Molecular Biological Responses to Drought in Maize

Almost all plants can withstand extremes of osmotic stress at some stages of their life cycle. This is the case in cereal seed whose embryos can tolerate reductions in water content of about 80%. Such severe desiccation kills cells from other parts of the plant. During growth, plants also endure mild water deficits (10–15% reduction of their water content) in dry periods. However, the molecular mechanisms contributing to tolerance of dehydration are largely unresolved. The hormone, abscisic acid (ABA), appears to modulate the responses of plants under conditions of water deficit. Developmental studies of seeds and physiological studies of the effects of water stress indicate that ABA controls quantitative and qualitative aspects of the accumulation of specific mRNAs and proteins. Despite considerable research in this field, the function of the ABA- responsive proteins remains elusive and the mode of action of the hormone is unknown. The rab genes of maize are expressed in different plant tissues in response to ABA and osmotic stress. Current evidence indicates that more than one mechanism determines the level of expression of the rab genes. One level of control involves transcription of the rab genes. Specific sequence elements directing hormone responsiveness are localised in discrete regions of the rab gene promoters, rab mRNAs are induced in all tissues by ABA. In contrast, the level of rab proteins differs between embryo and vegetative tissues since they are strongly accumulated in dry embryos but poorly represented in the overall protein pattern of ABA- treated or water-stressed vegetative tissues. These data indicate that different levels of control are involved in the regulation of rab mRNA stability and protein accumulation during ABA-induced responses.