PLANT NUCLEAR MIGRATIONS AS INDICATORS OF CRITICAL INTERACTIONS BETWEEN RESISTANT OR SUSCEPTIBLE COWPEA EPIDERMAL-CELLS AND INVASION HYPHAEOF THE COWPEA RUST FUNGUS

Living epidermal cells of cowpea (Vigna unguiculata (L.) Walp.) were e xamined by contrast-enhanced video microscopy during penetration by in vasion hyphae of the monokaryotic stage of the cowpea rust fungus (Uro myces vignae Barclay race 1). In resistant or susceptible host cvs, th e plant nucleus migrated to the penetration site before the fungus had fully penetrated the plant wall and, at sites of unsuccessful infecti on, remained during the formation of a callose-containing papilla. Nuc lear migration was also induced by applying hemicellulase, but not H2O 2, to localized sites of wall damage. Hemicellulase-induced migration was inhibited by calcium chelators and a protein kinase inhibitor, but not catalase. In both resistant and susceptible cvs, the plant nucleu s migrated away from successful infection sites at about the time that the fungal penetration peg made contact with the plant plasma membran e, and the epidermal cell showed no further cytological responses to t he growth of the fungal intraepidermal vesicle. In the susceptible cv. , the nucleus migrated back to the fungus when the latter initiated ti p growth. Inhibitors of transcription or translation did not affect th is migration and only slightly reduced fungal growth. In the resistant cv. in which the invaded cell exhibited a hypersensitive response (HR ), the plant nucleus changed its appearance before the cessation of cy toplasmic streaming and usually did not migrate to the fungus, even if the latter initiated tip growth. Nuclear DNA cleavage usually followe d the subsequent cessation of cytoplasmic streaming. Treatments that d elayed cell death and increased fungal growth also increased the frequ ency of nuclear migration to the fungus. It is argued that these and o ther data suggest that U. vignae negates nonspecific, penetration-indu ced, defence responses upon entering cells of both susceptible and res istant cultivars. The results also suggest that effects on the plant n ucleus are one of the earliest signs of the HR in this system, often p receding the cessation of cytoplasmic streaming and detectable changes in plasma membrane permeability.