Localization, ion channel regulation, and genetic interactions during abscisic acid signaling of the nuclear mRNA cap-binding protein, ABH1

Molecular tailoring of farnesylation for plant drought tolerance and yield protection

Protecting crop yield under drought stress is a major challenge for modern agriculture. One biotechnological target for improving plant drought tolerance is the genetic manipulation of the stress response to the hormone abscisic acid (ABA). Previous genetic studies have implicated the involvement of the beta-subunit of Arabidopsis farnesyltransferase (ERA1) in the regulation of ABA sensing and drought tolerance. Here we show that molecular manipulation of protein farnesylation in Arabidopsis, through downregulation of either the alpha- or beta-subunit of farnesyltransferase enhances the plant's response to ABA and drought tolerance. To test the effectiveness of tailoring farnesylation in a crop plant, transgenic Brassica napus carrying an ERA1 antisense construct driven by a drought-inducible rd29A promoter was examined. In comparison with the non-transgenic control, transgenic canola showed enhanced ABA sensitivity, as well as significant reduction in stomatal conductance and water transpiration under drought stress conditions. The antisense downregulation of canola farnesyltransferase for drought tolerance is a conditional and reversible process, which depends on the amount of available water in the soil. Furthermore, transgenic plants were more resistant to water deficit-induced seed abortion during flowering. Results from three consecutive years of field trial studies suggest that with adequate water, transgenic canola plants produced the same amount of seed as the parental control. However, under moderate drought stress conditions at flowering, the seed yields of transgenic canola were significantly higher than the control. Using protein farnesyltransferase as an effective target, these results represent a successful demonstration of engineered drought tolerance and yield protection in a crop plant under laboratory and field conditions.

Plant gene expression in the age of systems biology: integrating transcriptional and post-transcriptional events

The extensive mechanistic and regulatory interconnections between the various events of mRNA biogenesis are now recognized as a fundamental principle of eukaryotic gene expression, yet the specific details of the coupling between the various steps of mRNA biogenesis do differ, and sometimes dramatically, between the different kingdoms. In this review, we emphasize examples where plants must differ in this respect from other eukaryotes, and highlight a recurring trend of recruiting the conserved, versatile functional modules, which have evolved to support the general mRNA biogenesis reactions, for plant-specific functions. We also argue that elucidating the inner workings of the plant 'mRNA factory' is essential for accomplishing the ambitious goal of building the 'virtual plant'.

Regulatory Networks of the Phytohormone Abscisic Acid

Structurally similar to retinoic acid (RA), the phytohormone abscisic acid (ABA) controls many developmental and physiological processes via complicated signaling networks that are composed of receptors, secondary messengers, protein kinase/phosphatase cascades, transcription factors, and chromatin-remodeling factors. In addition, ABA signaling is further modulated by mRNA maturation and stability, microRNA (miRNA) levels, nuclear speckling, and protein degradation. This chapter highlights the identified regulators of ABA signaling and reports their homologues in dicotyledonous and monocotyledonous plants.

Lesions in the mRNA cap-binding gene ABA HYPERSENSITIVE 1 suppress FRIGIDA-mediated delayed flowering in Arabidopsis

Recessive mutations that suppress the late-flowering phenotype conferred by FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) and which also result in serrated leaf morphology were identified in T-DNA and fast-neutron mutant populations. Molecular analysis showed that the mutations are caused by lesions in the gene encoding the large subunit of the nuclear mRNA cap-binding protein, ABH1 (ABA hypersensitive1). The suppression of late flowering is caused by the inability of FRI to increase FLC mRNA levels in the abh1 mutant background. The serrated leaf morphology of abh1 is similar to the serrate (se) mutant and, like abh1, se is also a suppressor of FRI-mediated late flowering although it is a weaker suppressor than abh1. Unlike se, in abh1 the rate of leaf production and the number of juvenile leaves are not altered. The abh1 lesion affects several developmental processes, perhaps because the processing of certain mRNAs in these pathways is more sensitive to loss of cap-binding activity than the majority of cellular mRNAs.

A mutation in the Cap Binding Protein 20 gene confers drought tolerance to Arabidopsis

In a genetic screen for Arabidopsis mutants displaying pleiotropic alterations in vegetative development and stress responses we have identified a T-DNA insertion mutation in the Cap Binding Protein 20 (CBP20) gene, that encodes the 20kDa subunit of the nuclear mRNA cap binding complex (nCBC). Plants homozygous for the recessive cbp20 mutation show mild developmental abnormalities, such as serrated rosette leaves, delayed development and slightly reduced stature. Loss of the cbp20 function also confers hypersensitivity to abscisic acid during germination, significant reduction of stomatal conductance and greatly enhanced tolerance to drought. Expression of the wild type cDNA by CaMV35S promoter provides full genetic complementation of the pleiotropic cbp20 phenotype. Phenotypic characteristics of the cbp20 mutant are very similar to those of recently described abh1 mutant that is defective in the 80kDa subunit of nCBC. Our data thus confirm that both genes are dedicated to the same function. CBP20 provides a new target for breeding efforts that aim at the improvement of drought tolerance in plants. Our results also show that screening for pleiotropic phenotypes in mutant plant populations may be a fruitful strategy to isolate genes for agronomically important traits.

Differential mRNA translation contributes to gene regulation under non-stress and dehydration stress conditions in Arabidopsis thaliana

Translational 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 significantly 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-deficit stress, while 40% showed impaired ribosome loading (RL). PS association declined significantly 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.

Relay and control of abscisic acid signaling

Insights into the signal transduction of the phytohormone abscisic acid (ABA) have unfolded dramatically in the past few years and reveal an unanticipated complexity. Knockout lines and RNA-interference technology, together with protein interaction analyses, have been used to identify many of the cellular components that regulate or modulate ABA responses. ABA signaling is characterized by a plethora of intracellular messengers. This may reflect the function of ABA in integrating several stress responses and antagonizing pathways via cross-talk, but it hampers the establishment of a unifying concept. Transcriptome analyses have unraveled more than a thousand genes that are differentially regulated by ABA, and these ABA-mediated changes in gene expression translate to major changes in proteome expression. ABA-induced mechanisms that re-adjust cellular protein expression are just surfacing. ABA-response-specific transcription factors have a well-established function in that process and, recently, it has also become clear that phytohormone signaling enforces a sophisticated interference with protein expression at the posttranscriptional level. This interference includes both targeted proteolysis and the regulation of the translation of specific mRNAs by RNA-binding proteins.

Stress memory in plants: a negative regulation of stomatal response and transient induction of rd22 gene to light in abscisic acid-entrained Arabidopsis plants

All organisms, including plants, perceive environmental stress, and they use this information to modify their behavior or development. Here, we demonstrate that Arabidopsis plants have memory functions related to repeated exposure to stressful concentrations of the phytohormone abscisic acid (ABA), which acts as a chemical signal. Repeated exposure of plants to ABA (40 micro m for 2 h) impaired light-induced stomatal opening or inhibited the response to a light stimulus after ABA-entrainment under both dark/light cycle and continuous light. Moreover, there were transient expressions of the rd22 gene during the same periods under both the growing conditions. Such acquired information in ABA-entrained plants produced a long-term sensitization. When the time of light application was changed, a transient induction of the rd22 gene in plants after ABA-entrainment indicated that these were light-associated responses. These transient effects were also observed in kin1, rab18, and rd29B. The transient expression of AtNCED3, causing the accumulation of endogenous ABA, indicated a possible regulation by ABA-dependent pathways in ABA-entrained plants. An ABA immunoassay supported this hypothesis: ABA-entrained plants showed a transient increase in endogenous ABA level from 220 to 250 pmol g-1 fresh mass at 1-2 h of the training period, whereas ABA-deficient (aba2) mutants did not. Taking into account these results, we propose that plants have the ability to memorize stressful environmental experiences, and discuss the molecular events in ABA-entrained plants.

Impacts of altered RNA metabolism on abscisic acid signaling

The plant hormone abscisic acid (ABA) regulates many essential processes in growth and development. The recent characterization of ABA-sensitivity mutations in RNA-binding proteins has led to the recognition of a functional link between post-transcriptional mRNA processing and the ABA signal transduction machinery. By influencing transcript abundance, these RNA-binding proteins may modulate ABA signaling through the alteration of mRNA processing events such as splicing, 3' processing, nuclear export, transcript stability and RNA degradation.