AEROBIC FERMENTATION DURING TOBACCO POLLEN DEVELOPMENT

In vegetative organs of plants, the metabolic switch from respiration to fermentation is dictated by oxygen availability. The two genes dedi cated to ethanolic fermentation, pyruvate decarboxylase and alcohol de hydrogenase, are induced by oxygen deprivation and the gene products a re active under oxygen stress. in pollen, these two genes are expresse d in a stage-specific manner and transcripts accumulate to high levels , irrespective of oxygen availability. We have examined the expression pattern of pyruvate decarboxylase and alcohol dehydrogenase at the pr otein level in developing pollen and show that the active proteins are localized to the gametophytic tissue and begin to accumulate at micro spore mitosis. A flux through the ethanolic fermentation pathway could already be detected very early in pollen development, occurring in al l stages from premeiotic buds to mature pollen. This flux was primaril y controlled not by oxygen availability, but rather by sugar supply. A t a high rate of sugar metabolism, respiration and fermentation took p lace concurrently in developing and germinating pollen. We propose tha t aerobic fermentation provides a shunt from pyruvate to acetyl-CoA to accommodate the increased demand for energy and biosynthetic intermed iates during pollen development and germination. A possible undesirabl e side-effect is the potential accumulation of toxic acetaldehyde. Our results support a model for cms-T-type male sterility in maize, in wh ich degeneration of the tapetum is caused by the toxic effects of acet aldehyde on mitochondria weakened by the presence of the URF13 protein .