No abstract

Cellular water-deficit stress triggers many changes in gene expression which can be used to define the response of a plant to an environmental condition. Microarray technology permits the study of expression patterns of thousands of genes simultaneously, permitting a comprehensive understanding of the types and quantities of RNAs that are present in a cell in response to water-deficit stress. The expression of specific genes was compared in three different experiments designed to understand changes in gene expression in response to water-deficit stress. Surprisingly, there was a relatively small set of genes that were commonly induced or repressed. There were 27 genes commonly induced and three commonly repressed; 1.4% and 0.2% of the genes analysed in common to all three experiments. The induced genes fell into six different functional categories: metabolism, transport, signalling, transcription, hydrophilic proteins, and unknown. The three commonly repressed genes indicated that repression of gene expression supported a frequently observed response to water-deficit stress, decreased growth. A more detailed analysis of genes involved in cell wall metabolism, indicated that there was a global decrease in expression of genes that promote cell expansion.

SummaryTranslational regulation was evaluated for over 2000 genes by measurement of the proportion of individual mRNA species in polysomal (PS) complexes in leaves of non-stressed and moderately dehydration-stressed Arabidopsis. The amount of each mRNA in polysomes ranged from 23 to 97% in non-stressed leaves and was signi®cantly reduced for a large portion of the genes (71%) in response to dehydration. The effect of dehydration on translational status varied extensively between mRNA species. Sixty per cent of the dehydration-inducible mRNAs with twofold or greater increase in abundance maintained PS levels in response to water-de®cit stress, while 40% showed impaired ribosome loading (RL). PS association declined signi®-cantly for 92% of the mRNAs that displayed a strong decrease in abundance, indicating a relationship between translation and decreased gene transcription and/or mRNA stability. Interestingly, many mRNAs that encode proteins of similar biological function displayed coordinate translational regulation. Thus, the abundance of PS mRNA may provide a more accurate estimate of gene expression than total cellular mRNA because of extensive differential translational regulation.

The mechanisms by which plants respond to reduced water availability (low water potential) include both ABA-dependent and ABA-independent processes. Pro accumulation and osmotic adjustment are two important traits for which the mechanisms of regulation by low water potential, and the involvement of ABA, is not well understood. The ABA-deficient mutant, aba2-1, was used to investigate the regulatory role of ABA in low water potential-induced Pro accumulation and osmotic adjustment in seedlings of Arabidopsis thaliana. Low water potential-induced Pro accumulation required wild-type levels of ABA, as well as a change in ABA sensitivity or ABA-independent events. Osmotic adjustment, in contrast, occurred independently of ABA accumulation in aba2-1. Quantification of low water potential-induced ABA and Pro accumulation in five ABA-insensitive mutants, abi1-1, abi2-1, abi3, abi4, and abi5, revealed that abi4 had increased Pro accumulation at low water potential, but a reduced response to exogenous ABA. Both of these responses were modified by sucrose treatment, indicating that ABI4 has a role in connecting ABA and sugar in regulating Pro accumulation. Of the other abi mutants, only abi1 had reduced Pro accumulation in response to low water potential and ABA application. It was also observed that abi1-1 and abi2-1 had increased ABA accumulation. The involvement of these loci in feedback regulation of ABA accumulation may occur through an effect on ABA catabolism or conjugation. These data provide new information on the function of ABA in seedlings exposed to low water potential and define new roles for three of the well-studied abi loci.