Since the 1990s, genetic studies aimed at analyzing causal genes from mutants have been actively conducted, leading to the rapid elucidation of the biosynthesis and signaling pathways of phytohormones. My research has focused on analyzing mutants associated with the terpenoid phytohormones, brassinosteroids (BR), gibberellins (GA), and strigolactones (SL). In particular, I have elucidated the functions of cytochrome P450 enzymes, a type of oxidases that are encoded as the causal genes of the following mutants. In the BR studies, the analysis of the dwarf tomato mutant d^x resulted in the discovery of CYP85A3, which catalyzes the conversion of castasterone to brassinolide, representing the final step in the BR biosynthetic pathway. In the GA studies, CYP714D1, encoded by the causal gene for the tall rice mutant eui, was identified as a novel deactivation enzyme responsible for 16α,17-epoxidation of GAs. Additionally, I found that a C-13 oxidase in the GA₁ biosynthetic pathway is also encoded by CYP714. Concerning the SL studies, CYP711A1, encoded by the causal gene for the highly branching mutant max1 in Arabidopsis, was identified as the enzyme catalyzing the conversion of the SL precursor carlactone to carlactonoic acid. Furthermore, I investigated the function of CYP711As in various plant species and found that the biosynthetic pathways following carlactonoic acid are diverse among plant species.

Stomatal pores in the plant epidermis open and close to regulate gas exchange between leaves and the atmosphere. To uncover and manipulate the stomatal opening pathway, we have screened several a series of chemical libraries and identified compounds that affect stomatal opening. Here we show benzyl isothiocyanate (BITC), a Brassicales-specific metabolite, as a potent stomatal-opening inhibitor that suppresses the plasma membrane proton pump. We further developed BITC derivatives with multiple isothiocyanate groups (Multi-ITCs), which demonstrated at most 66-times higher inhibitory activity on stomatal opening, as well as a longer duration of the effect and negligible toxicity. Our research elucidates the biological function of BITC and its use as an agrochemical that confers drought tolerance on plants by suppressing stomatal opening.

Brassinosteroids (BRs) are essential phytohormones that regulate a broad range of physiological processes. BR signaling activates BIL1/BZR1 of the plant-specific bHLH-like transcription factor family. BR-responsive gene expression is directly regulated by BIL1/BZR1 in both activation and repression. Here we describe a non-canonical dimeric structure that gives rise to a unique DNA recognition mechanism for BIL1/BZR1. This structure fine-tunes the Glu(i) residue in the direction that loosens the recognition of the C₁A₂ bases of the G-box motif (5´-CACGTG-3´) and tightens a hydrogen bonding network with the phosphate groups of DNA. Consequently, BIL1/BZR1 binds to the NN-BRRE-core motif (5´-NNCGTG-3´) and exerts selectivity for two nucleobases flanking the motif by the DNA shape readout without direct recognition of the nucleobases. Our genome-wide data analysis also shows that the tight binding of BIL1/BZR1 alone to gene promoters is required for BR-responsive transcriptional repression but not for transcriptional activation.

Plants can generate pluripotent tissue, known as callus, by dedifferentiation of somatic cells. Pluripotent callus is typically induced artificially by culturing explants with a hormone mixture of auxin and cytokinin, and an entire plant body can be regenerated from the callus. A recent discovery introduces a small compound, PLU, that induces the formation of pluripotent callus without external hormone application. Although the callus induced by PLU shares common features with that induced by conventional callus-inducing media supplemented with external hormones, they have several distinct differences. PLU provides a novel approach to manipulate and study plant pluripotency from a different angle than the conventional method with the hormone application.

Allelopathy is one of the survival strategies employed by plants, and allelochemicals, which play a critical role, are expected to be useful compounds in plant physiology research tools, as well as lead compounds for pesticides and growth regulators. cis-Cinnamic acid, the active compound of cis-cinnamic acid glycosides isolated from Spiraea thunbergii, exhibits strong inhibitory effects on the elongation of plant roots. Our research group, consisting of plant physiologists and organic chemists, synthesized cis-cinnamic acid and developed a fluorescent probe through structure-activity relationship studies, revealing the root cap with high affinity for cis-cinnamic acid through molecular imaging experiments. Drawing inspiration from the inhibitory effect on gravitropism, we screened cis-cinnamic acid analogues and discovered ku-76 (2Z,4E-5-phenylpenta-2,4-dienoic acid), which specifically inhibits gravitropism. Using this as a lead compound, we further modified its structure and successfully developed BMA ((Z)-4-((Z)-benzylidene)-6-(4-methoxyphenyl)hex-2-en-5-ynoic acid), a potent inhibitor. Additionally, plant physiology experiments on auxin distribution suggest its involvement in the control of polar auxin transport.

This research note summarizes the findings reported in our paper, Cui et al. (2023), detailing our investigation into the question, “How can we increase rice yields through fertilization?” and our approach to addressing it through trial and error. Initially, we aimed to identify fertilization response genes of rice in real paddy fields rather than controlled growth chambers in laboratories. In these fields, we observed that fertilization increased tiller numbers, subsequently leading to increased rice yields through appropriate fertilization methods. We conducted an extensive field transcriptome analysis using paired t-tests on dozens of sample pairs with and without fertilization at distinct timings after fertilization, across several developmental stages of rice seedlings. Through this analysis, we identified over a hundred genes consistently showing a fertilization response and chose to focus on a gene termed Os1900, homologous to the Arabidopsis MAX1 gene, based on their homology. Following extensive analysis of rice MAX1 homologs, we identified Os1900 as a major contributor to controlling tiller numbers in fields. We discovered that Os1900 primarily catalyzes the conversion of carlactone to carlactonoic acid, in conjunction with another MAX1 homolog, Os5100. Additionally, we successfully obtained several promoter deletion lines aimed at maintaining tiller numbers and a deletion line to sustain yield under low fertilization conditions. These lines hold promise for future breeding and agricultural practices aligned with the Sustainable Development Goals (SDGs).

Grafting-induced flowering has the potential to enhance plant breeding and seed production by shortening the time needed for floral induction and expanding the environments where seed production is feasible. However, the application of this technique has been limited to some crop species due to the difficulty of inducing flowering through grafting. We have established a technique to rapidly induce flowering in cabbage (Brassica oleracea L.), a cruciferous vegetable for which no effective grafting method had previously been established. This was achieved by using specific radish (Raphanus sativus L.) rootstocks with a high capacity to supply florigen. We call this method the “GRAFT (Grafting onto Relatives capable of Abundant Florigen Transmission) method”. This paper outlines previous studies on grafting-induced flowering, and discusses key rootstock factors for floral induction in grafted cabbage and the potential application of this method to the breeding and seed production of cruciferous crops.

With the progression of global warming in recent years, high temperatures have produced various adverse impacts on satsuma mandarin production. Labor shortages have also necessitated the adoption of labor-saving cultivation techniques. We thus developed three techniques that utilize plant growth regulators to reduce widespread damage that currently occurs due to global warming and labor shortages. The first technique reduces peel puffing, the second improves upon handpicking to shorten harvesting time, and the third increases flowering, especially leafy flowers, in greenhouse cultivation. Our technique for reducing peel puffing is widely used in modern satsuma mandarin cultivation.

Pyraclonil is a potent inhibitor of protoporphyrinogen oxidase (PPO) and has been registered for use as paddy herbicide for preemergence and early postemergence control of grasses, sedges, and broadleaf weeds in Japan in 2007. Since the launch of the product, pyraclonil has contributed to the control of problematic weeds that have developed resistance to sulfonylurea herbicides in paddy fields. In addition, pyraclonil is highly safe to rice even in the application at transplanting. Furthermore, pyraclonil shows synergistic action with 4-hydroxyphenylpyruvate dioxygenase (4-HPPD) inhibitors. In the last decade, pyraclonil has become one of the most popular paddy herbicides in Japan.