Superoxide release is necessary for phytoalexin accumulation in Nicotiana tabacum cells during the expression of cultivar-race and non-host resistance towards Phytophthora spp.

Abstract

The relationship between superoxide radical generation, the accumulation of the phytoalexin capsidiol and hypersensitive cell death has been examined in Nicotiana tabacum following challenges by compatible and incompatible races of Phytophthora nicotianae, and the non-host species Phytophthora palmivora. Challenging suspension cell cultures of N. tabacum with zoospores of incompatible isolates of P. nicotianae elicits a biphasic burst of superoxide release. The maximum rate of capsidiol accumulation between 9 and 12 h after challenge coincides with the second oxidative burst and the onset of hypersensitive cell death. Addition of superoxide dismutase or Mn (III) desferal, which scavenge superoxide anions and quench the superoxide burst, suppresses both phytoalexin accumulation and hypersensitive cell death. Mevastatin, an inhibitor of the sesquiterpenoid biosynthesis enzyme HMG-CoA reductase, has no effect on the oxidative burst or hypersensitive cell death, but abolishes capsidiol accumulation. Zoospores of the non-host pathogen P. palmivora also elicit superoxide release, but in a single, broad burst between 3 and 12 h after challenge. Capsidiol accumulates to levels similar to those seen in incompatible host reactions, although the onset of capsidiol accumulation is more rapid in the non-host interaction. As in the incompatible interaction, phytoalexin accumulation and hypersensitive cell death are both inhibited by superoxide scavengers, although scavenging does not render host cells susceptible to infection by non-host zoospores. Our findings indicate that phytoalexin accumulation and hypersensitive cell death in both incompatible and non-host interactions are regulated by pathways that diverge downstream of superoxide release. While hypersensitive cell death and phytoalexin accumulation appear to be necessary for gene-for-gene resistance, other factors cause non-host infection to fail.