Plants recognize and respond to pathogen attack with specific defense mechanisms. The rapid production of Reactive Oxygen Intermediates (ROI), from molecular oxygen, in plants has been correlated with resistance responses against pathogens. These oxygen derivatives can be toxic to invading microbes and can serve as signals to the plant cells being invaded. ROI accumulation is frequently correlated with the Hypersensitive Response (HR), a kind of programmed cell death (PCD) which probably also requires nitric oxide (NO) formation. In addition, ROI play a role in morphogenetic phenomena like cell elongation, strengthening of the cell wall, lignification and response to other stresses such as wounding attack by insects, saline and cold stress. Examples of developmentally regulated cell 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 (the embryo suspensor, or the aleurone and the endosperm after the seed germination) and senescence. In flowers, the interior cell layer of the anther locule (the tapetum) undergoes PCD during pollen development. The role of ROI in these processes is largely unexplored.
Several biochemical mechanisms have been proposed to explain ROI generation by plants. However, various lines of evidence implicate an NADPH oxidase, analogous to that which generates the respiratory burst in mammalian phagocytes, as the source of the ROI detected in plants, at least upon infection.
This investigator has found transposon insertions in the coding region of 8 Arabidopsis Atrboh homologs of the mammalian gp91phox, the key membrane bound NADPH-binding flavocytochrome b558 so far identified. These mutant lines are an incomparable tool to dissect the role of specific gp91phox homologues, not only in plant defense but also in relation to developmental processes, PCD and other responses to the environment. Preliminary data demonstrate that there is redundancy in this gene family, and that ROI generated via the gp91-phox protein play a role in at least one critical developmental process.
The identification of the Atrboh genes enables the PI to look for more components in early steps of the signal transduction pathway to defense, and to study the mechanism that activates the plant NADPH oxidase.