Although the history of plant hormone research is less than 100 years old, the history of plant hormones spans hundreds of millions of years. In recent years, the advent of advances in genome analysis tools such as next-generation sequencers and CRISPR genome editing has led to rapid progress in phytohormone research in early-diverging land plants, including bryophytes and algae. This led to the accumulation of knowledge about the origin and evolution of phytohormones.
The functioning of plant hormones requires the matching of hormone substance production, reception, and signal transduction. Previous studies have shown that the establishment of plant hormone action during evolution is diverse for each plant hormone. In addition, it is becoming clear that ligand specificity and diversity of receptors have been acquired during evolution. The diversification of signal transduction systems has led to the diversification of actions, which has led to the establishment of elaborate mechanisms controlling plant growth and development.
Strigolactones (SLs) were first identified as compounds that are exudated from roots, inducing seed germination of parasitic plants and hyphal branching of arbuscular mycorrhizal (AM) fungi. Later, SLs were identified as a class of plant hormones that regulate plant growth and development. Thus, SLs play dual functions as hormones and rhizosphere signaling molecules. This remarkable feature makes SLs unique among plant hormones. In addition, the diversity of structures is another unique feature of SLs. Recently, a novel SL has been identified from bryophytes and named bryosymbiol (BSB). BSB is also present in ferns and seed plants, indicating it is likely to be the ancestral SL that originated in the common ancestor of land plants. BSB is required for symbiosis with AM fungi in Marchantia paleacea, a bryophyte, while it does not seem to play a role in controlling growth and development. These findings indicate that the SLs originated as rhizosphere signaling molecules, and thefunction of SLs as plant hormones established later after the occurrence of D14, the SL receptor in the evolution of vascular plants.
Gibberellins (GAs) are tetracyclic diterpenoid phytohormones that have diverse biological roles in plant growth and development, such as stem elongation, seed germination, floral induction and anther development in seed plant. Until now, more than 140 gibberellins have been identified, only a few, including GA4, GA1, GA3, and GA7, function as bioactive gibberellins. These bioactive gibberellins have structural common features, that is, a carboxyl group at the C6 position (C6-COOH), a hydroxyl group at the C3 position (C3-OH) of the ent-gibberellane skeleton, a γ-lactone ring, and a non-hydroxyl group at the C2 position (C2-non OH). Gibberellins are perceived and transduced for their specific responses through GID1 (receptor)-GA-DELLA (repressor) system. Both biosynthesis and the signaling of gibberellin were established in fern, and finetuned in gymnosperm and angiosperm, including the establishment of their inactivation systems. Here, I reviewed the co-evolution of synthesis/inactivation and signal transduction of gibberellin, especially in the structural aspects.
Jasmonates are plant hormones involved in the stress response, development, and growth of plants. The biosynthesis and signal transduction mechanism of jasmonates have been studied mainly using angiosperms such as Arabidopsis thaliana. In recent years, genomic analyses of many land plants have progressed, and functional analyses of jasmonates have been performed in plants other than angiosperms. Accordingly, bryophytes are shown to synthesize (+)-cis-12-oxo-phytodienoic acid (OPDA) but not jasmonic acid (JA) or (+)-7-iso-jasmonoyl-L-isoleucine (JAIle), unlike vascular plants. Basic components of signaling pathway of jasmonates are suggested to exist ubiquitously in land plants. This paper outlines the evolution of biosynthesis and signaling pathway of jasmonates that have been elucidated.
Growth and organogenesis in plants is sustained by pluripotent stem cells. Small endogenous peptides encoded by the CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) genes were originally identified as a regulator of stem cell activities in Arabidopsis thaliana. Subsequent studies in monocot and dicot model plants uncovered various developmental and physiological roles of CLEs as well as their involvement in cross-species communication. Homologues of CLEs and their receptors are widely conserved across land plant lineages. Over the last few years, functional and comparative genomic studies on non-flowering plant CLEs have accumulated and started to shed light on how CLE signalingsystems evolved. This review aims to serve as a primer on our growing understanding as well as outstanding questions about CLE signaling systems from an evolutionary perspective.
Prohexadione-calcium inhibits 3β- and 2β-hydroxylation steps in the biosynthetic pathway of gibberellin (GA), which involves activation and inactivation of GAs. It shows a lodging-reducing effect on rice, wheat and other crops as an inhibitor of GA activation, while it promotes flowering in stock and fruit development in Japanese pear as an inhibitor of GA inactivation. It also protects apple from a bacterial disease, the fire bright. Bispyribac-sodium, which potently inhibits acetolactate synthase in the biosynthetic pathway of branched-chain amino acids, has been originally developed as a rice herbicide, but it is also utilized as a plant growth regulator such as a weed suppressor in the paddy ridge and a promotor of sugarcane ripening.
In 1840, Justus von Liebig proposed the theory of mineral plant nutrition, through the invention of the Haber-Bosch process, leading to the industrialization of chemical fertilizer to feed the human population. Because the excessive use of chemical fertilizer has led to numerous environmental problems, understanding the agroecosystem that contains plants, microbes, and soils is necessary for sustainable agriculture. Our recent study digitalizing agroecosystems revealed the network structure of an agroecosystem established with different management practices and identified that organic nitrogen is a key component contributing to crop yield in an organic agriculture. We summarize the recent progress about organic nitrogen in plant growth promotion and discuss new research field for plant nutrition, which provides a potential solution to make crop production more sustainable.
Synthetic agrochemicals such as fungicides and insecticides have been widely used for disease and pest control in agriculture because of their highly effective and rapid action on targets. However, the repeated use of a synthetic agrochemical often causes the appearance of fungicideresistant strains or insecticide-resistant herbivore pests. Therefore, there are constant needs for the development of new agrochemicals that overcome resistant strains and pests. Some of these agrochemicals are compounds that induce the defense responses of plants to pathogens and herbivore pests and do not exhibit direct antimicrobial and insecticidal activities. Recently, amino acids and their related compounds have been shown to induce resistance of plants to pathogens and herbivore pests. In this article, we will describe recent advances of functions of amino acids as disease resistance-inducing compounds.
The plant hormone auxin regulates a wide range of plant growth processes. Auxin has been assumed to be involved in environmental adaptation through precise and flexible coordination among such multiple growth processes which results in various plant shapes. Existence of auxin in various plant species that have diverse morphologies supports this assumption. However, proving the involvement of auxin in environmental adaptation has been technically challenging, and the evidence for it has not been presented. Recently, full genome sequences of natural Arabidopsis thaliana populations became publicly available. These natural populations have adapted to considerably different environments and, therefore, provide us opportunities to study molecular mechanisms of adaptation using genetic approaches including genome wide association study （GWAS）. Using a combination of GWAS and a chemical biology approach, we have identified a gene, EXOCYST70A3, which regulates natural variation of root system architectures and is involved in drought resistance. This study proposes a model of studies to identify molecular bases of adaptation in plants in an integrated way that leverages chemical biology approaches.
Temperature is the most important physical parameter that fundamentally affects cellular metabolism and functions. Analysis of intracellular temperature is expected to provide new information of cellular condition and metabolism. We have developed a method to analyze intracellular temperature changes using fluorescent polymeric thermometer (FPT) and fluorescence lifetime imaging microscopy (FLIM). Using this method, we found that there is a temperature difference >1°C within a mammalian cultured cell, and the temperature in the nucleus and in the area near mitochondria and centrosome is higher than the other area in the cytoplasm. While we originally adapted FLIM for accurate intracellular temperature measurement, fluorescence microscope or confocal laser scanning microscope can also be used. In this technical note, we describe the brief protocol for intracellular imaging of mammalian cultured cells using FPT with fluorescence microscope or confocal laser scanning microscope. We also discuss the future research directions using intracellular temperature imaging.
Plants take up and translocate nutrients through transporters. Transporters which also act as sensors are called transceptors. In Arabidopsis thaliana, NRT1.1/CHL1 is known as a nitrate transceptor regulating primary nitrate responses, and IRT1 iron transporter acts as a metal transceptor to regulate degradation of itself. Here I discuss the function and regulation of a borate transceptor BOR1. BOR1 is a key transporter for boron (B) translocation and survival under B limitation in the soil. Upon sufficient-B supply, BOR1 undergoes ubiquitination and is transported to the vacuole for degradation, to avoid B toxicity. Our recent findings suggest that polyubiquitination of BOR1 relies on its conformational transitionduring the active transport cycle. In our model, BOR1 borate transceptor directly senses the B concentration and promotes its own polyubiquitination and vacuolar sorting for quick and precise maintenance of B homeostasis.
Structural determination of novel strigolactones (SLs) has been contributed to the elucidation of biosynthetic and metabolic pathways of SLs. Since SLs are only minor components of plant metabolites released to the rhizosphere, SLs must be separated from the other metabolites in root exudate with high purities and high recovery rates. Here, I would like to introduce some tips on how to improve the recovery rate of SLs from the root exudate of hydroponically grown plants．