Technical Abstract: Anaerobic treatment dramatically alters the pattern of gene expression in maize seedlings. During anaerobiosis there is an immediate repression of preexisting protein synthesis and the simultaneous selective synthesis of four polypeptides with a MW of approximately 33 kDa (the transition polypeptides). After 90 min, this is followed by specific transcription and mRNA accumulation as well as the selective synthesis of approximately twenty anaerobically-induced proteins. Among these anaerobic proteins (ANPs) are enzymes involved in glycolysis and related processes, such as alcohol dehydrogenase (ADH), aldolase, enolase, glucose-phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, pyruvate decarboxylase, and sucrose synthase. However, we also found two inducible genes that have different functions. One that has homology to xyloglucan endotransglycosylase may be involved in aerenchyma formation during flooding. The anaerobic response is most likely a plant's natural reaction to flooding. A similar condition has been observed in every plant species analyzed to date. We have been analyzing the anaerobic response and flooding tolerance in maize using a combination of molecular, biochemical and genetic approaches. Our studies include the characterization of anaerobically inducible genes, the identification of genes involved in flooding tolerance, and analysis of the signal transduction events that are involved in perception of oxygen deprivation and lead to the anaerobic response.