Indole acetic acid attenuates disease severity in potato-Phytophthora infestans interaction and inhibits the pathogen growth in vitro

Auxins are implicated in a wide variety of developmental and physiological processes in plants. Here, we studied the role of auxins on the interaction between a plant pathogen and its host. Excised potato leaves were pre-trea ted with different IAA concentrations and inoculated with a compatible race of the late blight agent Phytophthora infestans. A 10-muM IAA-treatment re sulted in a 50 % reduction of disease severity, the extent of the protectio n being dose-dependent. In potato leaves upon inoculation with P. infestans , beta -1,3 glucanase, chitinase and extensin mRNA levels were down-modulat ed by 10 and 100 muM IAA. IAA also showed an antimicrobial activity in vitr o. A 45 % growth inhibition of the P. infestans mycelia was observed in V8- agar medium containing 1 muM IAA. Again, the percentage of inhibition was d ose-dependent. A 60 % depletion of fungal beta -1,3 glucanase activity was found in extracts from P. infestans growing in a medium containing 1 muM IA A. Furthermore, the IAA-mediated inhibitory effect of P. infestans mycelium growth was overcome by supplying the media with 1 ng(.)mL(-1) external bet a -1,3 glucanase [(1,3;1,4)beta -D-glucan 3(4)-glucano hydrolase]. Thus, an IAA dose-dependent correlation can be established between inhibition of fu ngal growth, and down regulation of plant and fungal beta -1,3 glucanase. T hese results support a putative role of auxins in the potato-P. infestans p atho-system as natural defence for pathogen spread and disease development. (C) 2001 Editions scientifiques et medicales Elsevier SAS.