The Implication of Xyloglucan Endotransglucosylase/ Hydrolase (XTHs) in Tomato Fruit Infection by Penicillium expansum Link. A EVA MIEDES AND ESTER P. LORENCES* Dpto Biología Vegetal. Facultad de Farmacia. Universidad de Valencia. Av. Vicente Andrés Estellés s/n. 46100 Burjassot (Valencia). Spain. e-mail: eva.miedes@uv.es 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. KEYWORDS: Cell wall; fruit infection; hemicelluloses; Solanum lycopersicum L; Penicillium expansum Link; A. Xyloglucan endotransglucosylase/hydrolase INTRODUCTION Plant cell walls provide a physical barrier between pathogens and the internal content of the cells. The high molecular weight polysaccharides, which are the principal components of the cell walls, are cross-linked by both ionic and covalent bonds into a network that resists physical penetration (1–3). Many pathogens release enzymes such as polygalacturonase (4, 5), and pectin lyases (6) which degrade cell wall polysaccharides. The role of these cell wall-degrading enzymes in many aspects of patho- genicity, including penetration, tissue maceration, nutrient acquirement, symptom expression, and plant defense induction has been studied (2, 3). We have previously investigated cell wall metabolism during infection of apple and tomato fruits by P. expansum and reported pectin depolymerisation associated with P. expansum infection, mainly due to the activation of pectinases (7). Hemicelluloses and xyloglucan were also depolymerised by fungal -glucanases during infection (8), which confirms the importance of hemi- cellulose degradation in the breakdown of plant cell walls. In addition to the cell wall-degrading enzymes, another type of cell wall enzyme, involved in the cell wall hemicelluloses metabolism, the xyloglucan endotransglucosilase/hydrolase (XTH), has been reported (9, 10). This enzyme is encoded by the XTHs family, and some members of this family have been shown to act as transglucosylases (XET), catalysing the transfer of a xyloglucan molecule fragment to another xyloglucan molecule, other XTHs act preferentially as hydrolases (XEH), which hydrolyses one xyloglucan molecule, whereas some of XTH proteins present both activities (11, 12). Since xyloglucans comprise the framework of the cell wall, XET action might be significantly important, and its function is considered essential for cell wall remodelling, including cell wall architecture, reconstruction, strength, and extensibility (13–15). It has also been reported that PcXTH gene induction might be associated with cell wall maintenance during “Rocha” pear development and ripening (16), suggesting that XTHs could have a potentially crucial role in fruit texture maintenance. One preliminary and interesting result in our studies in fruit infected by Penicillium was the drastic decrease of the XET activity in infected fruits, and we suggested that this decrease in XET activity during infection could be specifically produced by the fungus by way of an unknown mechanism (8). To further investigate the possible implication of the XTH enzyme and the XET activity with the fungus attack mechanism, we characterized the time course of XET activity and the SlXTHs expression, associated with fruit infection, in two different tomato fruit cultivars. The fungus used was Penicillium expan- sum Link. A., which has a broad host range and infects different fruits, such as tomato. We particularly wanted to evaluate if * To whom correspondence should be addressed. ester.lorences@ uv.es. J. Agric. Food Chem. 2007, 55, 9021–9026 9021 10.1021/jf0718244 CCC: $37.00  2007 American Chemical Society Published on Web 10/24/2007