Cytological analysis of leaf lesions induced by Pyricularia oryzae and Tenuazonic acid

Leaf lesions induced by Pyricularia oryzae and by tenuazonic acid (TA) on one resistant and one susceptible rice varieties were examined cytologically. TA is a phytotoxic metabolic produced by P. oryzae during its infection of rice leaves. It was observed that the mode of spreading of brown margin lesions caused by both the pathogen and TA was similar either on compatible rice varieties (show-appearing light browning, after 96 h if it occurs) or on some varieties with high level of partial resistance (rapid and intense browning, in 48 h). Leaf sections in lesions of the susceptible variety Maratelli and the resistant variety IRAT 13 were examined four days after inoculation with a compatible isolate C141 or with TA. It was observed that the hyphae appeared in epidermal or in parenchyma cells, in intercellular spaces in the mesophyll and in vascular tissues. In the resistant variety hyphae were rare and located within the brown area which was formed by a deposit of pigments in the cell walls and the cells themselves constituting an impervious barrier which prevented the diffusion of hydrophobic molecules and the progression of hyphae. Also, it was shown that most of peroxidase activity induced by P. oryzae and TA, mainly parietal, was located in the brown area. It was greater in the resistant variety than in the susceptible one and more intense when induced by P. oryzae. Examination of ultra thin sections of infected leaves revealed that leaf cells of Maratelli did not present any structural change except for the presence of osmophilic inclusions in a few cells. In IRAT 13 inoculated with TA some damaged cells were observed due to bursting of chloroplasts and swelling of thylakoids. Intracellular green lipid globules were present in healthy tissues but did not seem to exist in younger leaves of Maratelli. These lipid globules were abundant, brownish and of a larger size in vacoule of the brown cells surrounding the lesions caused by P. oryzae and TA. These globules were fewer in number and of lighter color in Maratelli. The presence of these globular structures in the cytoplasm and the vacoule suggested that they were first produced in the cytoplasm and then accumulated in the vacuole followed by lysis of the cytoplasm membrane