L-cysteine inhibits the in vitro and in vivo growth of Alternaria alternata via disrupting cell membrane integrity and compromising cell wall structure.

Alternaria alternata, a ubiquitous saprophytic fungus, is the causative agent of black spot decay in postharvest fruit and vegetables. This study was conducted to evaluate the effects of L-cysteine (Cys) on the in vitro and in vivo growth of A. alternata and to elucidate the possible mechanisms involved. Results showed that Cys effectively alleviated the postharvest black spot disease of 'Zaosu' pear fruit. Cys at various concentrations distinctly suppressed both conidial germination and mycelial growth of A. alternata, with a half-maximal inhibitory concentration of 1.0 g L-1. Moreover, 1.0 g L-1 Cys reduced the relative conductivity of the cell membrane, which led to increased leakage of nucleic acids, soluble sugars, and soluble proteins, as well as elevated malondialdehyde levels. Additionally, Cys distinctly down-regulated the transcription levels of multiple genes involved in β-1,3-glucan metabolism, chitin, ergosterol, phosphatidylcholine, and 1,8-dihydroxynaphthalene (DHN)-melanin synthesis, including AaHxk, AaPgm, AaUgp, AaRHO1, AaGS, AaPagm, AaUap, AaCHS2/3/4/5/6, AaERG1/3/6/9/11/26, AaPsd, AaPem1/2, AaCct, AaCki, AaPKS, Aa4HNR, Aa3HNR, AaSCD, and AaLac. Conversely, it enhanced the transcription levels of Aaβ-1,3-glu. Our findings suggest that Cys compromised the integrity of the cell wall and cell membrane, thereby suppressing the growth of A. alternata and demonstrating promising effects against black spot disease.