PIOX, A NEW PATHOGEN-INDUCED OXYGENASE WITH HOMOLOGY TO ANIMAL CYCLOOXYGENASE

Changes in gene expression induced in tobacco leaves by the harpin Hrp N protein elicitor were examined, and a new cDNA, pier (for pathogen-i nduced oxygenase), with homology to genes encoding cyclooxygenase or p rostaglandin endoperoxide synthase (PGHS), was identified. In addition to the amino acid identity determined, the protein encoded by pier is predicted to have a structural core similar to that of ovine PGHS-1. Moreover, studies of protein functionality demonstrate that the PIOX r ecombinant protein possesses at least one of the two enzymatic activit ies of PGHSs, that of catalyzing the oxygenation of polyunsaturated fa tty acids. pier transcripts accumulated after protein elicitor treatme nt or inoculation with bacteria. Expression of pier was induced in tis sues responding to inoculation with both incompatible and compatible b acteria, but RNA and protein accumulation differed for both types of i nteractions. We show that expression of pier is rapidly induced in res ponse to various cellular signals mediating plant responses to pathoge n infection and that activation of pier expression is most likely rela ted to the oxidative burst that takes place during the cell death proc esses examined. Cyclooxygenase catalyzes the first committed step in t he formation of prostaglandins and thromboxanes, which are lipid-deriv ed signal molecules that mediate many cellular processes, including th e immune response in vertebrates. The finding of tobacco PIOX suggests that more similarities than hitherto expected will be found between t he lipid-based responses for plant and animal systems.