The physiological functions of abscisic acid (ABA) are triggered by interactions between PYR/PYL/RCAR receptors (PYL) and group-A protein phosphatases 2C (PP2C). PYL antagonists, chemical compounds, capable of disrupting these interactions would be valuable in investigating the regulatory mechanisms of ABA signaling, especially in plant systems lacking genetic resources. Based on the structural mechanism of the PYL-PP2C coreceptor system for ABA, we have generated 3'-modified ABA analogs, AS6, PAO4, and PAT3, and a 4'-modified ABA analog, PANMe, as PYL antagonists. These compounds bound PYL with an affinity comparable to that of ABA and inhibited ABA-induced PYL-PP2C interactions both in vitro and in vivo. Additionally, to facilitate the practical use of ABA in agricultural fields, we developed photostable ABA analogs, BP2A compounds series, in which the dienoic acid side chain of ABA was replaced with phenylacetic acid.

Natural auxin, indole-3-acetic acid (IAA), regulates various plant developmental processes and their response to environmental changes. Cellular IAA levels are modulated by three regulatory mechanisms, biosynthesis, polar transport, and inactivation. IAA is mainly synthesized from tryptophan (Trp) via indole-3-pyruvic acid by two-step reactions (IPA pathway) that are catalyzed by two enzymes, TAA1/TAR and YUCCA. The inactivation of IAA plays a crucial role in auxin homeostasis to maintain an appropriate cellular IAA level. Several metabolic enzymes have been implicated in auxin inactivation. GH3, UGT84B1, and IAMT1 enzymes catalyze the conjugation reaction of IAA, which is classified as reversible inactivation. DAO 2-oxoglutalate-dependent dioxygenase catalyzes the irreversible oxidative inactivation of auxin. This review summarizes the recent progress on IAA biosynthesis and inactivation pathways. We will discuss local IAA biosynthesis and the regulatory machinery modulating its homeostasis, and the main auxin oxidative inactivation pathway.

Plants are sessile organisms and therefore they have evolved molecular mechanisms to highly adapt to their environment;short- and long-term signal transduction. Short-term signal transduction includes responses to light, temperature, and ambient chemicals. Long-term signal transduction utilizes a complex network of interactions to coordinate biochemical and physiological responses such as flowering, fruit ripening, seed germination, photosynthetic regulation, and shoot or root development. The biological membrane serves not only as a partition that separates the inside and outside of cells and organelles, but also forms concentration gradients of compounds and electrical differences on either side of the membrane. This gives rise to both chemical and electrical gradients across the membrane, which establishes the membrane as a central role for signal transduction. This review focuses on the electric factor for plant hormone-mediated signal transduction. Recent studies have shown that anion channels and potassium channels coordinately modulate membrane potential and cellular ion strength in the response to environmental changes in various cells and tissues. We have described the similarities and dissimilarities with ion transport systems in neurotransmitter pathways and the molecular mechanisms involved in the maintenance of intracellular homeostasis in plants and in the differentiation of plant tissue growth.

Cytokinins are widely recognized as crucial phytohormones regulating plant growth and development by promoting cell division and differentiation. They are also known to be involved in inter-organism interactions, including the interaction between parasitic plants and host plants. Orobanchaceae parasitic plants form prehaustoria, primitive haustorial structures before host invasion, when in close proximity to the hosts. The induction of prehaustoria requires host-derived compounds, namely haustorium-inducing factors (HIFs), including quinones, phenolics and flavonoids, with structural similarity to lignin monomers. Notably, recent reports showed that cytokinins are capable to induce prehaustoria in the obligate parasitic plants in Orobanchaceae. Once prehaustoria invade into host roots, they become mature haustoria, which efficiently extract water and nutrients from the host. Cytokinins are also involved in this stage, as they are synthesized by parasitic plants and transported to host plants, causing hypertrophy and promoting efficient parasitism. In this review, we summarize the roles of cytokinins in the interaction between parasitic plants and host plants, with a specific focus on current understanding of prehaustorium induction, hypertrophy and germination of obligate parasites.

Strigolactones (SLs) are a group of plant hormones that regulate not only shoot branching but also various developmental processes in plants. Methyl carlactonoate (MeCLA) has been proposed to be a bioactive molecule because this compound could interact with the AtD14 receptor and rescue the SL-deficient mutants in Arabidopsis. We have recently identified a methyltransferase named CLAMT that converts carlactonoic acid (CLA), a biologically inactive biosynthetic intermediate, into MeCLA in Arabidopsis. Endogenous CLA levels were accumulated, but MeCLA levels were drastically reduced in the clamt mutants. The clamt mutants exhibited a hyper-branching phenotype. This phenotype was rescued by MeCLA but not CLA treatment, supporting the idea that the conversion of CLA to MeCLA is critical in producing the biologically active compound(s) in inhibiting shoot branching. Moreover, by using grafting, we demonstrated that carlactone and CLA are mobile from roots to shoots, but MeCLA is not. Our finding will accelerate the understanding of SL biosynthesis in plants.

Iron (Fe) is an essential nutrient for plant growth. Fe deficiency occurs frequently in alkaline or calcareous soils because of low solubility of Fe under high pH. Poaceae plants take up Fe(III) complex with mugineic acid family phytosiderophores (MAs) after their secretion. Insufficient secretion of MAs causes Fe deficiency and reduction of crop production. Application of 2’-deoxymugineic acid (DMA) into soil recovered Fe deficiency in rice plants, but the effect was not prolonged because of high biodegradability of DMA. Here, we developed a novel analog of phytosiderophore designated “proline 2’-deoxymugineic acid (PDMA)” by chemical synthesis. Soil application of Fe-PDMA or metal-free PDMA recovered Fe deficiency in rice plants. The effect of soil-applied Fe-PDMA was sustained longer than that of Fe-DMA. In addition, soil application of Fe-PDMA to non-poaceae plants also resulted in better recovery from Fe deficiency compared to other chelating complexes. The reducibility of Fe(III)-PDMA to ferrous ion was higher than that of other chelating complexes. The efficacy of PDMA was demonstrated in rice and peanuts in field experiments. In summary, PDMA is a promising chelate as a fertilizer not only for poaceae plants but also for non-poaceae plants, and its moderate biodegradability might be compatible with environment.

Drought stress mitigation and increasing plant productivity are two major goals for a sustainable future amidst growing populations and global climatic changes. Understanding inherent mechanisms adopted by plants to resist drought stress and to maintain optimal growth could act as fundamental tools which can be utilized for developing technologies for increasing the growth and survival potential of plants. Nicotinic acid (NA) participates in the nicotinamide adenine dinucleotide (NAD) biosynthesis and recycling pathways. Nicotinamidase 3 (NIC3), a root-specific and drought-responsive gene, generates NA from nicotinamide (NAM) during the NAD salvage pathway. Overexpressing NIC3 enhances plant survival ratio, increases plant growth, and accumulates NA as a metabolite. The exogenous application of NA mimics the effect of NIC3 overexpression. The fundamental biological potential of NA can be used to prime plants not only for better survival under drought stress conditions but also for increasing plant productivity.

Plant hormone, indole-3-acetic acid (IAA) is synthesized in a two-step enzymatic reactions involving tryptophan (Trp) aminotransferases (TAA1/TARs) and flavin monooxygenases (YUCs). In this study, we synthesized analogs of the reaction intermediate indole-3-pyruvic acid (IPyA), such as KOK2099, and demonstrated that IPyA regulates auxin biosynthesis at the level of enzyme regulation. Results from treatment of Arabidopsis seedlings indicated that the IPyA analogs are IAA biosynthesis inhibitor. Enzymatic studies with recombinant TAA1 showed that KOK2099 and IPyA inhibited TAA1 activity. When reacted sequentially of TAA1 and AtYUC10 to consume IPyA, the activity of TAA1 was significantly increased. Since aminotransferases are generally reversible, we tested the reverse reaction activity of TAA1. TAA1 has the activity to convert IPyA to Trp. IPyA levels are maintained by the push (by TAA1/TARs) and pull (by YUCs) of the two biosynthetic enzymes, in which TAA1 plays a key role in preventing the over- or under-accumulation of IPyA.

Ipfencarbazone exhibits excellent herbicidal activity against Echinochloa spp. and is highly safe for rice.

Although ipfencarbazone inhibited very long chain fatty acids (VLCFA) elongation in the microsomes prepared from late watergrass and rice at low concentrations, the inhibitory effect was higher in late watergrass than in rice. These results suggested that the primary site of action of ipfencarbazone is VLCFA elongase (VLCFAE) and ipfencarbazone has a differential affinity between VLCFAEs of the plants. The inhibitory activity of ipfencarbazone became higher in proportion to pre-incubation period with VLCFAE. The degree of inhibition did not decrease by dilution of VLCFAE-ipfencarbazone complex. These results suggested that ipfencarbazone binds to the VLCFAE irreversibly.