A postgermination developmental arrest checkpoint is mediated by abscisic acid and requires the ABI5 transcription factor in Arabidopsis

Abstract

Seed dormancy is a trait of considerable adaptive significance because it maximizes seedling survival by preventing premature germination under unfavorable conditions. Understanding how seeds break dormancy and initiate growth is also of great agricultural and biotechnological interest. Abscisic acid (ABA) plays primary regulatory roles in the initiation and maintenance of seed dormancy. Here we report that the basic leucine zipper transcription factor ABI5 confers an enhanced response to exogenous ABA during germination, and seedling establishment, as well as subsequent vegetative growth. These responses correlate with total ABI5 levels. We show that ABI5 expression defines a narrow developmental window following germination, during which plants monitor the environmental osmotic status before initiating vegetative growth. ABI5 is necessary to maintain germinated embryos in a quiescent state thereby protecting plants from drought. As expected for a key player in ABA-triggered processes, ABI5 protein accumulation, phosphorylation, stability, and activity are highly regulated by ABA during germination and early seedling growth.A bscisic acid (ABA) is a phytohormone regulating the initiation and maintenance of seed dormancy. It also plays an essential role in a plant's response to stress, particularly water deprivation, notably by regulating stomatal aperture (1). So far, ABA-insensitive screens have been widely used to identify molecular genetic components of the ABA signal transduction pathway (2, 3). In these screens, mutagenized Arabidopsis seeds were exposed to ABA concentrations that inhibit germination of wild-type (WT) seeds, and putative mutants that were able to germinate were isolated (2, 3). These screens have allowed the identification of several ABI (ABA-insensitive) genes (4-10) and recent studies have established that ABI1 and ABI3 are key players in vegetative and embryonic ABA responses, respectively (1, 11).Nonetheless, few reports have clarified the physiological role of ABA and mechanisms of action triggered by ABA during germination and early seedling growth. We were led to address these issues by the recent cloning and analysis of ABI5 by two independent groups (4, 5). The abi5 mutation is recessive and ABI5 encodes a putative transcription factor of the basic leucine zipper (bZIP) family (4, 5). The bZIP region of ABI5 shows extensive homology to previously characterized plant (bZIP) transcription factors capable of activating reporter genes containing ABA-responsive DNA elements (ABREs) (12)(13)(14). ABI5 also binds to ABREs in vitro (unpublished results) and dry seeds of abi5 show reduced transcript levels of ABA-responsive and ABRE-containing late embryonic genes such as AtEm1 and AtEm6 (4, 5). Together with the ABA insensitivity of abi5 mutants, these results show that ABI5 is the first bZIP plant factor found to be required in vivo to signal ABA-elicited responses.In the present work, we found that ABA regulates ABI5 accumulation and activity during a limited developmental window. On re...

ABA signalling is fine-tuned by antagonistic HAB1 variants

“…Seedlings with elongated radicles were counted every 24 days. For the root growth assay, seeds were plated on the MS medium and after 5 days the seedlings were transferred to plates containing different concentrations of ABA 37 . Root growth was then measured after 7 days.…”

Seed dormancy and germination

“…Surprisingly no dormancy or other seed maturation phenotype was observed in abi4 and abi5 mutants (Finkelstein, 1994; Finkelstein et al, 2008), except reduction of some seed maturation specific mRNAs (Finkelstein and Lynch, 2000; Söderman, et al, 2000). This may indicate that other genes are redundant in function to these seed specific transcription factors, which are members of the APETALA2 domain (ABI4, Finkelstein et al, 1998; Söderman, et al, 2000) and basic leucine zipper factor family (ABI5, Finkelstein and Lynch, 2000;Lopez-Molina et al, 2001). …”

E3 ubiquitin ligases and abscisic acid signaling

“…28 ABI5, a member of the Arabidopsis basic leucine zipper (bZIP) transcription factor family, functions downstream of ABI3 to execute ABA-dependent postgermination growth arrest. 29,30 The efficiency of the ABA mediated growth arrest is directly dependent on ABI5 protein accumulation through transcriptional activation and enhanced protein stability. 30,31 Accumulation of ABI5 protein in seedlings treated with 26S proteasome inhibitors amino acid motif that contains both a ubiquitin-binding site and an E2-binding site, enables the E3 to form a covalent thioesterlinked E3-ubiquitin intermediate before the transfer of ubiquitin to the substrate.…”

“…29,30 The efficiency of the ABA mediated growth arrest is directly dependent on ABI5 protein accumulation through transcriptional activation and enhanced protein stability. 30,31 Accumulation of ABI5 protein in seedlings treated with 26S proteasome inhibitors amino acid motif that contains both a ubiquitin-binding site and an E2-binding site, enables the E3 to form a covalent thioesterlinked E3-ubiquitin intermediate before the transfer of ubiquitin to the substrate. The second and third groups of E3s are Really Interesting New Gene (RING)-type and U-box-type E3 ligases.…”

“…Overexpression of AtCHIP leads to increased PP2A activity and and in RPN10 (a subunit of the 26S proteasome) mutant plants demonstrates that ABI5 turn-over is dependent on the UPS. 30,32 KEG, a multi-domain RING-type E3 ligase, has been shown to be required for maintaining low levels of ABI5 in the absence of ABA. 24,33 Disruption of KEG gene expression results in growth arrest immediately after germination, accumulation of extremely high levels of ABI5 protein and hypersensitivity to ABA.…”