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.

https://bioone.org/journals/the-arabidopsis-book/volume-2013/issue-11/tab.0166/Abscisic-Acid-Synthesis-and-Response/10.1199/tab.0166.pdf

Abscisic Acid synthesis and response.

The GAs (gibberellins) comprise a large group of diterpenoid carboxylic acids that are ubiquitous in higher plants, in which certain members function as endogenous growth regulators, promoting organ expansion and developmental changes. These compounds are also produced by some species of lower plants, fungi and bacteria, although, in contrast to higher plants, the function of GAs in these organisms has only recently been investigated and is still unclear. In higher plants, GAs are synthesized by the action of terpene cyclases, cytochrome P450 mono-oxygenases and 2-oxoglutarate-dependent dioxygenases localized, respectively, in plastids, the endomembrane system and the cytosol. The concentration of biologically active GAs at their sites of action is tightly regulated and is moderated by numerous developmental and environmental cues. Recent research has focused on regulatory mechanisms, acting primarily on expression of the genes that encode the dioxygenases involved in biosynthesis and deactivation. The present review discusses the current state of knowledge on GA metabolism with particular emphasis on regulation, including the complex mechanisms for the maintenance of GA homoeostasis.

Gibberellin biosynthesis and its regulation.

Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/jipb.12899

Abscisic acid dynamics, signaling, and functions in plants

Two Faces of One Seed: Hormonal Regulation of Dormancy and Germination

Plants exhibit a unique developmental flexibility to ever-changing environmental conditions. To achieve their profound adaptability, plants are able to maintain permanent stem cell populations and form new organs during the entire plant life cycle. Signaling substances, called plant hormones, such as auxin, cytokinin, abscisic acid, brassinosteroid, ethylene, gibberellin, jasmonic acid, and strigolactone, govern and coordinate these developmental processes. Physiological and genetic studies have dissected the molecular components of signal perception and transduction of the individual hormonal pathways. However, over recent years it has become evident that hormones do not act only in a linear pathway. Hormonal pathways are interconnected by a complex network of interactions and feedback circuits that determines the final outcome of the individual hormone actions. This raises questions about the molecular mechanisms underlying hormonal cross talk and about how these hormonal networks are established, maintained, and modulated throughout plant development.

Hormonal Interactions in the Regulation of Plant Development

Light absorbed by seed phytochromes of Arabidopsis thaliana modulates abscisic acid (ABA) and gibberellic acid (GA) signaling pathways at least partly via PHYTOCHROME-INTERACTING FACTOR3-LIKE5 (PIL5), a phytochrome-interacting basic helix-loop-helix transcription factor. Here, we report a new mutant, somnus (som), that germinates in darkness, independently of various light regimens. SOM encodes a nucleus-localized CCCH-type zinc finger protein. The som mutant has lower levels of ABA and elevated levels of GA due to expressional changes in ABA and GA metabolic genes. Unlike PIL5, however, SOM does not regulate the expression of GA-INSENSITIVE and REPRESSOR OF GA1 (RGA/RGA1), two DELLA genes encoding GA negative signaling components. Our in vivo analysis shows that PIL5 activates the expression of SOM by binding directly to its promoter, suggesting that PIL5 regulates ABA and GA metabolic genes partly through SOM. In agreement with these results, we also observed that the reduced germination frequency of a PIL5 overexpression line is rescued by the som mutation and that this rescue is accompanied by expressional changes in ABA and GA metabolic genes. Taken together, our results indicate that SOM is a component in the phytochrome signal transduction pathway that regulates hormone metabolic genes downstream of PIL5 during seed germination.

SOMNUS, a CCCH-Type Zinc Finger Protein in Arabidopsis, Negatively Regulates Light-Dependent Seed Germination Downstream of PIL5

Seeds monitor the environment to germinate at the proper time, but different species respond differently to environmental conditions, particularly light and temperature. In Arabidopsis thaliana, light promotes germination but high temperature suppresses germination. We previously reported that light promotes germination by repressing SOMNUS (SOM). Here, we examined whether high temperature also regulates germination through SOM and found that high temperature activates SOM expression. Consistent with this, som mutants germinated more frequently than the wild type at high temperature. The induction of SOM mRNA at high temperature required abscisic acid (ABA) and gibberellic acid biosynthesis, and ABA-INSENSITIVE3 (ABI3), ABI5, and DELLAs positively regulated SOM expression. Chromatin immunoprecipitation assays indicated that ABI3, ABI5, and DELLAs all target the SOM promoter. At the protein level, ABI3, ABI5, and DELLAs all interact with each other, suggesting that they form a complex on the SOM promoter to activate SOM expression at high temperature. We found that high-temperature-inducible genes frequently have RY motifs and ABA-responsive elements in their promoters, some of which are targeted by ABI3, ABI5, and DELLAs in vivo. Taken together, our data indicate that ABI3, ABI5, and DELLAs mediate high-temperature signaling to activate the expression of SOM and other high-temperature-inducible genes, thereby inhibiting seed germination.

/pdf/aba-insensitive3-aba-insensitive5-and-dellas-interact-to-10c1y1t3wr.pdf

ABA-INSENSITIVE3, ABA-INSENSITIVE5, and DELLAs Interact to Activate the Expression of SOMNUS and Other High-Temperature-Inducible Genes in Imbibed Seeds in Arabidopsis

Ginseng is a medicinal herb that requires cultivation under shade conditions, typically for 4–6 years, before harvesting. The principal components of ginseng are ginsenosides, glycosylated tetracyclic terpenes. Dammarene-type ginsenosides are classified into two groups, protopanaxadiol (PPD) and protopanaxatriol (PPT), based on their hydroxylation patterns, and further diverge to diverse ginsenosides through differential glycosylation. Three early enzymes, dammarenediol-II synthase (DS) and two P450 enzymes, protopanaxadiol synthase (PPDS) and protopanaxatriol synthase (PPTS), have been reported, but glycosyltransferases that are necessary to synthesize specific ginsenosides have not yet been fully identified. To discover glycosyltransferases responsible for ginsenoside biosynthesis, we sequenced and assembled the ginseng transcriptome de novo and characterized two UDP-glycosyltransferases (PgUGTs): PgUGT74AE2 and PgUGT94Q2. PgUGT74AE2 transfers a glucose moiety from UDP-glucose (UDP-Glc) to the C3 hydroxyl groups of PPD and compound K to form Rh ₂ and F2, respectively, whereas PgUGT94Q2 transfers a glucose moiety from UDP-Glc to Rh ₂ and F2 to form Rg ₃ and Rd, respectively. Introduction of the two UGT genes into yeast together with PgDS and PgPPDS resulted in the de novo production of Rg ₃. Our results indicate that these two UGTs are key enzymes for the synthesis of ginsenosides and provide a method for producing specific ginsenosides through yeast fermentation.

/pdf/two-ginseng-udp-glycosyltransferases-synthesize-ginsenoside-46bbatl9qh.pdf

Two Ginseng UDP-Glycosyltransferases Synthesize Ginsenoside Rg3 and Rd

A previous study showed that SOMNUS (SOM), which encodes a C3H-type zinc finger protein, is a key negative regulator of seed germination that acts downstream of PHYTOCHROME INTERACTING FACTOR3-LIKE5 (PIL5). However, it was not determined if PIL5 is the sole regulator of SOM expression. Public microarray data suggest that the expression of SOM mRNA is regulated also by ABSCISIC ACID INSENSITIVE3 (ABI3), another key regulator of seed germination. By analyzing abi3 mutants and ABI3 overexpression lines, we show here that ABI3 activates the expression of SOM mRNA collaboratively with PIL5 in imbibed seeds. Chromatin immunoprecipitation analysis coupled with electrophoretic mobility shift assay indicate that ABI3 activates the expression of SOM mRNA by directly binding to two RY motifs present in the SOM promoter in vivo, which is further supported by the greatly decreased expression of a reporter gene driven by a SOM promoter bearing mutated RY motifs. At the protein level, the ABI3 protein interacts with the PIL5 protein. The ABI3-PIL5 interaction, however, does not affect targeting of ABI3 and PIL5 to SOM promoters. Taken together, our results indicate that ABI3 and PIL5 collaboratively activate the expression of SOM mRNA by directly binding to and interacting with each other at the SOM promoter.

/pdf/abi3-and-pil5-collaboratively-activate-the-expression-of-2ui1rjpzb5.pdf

Soohwan Lim

Papers

SOMNUS, a CCCH-Type Zinc Finger Protein in Arabidopsis, Negatively Regulates Light-Dependent Seed Germination Downstream of PIL5

ABA-INSENSITIVE3, ABA-INSENSITIVE5, and DELLAs Interact to Activate the Expression of SOMNUS and Other High-Temperature-Inducible Genes in Imbibed Seeds in Arabidopsis

Two Ginseng UDP-Glycosyltransferases Synthesize Ginsenoside Rg3 and Rd

ABI3 and PIL5 Collaboratively Activate the Expression of SOMNUS by Directly Binding to Its Promoter in Imbibed Arabidopsis Seeds

Identification of three groups of ginsenoside biosynthetic UDP-glycosyltransferases from Gynostemma pentaphyllum.