Jasmonate is a well-known defence hormone for plants, but it is also necessary for growth and development. Indeed, the identification of the COI1 receptor was based on the jasmonate-triggered response of root growth inhibition. In this special issue, a collection of review papers and two research papers discuss the current state of progress in this field, covering areas from seed germination and flowering to the Jasminum sambac genome.

Seed plants have evolved mechanisms that maintain the dormancy of mature seeds until the time is appropriate for germination. Seed germination is a critical step in the plant life cycle, and it is an important trait in relation to agricultural production. The process is precisely regulated by various internal and external factors, and in particular by diverse endogenous hormones. Jasmonates (JAs) are one of the main plant hormones that mediate stress responses, and recent studies have provided evidence of their inhibitory effects on seed germination. In this review, we summarize our current understanding of the molecular mechanisms underlying the regulatory roles of JAs during the seed germination stage. We describe the crosstalk between JA and other phytohormones that influence seed germination, such as abscisic acid and gibberellic acid.

The phytohormone jasmonate is an essential endogenous signal in the regulation of multiple plant processes for environmental adaptation, such as primary root growth inhibition and root hair elongation. Perception of environmental stresses promotes the accumulation of jasmonate, which is sensed by the CORONATINE INSENSITIVE1 (COI1)-JASMONATE ZIM-DOMAIN (JAZ) co-receptor, triggering the degradation of JAZ repressors and induction of transcriptional reprogramming. The basic helix-loop-helix (bHLH) subgroup IIIe transcription factors MYC2, MYC3, and MYC4 are the most extensively characterized JAZ-binding factors and together stimulate jasmonate-signaled primary root growth inhibition. Conversely, the bHLH subgroup IIId transcription factors (i.e. bHLH3 and bHLH17) physically associate with JAZ proteins and suppress jasmonate-induced root growth inhibition. For root hair development, JAZ proteins interact with and inhibit ROOT HAIR DEFECTIVE 6 (RHD6) and RHD6 LIKE1 (RSL1) transcription factors to modulate jasmonate-enhanced root hair elongation. Moreover, jasmonate also interacts with other signaling pathways (such as ethylene and auxin) to regulate primary root growth and/or root hair elongation. Here, we review recent progress into jasmonate-mediated primary root growth and root hair development.

Flower development and fertility are coordinately regulated by endogenous developmental signals, including the phytohormones jasmonates (JAs), auxin, and gibberellin, and environmental cues. JAs regulate stamen development and fertility under basal conditions, affect root growth and trichome formation under stress conditions, and control defense responses against insect herbivores and pathogens. Since the 1990s, an increasing number of studies have revealed the essential roles of JA biosynthesis, signaling, and crosstalk in regulation of flower development and fertility. Here, we summarize and present an updated overview of the JA pathway and its crosstalk in modulating flower/sexual organ development and fertility in Arabidopsis, tomato, rice, maize, and sorghum.

Plants have remarkable abilities to regenerate in response to wounding. How wounding triggers rapid signal transduction to induce a cellular response is a key topic for understanding the molecular mechanism of plant regeneration. An increasing body of evidence indicates that jasmonate, a hormone that is produced rapidly in response to wounding, plays multiple roles in different plant regeneration processes. In this review, we summarize recent advances on the roles of jasmonate in tissue repair, the formation of wound-induced callus, de novo organ regeneration, and somatic embryogenesis. Physiological and molecular analyses indicate that jasmonate can regulate stem cell activities, cell proliferation, cell fate transition, and auxin production, thereby contributing to plant regeneration. In addition, jasmonate is strictly controlled in plant cells via restriction of the jasmonate concentration and its signalling pathway in a spatial and temporal manner during regeneration. Overall, jasmonate acts as the hormone linking wounding to distinct types of regeneration in plants.

Plants and microbial pathogens often engage in a fierce war that determines their survival. Host plants have evolved sophisticated regulatory mechanisms to fine-tune defense responses to counter attacks from pathogens, while pathogens often hijack the lipid-derived phytohormone jasmonate to cause hormonal signaling imbalances for efficient infection. This review focuses on the jasmonate-based warfare between host plants and pathogenic intruders, and further discusses approaches to uncouple plant growth and defense tradeoffs in crop breeding.

Phytohormones regulate plant growth and development by integrating various internal developmental cues with external environmental conditions. Jasmonic acid (JA) is an oxylipin-derived phytohormone that modulates the plasticity of plant responses against fluctuating environmental cues. An increasing number of studies have shown that it regulates a wide spectrum of plant physiological and biochemical processes, including reproductive development, primary root growth, root hair development, seed germination, senescence, regeneration, defense against biotic stress such as pathogen infection and herbivory, and mitigation of a number of abiotic stresses such as salinity, drought, high and low temperatures, wounding, excessive UV exposure, limited water availability, and metal(oid)-induced toxicity. Nutrient deficiency is an abiotic stress that adversely affects plant growth, development, and productivity, and JA also plays an important role in regulation of these processes under such conditions. In this review, we summarize recent advances relating to the role of JA and its methyl ester derivative (methyl jasmonate) in modulating responses to nutrient deficiency, to the impact of nutrient status on JA biosynthesis and signaling, and to the crosstalk of JA with other phytohormones in shaping plant growth and development under deficiencies of various mineral elements.

When attacked by herbivores, plants produce electrical signals which can activate the synthesis of the defense mediator jasmonate. These wound-induced membrane potential changes can occur in response to elicitors that are released from damaged plant cells. We list plant-derived elicitors of membrane depolarization. These compounds include the amino acid l-glutamate (Glu), a potential ligand for GLUTAMATE RECEPTOR-LIKE (GLR) proteins that play roles in herbivore-activated electrical signaling. How are membrane depolarization elicitors dispersed in wounded plants? In analogy with widespread turgor-driven cell and organ movements, we propose osmoelectric siphon mechanisms for elicitor transport. These mechanisms are based on membrane depolarization leading to cell water shedding into the apoplast followed by membrane repolarization and water uptake. We discuss two related mechanisms likely to occur in response to small wounds and large wounds that trigger leaf-to-leaf electrical signal propagation. To reduce jasmonate pathway activation, a feeding insect must cut through tissues cleanly. If their mandibles become worn, the herbivore is converted into a robust plant defense activator. Our models may therefore help to explain why numerous plants produce abrasives which can blunt herbivore mouthparts. Finally, if verified, the models we propose may be generalizable for cell to cell transport of water and pathogen-derived regulators.

Jasminum sambac is a well-known plant for its attractive and exceptional fragrance, the flowers of which are used to produce scented tea. Jasmonate (JA), an important plant hormone was first identified in Jasminum species. Jasmine plants contain abundant JA naturally, of which the molecular mechanisms of synthesis and accumulation are not clearly understood. Here, we report a telomere-to-telomere consensus assembly of a double-petal J. sambac genome along with two haplotype-resolved genomes. We found that gain-and-loss, positive selection, and allelic specific expression of aromatic volatile-related genes contributed to the stronger flower fragrance in double-petal J. sambac compared with single- and multi-petal jasmines. Through comprehensive comparative genomic, transcriptomic, and metabolomic analyses of double-petal J. sambac, we revealed the genetic basis of the production of aromatic volatiles and salicylic acid (SA), and the accumulation of JA under non-stress conditions. We identified several key genes associated with JA biosynthesis, and their non-stress related activities lead to extraordinarily high concentrations of JA in tissues. High JA synthesis coupled with low degradation in J. sambac results in accumulation of high JA under typical environmental conditions, similar to the accumulation mechanism of SA. This study offers important insights into the biology of J. sambac, and provides valuable genomic resources for further utilization of natural products.

Jasmonate (JA) has been found to be a relevant hormone in floral development in numerous species, but its function in cucurbit floral development and sex determination is unknown. Crosstalk between JA and ethylene (ET) in the differential regulation of male and female floral development was investigated by using the novel JA-deficient mutant lox3a, and the ET-deficient and -insensitive mutants, aco1a and etr2b, respectively, of Cucurbita pepo. The lox3a mutation suppresses male and female flower opening and induces the development of parthenocarpic fruit. A bulked-segregant analysis coupled with whole genome sequencing and fine mapping approach allowed the identification of lox3a mutation in CpLOX3A, a LIPOXYGENASE gene involved in JA biosynthesis. The reduced JA content and expression of JA-signalling genes in male and female flowers of lox3a, and the rescue of lox3a phenotype by external application of methyl jasmonate (MeJA), demonstrated that JA controls petal elongation and flower opening, as well as fruit abortion in the absence of fertilization. JA also rescued the phenotype of ET mutants aco1a and etr2b, which are both specifically defective in female flower opening and fruit abortion. ET, the sex determining hormone of cucurbits, is induced in female flowers towards anthesis, activating JA production and promoting the aperture of the female flower, and the abortion of the unfertilized ovary. Given the close association between flower closure and parthenocarpic fruit development, we propose that flower opening can act as a switch that triggers fruit set and development in fertilized ovaries, but may alternatively induce the abortion of the unfertilized ovary. Both ET and JA from mature and senescent petals can serve as remote signals that determine the alternative development of the ovary and fruit.