Differential spatiotemporal dynamics of cell wall-degrading enzymes underlie pathogenicity variation in two Alternaria species causing kiwifruit soft rot

Kiwifruit soft rot caused by fungal pathogens can result in substantial postharvest losses. While Alternaria alternata and A. tenuissima have been implicated in this disease, their pathogenic mechanisms remain poorly characterized. This study investigated the spatiotemporal dynamics of cell wall-degrading enzyme (CWDE) activities during infection of kiwifruit by A. alternata (strain P1-1W) and A. tenuissima (strain P1-2W). Using a zonal sampling approach (healthy, marginal, and lesioned tissues) over 0–6 days post-inoculation (dpi), we analyzed the activities of six CWDEs: polygalacturonase (PG), polymethylgalacturonase (PMG), PG trans-eliminase (PGTE), PMG trans-eliminase (PMTE), cellulase (Cx), and β-glucosidase (β-Glu). Our results demonstrated that infection time, rather than spatial proximity to lesions, was the primary determinant of CWDE activity profiles for both pathogens. Despite this shared temporal regulation, the two species exhibited distinct enzymatic strategies. A. alternata deployed a PMG-centric, biphasic infection strategy, characterized by early and sustained PMG induction (peaking at 37.08 U/mL at 4 dpi), with significant contributions from Cx, β-Glu and PG at later stages. In contrast, A. tenuissima adopted a cellulase-dominant strategy, where Cx and β-Glu were the principal drivers of tissue maceration (exhibiting sustained elevation from 3–6 dpi). Two pectinases (PG and PMG) played transient, early roles. Activities of PGTE and PMTE were negligible for both species. These findings reveal that closely related Alternaria species employ divergent, temporally programmed CWDE arsenals to infect kiwifruit, providing new insights into their pathogenic mechanisms.