Auxin plays multiple roles in plant growth and development. The auxin concentrations need to be strictly regulated by its biosynthesis, transport, and metabolism in a suitable manner for the plant parts, organs, and developmental events. I developed a chemical inhibitor of GH3 enzymes that catalyze the conjugation reaction of IAA, a dominant endogenous auxin, with amino acid and transform IAA into inactive form. This inhibitor, named Kakeimide (KKI), suppresses auxin metabolism and accumulates endogenous IAA. Endogenous IAA amount in Arabidopsis roots was increased twofold in 10 minutes by treatment of KKI. This result suggests that endogenous IAA is continuously synthesized and metabolized in a 10-minute turnover time. Strigolactone (SL) has been known as a plant hormone that suppresses axillary shoot branching, and its receptor D14 and signaling mechanism have been elucidated. The homological receptor of D14, HTL/KAI2, receives unidentified hormone-like endogenous compounds called KL. HTL/KAI2 mediates the signaling stimulated by karrikins derived from burnt plant material. I developed agonists of SL receptors to mimic plant hormonal action and the structure-activity relationships study elucidated the target preferences of derivatives. Then, I developed agonists of KL receptors with structural modification from the D14 agonist to investigate the biological roles of HTL/KAI2 receptor.

This special issue first focuses on the infection strategy of Ralstonia solanacearum, particularly on research into quorum sensing (QS), a bacterial density-dependent gene expression mechanism. Secondly, it discusses the use of beneficial microorganisms in crop cultivation, such as biological control. Thirdly, it explains how endophytes induce flower bud formation in strawberry plants, including how this process works. Finally, it discusses an integrated understanding of pathogenicity and symbiosis shown by the interaction between the endophytic fungus Colletotrichum tofieldiae and Arabidopsis thaliana.

Ralstonia solanacearum species complex (RSSC) strains are globally devastating plant pathogens. The cell density dependent gene expression system in RSSC strains is called the phc quorum sensing (QS) system. It regulates the expression of about 30% of all genes, including those related to cellular activity, primary and secondary metabolism, pathogenicity, and more. The phc regulatory elements encoded by the phcBSRQ operon and the phcA gene play critical roles. RSSC strains use methyl 3-hydroxymyristate (3-OH MAME) or methyl 3-hydroxypalmitate (3-OH PAME) as the QS signal. Each type of RSSC strain has specificity in generating and receiving its QS signal, but their signaling pathways may not differ significantly. In this review, I describe the genetic, biochemical, and chemical factors involved in QS signal input and the regulatory network, the control of the phc QS system, and QS-dependent interactions with soil fungi.

Soil-borne diseases are a major threat to global crop production, causing significant yield losses. This paper reviews recent advances in understanding the complex relationships between soil microbiome, rhizosphere microbiome, and soil-borne diseases, and highlights novel strategies for disease control. I discuss the concept of general and specific soil suppressiveness, the role of the microbiome in plant health, and the impact of agricultural practices on microbial communities. The review covers innovative approaches to disease management, including crop rotation, intercropping, organic amendments, biocontrol agents, and the use of compounds that enhance the disease-suppressing capacity of native microbiomes. Recent technological advances, particularly in next-generation sequencing, have revolutionized our understanding of soil microbial ecology and its implications for disease control. This comprehensive review provides insights into the complex interactions within the soil ecosystem and highlights promising avenues for the development of sustainable and effective soil-borne disease management strategies in agriculture.

I with my colleagues have focused on the use of Dark Septate Endophytic fungus (DSE) and demonstrated that these DSEs can be used to improve various plant functions, such as disease suppression. In other words, seedlings treated with DSE have improved absorption of nitrogen and phosphate, and improved resistance to environmental stresses such as high temperatures and acidic soil. In this paper, I will describe specific examples of plants that have been given unique functions and positive effects. I will also present some findings regarding the beneficial microbial network necessary to maintain these effects in the field, particularly the relationship with mycosphere bacteria.

Plants in nature interact with microbes exhibiting diverse lifestyles. The resulting plant phenotype reflects the lifestyles of microbes, with the effects ranging from beneficial, such as growth promotion or resilience to abiotic/biotic stress, commensal (neutral) to even pathogenic, i.e., growth inhibition or disease symptoms. It has been reported that microbes can convert their lifestyles depending on host environments; however, the genetic background, especially for microbes, remains elusive. Colletotrichum fungi are found in many plants as not only pathogens causing anthracnose disease but also endophytes showing beneficial or commensal effects on plant hosts. Since Colletotrichum fungi adopt diverse lifestyles and persist in various host environments, we utilize the fluctuating Colletotrichum lifestyles to unveil the molecular mechanisms that discriminate beneficial and pathogenic lifestyles under changing host environments. In this review, we first summarize previous findings on beneficial or pathogenic lifestyles in plant-microbe interactions, especially for the lifestyles of Colletotrichum fungi and the molecular background of the interaction. Additionally, we introduce our research on the “hidden” virulence of Colletotrichum tofieldiae endophytic isolates to investigate the mechanisms behind lifestyle transitions using Arabidopsis thaliana as a model. Finally, we discuss the potential ecological role of Colletotrichum fungi within its niche.

Cyst nematodes are among the most economically important species of root parasitic nematodes, cause extensive crop losses worldwide. Because the hatching of cyst nematodes requires host-derived molecules known as hatching factors (HFs), HFs are key targets for cyst nematode control. In this review, we introduce the history and future perspectives of research on HFs.

In angiosperms, gibberellins (GAs) function as a key growth promoting hormone that regulates various developmental processes, such as seed germination, stem elongation and floral development. In recent years, research in non-seed plants revealed a stepwise model for GA evolution. While the complete biosynthesis pathway for canonical GAs and perception via the GIBBERELLIN INSENSITIVE DWARF1 (GID1) receptor are established in vascular plants, the production of GA precursors and the transcriptional regulator DELLA emerged along with ancestral land plants. In particular, studies in the model bryophytes, Marchantia polymorpha and Physcomitrium patens showed that compounds derived from the GA precursor ent-kaurenoic acid actively regulates light responses in these species. Biochemical and genetic experiments also indicated that DELLA proteins have ancestral functions to interact with various transcriptional factors and modulate plant growth before recruited into the GA signaling pathway. In this review, we focus on these recent progresses, hoping to provide an updated perspective of GA evolution in land plants.

The plant hormone auxin controls a wide range of cellular and developmental processes in streptophyte plants. In angiosperms, auxin regulates gene expression through the nuclear auxin pathway (NAP), where redundant but distinct signaling components mediate diverse auxin responses via multiple layers of functional controls. Additionally, recent studies have identified the cytoplasmic auxin pathway (CAP) and the apoplastic auxin pathway (AAP), both of which induce rapid physiological changes in response to intracellular and extracellular auxin molecules, respectively. The presence of these distinct signaling pathways and their cooperative or antagonistic functions contribute to the extensive roles of auxin. Accumulating genomic information indicates that the evolutionary origin of the NAP can be traced back to the common ancestor of land plants. Moreover, several model species in bryophytes, the non-vascular lineage of land plants, have provided key evidence for the conservation of the NAP and offer unique experimental frameworks to test design principles of auxin signaling mechanisms. The AAP likely originated in streptophyte algae, and cross-species omics approaches have identified evolutionarily conserved key factors in the AAP and its physiological roles. In this review, we explore the diversity of auxin signaling pathways and their evolutionary backgrounds.

Brassinosteroids (BRs) have various physiological activities, such as promoting cell division and elongation, increasing abiotic stress resistance and disease resistance, and regulating chloroplast development. In comparison with wild-type (WT) plants, BR-deficient mutants such as de-etiolated 2 (det2) grown in the dark exhibit de-etiolated phenotypes with shortened hypocotyls, open cotyledons, and increased expression of photosynthesis-associated genes. In the light, det2 mutants produce dwarf, dark green leaves and exhibit promotion of chloroplast development. Based on these phenotypes, BR is considered to have significant functions in chloroplast development, but knowledge of the key factors to regulate chloroplast development under BR signaling is limited.

In the light, the greening and chloroplast development of cotyledon was promoted in the WT seedlings grown on medium supplemented with BR biosynthesis inhibitor Brz. To reveal the molecular mechanism for regulation of chloroplast development by the BR signaling, we performed chemical biology approach to screen the Brz-insensitive-pale green (bpg) mutants, that kept the pale green cotyledon against the Brz-inducible enhancement of greening conditions. By these trials, we identified BPG1/DVR as a chlorophyll biosynthesis-related enzyme, BPG2 as a chloroplastic GTPase, and BPG3 as a possible functional protein on the photosynthetic activity of PSII. Currently, we identified a novel regulator of chloroplast development, BPG4 which regulated GLK transcriptional activity and is involved in light and BR signaling. In this report, the molecular mechanisms of chloroplast development by BPG4, BPGs, and BR signaling would be introduced.

Plants have developed sophisticated defense mechanisms to respond to biotic stresses through a process known as plant-plant communication. This involves the transfer of signaling molecules, particularly volatile organic compounds (VOCs), between plants. When a plant is attacked by herbivores, it releases specific VOCs that are detected by neighboring plants. These nearby plants, although not directly attacked, respond by strengthening their own defense systems. Since around 2000, numerous chemical and ecological studies have focused on the role of VOCs in plant defense, revealing that the biosynthesis and emission of these compounds are crucial for plant survival. However, while significant progress has been made in understanding the molecular mechanisms of VOC biosynthesis and emission, the study of how plants receive and respond to VOCs has lagged behind. Research into receptors for volatile phytohormones, such as methyl jasmonate and methyl salicylate, have been shown, but the molecular mechanisms underlying the perception of VOCs remain less understood due to the vast structural diversity of these compounds. Recent discoveries have shed light on this area. For example, when tomato plants are attacked by insects, they release (Z)-3-hexenol known as a green leaf volatile. This compound is absorbed by neighboring tomato plants and is then glycosylated into (Z)-3-hexenyl β-vicianoside (HexVic). HexVic functions as a poison, with lethal and growth-retard effects on herbivorous insects. This finding indicates that glycosylation of VOCs is one of the molecular mechanisms for the perception in plant-plant communication.

This research note shows the chemical and ecological significance of VOC glycosylation in plant-plant communication, highlighting the recently identified glycosyltransferase UGT91R1 responsible for the production of the insecticidal HexVic. This discovery provides key insights into how plants use VOCs not only to communicate but also to protect themselves from potential threats in near future, offering new perspective for understanding plant resilience and adaptability in dynamic environments.

Plant hormones are functioning in various phenomena, such as plant growth, development, and stress response. Sometimes plant hormones act where they are synthesized, and sometimes they are transported to act after synthesis. Transport through vascular is known as long-distance transport, while apoplastic transport, transport through influx/efflux carrier proteins, and transport through plasmodesmata (PD), is known as local transport. Recent papers report that plant hormone transport through PD plays an important role in several plant development. PD are plant-specific structures and are tubular with a diameter of 30–50 nm, traverse the cell wall, and connects the cytoplasms of adjacent cells. The transcription factors, mRNA, and ions are transported in lateral or longitudinal direction through PD. 27–81 kDa molecules are transported through PD. Therefore, plant hormones that are small molecules are also thought to be distributed through PD, indicating the importance. Thus, in this review, the recent studies on the relationship between transport through PD and plant hormones are introduced, and the issues and prospects of the study of plant hormone transport are described.

We have been investigating the tolerances of a terrestrial cyanobacterium, Nostoc sp. HK-01, to space environments. The cyanobacterium was selected as a species for the astrobiology experiment, Tampopo mission, and the experiment showed that the cyanobacterium has the high tolerance to the space environment outside ISS (International Space Station). A part of the mechanisms of the tolerance to heat and ultraviolet (UV) was investigated. In the heat tolerance of the species, some chemicals were accumulated in the akinete and the chemicals were found to be compatible solutes. In the UVC tolerance, some chemicals were found to be shielding from the UV exposure. We also describe the several lessons learned during the space experiment. This article includes a memorial for Prof. Dr. Teruko Nakamura.

Isoprothiolane has been developed as a plant growth regulator for healthy rice seedlings, rice ripening enhancement, root stimulation in flowers, citrus coloring, etc., as well as fungicide. It has a wide variety of physiological effects on plants and mechanisms for these bioactivities in plant have been studied since 1980s. Analysis in rice plants indicated that isoprothiolane induces root elongation, prevention of leggy shoot and anti-aging thorough interaction with plant hormones such as auxins, ethylene, and cytokinins. Genetic analysis in Arabidopsis revealed that isoprothiolane activates signal transduction of auxins, jasmonates and ethylene which are essential hormones for root elongation by isoprothiolane. More recently, it was reported isoprothiolane promotes cell division of quiescent center in root meristem in an ethylene- and jasmonate-dependent manner. Studies on citrus fruit demonstrated isoprothiolane reduces gibberellin content and promotes fruit maturity, which result in peel color change through reduction of total chlorophyll and increase of β-cryptoxanthin content.