Hypersensitive death, autofluorescence, and ultrastructural changes in cells of leaf sheaths of susceptible and resistant near-isogenic lines of rice (Pi-zt) in relation to penetration and growth of Pyricularia oryzae

Leaf sheaths of susceptible and resistant (Pi-zt) near-isogenic lines of rice were inoculated with Pyricularia oryzae (Magnaporthe grisea) and the host – parasite interactions in living tissues were continuously observed using fluorescence and bright field microscopy. Growth of invading hyphae in the resistant line was restricted within cells that underwent hypersensitive death during fungal penetration. Experiments using heat shock and NaAsO2 treatments suggested that cell death alone is not responsible for restriction of hyphal growth within the penetrated cell. Fungal hyphae grew unimpeded in dead cells of treated tissue unless the dead cells had also developed the capacity to autofluoresce under blue light. Electron microscopic studies showed that hypersensitive cell death occurred specifically in the resistant line after penetration of the cuticle, and in some cases it occurred before cell walls were completely penetrated. In the susceptible line, wall appositions were induced in the epidermal cells at a similar stage of fungal penetration (i.e., before infection hyphae had reached the cell lumen). Inward indentation of the cuticle and cell wall and inward folding of the cuticle at the penetration site suggested that the cuticle was breached mechanically. Wall penetration appeared to be facilitated by enzyme action because the microfibrils of host cell walls around infection pegs were disorganized and nonhomogeneous in electron density. Key words: resistance, penetration, ultrastructure, autofluorescence, rice blast, Pyricularia oryzae (Magnaporthe grisea).