Jasmonate Signaling in Arabidopsis
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Jasmonate Signaling in Arabidopsis

JAs (Jasmonates) are Fatty Acid–derived Signaling molecules involved in the regulation of many Physiological and Developmental Processes in Plants, including Root Growth, Tuberization, Fruit Ripening, Senescence, Tendril Coiling, and Pollen Development. They are also important Regulators of Plant responses to Environmental Stress, such as Ozone Exposure, Wounding, Water Deficit, and Pathogen and Pest Attack. For most of these Stress situations, the precise plant response is not activated only by JAs but is the result of a network of interactions between different signaling pathways. Several examples of cross talk between JAs and other hormonal pathways, such as ET (Ethylene), SA (Salicylic Acid), Auxins, or ABA (Abscisic Acid), have been reported. JA also inhibits Seed Germination in several species and is believed to have a Synergistic effect with ABA in this process in Arabidopsis thaliana. In Arabidopsis spp., the JA response pathway is generally required for Defenses against Necrotrophic Pathogens and Chewing Insects (Ref.1).

The JA signaling pathway involves several Signal transduction events: the Perception of the primary wound or stress stimulus and transduction of the signal locally and systemically; the perception of this signal and induction of JA biosynthesis; the perception of JA and induction of responses; and finally, integration of JA signaling with outputs from the SA, Ethylene, and other signaling pathways.  Signaling in the Jasmonate pathway depends on at least two massive signaling machines that interact to form a complex of 0.7 mDa. The first of these Complexes is the SCF-COI1 complex, which is an E3-ubiquitin ligase. The second multimolecular complex involved in JA signaling is the CSN (COP9 Signalosome Complex). The SCF-complex is composed of four subunits ( Cul1 (Cullin-1), Skp1, RBX1 (RING-Box Protein-1) and an F-box protein). The Cul1 requires RUb (Ubiquitin-Related Protein) modification mediated by AxR1 (Auxin Resistant-1)-Ecr1 (RUb-activating enzyme-1) for normal activity of the complex. AxR1 links the Auxin and JA signaling pathways. The SCF -complex physically associates with a protein COI1 (Coronatine-Insensitive Protein-1) to form active SCF-COI1. Sequence analysis of the deduced Amino Acid sequence of the COI1 protein indicates the presence of an F-box motif and several Leucine rich repeats. Leucine rich repeats are short sequence motifs believed to be involved in protein-protein interactions (Ref.2). Skp1 and F-box containing proteins function by connecting R (Regulatory) proteins with Ubiquitin-mediated proteolysis by serving as components of Ubiquitin-protein ligase to facilitate transfer of Ubiquitin from E2 to a targeted R-protein. COI1 may act by promoting the degradation of R-protein that exerts a negative regulatory effect in JA perception. R-proteins are yet unidentified. SCF-COI1 interacts with CSN and together they form the core of the canonical signal pathway and control almost all well-characterized JA responses. The CSN/ SCF-COI1 machine probably targets transcriptional repressors for ubiquitination. This leads to their destruction by the Proteasome or at least to modification of their activities. The putative Repressors have yet to be identified.  Another gene strongly influencing the JA pathway is Sgt1b. A mutation in this gene, which is a known regulator of R-gene mediated resistance, reduced the inhibitory effects of MJ (Methyl Jasmonate) on root growth (Ref.3).

Herbivore and pathogenic attack promote a transient increase of both JA and Ethylene. In Arabidopsis, wounding activates independent signaling pathways regulating different sets of target genes, such as VSP (Vegetative Storage Protein), JR1, or Thi1.2, either at the wound site or in distal leaves. Myc2 is involved in activation of JA-regulated genes. The Myc2 gene encodes a bHLH-LZ (basic Helix-Loop-Helix-Leucine Zipper) transcription factor. The expression of this gene is JA-inducible (Ref.4). Ethylene, on the other hand, prevents local expression of these genes. WRKY70 (WRKY Transcription Factor-70) is a transcription factor that is a repressor of JA-inducible gene VSP. Overexpression of WRKY70 in Arabidopsis leads to the constitutive activation of the PR (Pathogenesis-Related) genes PR2 (Pathogenesis-Related gene-2) and PR5 (Pathogenesis-Related gene-5). The expression of WRKY70 is repressed by JA, but is activated by SA. The protein thus acts as a node between JA and SA signaling. JA and Ethylene synergistically cooperate to activate expression of basic PR proteins such as ChiB (Acidic Endochitinase), PR3 (Pathogenesis-Related gene-3), and PDF1.2 (Plant Defensin Protein-1.2). ERF1 (Ethylene Response Factor-1) integrates Ethylene and JA signals to regulate the expression of these defense genes. Arabidopsis ERF1 is an AP2 and ERF domain transcription factor. Whereas ERF1 activates JA/ET-regulated gene expression, Myc2 is involved in repression of JA/ET-induced gene expression.  Another protein, NPR1(Nonexpresser of PR genes-1), mediates inhibition of JA responses by the SA signal pathway, which is important in defense gene regulation and in particular in active resistance to many pathogens. A second link to the SA in Arabidopsis is the MPK4 (Mitogen-activated Protein Kinase homolog-4) [a plant homolog of MAPK (Mitogen-Activated Protein Kinase)]. MPK4 is a positive regulator of JA-inducible gene expression and a negative regulator of SA-dependent systemic acquired resistance (Ref.2 and Ref.5).

Although it has become clear in the last decade that JA is a key regulator in the development, physiology, and defense of plants, the complexity of the signaling network in which JA evolves is just emerging. JA is involved in Carbon partitioning, in mechanotransduction, and the ability of plants to synthesize and perceive JA is absolutely essential for the correct development and release of pollen in Arabidopsis. Intensive studies with hormone mutants have indicated that plant hormone signaling pathways are not linear but rather a network interacting with each other to make a coordinated plant response(s) during growth and development. In addition to forward genetics approaches, the recent availability of the whole Arabidopsis genome sequence now provides another opportunity to use reverse genetics to dissect these complex-signaling pathways. Current challenges would be to define those networks and understand how plants use JA pathway to respond to biotic and abiotic stresses (Ref.6).