Characterization of Major Cell-Wall-Degrading Enzymes Secreted by Diaporthe spp. Isolate Z1-1N Causing Postharvest Fruit Rot in Kiwifruit in China

Abstract

Pathogen-induced fruit decay is a significant threat to the kiwifruit industry, leading to considerable economic losses annually. The cell-wall-degrading enzymes (CWDEs) secreted by these pathogens are crucial for penetrating the cell wall and accessing nutrients. Among them, Diaporthe species are recognized as major causal agents of soft rot in kiwifruit, yet their pathogenic mechanisms are not well understood. In this study, we explored the production of various CWDEs secreted by Diaporthe Z1-1N, including polygalacturonase (PG), polymethylgalacturonase (PMG), polygalacturonic acid transeliminase (PGTE), pectin methyltranseliminase (PMTE), endoglucanase (Cx), and β-glucosidase (β-glu), both in liquid cultures and within infected kiwifruit tissues. Our findings revealed significant activities of two pectinases (PG and PMG) and cellulases (Cx and β-glu) in the infected tissues. In contrast, very low levels of PMTE and PGTE activities were observed under the same conditions. When orange pectin served as the carbon source, PG and PMG showed notable activities, while PMTE and PGTE remained inactive. Moreover, the activities of Cx and β-glu significantly decreased by more than 63 times in the liquid medium with carboxymethyl cellulose (CMC) as the carbon source compared to their levels in infected kiwifruit. A further analysis indicated that the necrotic lesions produced by pectinase extracts were larger than those produced by cellulase extracts. Notably, four enzymes—PG, PMG, Cx, and β-glu—exhibited high activities on the third or fourth day post-infection with Diaporthe Z1-1N. These results suggest that Diaporthe Z1-1N secretes a range of CWDEs that contribute to kiwifruit decay by enhancing the activities of PG, PMG, Cx, and β-glu. This study sheds light on the pathogenicity of Diaporthe in kiwifruit and highlights the importance of these enzymes in the decay process.