Functional Genomics of the Arabidopsis gp91-phox NADPH Oxidase Family

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Plants recognize and respond to pathogen attack with specific defensemechanisms. The rapid production of Reactive Oxygen Intermediates (ROI),from molecular oxygen, in plants has been correlated with resistanceresponses against pathogens. These oxygen derivatives can be toxic toinvading microbes and can serve as signals to the plant cells beinginvaded. ROI accumulation is frequently correlated with the HypersensitiveResponse (HR), a kind of programmed cell death (PCD) which probably alsorequires nitric oxide (NO) formation. In addition, ROI play a role inmorphogenetic phenomena like cell elongation, strengthening of the cellwall, lignification and response to other stresses such as wounding attackby insects, saline and cold stress. Examples of developmentally regulatedcell death include the formation of specialized structures like xylem,organ sculpting (formation of certain leaf lobes, floral organ abortion),the degeneration of organs once they have accomplished their function (theembryo suspensor, or the aleurone and the endosperm after the seedgermination) and senescence. In flowers, the interior cell layer of theanther locule (the tapetum) undergoes PCD during pollen development. Therole of ROI in these processes is largely unexplored.Several biochemical mechanisms have been proposed to explain ROI generationby plants. However, various lines of evidence implicate an NADPH oxidase,analogous to that which generates the respiratory burst in mammalianphagocytes, as the source of the ROI detected in plants, at least uponinfection.This investigator has found transposon insertions in the coding region of 8 Arabidopsis Atrbohhomologs of the mammalian gp91phox, the key membrane bound NADPH-bindingflavocytochrome b558 so far identified. These mutant lines are anincomparable tool to dissect the role of specific gp91phox homologues, notonly in plant defense but also in relation to developmental processes, PCDand other responses to the environment. Preliminary data demonstratethat there is redundancy in this gene family, and that ROI generated viathe gp91-phox protein play a role in at least one critical developmentalprocess.The identification of the Atrboh genes enables the PI to look for morecomponents in early steps of the signal transduction pathway to defense,and to study the mechanism that activates the plant NADPH oxidase.