ARP2/3 complex-mediated actin dynamics is required for hydrogen peroxide-induced stomatal closure in Arabidopsis.

Multiple cellular events like dynamic actin reorganization and hydrogen peroxide (H(2)O(2)) production were demonstrated to be involved in abscisic acid (ABA)-induced stomatal closure. However, the relationship between them as well as the underlying mechanisms remains poorly understood. Here, we showed that H(2)O(2) generation is indispensable for ABA induction of actin reorganization in guard cells of Arabidopsis that requires the presence of ARP2/3 complex. H(2)O(2) -induced stomatal closure was delayed in the mutants of arpc4 and arpc5, and the rate of actin reorganization was slowed down in arpc4 and arpc5 in response to H(2)O(2), suggesting that ARP2/3-mediated actin nucleation is required for H(2)O(2) -induced actin cytoskeleton remodelling. Furthermore, the expression of H(2)O(2) biosynthetic related gene AtrbohD and the accumulation of H(2)O(2) was delayed in response to ABA in arpc4 and arpc5, demonstrating that misregulated actin dynamics affects H(2)O(2) production upon ABA treatment. These results support a possible causal relation between the production of H(2)O(2) and actin dynamics in ABA-mediated guard cell signalling: ABA triggers H(2)O(2) generation that causes the reorganization of the actin cytoskeleton partially mediated by ARP2/3 complex, and ARP2/3 complex-mediated actin dynamics may feedback regulate H(2)O(2) production.

AtHAP3b plays a crucial role in the regulation of flowering time in Arabidopsis during osmotic stress.

The HAP complex has been found in many eukaryotic organisms. HAP recognizes the CCAAT box present in the promoters of 30% of all eukaryotic genes. The HAP complex consists of three subunits--HAP2, HAP3 and HAP5. In this paper, we report the biological function of the AtHAP3b gene that encodes one of the HAP3 subunits in Arabidopsis. Compared with wild-type plants, hap3b-1 and hap3b-2 mutants exhibited a delayed flowering time under long-day photoperiod conditions. Moreover, the transcription levels of FT were substantially lower in the mutants than in the wild-type plants. These results imply that AtHAP3b may function in the control of flowering time by regulating the expression of FT in Arabidopsis. In a subsequent study, AtHAP3b was found to be induced by osmotic stress. Under osmotic stress conditions, the hap3b-1 and hap3b-2 mutants flowered considerably later than the wild-type plants. These results suggest that the AtHAP3b gene plays more important roles in the control of lowering under osmotic stress in Arabidopsis.

A novel drought-inducible gene, ATAF1, encodes a NAC family protein that negatively regulates the expression of stress-responsive genes in Arabidopsis.

NAC proteins are plant-specific transcriptional regulators. ATAF1 was one of the first identified NAC proteins in Arabidopsis. In present study, we characterized the ATAF1 expression and biological function in response to water deficit stress. ATAF1 mRNA expression was strongly induced by dehydration and abscisic acid (ABA) treatment, but inhibited by water treatment, suggesting a general role in drought stress responses. Transient expression analysis in onion epidermal cells indicated the nuclear localization for the ATAF1::GFP fusion protein. Yeast transactivation analysis showed that ATAF1 had ability to activate reporter gene expression. Furthermore, domain deletion analysis revealed that the ATAF1 transactivation activity was conferred by its C-terminal domain. When ATAF1 gene was knocked out by T-DNA insertions, Arabidopsis ataf1-1 and ataf1-2 mutants displayed a recovery rate about seven times higher than wild-type plants in drought response test. This ataf1 phenotype was coincident with the enhanced expression of stress responsive marker genes, such as COR47, ERD10, KIN1, RD22 and RD29A under drought stress. Above evidences suggest that ATAF1, as a transcriptional regulator, negatively regulates the expression of stress responsive genes under drought stress in Arabidopsis.