Cytokinins (CKs) play a crucial role in various phases of plant growth and development, but the basic molecular mechanisms of their biosynthesis and signal transduction only recently became clear. The progress was achieved by identifying a series of key genes encoding enzymes and proteins controlling critical steps in biosynthesis, translocation, and signaling. Basic schemes for CK homeostasis and root/shoot communication at the whole-plant level can now be devised. This review summarizes recent findings on the relationship between CK structural variation and activity, distinct features in CK biosynthesis between higher plants and Agrobacterium infected plants, CK translocation at whole-plant and cellular levels, and CKs as signaling molecules for nutrient status via root-shoot communication.

CYTOKININS: Activity, Biosynthesis, and Translocation

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.

Molecular activities, biosynthesis and evolution of triterpenoid saponins.

The role of ABA and MAPK signaling pathways in plant abiotic stress responses.

A comparative transcriptome analysis for successive stages of Arabidopsis (Arabidopsis thaliana) developmental leaf senescence (NS), darkening-induced senescence of individual leaves attached to the plant (DIS), and senescence in dark-incubated detached leaves (DET) revealed many novel senescence-associated genes with distinct expression profiles. The three senescence processes share a high number of regulated genes, although the overall number of regulated genes during DIS and DET is about 2 times lower than during NS. Consequently, the number of NS-specific genes is much higher than the number of DIS- or DET-specific genes. The expression profiles of transporters (TPs), receptor-like kinases, autophagy genes, and hormone pathways were analyzed in detail. The Arabidopsis TPs and other integral membrane proteins were systematically reclassified based on the Transporter Classification system. Coordinate activation or inactivation of several genes is observed in some TP families in all three or only in individual senescence types, indicating differences in the genetic programs for remobilization of catabolites. Characteristic senescence type-specific differences were also apparent in the expression profiles of (putative) signaling kinases. For eight hormones, the expression of biosynthesis, metabolism, signaling, and (partially) response genes was investigated. In most pathways, novel senescence-associated genes were identified. The expression profiles of hormone homeostasis and signaling genes reveal additional players in the senescence regulatory network.

Transcription Analysis of Arabidopsis Membrane Transporters and Hormone Pathways during Developmental and Induced Leaf Senescence

N-glucosylation of cytokinins by glycosyltransferases of Arabidopsis thaliana.

Abscisic acid (ABA) can exist as two enantiomers, with (+)-ABA as the naturally occurring form. Typically, both enantiomers occur in chemical preparations and both can be modified in the plant to their respective glucose esters. To identify glycosyltransferases capable of discriminating between the different forms of ABA, the Family 1 enzymes of Arabidopsis thaliana were screened for activity towards (±)-ABA. Eight enzymes were found to recognise the plant hormone, with one UGT71B6 showing enantioselective glucosylation towards (+)-ABA. UGT71B6 was used in a whole-cell biocatalysis system as a means of separating (+)- and (−)-ABA, thereby offering an alternative to chemical synthesis for the production of pure (+)-ABA.

Resolution of (+)-abscisic acid using an Arabidopsis glycosyltransferase

Regioselectivity of glycosyltransferases offers an important means to overcome the limitations of chemical synthesis of small molecule glycosides. In this study we explore a large multigene family of UDP-glucose:glycosyltransferases of Arabidopsis for their potential as novel biocatalysts for in vitro synthesis and whole-cell catalysis. We used quercetin as a substrate for this study because the flavonol and its glycosides have important medicinal properties and the metabolite provides a complex structure for regioselective glucosylation. We analyzed the activity of 91 recombinant enzymes for in vitro activity toward quercetin and discovered 29 that are capable of glucosylating the substrate. We demonstrate the first enzymic synthesis of a range of glucosides in vitro, including the 3-O-, 7-O-, 3'-O-, and 4'-O-monoglucosides, 3,7-di-O-glucoside, and 7,3'-di-O-glucoside. We also show that the regioselectivity of glucosylation can be maintained when the enzymes are used as whole-cell biocatalysts in Escherichia coli.

Arabidopsis glycosyltransferases as biocatalysts in fermentation for regioselective synthesis of diverse quercetin glucosides.

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N-glucosylation of cytokinins by glycosyltransferases of Arabidopsis thaliana.

Resolution of (+)-abscisic acid using an Arabidopsis glycosyltransferase

Arabidopsis glycosyltransferases as biocatalysts in fermentation for regioselective synthesis of diverse quercetin glucosides.