Antifungal effects of cysteine towards Eutypa lata, a pathogen of vineyards

Cysteine inhibited mycelial growth of the pathogenic fungus affecting grapevines Eutypa lata Pers. Fr. Tul. and C. Tul. in a concentration-dependent manner. The threshold value (defined by the concentration inducing a growth inhibition higher than 5%) was 0.5 mM. A 10 mM concentration induced a complete inhibition of growth and triggered necrotic processes as evidenced by an increasing number of nuclei stained by propidium iodide. In conditions mimicking the plant environment (in particular, a pH near the apoplastic value, i.e. 5.5), 6 mM cysteine induced dramatic modifications in the structural organization of the mycelium (wall, mitochondria, vacuoles and nucleus) leading to death of the hyphae. The antifungal effect of the molecule increased at the acidic experimental pH (pH 4.1). The effect was highly specific to cysteine since modifying the molecular arrangement or masking the SH-function hindered the antifungal efficiency. Cysteine spectrum of action was broad among the various strains of E. lata tested. However, a lower efficiency was observed against fungal species intervening in other grapevine diseases (esca, black dead arm). Besides its direct antifungal effect, the role of cysteine presents particular interest in the fight against fungal pathogens since it triggered an excretion of ergosterol, a compound with elicitor properties. Therefore, cysteine may indirectly increase plant defense reactions.