Although jasmonate (JA) regulates many different processes in the plant, its role in wound response is best understood. Following mechanical wounding or herbivory, JA biosynthesis is rapidly activated, leading to expression of the appropriate response genes. For example, in the
tomato, wounding produces defense molecules that inhibit leaf digestion in
guts of insects. Another indirect result of JA signaling is the volatile emission of JA-derived compounds. MeJA on leaves can travel airborne to nearby plants and
elevate levels of transcripts related to wound response. JA and its derivatives have also been implicated in plant development,
symbiosis, and a host of other processes included in the list below. • By studying mutants overexpressing JA, one of the earliest discoveries made was that JA inhibits root growth. The mechanism behind this event is still not understood, but mutants in the COI1-dependent signaling pathway tend to show reduced inhibition, demonstrating that the COI1 pathway is somehow necessary for inhibiting root growth. • JA plays many roles in
flower development. Mutants in JA synthesis or in JA signaling in Arabidopsis present with male sterility, typically due to delayed development. The same genes promoting male fertility in Arabidopsis promote female fertility in tomatoes.
Overexpression of 12-OH-JA can also delay flowering. • High levels of JA encourage the accumulation of storage proteins; genes encoding vegetative storage proteins are JA responsive. Specifically, tuberonic acid, a JA derivative, induces the formation of tubers. • JAs also play a role in symbiosis between plants and microorganisms; however, its precise role is still unclear. JA currently appears to regulate signal exchange and nodulation regulation between legumes and rhizobium. On the other hand, elevated JA levels appear to regulate carbohydrate partitioning and stress tolerance in mycorrhizal plants. carnivory process, with JA signaling
Knowable Magazine Role in pathogenesis Pseudomonas syringae causes bacterial speck disease in tomatoes by hijacking the plant's jasmonate (JA) signaling pathway. This bacteria utilizes a
type III secretion system to inject a cocktail of viral effector proteins into host cells. One of the molecules included in this mixture is the phytotoxin
coronatine (COR). JA-insensitive plants are highly resistant to
P. syringae and unresponsive to COR; additionally, applying MeJA was sufficient to rescue virulence in COR mutant bacteria. Infected plants also expressed downstream JA and wound response genes but repressed levels of
pathogenesis-related (PR) genes. All these data suggest COR acts through the JA pathway to invade host plants. Activation of a wound response is hypothesized to come at the expense of pathogen defense. By activating the JA wound response pathway,
P. syringae could divert resources from its host's
immune system and infect more effectively. Plants produce
N-acylamides that confer
resistance to
necrotrophic pathogens by activating JA biosynthesis and signalling.
Arachidonic acid (AA), the counterpart of the JA precursor α-LeA occurring in
metazoan species but not in plants, is
perceived by plants and acts through an increase in JA levels concomitantly with resistance to necrotrophic pathogens. AA is an evolutionarily conserved
signalling molecule that acts in plants in response to stress similar to that in
animal systems.
Cross talk with other defense pathways While the jasmonate (JA) pathway is critical for wound response, it is not the only signaling pathway mediating defense in plants. To build an optimal yet efficient defense, the different defense pathways must be capable of cross talk to fine-tune and specify responses to abiotic and biotic challenges. One of the best studied examples of JA cross talk occurs with
salicylic acid (SA). SA, a hormone, mediates defense against pathogens by inducing both the expression of pathogenesis-related genes and
systemic acquired resistance (SAR), in which the whole plant gains resistance to a pathogen after localized exposure to it. Wound and pathogen response appear to be interact negatively. For example, silencing phenylalanine ammonia lyase (PAL), an enzyme synthesizing precursors to SA, reduces SAR but enhances herbivory resistance against insects. Similarly, overexpression of PAL enhances SAR but reduces wound response after insect herbivory. Generally, it has been found that pathogens living in live plant cells are more sensitive to SA-induced defenses, while herbivorous insects and pathogens that derive benefit from cell death are more susceptible to JA defenses. Thus, this trade-off in pathways optimizes defense and saves plant resources. Cross talk also occurs between JA and other plant hormone pathways, such as those of
abscisic acid (ABA) and
ethylene (ET). These interactions similarly optimize defense against pathogens and herbivores of different lifestyles. For example, MYC2 activity can be stimulated by both JA and ABA pathways, allowing it to integrate signals from both pathways. Other transcription factors such as ERF1 arise as a result of JA and ET signaling. All these molecules can act in combination to activate specific wound response genes. Finally, cross talk is not restricted for defense: JA and ET interactions are critical in development as well, and a balance between the two compounds is necessary for proper apical hook development in
Arabidopsis seedlings. Still, further research is needed to elucidate the molecules regulating such cross talk. == Mechanism of signaling ==