Four Fusarium oxysporum formae speciales can cause yellows disease of the economically important species in Brassicaceae family. Among these, F. oxysporum f. sp. rapae induces yellows in leaf mustard and Chinese cabbage. SGE1 (SIX gene expression 1) is a transcription factor characterized by the presence of the WOPR box domain. Homologs of SGE1 in other Fusarium species play a crucial role in virulence and regulate the expression of SIX (secreted-in-xylem) effector genes. However, the role of the SGE1 homolog in F. oxysporum f. sp. rapae (FoRP-SGE1) in pathogenesis and fungal development remains unexplored. To investigate its function in regulating pathogenicity and fungal development, gene knockout mutants of FoRP-SGE1 (ΔFoRP-SGE1) were generated and validated. ΔFoRP-SGE1 showed a reduction in conidiation, but normal colony growth and conidial germination. Notably, ΔFoRP-SGE1 completely lost pathogenicity, but it retained the ability to colonize leaf mustard plants, indicating that FoRP-SGE1 is a key pathogenicity factor. Expression of SIX9 and SIX14 was significantly diminished in ΔFoRP-SGE1. Furthermore, most chlamydospores of ΔFoRP-SGE1 lacked the outermost fibrillose coat. Germination of ΔFoRP-SGE1 chlamydospores was also impaired under various stress conditions, including osmotic stress, drought, UV exposure, and fluazinam toxicity. This study presents, for the first time, the role of a Fusarium SGE1 homolog in the morphology and persistence of chlamydospores. Collectively, our findings suggest that FoRP-SGE1 is a critical pathogenicity factor in the leaf mustard-F. oxysporum f. sp. rapae pathosystem and is involved in the development of the fibrillose coat of chlamydospores and their resistance to environmental stresses.
Tsai et al. (2026) studied this question.