The transcription factor CsAtf1 regulates CsErg5B to modulate conidial germination and fludioxonil sensitivity in Colletotrichum siamense

Abstract

Colletotrichum siamense is a predominant causal agent of anthracnose in rubber tree and numerous economically important crops, causing severe yield losses worldwide. Conidial germination represents a critical early step for successful infection, while the high-osmolarity glycerol (HOG) MAPK pathway and ergosterol biosynthesis individually govern fungal development, stress adaptation, and fungicide responses. However, the molecular cross-talk between these two modules remains largely elusive in phytopathogenic fungi. Here, we show that CsErg5B, a sterol C-22 desaturase homolog, functions downstream of the HOG-regulated transcription factor CsAtf1 in C. siamense. CsErg5B was indispensable for ergosterol biosynthesis, conidial germination, appressorium formation, and full virulence. The ΔCsErg5B mutant showed increased conidiation but severely impaired germination, and exhibited elevated resistance to fludioxonil while showing hypersensitivity to azole fungicides. Epistasis analysis using the ΔCsErg5B/ΔCsCyp51G1 double mutant, in which CsCyp51G1 is another downstream target of CsAtf1, revealed that CsErg5B is epistatic to CsCyp51G1 in regulating conidial development and fludioxonil sensitivity. Furthermore, overexpression of CsErg5B significantly suppressed defects in conidial germination and fludioxonil sensitivity in both ΔCsAtf1 and ΔCsPbs2 mutants. Taken together, our findings establish the CsAtf1-CsErg5B axis as a link between HOG MAPK signaling and ergosterol homeostasis, thereby governing conidial germination and fungicide sensitivity in C. siamense. This study provides mechanistic insights into the regulatory network underlying fungal development and fungicide response, and offers promising molecular targets for the integrated management of plant anthracnose.