As sessile organisms, plants have evolved many mechanisms to mitigate the potential damage of abiotic stresses, such as drought. In general, plant hormones play roles in either sensing change or participating in molecular signalling events that cause changes in cellular physiology in response to the specific stress. The hormone abscisic acid (ABA) has been implicated as a key mediator of stress responsiveness, slowing metabolism when growth conditions are unfavourable, inducing bud and seed dormancy, and regulating stomatal movements.
When conditions become favourable for seed germination (temperature and water availability are the critical factors), dormancy is broken and the seedling emerges from the seed. This process is mediated by an increase in the level of gibberellic acid, which promotes germination, and a concomitant decrease in the levels of ABA, which maintains the dormant state. Part of the signalling networks involve perception of the hormone level, and therefore it should be possible, by using genetic approaches, to identify mutants that are either insensitive or supersensitive to a hormone.
Peter McCourt and his colleagues at the University of Toronto embarked on such a mutant hunt, looking for Arabidopsis seeds that displayed abnormal responses to ABA. Normally, wild-type seeds will germinate efficiently on a minimal medium containing low concentrations of ABA (0.3 ?M), but mutants that exhibit an enhanced response to ABA should not. From a population of nearly 200,000 mutagenized seeds, seven lines were selected that could not germinate on 0.3 ?M ABA but that did germinate when the seeds were transferred onto a minimal medium without added ABA. One of these, designated era-1, showed recessive Mendelian inheritance and was selected for further molecular and physiological characterization.
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