Rapid necrosis of guard cells is associated with the arrest of fungal growth in leaves of Indian mustard (Brassica juncea) inoculated with avirulent isolates of Leptosphaeria maculans

Germ tubes of both virulent and avirulent isolates of Leptosphaeria maculans penetrate leaves oi Brassica juncea cultivar Stoke through stomatal pores. There is a spectrum of responses at different times after inoculation, ranging from necrosis of guard or mesophyll cells, to the presence of small non-sporulating colonies and large sporulating colonies. In Brassica napus, the same spectrum of events also occurred in compatible and incompatible interactions with L. maculans. However, in incompatible interactions in B. juncea only one response occurred-rapid necrosis of guard cells associated with the arrest of hyphal growth at the stomatal pore. Injection of pycnidiospores of an avirulent isolate into the mesophyll layer of B. juncea leaves, leads to the expression of resistance responses such as deformation of hyphae, accumulation of dark coloured material and of callose. This suggests that B. juncea can express resistance responses in the mesophyll layer in addition to those of the stomata. (C) 1996 Academic Press Limited