Xyloglucan endotransglucosylase/hydrolase (XTH) overexpression affects growth and cell wall mechanics in etiolated Arabidopsis hypocotyls.

Growth and biomechanics of etiolated hypocotyls from Arabidopsis thaliana lines overexpressing xyloglucan endotransglucosylase/hydrolase AtXTH18, AtXTH19, AtXTH20, and PttXET16-34 were studied. Overexpression of AtXTH18, AtXTH19, and AtXTH20 stimulated growth of hypocotyls, while PttXET16-34 overexpression did not show this effect. In vitro extension of frozen/thawed hypocotyls measured by a constant-load extensiometer started from a high-amplitude initial deformation followed by a slow time-dependent creep. Creep of growing XTH-overexpressing (OE) hypocotyls was more linear in time compared with the wild type at pH 5.0, reflecting their higher potential for long-term extension. XTH-OE plants deposited 65-84% more cell wall material per hypocotyl cross-sectional area than wild-type plants. As a result, their wall stress under each external load was lower than in the wild-type. Growing XTH-OE hypocotyls had higher values of initial deformation·stress(-1) compared with the wild type. Plotting creep rates for each line under different loads against the respective wall stress values gave straight lines. Their slopes and intercepts with the abscissa correspond to ϕ (in vitro cell wall extensibility) and y (in vitro cell wall yield threshold) values characterizing cell wall material properties. The wall material in XTH-OE lines was more pliant than in the wild type due to lower y values. In contrast, the acid-induced wall extension in vitro resulted from increasing ϕ values. Thus, three factors contributed to the XTH-OE-stimulated growth in Arabidopsis hypocotyls: their more linear creep, higher values of initial deformation·stress(-1), and lower y values.

The implication of xyloglucan endotransglucosylase/hydrolase (XTHs) in tomato fruit infection by Penicillium expansum Link. A.

In general, cell wall-degrading enzymes produced by plant pathogenic fungi are considered important pathogenicity factors. In this work, we evaluate the implication of xyloglucan endotransglucosylase/hydrolase (XTHs), a potential hemicellulosic repairing enzyme, in the infection mechanism process by the fungus. This study investigated the SlXTHs expresion and xyloglucan endotransglucosylase (XET) activity during infection of two tomato fruit cultivars by Penicillium expansum Link. A. In infected fruits, XET specific activity decreased drastically after long infection periods, 24 and 48 h for Canario and Money Maker tomato fruits, respectively. Real Time RT-PCR of eleven SlXTHs also showed a decrease in expression as the infection progressed in both tomato fruit cultivars. Results suggest that the reduction in SlXTHs expression during infection might be related with the fungus attack mechanism. We suggest a possible transcriptional control of the SlXTHs expression by the fungus, causing a decrease in XET activity and, consequently, lower xyloglucan endotransglucosylation, which changes the xyloglucan structure. These changes might increase the fruit softening and wall disassembly, facilitating the fungus colonization and the progress of the infection.

Xyloglucan endotransglycosylase activity in pine hypocotyls. Intracellular localization and relationship with endogenous growth.

Since xyloglucan depolymerization has been proposed as one of the biochemical bases for cell wall-loosening in gymnosperms, we characterized xyloglucan endotransglycosylase (XET) activity during pine hypocotyl growth to establish a possible relationship. XET activity was measured as the incorporation of [3 H]XXXGol into partially purified pine hypocotyl xyloglucan. XET specific and total activity was determined in the subapical and basal segments of pine hypocotyls at two different stages of growth in different subcellular fractions. XET activity was found in the apoplastic fluid, the symplastic fluid, and in the fraction of proteins ionically and covalently bound to the cell walls with different distribution profiles. The results showed a relationship between XET activity and hypocotyl growth in all the fractions, suggesting an important role for XET during growth. Consequently, the suggested growth-promoting effect of XET in angiosperms can also be extended to gymnosperms. Also, the results demonstrate that XET bound to the cell wall is able to act on endogenous wall-bound xyloglucan as well as soluble polymeric xyloglucan, using them as substrates for the endotransglycosylation reaction.

Apple (Malus domestica) and tomato (Lycopersicum esculentum) fruits cell-wall hemicelluloses and xyloglucan degradation during Penicillium expansum infection.

We characterized the changes in cell-wall hemicellulosic polysaccharides and the hemicellulose-degrading enzymes associated with apple and tomato fruits infected by Penicillium expansum. Our results showed a reduction in the molecular mass of hemicelluloses, with this reduction being particularly notable in the xyloglucan associated with P. expansum infection. The activation of fungal beta-glucanases was also highlighted. Fruit xyloglucan endotransglucosylase/hydrolase (XTH)-specific activity decreased drastically during the infection process in both apple and tomato fruits. We suggest that XTH reduction during the infection might be related with the fungus attack mechanism. We also suggest that the decrease in activity and the consequent lower xyloglucan endotransglucosylation, together with the increase in endoglucanases, would permit fungal access to the cellulose-xyloglucan network, increase the efficiency of cellulose hydrolysis, and thus facilitate the progress of the fungal infection. The results confirm the importance of hemicellulose degradation in the breakdown of plant cell walls, causing cell-wall loosening, increasing the porosity of the wall, and allowing the colonization of plant tissue.

Overexpression of a cell wall enzyme reduces xyloglucan depolymerization and softening of transgenic tomato fruits.

Xyloglucan xyloglucosyltransferase/endohydrolase (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151) has been proposed to have a dual role integrating newly secreted xyloglucan chains into an existing wall-bound xyloglucan and restructuring existing cell wall material by catalyzing transglucosylation between previously wall bound xyloglucan molecules. In this work we generated transgenic tomatoes with altered levels of an XTH gene. These transgenic fruits showed significant overexpression of the XTH proteins in comparison with the wild type. Specific XET activity was approximately 4.33 fold higher in the transgenic fruits compared with the wild type fruits, although in both cases the activity decreased during fruit development. Cell wall hemicelluloses extracted with 24% KOH showed a depolymerization of total sugar and xyloglucan during ripening, although this depolymerization was much lower in the transgenic than in the wild type tomatoes. These results suggest that the increased XET activity in the transgenic plants was responsible for the lower xyloglucan depolymerization. Fruit softening, during ripening, was lower in the transgenic tomatoes, indicating that the xyloglucan structure is related with the softening mechanism and that XET is one of the enzymes involved in the process. We suggest that the role of XET during fruit growth and ripening could be related to the maintenance of the structural integrity of the cell wall and the decrease in activity during ripening might contribute to the fruit softening.

Xyloglucan endotransglucosylase/hydrolases (XTHs) during tomato fruit growth and ripening.

Depolymerization of cell wall xyloglucan has been proposed to be involved in tomato fruit softening, along with the xyloglucan modifying enzymes. Xyloglucan endotransglucosylase/hydrolases (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151) have been proposed to have a dual role integrating newly secreted xyloglucan chains into an existing wall-bound xyloglucan, or restructuring the existing cell wall material by catalyzing transglucosylation between previously wall-bound xyloglucan molecules. Here, 10 tomato (Solanum lycopersicum) SlXTHs were studied and grouped into three phylogenetic groups to determine which members of each family were expressed during fruit growth and fruit ripening, and the ways in which the expression of different SlXTHs contributed to the total XET and XEH activities. Our results showed that all of the SlXTHs studied were expressed during fruit growth and ripening, and that the expression of all the SlXTHs in Group 1 was clearly related to fruit growth, as were SlXTH12 in Group 2 and SlXTH6 in Group 3-B. Only the expression of SlXTH5 and SlXTH8 from Group 3-A was clearly associated with fruit ripening, although all 10 of the different SlXTHs were expressed at the red ripe stage. Both total XET and XEH activities were higher during fruit growth, and decreased during fruit ripening. Ethylene production during tomato fruit growth was low and experienced a significant increase during fruit ripening, which was not correlated either with SlXTH expression or with XET and XEH activities. We suggest that the role of XTH during fruit development could be related to the maintenance of the structural integrity of the cell wall, and the decrease in XTHs expression, and the subsequent decrease in activity during ripening may contribute to fruit softening, with this process being regulated through different XTH genes.