Enlarged analogues of uniconazole, new azole containing inhibitors of ABA 8′-hydroxylase CYP707A

B2, an abscisic acid mimic, improves salinity tolerance in winter wheat seedlings via improving activity of antioxidant enzymes

Salinity severely inhibits growth and reduces yield of salt-sensitive plants like wheat, and this effect can be alleviated by plant growth regulators and phytohormones, among which abscisic acid (ABA) plays a central role in response to various stressful environments. ABA is highly photosensitive to light disruption, which this limits its application. Here, based on pyrabactin (a synthetic ABA agonist), we designed and synthesized a functional analog of ABA and named B2, then evaluated its role in salt resistance using winter wheat seedlings. The phenotypes showed that B2 significantly improved the salt tolerance of winter wheat seedlings by elevating the biomass. The physiological analysis found that B2 treatment reduced the generation rate of O₂ ^-, electrolyte leakage, the content of proline, and the accumulation of malonaldehyde (MDA) and H₂O₂ and also significantly increased the contents of endogenous hormones zeatin riboside (ZA) and gibberellic acid (GA). Further biochemical analysis revealed that the activities of various antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), were enhanced by B2, and the activities of antioxidase isozymes SOD3, POD1/2, and APX1/2 were particularly increased, largely resembling ABA treatment. The abiotic stress response-related gene TaSOS1 was significantly upregulated by B2, while the TaTIP2;2 gene was suppressed. In conclusion, an ABA analog B2 was capable to enhance salt stress tolerance in winter wheat seedlings by stimulating the antioxidant system, providing a novel regulator for better survival of crops in saline soils and improving crop yield.

Agonist, antagonist and signaling modulators of ABA receptor for agronomic and post-harvest management

Abscisic acid (ABA) is a ubiquitous phytohormone, plays important roles in several physiological processes, including stress adaptation, flowering, seed germination, fruit ripening, and leaf senescence etc. ABA binds with START domain proteins called Pyrabactin Resistance1 (PYR1)/PYR1-like (PYL)/Regulatory Components of ABA Receptors (RCARs) and controls the activity of PP2C phosphatase proteins and in turn the ABA-dependent signaling pathway. Fourteen ABA receptors have been identified in the model plant Arabidopsis thaliana and have shown to be involved in various biological functions. Under field conditions, exogenous application of ABA produces inadequate physiological response due to its rapid conversion into the biologically inactive metabolites. ABA shows selective binding preferences to PYL receptor subtypes and hence produces pleiotropic physiological and phenotypic effects which limit the usage of ABA in agriculture. An agrochemical meant for ameliorating the undesirable physiological effect of the plant should ideally have positive biological attributes without affecting the normal growth, development, and yield. Therefore, to overcome the limitations of ABA for its usage in various agricultural applications, several types of ABA-mimicking agents have been developed. Many compounds have been identified as having significant ABA-agonist/antagonist activity and can be employed to reverse the excessive/moderate ABA action. The present review highlights the potential usage of ABA signaling modulators for managing agronomic and postharvest traits. Besides, designing, development and versatile usage of ABA-mimicking compounds displaying ABA agonists and antagonist activities are discussed in detail.

Highly Enantioselective One-Pot Synthesis of Chiral β-Heterosubstituted Alcohols via Ruthenium-Prolinamide-Catalyzed Asymmetric Transfer Hydrogenation

The utility of a chiral Ru-prolinamide catalytic system has been demonstrated in one-pot synthesis of optically active β-triazolylethanol and β-hydroxy sulfone derivatives. The said methodology proceeds through asymmetric transfer hydrogenation of in situ formed ketones of the corresponding chiral products. Various chiral prolinamide ligands were screened, and ligand L6 with isopropyl groups substituted at the ortho position has shown excellent activity at 60 °C in aqueous medium producing up to 95% yield and 99.9% enantioselectivity.

Chemical regulators of plant hormones and their applications in basic research and agriculture*

Plant hormones are small molecules that play versatile roles in regulating plant growth, development, and responses to the environment. Classic methodologies, including genetics, analytic chemistry, biochemistry, and molecular biology, have contributed to the progress in plant hormone studies. In addition, chemical regulators of plant hormone functions have been important in such studies. Today, synthetic chemicals, including plant growth regulators, are used to study and manipulate biological systems, collectively referred to as chemical biology. Here, we summarize the available chemical regulators and their contributions to plant hormone studies. We also pose questions that remain to be addressed in plant hormone studies and that might be solved with the help of chemical regulators.

Abscinazole-E3M, a practical inhibitor of abscisic acid 8'-hydroxylase for improving drought tolerance

Abscisic acid (ABA) is an essential phytohormone that regulates plant water use and drought tolerance. However, agricultural applications of ABA have been limited because of its rapid inactivation in plants, which involves hydroxylation of ABA by ABA 8′-hydroxylase (CYP707A). We previously developed a selective inhibitor of CYP707A, (−)-Abz-E2B, by structurally modifying S-uniconazole, which functions as an inhibitor of CYP707A and as a gibberellin biosynthetic enzyme. However, its synthetic yield is too low for practical applications. Therefore, we designed novel CYP707A inhibitors, Abz-T compounds, that have simpler structures in which the 1,2,3-triazolyl ring of (−)-Abz-E2B has been replaced with a triple bond. They were successfully synthesised in shorter steps, resulting in greater yields than that of (−)-Abz-E2B. In the enzymatic assays, one of the Abz-T compounds, (−)-Abz-E3M, acted as a strong and selective inhibitor of CYP707A, similar to (−)-Abz-E2B. Analysis of the biological effects in Arabidopsis revealed that (−)-Abz-E3M enhanced ABA’s effects more than (−)-Abz-E2B in seed germination and in the expression of ABA-responsive genes. Treatment with (−)-Abz-E3M induced stomatal closure and improved drought tolerance in Arabidopsis. Furthermore, (−)-Abz-E3M also increased the ABA response in rice and maize. Thus, (−)-Abz-E3M is a more practical and effective inhibitor of CYP707A than (−)-Abz-E2B.

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.

Abscinazole-E1, a novel chemical tool for exploring the role of ABA 8'-hydroxylase CYP707A

We developed abscinazole-E1 (Abz-E1), a specific inhibitor of abscisic acid (ABA) 8'-hydroxylase (CYP707A). This inhibitor was designed and synthesized as an enlarged analogue of uniconazole (UNI), a well-known plant growth retardant, which inhibits a gibberellin biosynthetic enzyme (ent-kaurene oxidase, CYP701A) as well as CYP707A. Our results showed that Abz-E1 functions as a potent inhibitor of CYP707A and a poor inhibitor of CYP701A both in vitro and in vivo. Abz-E1 application to plants resulted in improved desiccation tolerance and an increase in endogenous ABA.

Selectivity improvement of an azole inhibitor of CYP707A by replacing the monosubstituted azole with a disubstituted azole

The plant growth-retardant uniconazole (UNI), a triazole inhibitor of gibberellin biosynthetic enzyme (CYP701A), inhibits multiple P450 enzymes including ABA 8'-hydroxylase (CYP707A), a key enzyme in ABA catabolism. Azole P450 inhibitors bind to a P450 active site by both coordinating to the heme-iron atom via sp2 nitrogen and interacting with surrounding protein residues through a lipophilic region. We hypothesized that poor selectivity of UNI may result from adopting a distinct conformation and orientation for different active sites. Based on this hypothesis, we designed and synthesized novel UNI analogs with a disubstituted azole ring (DSI). These analogs were expected to have higher selectivity than UNI because the added functional group may interact with the active site to restrict orientation of the molecule in the active site. DSI-505ME and DSI-505MZ, which have an imidazolyl group with a methyl 5-acrylate, strongly inhibited recombinant CYP707A3, with no growth-retardant effect.