Plant development and hormone action

“WEB” Signal Transduction: From Circumstantial to Conceptual State of ABA Signaling Pathways. Complied/reviewed by Hemayet Ullah Assistant Professor of Biology Howard University Washington, DC 20059 hullah@howard.edu August 23, 2007 Last decade has seen tremendous scientific efforts in unlocking the plant stress hormone ABA signal transduction pathways. However, the advancement in the understandings of the cellular signal transduction mechanism of ABA, compared to the same in other plant hormones, has not kept up the pace. The apparent lack in the advancement stems from the fact that identification of an ABA receptor has been very elusive. The scenario has significantly changed with the recent identification of three potential ABA receptors that offers a shift from the circumstantial to a conceptual understanding of ABA signaling (for a recent review see Hirayama and Shinozaki 2007; this compilation is based on the review). In addition to receptors, the identification of diverse downstream signaling factors indicates a Wide Efficient Branched (WEB) type interconnected ABA signaling mechanism. Three independent labs recently reported the identification of three ABA-binding proteins. They are: the flowering-time control protein FCA (Razem et al., 2006), the Mg-chelatase H subunit (Shen et al., 2006), and the G protein coupled receptor GCR2 (Liu et al., 2007). Studies using ABA conjugated to a protein or chemical have indicated ABA recognition sites at both the cell surface and intracellular space. The FCA Receptor: At first, using anti-idiotypic antibodies (see below for explanation) an ABA binding protein was identified from Barley. Arabidopsis homolog of that protein codes for a flowering time controlling protein (FCA). Together with FY protein, FCA negatively regulates expression of the major flowering time control protein FLC. As ABA binds with FCA, in the presence of ABA the protein (FCA) cannot negatively regulate the FLC protein. However, the ABA responses of guard cells and seeds in the fca-1 mutant are normal, suggesting that FCA is not involved in these representative ABA responses. Mg-chelatase H subunit (ABAR): Contrary to FCA, perturbation of ABAR gene expression lead to measurable ABA related phenotypes in guard cells and seed germination. The ABAR codes for Mg-chelatase H subunit that is involved in chlorophyll biosynthesis and plastid to nucleus signaling pathways. As the functions of the Mg-chelatase and ABA receptor are reported to be separable, it is postulated that this ABA receptor is not involved in the plastid to nuclear signaling. Although the connection between ABA signaling and plastid-to-nuclear retrograde signaling is obscure, these two signaling pathways might mutually affect plant adaptation to environmental stresses. G-Protein Coupled Receptor (GCR2): Membrane localized GCR2 gene knock out confers ABA insensitivity in germination, stomatal movement, and ABA inducible genes. In addition, GCR2 is found to bind to ABA in a dose dependent manner. A canonical G-protein subunit- GPA1 is found to bind to, like in metazoan counterpart system, GCR2 and ABA disrupts this interaction. These observations are consistent with GCR2 being an ABA receptor. However, the evidence for a ‘conventional’ G-protein coupled Receptor mediated signaling is still elusive in the plant kingdom. It remains to be seen whether binding of ABA to GCR2 elicits a novel and/or a conserved signaling pathways to educe ABA responses. Second Messengers: Diverse groups of second messengers (downstream from ABA binding to receptor) belonging to distinct signaling pathways have been identified. Along with the three receptors, these molecules represent a network of signaling pathways with nodes and internodes that define dedicated pathways. Calcium, Phosphatidic acid, Reactive oxygen species, PPC2 (protein phosphotase), diverse protein kinases including MAPK (Mitogen Activated Protein Kinase) are well known mediators of ABA signaling. RNA metabolism: Out of many widely studied ABA signaling pathways, the role of RNA metabolism in ABA signaling is attracting much attention. In addition to the FCA receptor (involved in RNA stability), many ABA mutants are isolated that are altered in the RNA processing pathways like splicing, capping, splicing, transportation from nucleus to cytoplasm, and degradation. This indicates that ABA signaling requires a fine regulation of RNA processing of ABA responsive genes. In addition, ABA biosynthesis and degradation pathways also regulate ABA responses as well. This area is beyond the scope of the present discussion and readers are requested to read an excellent review on this matter (Nambara and Marion-Poll, 2005). The ‘WEB’ signaling network: Three putative receptors with different structure, different subcellular localizations (Nuclear, plastid, and plasma membrane) and possibly different downstream pathways form the core of the ABA signaling networks. The multiple perception sites of ABA reflect the broadness and versatility of its physiological functions. In contrast to a linear signaling pathways identified in plant hormone ethylene and cytokinin, ABA appears to act simultaneously and independently at multiple sites in the cell. The second messengers are arranged in a non-linear pathways where they affect each other positively and/or antagonistically manner. Such a mode of signal perception might represent a complicated multi input signaling pathway. A web-like network with many nodes is suitable for gathering information and allowing cross-talk with other signaling pathways, and it contributes to the efficient propagation and fine tuning of the signal. Presumably, the ABA signaling system has developed in complexity because it enables plants to respond to and adapt to stresses efficiently. Hirayama, T and Shinozaki, K. (2007) Perception and transduction of abscisic acid signals: keys to the function of the versatile plant hormone ABA. Trends in Plant Science 12 (8): 343-351. Liu, X. et al. (2007) A G protein-coupled receptor is a plasma membrane receptor for the plant hormone abscisic acid. Science 315, 1712–1716. Nambara, E. and Marion-Poll, A. (2005) Abscisic acid biosynthesis and catabolism. Annu. Rev. Plant Biol. 56, 165–185 Razem, F.A. et al. (2006) The RNA-binding protein FCA is an abscisic acid receptor. Nature 439, 290–294 Shen, Y.Y. et al. (2006) The Mg-chelatase H subunit is an abscisic acid receptor. Nature 443, 823–826 Anti-idiotypic antibodies: An antigen, for example ABA, when injected into a mouse, antibodies are elicited. The purified antified can then be injected to a second mouse. The second mouse will recognize the antibody as its own (except the ABA binding epitope region). Therefore, the second mouse will produce antibody specific to the ABA binding epitope. This antibody is called the anti-idiotypic antibody. Posted on: 9/3/200711:59:11 PM