Abscisic Acid synthesis and response

Abstract

Abscisic acid (ABA) is one of the "classical" plant hormones, i.e. discovered at least 50 years ago, that regulates many aspects of plant growth and development. This chapter reviews our current understanding of ABA synthesis, metabolism, transport, and signal transduction, emphasizing knowledge gained from studies of Arabidopsis. A combination of genetic, molecular and biochemical studies has identified nearly all of the enzymes involved in ABA metabolism, almost 200 loci regulating ABA response, and thousands of genes regulated by ABA in various contexts. Some of these regulators are implicated in cross-talk with other developmental, environmental or hormonal signals. Specific details of the ABA signaling mechanisms vary among tissues or developmental stages; these are discussed in the context of ABA effects on seed maturation, germination, seedling growth, vegetative stress responses, stomatal regulation, pathogen response, flowering, and senescence.

Figure 1.

Growth of Ler-0 “wild-type” and ABA-deficient plants (aba1-1, ZEP-deficient), without or with ABA treatment. For ABA treatment, plants were sprayed weekly with 5 mM ABA, starting at the rosette stage and continuing until the siliques started to brown. Photograph courtesy of the Arabidopsis Biological Resources Center.

Figure 2.

Structure of ABA. Potentially interacting portions identified by structure-function studies are circled. Critical regions indicated with solid lines and less critical regions with dotted lines. From Plant Cell 18:786–791.

Figure 3.

ABA metabolic pathways. ABA biosynthesis, degradation and conjugation pathways are shown in relation to the cellular compartments where these events occur. Carotenoid intermediates are highlighted in yellow. Enzymes regulating key regulatory steps are shown in bold. Individual loci identified based on ABA deficiency are shown in italics.

Figure 4.

Core ABA signaling pathway interactions across a broad range of ABA concentrations. Monomeric receptors (PYL4–10) weakly interact with PP2Cs in the absence of ABA and interact strongly at low ABA concentrations when ABA binding alters their conformation. Dimeric receptors (PYR1/PYL1–3) have a lower affinity for ABA, so higher concentrations are needed for them to dissociate and then strongly interact with the PP2Cs. All receptor interactions with PP2Cs result in inactivation of the PP2Cs and derepression of the SnRK2s, which then phosphorylate numerous proteins involved in ABA response. Green arrows indicate activation and red bars indicate repression.

Figure 5.

ABA signaling mediated by the plastid-localized CHLH protein. The H subunit of MgCheletase spans the outer chloroplast membrane. It can associate with 3 other subunits (CHLI, CHLD, and GUN4) to function in synthesis of Mg-ProtophyrinIX (Mg-ProtoIX) within the plastid. Alternatively, in association with CHLI, the C-terminus of CHLH can bind WRKY40 in the cytosol (and, to a lesser extent, WRKY 18 and 60). CHLH/WRKY binding is enhanced by ABA, such that these WRKYs are greatly depleted in nuclei, thereby derepressing several transcription factors that positively regulate ABA-induced gene expression.

Figure 6.

Interactions among some of the hormonal and developmental signals and regulatory elements controlling seed maturation. In the upper portion (“Accumulation”), factors regulating gene expression during the progression through seed development are shown in boxes representing the relative timing and abundance of their expression. ABA accumulation is depicted in pale red triangles. PYL5 is the ABA receptor that is most highly expressed in mid-seed development, and therefore likely to mediate ABA signaling during this stage. To simplify the diagram, intermediates in this signaling pathway are not shown (see Fig.4 for details). In the lower portion (“Roles”), the events regulated by specific factors are shown. In both parts, arrows represent positive regulation and red bars indicate repression.

Figure 7.

BiFC interaction between ABI4 and ABI5. N. benthamiana leaf co-infiltrated with Agrobacterium tumefaciens carrying plasmids encoding nYFP-ABI4 and either CYFP-ABI5 (left) or cYFP (right), and the P19 protein of tomato bushy stunt virus to enhance transient expression. Fluorescence was scored 3 days later with an Olympus AX70 microscope. (Finkelstein, unpublished)

Figure 8.

Interactions among some of the hormonal and environmental signals and regulatory elements controlling the transition from dormancy to germination. ABA receptors listed are those that are most abundantly expressed in dry and imbibing seeds. Red triangles represent ABA accumulation during either dormancy maintenance or in response to dehydrating stresses following dormancy release. Arrows indicate positive regulation and red bars represent repression, illustrating substantial cross-talk controlling ABA/GA balance and feedback within each hormone's signaling network.

Figure 9.

Stress signaling pathways Summary of ABA-dependent and ABA-independent pathways mediating response to abiotic stresses. Overlaps among stress-regulated transcriptomes are based on data from ATH1 and tiling arrays presented in (Zeller et al., 2009).