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Jasmonates (JA) are cellular signal molecules regulating the plant innate immunity. The concentrations of JA in healthy unperturbed plant tissues are very low. JA concentration can be increased by enhancing the activities of the enzymes involved in JA biosynthesis. The key enzymes involved in the biosynthesis involve phospholipases, lipoxygenases (LOXs), allene oxide synhases (AOS), allene oxide cyclase (AOC), and OPDA reductase (OPR). The genes encoding these enzymes have been cloned and engineered to enhance the JA biosynthesis and JA accumulation will activate plant immune system. LOX genes have been cloned from different plants. Constitutive overexpression of LOX genes has been shown to be required for conferring resistance against diseases. A 9-lipoxygenase gene, DkLOX3, has been cloned from persimmon plants. The Arabidopsis plants overexpressing the persimmon DkLOX3 gene showed resistance to both bacterial and fungal pathogens. An LOX gene, LOX1, has been cloned from tobacco. The transgenic tobacco plants constitutively expressing LOX1 showed increased resistance against the oomycete pathogen Phytophthora parasitica var nicotianae. A pepper 9-LOX gene, CaLOX1, which encodes a 9-lipoxygenase, was isolated from pepper leaves. Overexpression of CaLOX1 in Arabidopsis thaliana conferred enhanced resistance to bacterial, oomycete and fungal pathogens. An allene oxidase synthase gene, OsAOS2 was cloned from rice plants. Transgenic rice plants overexpressing OsAOS2 showed increased resistance against the blast disease. Alkamide treatment enhanced the expression of genes encoding enzymes for jasmonic acid biosynthesis. It enhanced the expression of lipoxygenase genes (LOX2 and LOX3), allene oxide synthase gene (AOS), allene oxide cyclase2 gene (AOC2), and OPDA reductase3 (OPR3) gene. It induces resistance against Botrytis cinerea infection in Arabidopsis. The alkamide-induced resistance depends on activation of JA signaling system. The alkamide treatment induced the expression of the JA-inducible genes JAZ8, VSP2 and ERF2. The alkamide has great potential to combat pathogens by triggering JA biosynthesis pathway. Chitosan triggers lipoxygenase activity and induces accumulation of jasmonic acid. Chitosan activates plant innate immune system and controls several crop diseases. Arachidonic acid isolated from microbes is an elicitor of plant defense responses. Bioengineering technology has been developed to make the plants themselves to produce arachidonic acid without any pathogen invasion. Production of arachidonic acid in a higher plant was achieved using genes encoding enzymes participating in the 3/6∆8-desaturation biosynthetic pathways for the formation of C20 polyunsaturated fatty acids. The arachidonic acid-containing transgenic plants showed increased levels of jasmonic acid. The transgenic plants showed enhanced expression of LOX3 gene encoding lipoxygenase and AOS gene encoding allene oxide synthase. The transgenic plants showed increased expression of DGL gene encoding phospholipase A. Developing transgenic plants constitutively producing arachidonic acid may be a potential approach to activate JA pathway for management of plant diseases. Hexanoic acid is a nine carbon dicarboxylic acid that acts as an inducer of plant defenses by means of a priming mechanism. Hexanoic acid induces JA-dependent defense responses by priming. Hexanoic acid has been reported to be a potent natural priming agent of proven efficacy in a wide range of host plants and pathogens. Priming for JA-dependent defenses using hexanoic acid appears to be an effective tool for management of crop diseases. Ulvan is a plant defense activator derived from the green algae belonging to the Ulva genus. Ulvan treatment induces elevation of JA content in plants. It induces the expression of well-known jasmonic acid-responsive genes including lipoxygenase, hydroxyproline-rich glycoproteins, proline-rich proteins, defensin and wound-induced protein. Ulvan activates JA-amino acid synthase, which is involved in Jasmonoyl-isoleucine (JA-Ile) biosynthesis. JA-Ile has been shown to be involved in defense signaling. Ulvan effectively controlled several diseases. Several transcription factor genes have been engineered to manipulate JA signaling system for crop disease management. Application of Trichoderma asperellum, Trichoderma virens, T. harzianum, and Pseudomonas putida strains activates JA signaling system and induces systemic resistance. |
