Abscisic Acid and abiotic stress signaling

Abstract

Abiotic stress is severe environmental stress, which impairs crop production on irrigated land worldwide. Overall, the susceptibility or tolerance to the stress in plants is a coordinated action of multiple stress responsive genes, which also cross-talk with other components of stress signal transduction pathways. Plant responses to abiotic stress can be determined by the severity of the stress and by the metabolic status of the plant. Abscisic acid (ABA) is a phytohormone critical for plant growth and development and plays an important role in integrating various stress signals and controlling downstream stress responses. Plants have to adjust ABA levels constantly in responce to changing physiological and environmental conditions. To date, the mechanisms for fine-tuning of ABA levels remain elusive. The mechanisms by which plants respond to stress include both ABA-dependent and ABA-independent processes. Various transcription factors such as DREB2A/2B, AREB1, RD22BP1 and MYC/MYB are known to regulate the ABA-responsive gene expression through interacting with their corrosponding cis-acting elements such as DRE/CRT, ABRE and MYCRS/MYBRS, respectively. Understanding these mechanisms is important to improve stress tolerance in crops plants. This article first describes the general pathway for plant stress response followed by roles of ABA and transcription factors in stress tolerance including the regulation of ABA biosynthesis.

Keywords: ABA; ABA-responsive element; ABA-responsive genes; cis-acting elements; environmental stress; plant stress hormone; signal transduction; transcription factors.

Figure 1

ABA biosynthesis pathway and its regulation. ABA is synthesized from β- carotene via the oxidative cleavage of neoxanthin and conversion of xanthoxin to ABA via ABA-aldehyde. Abiotic stresses (Dehydration, cold, salinity) stimulate ABA biosynthesis and accumulation by activating genes involved in the ABA biosynthetic pathway, which itself could be mediated by calcium-dependent phosphorylation cascade. ABA could also upregulate expression of ABA biosynthetic genes via calcium signaling pathways (see Xiong et al., 2002; Zhu, 2002). AAO, ABA-aldehyde oxidase; MCSU, molybdenium cofactor sulfurase, NCED, 9-cis-epoxycarotenoid dioxygenase, ZEP, zeaxanthin epoxidase.

Figure 2

Transcriptional regulatory network of cis-acting elements and ABA-dependent transcription factors involved in drought, cold and salinity stress gene expression. Abiotic stress signaling seems to be mediated by transcription factors such as NAC, ZF-HD, DREB2A/DREB2B and DREB1/CBF, AREB1, RD22BP1 and MYC/MYB transcription activators, which interact with NACR, ZF-HDR, DRE/CRT, ABRE and MYCRS/MYBRS elements in the promoter of the stress genes, respectively. AtMYC2 and AtMYB2 act cooperatively to activate the expression of ABA-inducible genes such as RD22. Cis-acting elements that are involved in transcription of stress-responsive gene are shown in boxes. Transcription factor that regulate stress-inducible gene expression are shown in ovals. ABA, abscisic acid; ABRE, ABA-responsive element; AREB, ABRE-binding protein; CBF, C-repeat-binding factor; COR, cold regulated genes; CRT, C-repeat; DRE, dehydration-responsive element; DREB, DRE-binding protein; ERD, early responsive to dehydration, MYB, myeloblastosis; MYBRS, MYB-recognition sequence; MYC, myelocytomatosis; MYCRS, MYC-recognition sequence; NACR, NAC-recognition site; RD, genes responsive to dehydration; ZF-HD; zinc-finger homeodomain.