Responses to Oxygen Deprivation and Potential for Enhanced Flooding Tolerance in Maize

Abstract

Although plants release oxygen as a byproduct during the process of photosynthesis, they are obligatory aerobes requiring the gas for their survival, growth and productivity. Oxygen limitation, the predominant stress in flooded plants, dramatically affects the gene expression, development and productivity of maize. Serious efforts are being made to improve flooding-tolerance of the crop across the globe. Here, we present an overview of gene expression changes in response to oxygen deprivation. We also discuss the early cellular events that lead to altered gene expression and how the sub-cellular responses, in turn, may shape the organismal responses to flooding stress. Complete lack of O 2 (anoxia) leads to an immediate cessation of protein synthesis followed by a selective synthesis of about twenty anaerobic proteins in maize seedlings. Among these are enzymes involved in glycolytic-fermentative pathways needed for rescuing the cell from the resulting energy crisis and other genes that appear to function in longer-term responses, such as aerenchyma formation and root tip death. Although 'aerobic' proteins continue to be synthesized under hypoxia, the majority of the 'anaerobic' genes are transcriptionally and translationally induced even by a partial depletion of oxygen (hypoxia). This indicates the presence of an exquisite oxygen-sensing system in plants that may be optimizable for enhanced tolerance. Research in this area has shown that transient cytosolic Ca 2+ perturbations are essential to trigger adaptive gene expression. Although glycolysis/fermentation enzymes are necessary for adaptation, their activities do not correlate with flooding tolerance. Instead, tolerance to prolonged stress seems to depend on the capacity to quickly restore cellular ionic homoeostasis and whole plant modifications for recouping O 2 supply. Current genetic or molecular breeding efforts are aimed at exploiting the genetic variability for flooding tolerance available in maize and its wild relatives. In addi-tion, we suggest novel molecular strategies based on our understanding of early events and molecular responses.