6 ARABIDOPSIS-THALIANA HOMOLOGS OF THE HUMAN RESPIRATORY BURST OXIDASE (GP91(PHOX))

An NADPH oxidase analogous to that in mammalian phagocytes has been hy pothesized to produce reactive oxygen species (ROS) in the plant defen ce response. A. thaliana contains at least six gp91(phox) homologues, designated AtrbohA-F (A. thaliana Respiratory Burst Oxidase Homologues ), which map to different positions. Transcripts of three of these gen es can be detected in healthy plants by RNA gel blot analyses. The Atr boh gene products are closely related to gp91(phox) and the intron loc ations suggest a common evolutionary origin. A putative EF-hand Ca2+-b inding motif in the extended N-terminal region of the Atrboh proteins suggests a direct regulatory effect of Ca2+ on the activity of the NAD PH oxidase in plants.