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The HOG MAPK pathway mediates diverse cellular and physiological processes, including osmoregulation and fungicide sensitivity, in phytopathogenic fungi. However, the molecular mechanisms underlying HOG MAPK pathway-associated stress homeostasis and pathophysiological developmental events are poorly understood. Here, we demonstrated that the oxalate decarboxylase CsOxdC3 in Colletotrichum siamense interacts with the protein kinase kinase CsPbs2, a component of the HOG MAPK pathway. The expression of the CsOxdC3 gene was significantly suppressed in response to phenylpyrrole and tebuconazole fungicide treatments, while that of CsPbs2 was upregulated by phenylpyrrole and not affected by tebuconazole. We showed that targeted gene deletion of CsOxdC3 suppressed mycelial growth, reduced conidial length, and triggered a marginal reduction in the sporulation characteristics of the Δ CsOxdC3 strains. Interestingly, the Δ CsOxdC3 strain was significantly sensitive to fungicides, including phenylpyrrole and tebuconazole, while the CsPbs2 -defective strain was sensitive to tebuconazole but resistant to phenylpyrrole. Additionally, infection assessment revealed a significant reduction in the virulence of the Δ CsOxdC3 strains when inoculated on the leaves of rubber tree (Hevea brasiliensis). From these observations, we inferred that CsOxdC3 crucially modulates HOG MAPK pathway-dependent processes, including morphogenesis, stress homeostasis, fungicide resistance, and virulence, in C. siamense by facilitating direct physical interactions with CsPbs2. This study provides insights into the molecular regulators of the HOG MAPK pathway and underscores the potential of deploying OxdCs as potent targets for developing fungicides. [ABSTRACT FROM AUTHOR]

• CsAtf1 affects the regulation of fludioxonil resistance. • CsAtf1 deletion attenuates the virulence of C. siamense. • CsAtf1 deletion mutants showed slightly retarded mycelial growth, small conidia, slow spore germination, and abnormal appressorium formation. In phytopathogenic fungi, the HOG MAPK pathway has roles in osmoregulation, fungicide sensitivity, and other processes. The ATF1/CREB-activating transcription factor Atf1 is a regulator that functions downstream of the HOG MAPK pathway. Here, we identified a gene, designated CsAtf1 , that encodes a bZIP transcription factor in Colletotrichum siamense , which is the main pathogen that causes Colletotrichum leaf fall disease in rubber trees in China. CsAtf1 localizes to the nucleus. Its mRNA expression correlates positively with that of CsPbs2 and CsHog1 in the HOG MAPK pathway in response to activator (anisomycin), inhibitor (SB203580) and fludioxonil treatments. The CsAtf1 deletion mutant showed slightly retarded mycelial growth, small conidia, slow spore germination, and abnormal appressorium formation. This mutant showed the increased spore germination rate after fludioxonil treatment and more resistance to the fungicide fludioxonil than did the wild-type fungus. However, unlike deletion of Pbs2 or Hog1 , which resulted in greater sensitivity to osmotic stress, the CsAtf1 deletion induced slightly increased resistance to osmotic stress and the cell wall stress response. The ΔCsAtf1 strain also exhibited significantly reduced virulence on rubber tree leaves. These data revealed that CsAtf1 plays a key role in the regulation of fludioxonil sensitivity and in pathogenicity regulation in C. siamense. [ABSTRACT FROM AUTHOR]