Ultrastructural evidence of differential changes in transcription, translation, and cortical microtubules during in planta penetration of cells resistant or susceptible to rust infection

Abstract

Resistance to race 1 of the cowpea rust fungus (Uromyces vignae) in cowpea (Vigna unguiculata) plants results from a resistance gene-dependent cell death process (the hypersensitive response), which involves the execution of a defined progression of cytological events. To reveal the cellular changes which occur prior to the commitment to hypersensitive cell death in resistant plants, we performed a stereological ultrastructural analysis of infected midvein epidermal cells in both susceptible and resistant leaves. Analyses of nuclear pore and polyribosome density, and nucleolus structure, suggest that levels of transcription and localized translation significantly increase prior to fungal contact with the plant plasma membrane in resistant cells whereas they decrease in susceptible cells, which remain alive for days after infection.Transcriptional activity appears to cease in resistant cells, and the nucleus starts to shrink, at the time when cytoplasmic streaming stops and the cell becomes committed to the death process. By the time protoplast collapse has begun, 31.6 nm electron-opaque particles containing nucleic acid are visible within the nucleus. The data suggest that transcriptional and translational activity leading to hypersensitive cell death begins as the fungus grows through the outer epidermal wall.