Histological Observation of Cellular Responses of Barley Leaves to Powdery Mildew Infection by UV-Fluorescence Microscopy

Abstract

The hypersensitive response was demonstrated as fluorescent collapsed cells in powdery-mildewed barley leaves with an aid of fluorescence microscopy. The fluorescent cells were formed in both epidermal and mesophyll cells, and these were exclusively found in incompatible barley. Papilla, halo, and lateral wall in the penetration sites were also detected with fluorescence, although, their response seemed to be nonspecific for resistance. When the parasite ceased growth at papillar stage, bright yellow autofluorescence was observed only in papilla and the adjacent cell wall within a radius of halo in infected epidermal cells. Those cells were not collapsed. When fluorescence was detected in entire areas of infected epidermal cells, the cells were collapsing, though mature haustoria were never found in those cells. The collapsed mesophyll cells shrinked and became multiangular, forming larger intercellular space in the infected leaf tissues. They were always fluorescent and considered to be dead because of the structural disintegration. In these cases, haustoria were usually formed in the epidermal cells above those collapsed cells. Cellular fluorescence observed in the transverse sections was emitted without any fluoresceins. Thus, it was suggested that the fluorescing compounds in papillae and these collapsed cells could be attributed to compounds other than callose which has been reported to accumulate in papillae.