Various phytohormones play important roles in regulation of induced resistance systems that are important self-protection systems to protect the whole plant body. Systemic acquired resistance (SAR) is one such immune system governed by salicylic acid, which is utilized to control rice blast disease in rice field by exploiting plant activators that stimulate SA-mediated signaling. The action mechanism studies revealed that probenazole (PBZ) stimulates the point upstream of SA biosynthesis, whereas other stimulate the downstream. The effects of plant activators were found to be suppressed by ABA-mediated signaling, which is activated by environmental stresses. Conversely, activation of SA-mediated signaling suppressed ABA signaling. Thus, there is an antagonistic crosstalk between SA-mediated signaling for disease resistance and ABA-mediated environmental stress responses. Whereas the SA-mediated signaling has a growth-inhibiting effect, priming of plant immunity, induced by stimuli of certain chemicals and microbes, has protective effects without growth inhibition. We demonstrated that strigolactone-mediated signaling and interaction with symbionts induced priming in Arabidopsis and tomato, respectively. Further studies from these findings are expected to contribute to the development of effective disease control technology for crops.

The study of strigolactones (SLs) began in 1966 with the isolation of strigol from the root exudate of cotton (Gossypium hirsutum>) as a seed germination stimulant for the root parasitic plants, Striga spp. (Cook et al. 1966). Subsequently, SLs with a wide variety of structures have been isolated from various plant species. In 2005, It was found that the role of SLs exuded from host roots are symbiotic signals towards arbuscular mycorrhizal (AM) fungi that supply phosphate to host plants (Akiyama et al. 2005). Three years later, it was further discovered that SLs also function as a phytohormone that suppresses shoot branching in plants, thereby accelerating SL research (Gomez-Roldan et al. 2008; Umehara et al. 2008). Mutants with excessive shoot branching had been analyzed since the 1990s, and when they were found to be defective in SL biosynthetic enzymes and signaling components, the major biosynthetic and signaling pathways were elucidated in the last 15 years (Mashiguchi et al. 2021). In this special issue, young and mid-career researchers who will lead the next generation of SL research have provided four topics on new development in SL studies. Two of them are about the discovery of new physiological effects of SL as rhizosphere signals, and the other two are about the development of selective agents of various SL effects. I hope that these discoveries will attract more and more students and researchers to the ever-expanding fascination of the SL world.

Strigolactones (SLs) are a group of carotenoid derived plant hormones that regulate various developmental processes including shoot branching. In addition, SLs are secreted into the rhizosphere and act as host recognition signals for root parasitic weeds and AM fungi. SLs in rice are synthesized via carlactone produced by a carotenoid isomerase and two carotenoid cleavage dioxygenases. Four CYP711As (Os900, Os1400, Os1900, and Os5100) are known to convert carlactone. Os900 is involved in the synthesis of 4-deoxyorobanchol and Os1400 in the synthesis of orobanchol. In addition, all these enzymes catalyze the conversion of CL to CLA. However, the function of these isozymes in planta was not elucidated. We have identified specific inhibitor of CYP711A isozymes and found that each isozyme has a distinct function. In this review, we discuss the history of development of isozyme specific biosynthesis inhibitor and its function.

Zaxinone is a carotenoid-derived bioactive molecule required for normal rice growth and development. Zaxinone functions by enhancing sugar metabolism and regulating SL and cytokinin homeostasis in rice roots. Zaxinone has great agricultural potential due to its growth-promoting activity and its ability to reduce infestation by the root parasitic plant Striga by inhibiting strigolactone (SL) production. However, zaxinone is unstable in the environment and its organic synthesis is laborious. This makes it difficult to use in both basic research and applications. To overcome these inconveniences, easy-to-synthesise and highly effective zaxinone mimics, namely MiZax, have been developed. The conjugated isoprenoid chain of zaxinone has been substituted with aromatic structures, inspired by several successful examples such as the development of fungicides and insecticides from natural isoprenoid bioactive compounds. The β-ionone ring of zaxinone was also replaced by a phenyl ring, which is a common approach in the design of SL analogs. The resulting compounds showed biological activities comparable to those of zaxinone, i.e., they promoted rice growth and mitigated Striga infection by reducing SL levels. In addition, application of these compounds to horticultural crops improved plant growth and performance, as well as fruit yield and quality, demonstrating their usefulness as plant growth regulators. MiZax may have future applications as probes for receptor/binding protein discovery and in chemical genetics to elucidate zaxinone biology.

Strigolactones (SLs) act as both plant growth regulators and as chemoattractants for arbuscular mycorrhizal fungi. Secreted SLs are also sensed by Orobanchaceae root parasitic plants as germination stimulants. Here we focus on the impact of exogenous SLs on Phtheirospermum japonicum, a facultative hemiparasite in the Orobanchaceae. The genome of P. japonicum encodes SL receptor candidates, and its roots exhibit chemotropism toward host-derived SLs. This chemotropism was also observed in the obligate hemiparasite Striga hermonthica but not in non-parasitic plants, suggesting that chemotropism to SLs might be limited to Orobanchaceae parasitic plants. Interestingly, under ammonium-rich conditions, where P. japonicum can grow without hosts, its chemotropism to SLs does not occur. Furthermore, SLs induced asymmetric auxin response in the root epidermis in P. japonicum, leading to asymmetric root growth. Our findings shed new light on the function of SLs as host-derived chemoattractants, providing valuable insights into the mechanisms behind parasitic plant interactions with their hosts.

Plants can sense the presence of neighboring plants to compete with them, to avoid competition with them, or to utilize them as one of survival strategies. The relationships between root parasitic plants and host plants are one of representative examples of plant-plant interactions. Then, what happens between general, non-parasitic plants? It is well known that plants actively interact with other organisms including microbes, but general plant-plant interactions remain poorly understood. In this review, I would like to show that plants detect and respond to neighboring plants by sensing environmental strigolactones.

It has recently become clear that a wide range of insects contains phytohormones such as cytokinin, auxin, and abscisic acid. Gall induction by insects has attracted attention as a phenomenon that may involve phytohormones synthesized by the insects themselves. Since the induction of insect galls is accompanied by extensive cell proliferation and vascular bundle formation, and the gall tissue exhibits high sink function, the involvement of auxin and cytokinin has long been postulated. In this review, we will summarize information on insect phytohormones, including their distribution, biosynthesis, and localization, which is necessary to consider their physiological functions in insect gall induction. Additionally, we will discuss the gaps in knowledge that need to be addressed to further understand their importance.

The pungent components in chili pepper fruits (Capsicum) are capsaicin and its analogs, collectively called as capsaicinoids. The chemical structure of capsaicinoids comprises an acid amide of vanillylamine with a fatty acid. Capsaicinoids are reported to have beneficial bioactivities in human. Capsinoids are low-pungent capsaicinoid analogs, which were first isolated from a low pungent mutant ‘CH-19 Sweet’ (C. annuum). They are structurally similar to capsaicinoids, but have an ester group instead of the amide moiety. Because of their low pungency, capsinoids are valuables in food industry. We demonstrated that loss-of-function mutation of the putative aminotransferase (pAMT) gene leads to low-pungency and accumulation of capsinoid. The screening of Capsicum bioresources revealed that multiple loss-of-function mutant alleles of pamt had occurred in C. chinense via mobilization of Tcc transposon. Further study identified two leaky pamt alleles (pamt^L1 and pamt^L2) with different levels of pAMT activity. The both alleles had a Tcc transposon insertion in intron 3, but the locations of the insertions within the intron were different. The analysis of pAMT transcripts suggested that the position of the intronic transposons could change splicing efficiency, which led to different pAMT activities and reduced capsaicinoid content to different levels. The pAMT mutations will be useful for genetic manipulation for pungency and capsinoid content in chili pepper fruits.

Long-chain base phosphates (LCBPs), such as sphingosine-1-phosphate and phytosphingosine-1-phosphate, are known to function as abscisic acid-mediated signaling molecules that regulate stomatal closure in plants. Recently, Os3BGlu6, a glycoside hydrolase family 1 (GH1) β-glucosidase, was shown to improve drought tolerance by stomatal closure in rice, but the biochemical functions of Os3BGlu6 remained unclear. Here we identified Os3BGlu6 as a novel GH1 glucocerebrosidase (GCase) that catalyzes the hydrolysis of glucosylceramide to ceramide. Phylogenetic and enzymatic analyses showed that GH1 GCases are widely distributed in seed plants and that pollen or anthers of all seed plants tested had high GCase activity, but activity was very low in ferns and mosses. The levels of LCBPs synthesized from ceramides, especially the levels of sphingadienine-1-phosphate, correlated with GCase activity in each rice organ and were significantly lower in Os3BGlu6-deficient rice mutants than in the wild type. These results indicate that Os3BGlu6 increases the level of sphingadienine-1-phosphate and subsequently improves drought tolerance via stomatal closure in rice.

Laser microdissection (LMD), also called Laser capture microdissection (LCM), is a method for isolating different tissue cells or specific single cells from various tissue samples under direct microscopic observation. The LMD method enables isolating the cells of interest region or specific cells for several analyses such as DNA/RNA analysis, proteomics, metabolomics, and other molecular analysis. This approach can be used to study various biological events at the tissue or cellular level, and the LMD method has also been used for a wide range of purposes in the plant sciences. This paper describes a technique for spatiotemporal phytohormone analysis using LMD to isolate different tissues/specific cells from cryosections of incised flowering stems of Arabidopsis thaliana.

Jasmonate (JA) and gibberellins (GAs) exert antagonistic effects on plant growth and development in response to environmental and endogenous stimuli. This study investigated the mechanism underlying JA-mediated regulation of endogenous GA levels. JA treatment increased DELLA accumulation with reduction of endogenous GA levels. JA treatment decreased bioactive GA, significantly decreased the expression of GA biosynthesis genes, and increased the expression of AtGA2ox, a GA inactivation gene. Conversely, JA treatment did not significantly affect gene expression in the myc2 myc3 myc4 triple mutant, and MYC2 directly activates some AtGA2ox genes. Additionally, JA post-transcriptionally regulated AtGA3ox1 expression. We identified microRNA miR5998 as a GA3ox1-associated miRNA;its overexpression inhibited plant growth by suppressing GA3ox1 expression. Overall, our findings indicate that JA treatment inhibits endogenous GA levels and plant growth by decreasing the expression of GA biosynthesis genes and increasing the expression of GA inactivation genes via miR5998 and MYC2 activities.

Jasmonic acid and its biologically active form, jasmonoyl isoleucine (JA-Ile), plays a pivotal role in regulating both plant growth and defense responses to stress. JA-Ile is recognized by the COI1-JAZ receptor complex. In rice, there are three COI1 proteins (OsCOI1a, OsCOI1b, and OsCOI2), yet the specifics of their functions still require further elucidation. Recently, we and two other research groups independently reported that OsCOI2 functions primarily in the JA response in rice using genome editing. The oscoi2 mutant displayed extremely reduced fertility, indicating that OsCOI2 plays a central role in regulating rice fertility. JA-induced senescence and defense responses were also suppressed in the oscoi2 mutant, showing that OsCOI2 plays a primary role in JA response in rice leaves. Additionally, two rice JAZ proteins, OsJAZ2 and OsJAZ5, were found to interact specifically with OsCOI2. This review provides an overview of the specialized functions of OsCOI2 and sheds light on the functional distinctions among rice COI1 proteins.

Mimosa pudica, commonly known as the sensitive plant, possesses the ability to perceive various external stimuli, including touch and injury, and subsequently conveys this information to its motor organ, pulvinus, resulting in rapid leaf movements within seconds. However, the molecular mechanisms and physiological significances behind the long-range signal transmission and the rapid leaf movements remain elusive. Using genetically-encoded Ca²⁺ biosensors, a wide-field fluorescent microscope and an electrophysiological system, we have simultaneously observed changes in cytosolic Ca²⁺ concentrations and surface potentials in Mimosa pudica in response to mechanical stimuli. Our findings revealed that ²⁺ signals coupled with electrical signals function as long-range signals triggering the rapid leaf movements in Mimosa pudica. Furthermore, using pharmacological and CRISPR-Cas9 genome editing techniques, we generated an immotile Mimosa pudica and found that these immotile leaves are more susceptible to insect attacks compared to the motile wild type. Based on these results, we propose that Mimosa pudica possesses a motion-based antiherbivory defense mechanism mediated by Ca²⁺ and electrical signals.

Cyclopyrimorate is a novel herbicide for paddy fields. It inhibits homogentisate solanesyltransferase (HST) in plastoquinone (PQ) biosynthesis pathway and is classified as the only HST inhibiting herbicide by Herbicide Resistance Action Committee. One of the remarkable herbicidal characteristics of this compound is that it has strong synergistic action with 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors. The combinations with three herbicides;pyrazolate, tefuryltrione and benzobicyclon, HPPD inhibitors, were confirmed to have the synergistic action, respectively. These three combinations show different herbicidal characteristics each other, therefore by combining with these herbicides, cyclopyrimorate can offer various weed control measures to paddy rice producers.