Jasmonate: a potent phytohormone
Jasmonates [jasmonic acid and its methyl ester methyl jasmonate] the cyclopentanone phytohormones are a class of oxidized lipids (oxylipins) derived from α-linolenic acids (α-LAs) through lipoxygenase-dependent manner. JA was first isolated from culture filtrate of the fungus Lasiodiplodia (Botryodiplodia) theobromae, a plant pathogen, was identified as a plant growth inhibitor, whereas its derivative methyl jasmonate was first isolated from Jasminum grandiflorum (jasmine) petal extract. 7-iso-Jasmonoyl-L-isoleucine (JA-Ile) is the best-described bioactive JA till date, but other JAs like cis-jasmone, jasmonoyl ACC (JA-ACC), and jasmonoyl isoleucine (JA-Ile) are also studied by scientists with multiple biological functions. Various developmental and environmental factors are responsible for the production of JA in membranes and resulted in expression of stress tolerant genes. JA is ubiquitously found in the plant kingdom and results in the expression of genes at the transcriptional and post transcriptional levels. An imperative role of jasmonate (when applied exogenously in low concentrations) is reported in enhanced pathogen resistance. Cross talks of Jasmonates with signaling pathways generated by various phytohormones like ABA, salicylic acid, ethylene, etc. result in diverse developmental processes like seed germination, seedling growth, pollen fertility, fruit ripening, senescence and tolerance. However, its extent of effectiveness entirely depends on the type of plant species tested or its concentration. MeJA is more volatile than JA, so exposure to it either in solution or in the gaseous phase can elicit plant responses. Apart from its significant role in plants, derivatives of Jasmonates, e.g., methyl jasmonate are used as a fragrant constituent in many aromatic mixtures.
Chemical structure of methyl jasmonate [3-oxo-2-(2-pentenyl)-, methyl ester]
Chemical structure of Jasmonic acid [3-oxo-2-(2-pentenyl) cyclopentaneacetic acid]
Various developmental and environmental factors are shown that signals membrane for the production of endogenous jasmonates which further results in the regulation of certain genes and defense mechanisms against stressful environment.
Biosynthesis of jasmonic Acid: the octadecanoid pathway
The biosynthetic pathway of jasmonates was identified in Year 1984. The scientists Vick and Zimmerman were the first to illustrate the biosynthetic pathway in a simplified manner which indicated that linolenic acid could be converted into the cyclopentanone 12-oxo-phytodienoic acid (12-oxo-PDA) through lipoxygenase enzyme. Methodically the pathway was studied in model plants like Arabidopsis and tomato. The octadecanoid pathway of Jasmonates completes in two cellular organelles such as chloroplasts and peroxisomes and are considered to be the primary sites . α-Linolenic acid (α-LeA) released from galactolipids (due to wounding or pathogens attack) of chloroplast membranes is found to be the main player of MeJA and JA production. Phospholipases1 (PL1), lipoxygenase, allene oxide synthase, and allene oxide cyclase (AOC) are significant enzymes involved in biosynthesis of jasmonates.
Step wise pathway of jasmonates biosynthesis
Role of enzymes involved in jasmonates biosynthetic pathway
• Phospholipases: formation of α-LeA from chloroplastic membrane lipids
• 13-Lipoxygenase (LOX): addition of oxygen molecule to α-LeA and results in formation of an intermediate compound 13-hydroperoxy-9,11,15-octadecatrienoic acid (13-HPOT).
• Allene oxide synthase (AOS): oxidation of 13-HPOT to allene oxide
• Allene oxide cyclase (AOC): formation of 12-oxo-phytodienoic acid (12-OPDA) an unstable compound from allene oxide.
The AOS and AOC are present in plastids and they act in concert. 12-OPDA is the final product of biosynthetic pathway formed in chloroplast and undergoes three cycles of β-oxidation in the peroxisomes. A methylation reaction by JA methyl transferase results in the formation of jasmonate methyl ester derivative, i.e., methyl jasmonate. Among the six 13-LOXs of Arabidopsis, four of them are (LOX2, LOX3, LOX4, and LOX6) but LOX2 is a vital lipoxygenase in JA biosynthesis.
· Key roles of LOX2
• Responsible for the bulk of JA formation upon wounding
• Oxylipin generation during natural and dark-induced senescence
Jasmonates signal perception and transduction pathway
Several transcription factors (TFs), repressors, up- regulation and down-regulation of certain genes, and members of ubiquitin-proteasome complexes are involved in JA signal perception and transduction pathway. Different independent studies with GC-MS and HPLC analyses in A. thaliana revealed that (+)-7-iso-JA-L-Ile is the only natural and direct JA-signaling ligand in plants.
The Skp1/Cullin/F-box (SCF) complex is of proteinous type and is the ubiquitin proteasome complex. In jasmonate signal transduction pathway coronatine insensitive1 (COI1), the locus encodes an F-box protein that associates with its other counterparts, SKP1, Cullin, and Rbx proteins, to form an E3 ubiquitin ligase. So COI1 is found to be the jasmonate receptor in signaling pathway. In 2007, scientists in independent research found new family of proteins called jasmonate zim-domain (JAZ) proteins in Arabidopsis, which were found to be the key negative regulator of JA signaling and act as substrate for SCFCOI1 E3 ubiquitin ligase complex. About 12 JAZ proteins were present in A. thaliana and contain N-terminal domain, a highly conserved C-terminal Jas domain that mediates the interaction with the COI1 and several transcription factors, and the conserved protein–protein interaction domain, the ZIM (TIFY) domain that helps in JAZ dimerization and interaction with novel interactor of JAZ (NINJA).
Moreover, the interaction of COI1 with the Jas domain of JAZ proteins in the presence of JA-Ile forms the co-receptor complex SCFCOI1, E3 ubiquitin ligase complex, JAZ degrons (JAZ1 to JAZ12) and IP5 form the co-receptor complex and found to be true jasmonates receptors.
A number of co-repressors also play their role simultaneously in signal transduction pathway. Corepressors in transcriptional regulators inhibit transcription initiation. TOPLESS (TPL) and TPL-related proteins (TPRs) are members of Groucho/ Tup1 co-repressor family that causes histone deacetylation and demethylation which resulted in chromatin modification, eventually mediated repression of genes. Interaction of TPL with JAZ proteins takes place via ethylene response factor (ERF)-associated amphiphilic repression (EAR) motif. Those JAZ proteins having absence of repression motif recruit TPL through an adapter protein called novel interactor of JAZ (NINJA).
MYC2 the master regulator of many biological processes is a bHLH transcription factor mediating the transcriptional regulation of JA. Other transcription factors that controls JA mediated signaling are MYC3, MYC4, MYB, GL3, EGL3 AP, GL1, etc.