XTH31’s hydrolase activity increased almost linearly with decreasing pH in the apoplastic range,6.2-4.5, consistent with a possible role in ’acid growth’. In conclusion, these two Al3+-repressed XTHs differ, in several important enzymic features, from other members of the Arabidopsis XTH family.