January 10th, 2018
Signalling of Abscisic Acid in Plants
ABA is implicated in various signalling functions in plants such as responses to environmental stresses (e.g. drought and salinity) and in the induction of dormancy in seeds (Finkelstein, Gampala & Rock, 2002). Such signalling role proceeds via a variety of intermediaries with G proteins, phospholipases, protein phosphatases and protein kinases being implicated (Finkelstein, Gampala & Rock, 2002). When perceived by various receptors, ABA induces either the phosphorylation or dephosphorylation of such intermediaries thus generating a signal that is conveyed via an array of second messengers (Paradis et al., 2011). Some of the second messengers involved in ABA’s signalling mechanisms are diacylglycerol pyrophosphate (DGPP), inositol triphosphate (IP3) and phosphatidic acid (PA) (Finkelstein, Gampala & Rock, 2002; Paradis et al., 2011).
IP3, which is generated in a reaction catalysed by phospholipase C (PLC), functions in ABA signalling in the regulation of stomatal function and gene expression (Finkelstein, Gampala & Rock, 2002). Such role has been evaluated using Arabidopsis, with ABA being shown to induce the expression of one of the six Arabidopsis PLC genes – AtPLC1 (Finkelstein, Gampala & Rock, 2002). AtPLC1 is required for ABA’s effects on germination, growth and expression of vegetative genes; however, AtPLC1 is not the sole requirements for such effects (Finkelstein, Gampala & Rock, 2002). Accordingly, ABA’s effect would be inducing the expression of AtPLC1, enhancing the formation of PLC, which would then lead to formation of IP3 to signal, for instance, for the closing of the stoma due to drought conditions that induced ABA’s production.
Another secondary messenger implicated in ABA’s signalling is PA. PA results from the activity of phospholipase D (PLD) and ABA has been noted to stimulate the activity of PLD using microsomes derived from barley aleurone in a process mediated by G protein activity (Finkelstein, Gampala & Rock, 2002; Shao et al., 2007). ABA in this respect, enhances the expression of PLDα, the most prevalent of the the PLD genes (Finkelstein, Gampala & Rock, 2002). Such activity of ABA in enhancing genes resulting into higher levels of PA has also been noted in Arabidopsis thaliana cells with respect to Atlpp2 gene (Paradis et al., 2011).
Physiologic responses mediated by ABA could also involve the enhancement of activities of enzymes such as superoxide dismutase (SOD) and catalase (CAT). For instance, Lu, Su, Li and Guo (2009) found out that SOD and CAT increased more in ABA-treated plants as compared to untreated plants under drought conditions. Such increases were noted to subside when H2O2 and NO levels were reduced by using either the molecules’ scavengers or their inhibitors (Lu et al., 2009). H2O2 and NO are involved in physiologic responses to drought e.g. stomata closure (Shao et al., 2007). ABA’s role in this respect is to induce the production of H2O2, which regulates NO production, with the resultant NO activating a protein kinase that up-regulates the expressions of genes for anti-oxidant enzymes (Lu et al., 2009; Shao et al., 2007).
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Lu, S., Su, W., Li, H. & Guo, Z. (2009). Abscisic acid improves drought tolerance of triploid bermudagrass and involves H2O2- and NO-induced antioxidant enzyme activities. Plant physiology and Biochemistry, 47, 132-138. doi:10.1016/j.plaphy.2008.10.006
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Paradis S., Villasuso, A.L., Aguayo, S. S., Maldiney, R., Habricot, Y., Zalejski, C., …, Jeannette, E. (2011). Arabidopsis thaliana lipid phosphate phosphatase 2 is involved in abscisic acid signalling in leaves. Plant Physiology and Biochemistry, 49, 357-362, doi:10.1016/j.plaphy.2011.01.010
Shao, H. B., Jiang, S.Y., Li, F. M., Chu, L. Y., Zhao, C. X., Shao, M. A., Zhao, X. N. & Li, F. (2007). Some advances in plant stress physiology and their implications in the systems biology era. Colloids and surfaces B: Biointerfaces, 54, 33-36doi:10.1016/j.colsurfb.2006.05.011