ROLE OF CALCIUM IN SIGNAL-TRANSDUCTION DURING THE HYPERSENSITIVE RESPONSE CAUSED BY BASIDIOSPORE-DERIVED INFECTION OF THE COWPEA RUST FUNGUS

The hypersensitive response (HR) of disease-resistant plant cells to f ungal invasion is a rapid cell death that has some features in common with programmed cell death (apoptosis) in animals. We investigated the role of cytosolic free calcium ([Ca2+](i)) in the HR of cowpea to the cowpea rust fungus. By using confocal laser scanning microscopy in co njunction with a calcium reporter dye, we found a slow, prolonged elev ation of [Ca2+](i) in epidermal cells of resistant but not susceptible plants as the fungus grew through the cell wall. [Ca2+](i) levels dec lined to normal levels as the fungus entered and grew within the cell lumen. This elevation was related to the stage of fungal growth and no t to the speed of initiation of subsequent cell death. Elevated [Ca2+] (i) levels also represent the first sign of the HR detectable in this cowpea-cowpea rust fungus system. The increase in [Ca2+](i) was preven ted by calcium channnel inhibitors. This effect was consistent with ph armacological tests in which these inhibitors delayed the HR. The data suggest that elevation of [Ca2+](i) is involved in signal transductio n leading to the HR during rust fungal infection.