Genome-wide analysis of mRNA decay rates and their determinants in Arabidopsis thaliana

Two different mechanisms control ethylene sensitivity in Arabidopsis via the regulation of EBF2 expression

EIN3 is a key transcription factor in the ethylene signaling pathway in Arabidopsis and its action is controlled by SCFEBF1/2 complex-mediated ubiquitination and subsequent 26S proteasome-dependent degradation. Our recent study revealed that ethylene signaling is regulated by a negative feedback system involving the direct activation of the EBF2 promoter by EIN3. This negative feedback loop likely modulates both the magnitude of the response to ethylene and the recovery after its withdrawal. The sequence downstream of the EBF2 stop codon was also found to play a role in determining the EBF2 expression levels independently of ethylene signaling. The reduction in EBF2 expression via the function of this sequence was suggested to be important for coupling the EBF2 expression levels to growth control. Two different mechanisms thus appear to be involved in determining ethylene sensitivity in Arabidopsis.

Ethylene signaling in Arabidopsis involves feedback regulation via the elaborate control of EBF2 expression by EIN3

EIN3 is a key transcription factor in the signaling pathway of the plant hormone ethylene in Arabidopsis. Ethylene signaling suppresses the 26S proteasome-mediated proteolysis of EIN3, the accumulation of which induces ethylene-responsive gene expression. The proteolysis of EIN3 has been suggested to be mediated by the E3 ubiquitin ligase SCF(EBF1/2) that contains either of the two closely related F-box proteins, EBF1 or EBF2. Here, we demonstrate feedback regulation of ethylene signaling that results from the direct upregulation of the EBF2 gene by EIN3. Although EBF1 and EBF2 show comparable activities as repressors of EIN3-dependent transcription, our analysis of transgenic Arabidopsis plants has revealed that the EBF2 promoter, but not the EBF1 promoter, is activated by ethylene. Furthermore, the results of protoplast transient assays in vivo and electrophoretic mobility shift assays in vitro have revealed that EIN3 can bind and activate the EBF2 promoter, indicating that EIN3 modulates EBF2 gene expression in planta. An ebf2 mutant transformed with the EBF2 gene under the control of a mutant promoter in which the EIN3-binding site was disrupted still showed an ethylene hyper-responsive phenotype, indicating the physiological relevance of EIN3-mediated activation of the EBF2 gene in the downregulation of ethylene signaling. We show an additional finding that a sequence downstream of the EBF2 coding region is also involved in modulations of both the EBF2 expression level and sensitivity to ethylene. These results thus indicate that the fine-tuning of ethylene signaling involves the intricate regulation of EBF2 expression in Arabidopsis.

State of decay: an update on plant mRNA turnover

Proper degradation of plant messenger RNA is crucial for the maintenance of cellular and organismal homeostasis, and it must be properly regulated to enable rapid adjustments in response to endogenous and external cues. Only a few dedicated studies have been done so far to address the fundamental mechanisms of mRNA decay in plants, especially as compared with fungal and mammalian model systems. Consequently, our systems-level understanding of plant mRNA decay remains fairly rudimentary. Nevertheless, a number of serendipitous findings in recent years have reasserted the central position of the regulated mRNA decay in plant physiology. In addition, the meteoric rise to prominence of the plant small RNA field has spawned a renewed interest in the general plant mRNA turnover pathways. Combined with the advent of widely accessible microarray platforms, these advances allow for a renewed hope of rapid progress in our understanding of the fundamental rules governing regulated mRNA degradation in plants. This chapter summarizes recent findings in this field.

Transient transcriptional responses to stress are generated by opposing effects of mRNA production and degradation

The state of the transcriptome reflects a balance between mRNA production and degradation. Yet how these two regulatory arms interact in shaping the kinetics of the transcriptome in response to environmental changes is not known. We subjected yeast to two stresses, one that induces a fast and transient response, and another that triggers a slow enduring response. We then used microarrays following transcriptional arrest to measure genome-wide decay profiles under each condition. We found condition-specific changes in mRNA decay rates and coordination between mRNA production and degradation. In the transient response, most induced genes were surprisingly destabilized, whereas repressed genes were somewhat stabilized, exhibiting counteraction between production and degradation. This strategy can reconcile high steady-state level with short response time among induced genes. In contrast, the stress that induces the slow response displays the more expected behavior, whereby most induced genes are stabilized, and repressed genes are destabilized. Our results show genome-wide interplay between mRNA production and degradation, and that alternative modes of such interplay determine the kinetics of the transcriptome in response to stress.