Alex A. R. Webb

ABI1 Protein Phosphatase 2C Is a Negative Regulator of Abscisic Acid Signaling

The plant hormone abscisic acid (ABA) is a key regulator of seed maturation and germination and mediates adaptive responses to environmental stress. In Arabidopsis, the ABI1 gene encodes a member of the 2C class of protein serine/threonine phosphatases (PP2C), and the abi1-1 mutation markedly reduces ABA responsiveness in both seeds and vegetative tissues. However, this mutation is dominant and has been the only mutant allele available for the ABI1 gene. Hence, it remained unclear whether ABI1 contributes to ABA signaling, and in case ABI1 does regulate ABA responsiveness, whether it is a positive or negative regulator of ABA action. In this study, we isolated seven novel alleles of the ABI1 gene as intragenic revertants of the abi1-1 mutant. In contrast to the ABA-resistant abi1-1 mutant, these revertants were more sensitive than the wild type to the inhibition of seed germination and seedling root growth by applied ABA. They also displayed increases in seed dormancy and drought adaptive responses that are indicative of a higher responsiveness to endogenous ABA. The revertant alleles were recessive to the wild-type ABI1 allele in enhancing ABA sensitivity, indicating that this ABA-supersensitive phenotype results from a loss of function in ABI1. The seven suppressor mutations are missense mutations in conserved regions of the PP2C domain of ABI1, and each of the corresponding revertant alleles encodes an ABI1 protein that lacked any detectable PP2C activity in an in vitro enzymatic assay. These results indicate that a loss of ABI1 PP2C activity leads to an enhanced responsiveness to ABA. Thus, the wild-type ABI1 phosphatase is a negative regulator of ABA responses.

Stimulus-Induced Oscillations in Guard Cell Cytosolic Free Calcium.

Ca2+ is implicated as a second messenger in the response of stomata to a range of stimuli. However, the mechanism by which stimulus-induced increases in guard cell cytosolic free Ca2+ ([Ca2+]i) are transduced into different physiological responses remains to be explained. Oscillations in [Ca2+]i may provide one way in which this can occur. We used photometric and imaging techniques to examine this hypothesis in guard cells of Commelina communis. External Ca2+ ([Ca2+]e), which causes an increase in [Ca2+]i, was used as a closing stimulus. The total increase in [Ca2+]i was directly related to the concentration of [Ca2+]e, both of which correlated closely with the degree of stomatal closure. Increases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of [Ca2+]e. At 0.1 mM, [Ca2+]e induced symmetrical oscillations. In contrast, 1.0 mM [Ca2+]e induced asymmetric oscillations. Oscillations were stimulus dependent and modulated by changing [Ca2+]e. Experiments using Ca2+ channel blockers and Mn2+-quenching studies suggested a role for Ca2+ influx during the oscillatory behavior without excluding the possible involvement of Ca2+ release from intracellular stores. These data suggest a mechanism for encoding the information required to distinguish between a number of different Ca2+-mobilizing stimuli in guard cells, using stimulus-specific patterns of oscillations in [Ca2+]i.

The Responses of Stomata to Abscisic Acid and Temperature are Interrelated

The usual response of stomata to abscisic acid (ABA) is a promotion of closure or an inhibition of opening. There are, however, a few reports that at low temperatures the stomata of chill-sensitive species show a reversal of this normal effect, i.e. ABA causes stomatal opening. We have reinvestigated the interactions between ABA and temperature on stomatal movements in detached epidermis of two species which are not chill sensitive, Bellis perennis and Cardamine pratensis, and, for comparison, the subtropical plant Commelina communis. A major effect of low temperatures was to reduce the stomatal response to ABA. This applied to all species (i. e. whether chill sensitive or not), suggesting that it is a widespread occurrence which may be of physiological and ecological significance. It was also demonstrated that, above approximately 15 °C, the magnitude of the stomatal response to ABA tends to increase with temperature and hence temperature-induced stomatal opening is moderated by the presence of ABA. These data suggest that some reinterpretation is required of the role of ABA during periods of water shortage. We propose that an important regulatory role of the hormone is to limit stomatal opening at periods when the evapotranspirational demand is greatest at higher temperatures, but to allow opening when temperatures are low and thereby facilitate the uptake of carbon dioxide.

How do stomata work

This paper discusses in detail new theories of the mechanisms of stomatal movements. Recent evidence is presented that suggests that stomata respond to changes in the soil water content, as occur during drought. Such a system requires root-to-shoot communication, and it has been suggested that the plant hormone abscisic acid (ABA) is synthesized in the roots in response to drought, and acts as the root-to-shoot messenger. Also discussed are the recent advances that have been made in determining how ABA can induce stomatal closure. The physiology and biochemistry of stomata can be studied at different levels of organization, from the whole plant to the genome. The techniques involved in such studies are described. Reference is made to source material which will allow simple experiments to be carried out in a school laboratory.