ABI3 thaliana. ABI3 was found to mediate
ABI3 (Abscisic acidinsensitive 3), a homologue of maize viviparous1 (VP1), was identified in mutant background insensitive to ABA hormone.Earlier designated as a seed specific transcription factor 1-3, ABI3 has now emerged as a general regulator incellular maturation and plant developmental processes such as quiescence ofshoot apex meristem, plastid differentiation, regulation of flowering time,etc.4, 5.
ABI3 was also found at the juncture of ABA-auxinsignaling crosstalk during seed germination, lateral root initiation and stressphysiology 6-8. In recent years, role of ABI3 has stretched beyonddevelopmental aspects to abiotic stress response. Ectopic expression of ABI3 inArabidopsis thaliana imparted highertolerance to freezing temperature 9 A breakthrough revelation of ABI3 mediated stressresponse was the finding that it can impart desiccation stress tolerance to thenon-seed bearing system of Physcomitrellapatens 10. In consonance, our previous work has established roleof ABI3 as a key player in dehydration stress response in Arabidopsis thaliana11. ABI3was found to mediate dehydration stress response by regulating expression ofseveral genes including the CRUCIFERINgroup of genes and the osmo-tolerance imparting Late Embryogenesis Abundant(LEA) genes.
ABI3 is a multi-domain proteinbelonging to the AFL (ABI3/ FUSCA3/LEAFY COTYLEDON 2) family of B3-domain containing transcriptionfactors. ABI3, FUS3 and LEC2 are known to form a signallingnetwork which regulates seed maturation and dormancy 12, 13. In addition to the B3 DNA binding domain, the ABI3protein has three other domains: the acidic A1 domain and two other basicdomains B1 and B2 which facilitate nuclear localization and mediateprotein-protein interaction especially with bZIP factors 14-17. Interestingly, the B3 domain of ABI3 is knownto bind to three different consensus sequences, the Sph/RY element (CATGCA), toG-box/ABREs (GACGTG) and the AuxRE (TGTCTC) depending on differentphysiological and developmental cues 8, 14, 18, 19 .
It is possible that the ability of B3 domain ofABI3 to bind to different cis-elementsunder various physiological conditions imparts the multifaceted roles of ABI3in plant stress and developmental responses. Such diversity in the role of ABI3requires a robust and well-orchestrated signalling network which maintains thespatio-temporal regulation of ABI3.During seed dormancy, ABI3is interdependently regulated by phyto-hormones auxin and ABA. ABA is known todirectly regulate ABI3, whereas auxin mediates its function through auxinresponse factor 10 (ARF10) and 16 (ARF16) 7. In additionto ARF mediated regulation in seed physiology, ABI3 is known to be regulated byFUS3, LEC1 and LEC2 during seed maturation 20-22. Interestingly, apart from being regulated by aplethora of other factors, ABI3 along with FUS3 is known to form a feedbackloop, regulating its own expression during seed development 23. Additionally, ABI3 is also regulated by CHD1-likeprotein CHR5 and CHD3-like protein PICKLE (PKL) where former induces activechromatin state by reducing nucleosome occupancy during transcription, whilelatter represses ABI3 expression via H3 K27 trimethylation post germination 24, 25.
Apart from transcriptional regulation, ABI3 isalso known to be regulated post-transcriptionally and post-translationally.ABI3 protein level is regulated by ABI3 Interacting Protein (AIP2), an E3ligase, which ubiquitinates and represses ABI3 activity 26. Post-transcriptional regulation of ABI3 involvesthe presence of lengthy 5′-UTR in the ABI3 gene, which negatively affects ABI3expression 27. Moreover, splice variants of ABI3 homologs wereidentified in both dicotyledonous and monocotyledonous plants suggesting animportant role of alternative splicing in ABI3 expression 28-30 .The diverse modes of ABI3 regulation mentionedabove, indicate the significance of this gene product in various plantsignalling pathways and necessitates understanding the mechanisms of ABI3regulation in utmost details. Understanding gene regulation in a eukaryoticsystem has the additional complexity at the level of chromatin.
Chromatinorganization at the upstream regulatory region of genes is one of the ratedetermining step for transcriptional activity in eukaryotes 31-33. Chromatin structure is regulated by twoaspects- i) the physical signature and ii) the chemical state of thenucleosomes. Physical signature of chromatin implies the distribution ofnucleosomes at a gene locus, which can either be strongly or looselypositioned. Whereas chemical states of nucleosome signifies presence of posttranslational modifications on histone residues such as acetylation, methylationand phosphorylation, among others 34.Such chemical modifications can prime the locus to either a transcriptionallyrepressed or transcriptionally activated state. A transcriptionally active geneis defined by presence of loosely compacted chromatin structure and presence ofhistone modifications associated with active chromatin such as H3 K9ac, H3K27ac, H3 K4me3 etc. For transcription activation the physical signature of chromatinneeds to alter as well and such alterations are often based on the chemicalmodifications. Nucleosomes occluding important regulatory elements at thepromoter region of a gene need to expose the required sequences fortranscriptional activation in presence of inducible cues 35, 36.
In the present work we have aimed to decode thegenetic aspects and the chromatin modifications that are essentially involvedin regulating ABI3 gene expression in response to dehydration stress and stressrecovery. We have shown how nucleosomes at the ABI3 locus occlude important regulatorycis-elements during transcriptionallyrepressed state of the gene, which get repositioned during transcriptionactivation. This process involves several histone modifications that bring inthe necessary alterations in the chromatin landscape. We have further dissectedout two important cis-elements thatplay a regulatory role in ABI3 expression during dehydration stress and subsequentrecovery phases.