HARPIN AND HYDROGEN-PEROXIDE BOTH INITIATE PROGRAMMED CELL-DEATH BUT HAVE DIFFERENTIAL-EFFECTS ON DEFENSE GENE-EXPRESSION IN ARABIDOPSIS SUSPENSION-CULTURES

Programmed cell death is increasingly viewed as a key component of the hypersensitive disease resistance response of plants. The generation of reactive oxygen species (ROS) such as H2O2 triggers a cell death pr ogramme in Arabidopsis suspension cultures following challenge with th e bacterial elicitor harpin. Both harpin and exogenous H2O2 initiate a cell death pathway that requires gene expression, and also act as sig nalling molecules to induce the expression of plant defence genes enco ding enzymes such as phenylalanine ammonia-lyase (PAL), glutathione S- transferase (GST) and anthranilate synthase (ASA1), an enzyme of phyto alexin biosynthesis in Arabidopsis. H2O2 induces the expression of PAL I and GST but not that of ASA1. Harpin initiates two signalling pathwa ys, one leading to increased ROS generation and expression of PALI and GST mRNA, and another leading to increased GST and ASA1 expression, i ndependent of H2O2.