Abscisic Acid also referred to as ABA is synthesized from melavonic acid. Sites of Biosynthesis: In mature leaves, especially in response to water stress. May be synthesized in seeds. Transport: ABA is exported from leaves in the phloem. Effects: ABA is important in aspects of plant development: including the synthesis of seed storage proteins and lipids, the promotion of seed storage desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from vegetative to reproductive growth.
Also ABA is important in some aspects of physiological responses to environmental stresses such as drought or osmotically-induced stomatal closure, the induction of tolerance of water, salt, hypoxic, and cold stress, and wound or pathogen response. A difference between these responses is that of speed. Stomatal responses are very quick whereas the other responses are quite slower. But these responses require the action of common signaling elements.
Perception: FCA an RNA binding protein involved in flowering binds ABA with a high affinity â€“ it is the ABA receptor which is involved in RNA metabolism and controls flowering time. WW domain that FCA possesses allows it to interact with FY, which is a mRNA processing factor. This FCA-FY complex negatively regulates expression of FLC (repressor of flowering). In addition it also reduces the amount of functional FCA protein through a negative feedback loop by adding a premature poly A tail to a truncated form of the FCA precursor mRNA, therefore resulting in a non-functional FCA protein. With the addition of ABA interacting to FCA this complex dissociates therefore causing accumulation of full length mRNA which in turn would delay the transition of flowering. - There appears to be both an intracellular and extracellular site of ABA action. - Intracellular site of ABA action observed in stomatal regulation - Recognition on the cytoplasmic side of the plasma membrane and a close physical association between the presumed ABA receptor and Calcium channels has been hypothesized. - ABA may also act as analogous to the lipophilic vitamins- both vitamin A and ABA are synthesized from B-carotene therefore thought that there may be a similar perception mechanism that may be conserved: binding to an intracellular receptor capable of acting as a transcription factor. - ABA has direct effects on membrane fluidity and thermal behavior. - Antagonisms between ABA and GA, cytokinins or auxins. - Signaling interactions between ABA and ethylene, brassinosteroid, light and sugars.
Correlated with the ABA response are gene promoter elements, kinases, kinase inhibitors, phosphotases, phospholipases and transcription factors.
-Ability to reconstitute ABA perception pathways in oocytes therefore useful in characterizing ABA response mechanisms.
-Only some hormone response loci appear to regulate multiple signaling pathways therefore suggesting that interactions among these pathways are relatively specific. - ABA regulated genes range from relatively high-abundance transcripts, which are required for adaptation to stress or reserve synthesis to low abundance transcripts which encodes signaling components. -Four main groups of cis-acting sequences that are known to be required or sufficient for ABA inducibility: ABRE, CE3, RY/Sph elements and the recognition sequences for MYB and MYC class transcription factors -speculated that there is at least 2000 ABA responsive genes in Arabidopsis. Taken all together in combination with many components, provides an enormous potential for redundancy and combinatorial controls in ABA-regulated gene expression. -studies done have observed that G proteins, phospholipases, protein kinases, and protein phosphotases participate in early events in ABA signalingâ€¦ but the question to ask is because these classes of proteins function in a wide range of signaling events how is specificity conferred?
Secondary messengers in Plants
-IP3 which is produced by PLC acts as a secondary messenger in ABA signaling regulating stomatal function and gene expression. -PA (Phosphatidic acid) produced by PLD mediates also the regulation of stomatal aperture and gene expression - ABA sensitizes these calcium channels by enhancing the production if reactive oxygen species that can serve as secondary messengers leading to channel activation.
-during seed maturation there are two peaks of ABA accumulation. The first peak is maternally derived and immediately precedes the maturation phase. While the second peak depends on synthesis in the embryo itself. - observed that endogenous ABA is not the only signal for dormancy maintenance in mature seed but dormancy is correlated with de novo synthesis of ABA duri