Electron Microscopy of Early Infection Processes in the Panicle Neck of Rice Inoculated with Pyricularia oryzae.

Abstract

Penetration and colonization of panicle neck tissue of a susceptible line, ZTS of rice by Pyricularia oryzae was investigated by electron microscopy. Embedding of the hard impermeable panicle neck tissue was improved by extending the period of resin infiltration. Penetration pegs pierced the outer cell wall of epidermal cells in such a way which suggested that they produced no more than small quantities of extracellular enzymes. Invading hyphae were present in cells 24hr after inoculation. Successful penetration through stomata was not observed. Collar-like wall appositions (papillae) were observed between the host cell wall and plasma membrane beneath the penetration peg in compartmentalized epidermal cells, but not in decompartmentalized ones. Invading hyphae had colonized sclerenchyma, parenchyma and vascular bundles by 48hr after inoculation. Invasion of the sclerenchyma mainly occurred through pit-pairs, where the host cell wall is especially thin. The host cytoplasm in invaded cells showed signs of degeneration, particularly of chloroplasts in parenchyma cells. Such degeneration would seem to be the cause of the panicle neck rot symptoms of infection by P. oryzae. Conidiophores were formed on the panicle necks 4 days after inocculation.