Oxygenases are enzymes that oxidize a substrate by transferring the oxygen from molecular oxygen O₂ to the substrate. They are classified into monooxygenases, which bind only one oxygen atom from O₂ to the substrate while the other oxygen atom combines with hydrogen to form water molecule, and dioxygenases, which bind both oxygen atoms to the substrate. In plants, two oxygease families—cytochrome P450 monooxygenases (hereafter CYP) and 2-oxoglutarate-dependent dioxygenases (hereafter DOX)—play crucial roles in the biosynthesis of plant hormones and secondary metabolites. Fortunately, the author had the opportunity to participate in pioneering research on these two types of oxygenases in plants. This review paper aims to introduce CYP and DOX as key enzymes that confer structural diversity to terpenoid compounds with various physiological activities, incorporating anecdotes from the author’s own research.

Plants acclimate to wetland environments by establishing a barrier to radial oxygen loss (ROL) at the root base, facilitating long-distance oxygen transport to the root apex and contributing to survival under anoxic and reduced soil conditions. This review summarizes the physiological functions and compositions of an ROL barrier, with particular emphasis on the regulatory mechanisms mediated by plant hormones that underlie apoplastic barrier formation in the exodermis.

Inter-kingdom interactions between plants and fungi in the rhizosphere are essential for their co-existence. Of the many small molecules mediating their interactions, strigolactones (SLs) stand out as a key group, facilitating the interaction between host plants and arbuscular mycorrhizal fungi (AMF). The complexity of soil fungi like AMF has made it challenging to elucidate how fungi perceive important plant signals in soil to mediate symbiosis. In this review, we outline an approach centred on using a model fungal system, baker’s yeast (Saccharomyces cerevisiae), to determine that strigolactones modulate phosphate homeostasis in fungi by targeting the high-affinity phosphate transporter, Pho84. We discuss the implications of SLs impinging on fungal phosphate transporters as well as the utility of using S. cerevisiae to study interactions in the rhizosphere.

Why plants need fertilization to produce seeds has long been discussed. We hereby identified a new specialized tissue required for seed formation at ovule’s chalazal end, showing the final form of the phloem end (Kasahara Gateway: KGW), supporting its transport function but is blocked by callose deposition. Callose was removed after central cell fertilization (open state), allowing nutrients to be transported to the seed. However, if fertilization fails, callose deposition persists (closed state), preventing the phloem end from transporting nutrients into the ovule. A β-1,3-glucanase gene, AtBG_ppap, was identified and Atbg_ppap mutant showed the closed state, producing smaller seeds due to incomplete callose degradation. Contrary, the AtBG_ppap overexpression line produced larger seeds due to continuous callose degradation, showing that the newly identified tissue is the ‘gateway’ for the seed nutrients. The mechanism was also identified in rice and could be applied widely to angiosperms to increase seed size.

Transition from seed dormancy to germination is a key trait that determines individual survival and population persistence. For crops, the dormancy–germination balance is directly linked to yield; for example, high germinability can increase the risk of pre-harvest sprouting. Assuming that rice cultivars grown across Japan’s north–south latitudinal range harbor diverse germination traits, we conducted a genotype-by-environment GWAS using 164 Japanese cultivars. We identified the 14-3-3 protein gene GF14h as a promoter of germination under optimal temperature conditions. Functional GF14h reduces ABA sensitivity and promotes germination by forming a nuclear complex with the bZIP transcription factor OREB1 and reducing its transactivation activity, leading to reduced expression of ABA-responsive genes. Moreover, MFT2 interacts with the OREB1–GF14h complex and fine-tunes rice germination by attenuating GF14h-mediated suppression of OREB1 function. I also summarize recent advances on GF14h function and highlight its importance in hormone signaling and breeding applications in rice.

Peptide signals play important roles in various physiological processes in plants. Proteolytic release of the mature peptide from its precursor is an important step in the generation of peptide signals. However, our understanding of the proteases that generate peptide signals remains limited compared with that of ligand-receptor interactions and downstream signaling pathways. Plant genomes contain hundreds of genes encoding proteases, many of which function redundantly. This redundancy can make it challenging to identify proteases in genetic studies. Recently, we established an efficient identification system for the target proteases in Arabidopsis apoplastic fluid. This system employs biochemical methods, including native two-dimensional electrophoresis for protein fractionation, an in-gel peptide cleavage assay using a fluorescence-quenching peptide substrate, and proteomic analysis with nano-LC-MS/MS. This paper describes the use of this system to identify proteases involved in the release of the immunogenic peptide flg22 from the bacterial flagellar protein flagellin.

KARRIKIN INSENSITIVE 2 (KAI2), a receptor for smoke-derived karrikins, is proposed to perceive an unknown endogenous signal, the KAI2 ligand (KL). Genetic studies have suggested that ligand hydrolysis is required for KAI2 signaling, but direct experimental evidence has remained limited. Here, we describe chemical tools designed to bind the KAI2 ligand-binding pocket while resisting hydrolysis, enabling mechanistic dissection of ligand-dependent receptor activation. Using these tools, we show that the highly bioactive ligand (+)-dMGer is hydrolyzed by KAI2 and transiently trapped in the catalytic pocket, whereas structurally related analogs with reduced hydrolyzability display weaker or no signaling activity. Structure–activity analyses further demonstrate that ligand hydrolysis alone is insufficient; subsequent covalent adduct formation with the catalytic triad is critical for full receptor activation. These findings provide direct experimental support for hydrolysis-dependent KAI2 signaling and offer new insight into the probable chemical features of the endogenous KAI2 ligand.

Plant growth regulators—including classical phytohormones such as auxin—have been intensively studied for decades. However, mutants exhibiting pleiotropic phenotypes continue to suggest the presence of unidentified hormone-like molecules. In this review, we highlight recent insights into Arabidopsis thaliana HOOKLESS1 (HLS1), together with ALTERED MERISTEM PROGRAM 1 (AMP1) and the MATE transporter DETOXIFICATION 51 (DTX51). Although HLS1 encodes a GNAT-domain protein and has long been proposed to function as a nuclear histone acetyltransferase, accumulating evidence increasingly challenges this view. Functional fusion analyses show that HLS1 localized outside the nucleus can fully complement the hls1 mutant phenotype, arguing against histones as its primary substrate. Furthermore, tissue-specific complementation analyses reveal that HLS1 regulates hook formation, leaf senescence, and shoot development in a non–cell-autonomous manner. Genetic interactions with AMP1 and transcriptional feedback involving DTX51 further support the idea that these factors participate in a shared regulatory pathway. We discuss the emerging possibility that HLS1 contributes to the biosynthesis of a mobile, yet unidentified, bioactive compound.

Plant cells exhibit high plasticity, enabling whole-plant regeneration through de novo organogenesis. While most plant species require exogenous phytohormones for shoot induction, ipecac (Carapichea ipecacuanha (Brot.) L. Andersson) spontaneously forms adventitious shoots from the epidermal cells of internodal segments cultured on phytohormone-free medium. This study elucidates the regulatory mechanisms underlying endogenous cytokinin (CK) biosynthesis during adventitious shoot formation. Our results demonstrate that shoot formation requires interactions between distinct tissues: the pith functions as a source of CK precursors, while the epidermis serves as the site of CK activation and organogenesis. Gene expression analysis revealed that CiIPT3 was primarily expressed in the pith, whereas CiLOG7 was specifically localized to the epidermis of the apical region. Furthermore, exogenous auxin suppressed CiLOG7 expression, and endogenous auxin predominantly accumulates in the basal region, where shoots do not form. These findings suggest that an autonomous “high CK/low auxin” gradient, established through tissue-specific CK biosynthesis, is a key determinant of the high regenerative capacity of ipecac.

Dimesulfazet (Code name: NC-653) is a novel rice herbicide developed by Nissan Chemical Corporation to control sedge weeds with high crop safety. Resistance to acetolactate synthase (ALS) inhibitors and the rise of perennial sedges created a need for new solutions beyond existing ALS, 4-hydroxyphenylpyruvate dioxygenase (HPPD), and fatty acid thioesterase (FAT) inhibitors. Through structural optimization of haloalkylsulfonanilide derivatives, dimesulfazet was discovered and registered in Japan in 2024. Mode-of-action studies revealed that it inhibits very long-chain fatty acid (VLCFA) biosynthesis. Arabidopsis treated with dimesulfazet showed phenotypes typical of VLCFA inhibition, and metabolomic profiling grouped it with Herbicide Resistance Action Committee (HRAC) Group 15 herbicides. Fatty acid analysis confirmed that dimesulfazet blocks elongation from C22:0 to C24:0, likely by targeting β-ketoacyl-CoA synthases (KCS). This unique mode of action expands herbicide options for rice cultivation and addresses challenges posed by resistant and perennial weeds, offering a safe and effective solution for sustainable paddy rice production.