Jasmonates (JAs) are lipid-derived phytohormones found across the plant kingdom that regulate plant physiology. These compounds serve as internal communication signals within the plant body. Jasmonates have a dual function, regulating a plant’s response to environmental stress while also governing fundamental processes of growth and development. This signaling system allows plants to coordinate a biological response, balancing the need for defense with the requirements of the life cycle.
The Jasmonate Signaling Pathway
The active jasmonate molecule that initiates the signaling cascade is Jasmonoyl-isoleucine (JA-Ile). JA-Ile is biosynthesized from linolenic acid via the octadecanoid pathway, which starts in the chloroplasts and ends in the peroxisome, generating Jasmonic Acid (JA). JA is then conjugated with the amino acid isoleucine by the enzyme JAR1 to form the active JA-Ile.
The signaling mechanism centers on the COI1 receptor, an F-box protein that forms part of the SCF\(^{\text{COI1}}\) E3 ubiquitin ligase complex. In the absence of stress, the pathway is inactive, maintained by repressor proteins called JAZ (Jasmonate ZIM-domain) proteins. JAZ proteins bind to and inhibit transcription factors responsible for activating JA-responsive genes.
When a cue triggers JA-Ile production, JA-Ile acts as a molecular “glue” facilitating the binding of JAZ proteins to the SCF\(^{\text{COI1}}\) complex. This binding marks the JAZ repressors for ubiquitination and rapid degradation by the 26S proteasome. Degradation of JAZ proteins releases inhibited transcription factors, such as MYC2, allowing them to move to the nucleus and activate defense and growth-related genes.
Activating Plant Defense Responses
Jasmonates mediate plant protection against biotic stresses, particularly herbivory and necrotrophic pathogens. Upon attack by chewing insects, the plant rapidly synthesizes JA-Ile, leading to an induced defense response. This mobilization includes specialized defense molecules that interfere with the herbivore’s feeding or digestion.
A significant defense strategy involves the production of anti-digestive proteins, such as proteinase inhibitors. Once ingested, these proteins interfere with the enzymes in the pest’s gut, preventing nutrient absorption. The JA pathway also triggers the synthesis of secondary metabolites, including alkaloids and terpenes. Alkaloids deter pests with repellent tastes, while volatile terpenes are released into the air as an indirect defense.
These volatile compounds signal nearby plants to prepare their defenses. The JA signaling pathway is also a defense regulator against necrotrophic pathogens, which kill host tissue to feed on dead cells. In this role, the JA pathway often works synergistically with the ethylene pathway to activate resistance genes against fungal and bacterial necrotrophs. This contrasts with the salicylic acid pathway, which governs defenses against biotrophic pathogens requiring living host tissue.
The JA signal can move from the initial attack site to distant, undamaged parts of the plant, establishing a broad protective state called induced systemic resistance. This systemic signaling primes the distal tissues, allowing them to mount a faster and stronger defense response if subsequently attacked.
Regulation of Growth and Reproductive Development
Beyond defense, jasmonates regulate plant growth and reproductive development. JAs mediate plant architecture, particularly by inhibiting primary root elongation. This function is part of a trade-off, diverting resources away from vegetative growth and toward defense when JA levels rise.
Jasmonates are important for the development of specialized epidermal structures, such as trichomes (small, hair-like outgrowths). These structures act as a physical defense or store defensive compounds. The hormone also influences the timing of senescence, the programmed aging and death of leaves.
In reproductive development, jasmonates are essential for fertility. They are required for the proper development of the stamen, the male reproductive organ of the flower. Specifically, JA-Ile signaling is necessary for anther dehiscence, the physical splitting of the anther to release pollen.
Without proper jasmonate signaling, plants often exhibit male sterility because the pollen cannot be released or is non-viable. The hormone helps regulate the transition from vegetative to reproductive growth, sometimes acting to delay flowering.