The Role of Salicylic Acid, Jasmonic Acid and Ethylene in Plant Defense
Abstract
A complex network of cross – talk between the salicylic acid (SA) and jasmonic acid (JA) pathways further fine tunes plant defense responses. SAcan be formed from cinnamate via o – coumarate or benzoate depending on whether the hydroxylation of the aromatic ring takes place before or after the chain – shortening reactions. SAnot only functions against biotrophs, but also activates plant resistance against the below – ground disease such as root – knot nematodes. The synthesis of jasmonates and many other oxylipins is initiated by lipoxygenases (LOXs), which catalyze the regio – and stereoselective dioxygenation of polyunsaturated fatty acids. JAactivates plant immune responses to necrotrophic pathogens, some phloem – feeding insects and chewing herbivores. Also JA is also involved in other aspects of plant – pathogen interactions, including systemic acquired resistance (SAR). The role of ethylene (ET) in plant diseases resistance is dramatically different duo to type of pathogene and plant species. There are many evidence that show ethylene response is linked to gene for gene resistance. It is proven that there are a strong connection between different pathways related to SA, JAand ET for plant diseases resistance. So that SA – dependent and JA/ethylene – dependent pathwa ysinduce expression of different sets of PRgenes and result in plant resistance to different pathogens.
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Introduction
Plant resistance to pathogenic agents usually operates through a complex defense mechanism network. Compounds such as salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) regulate plant defense pathways to trigger appropriate responses to different pathogens [4]. Whereas the SAsignaling pathway is mainly activated against biotrophic pathogens, the JA/ET signaling pathway is activated against necrotrophic pathogens [28]. So SArely on living plant tissue for nutrients [34]; [79] and [13]. In contrast, plants produce jasmonic acid (JA) in response to wounding caused by insects and to necrotrophic microbes which obtain nutrients from dead host cells [13]; [118] and [102]. It can be said that the SA – mediated defenses have a major role in the basal resistance to the bacterial and oomycete pathogens, Pseudomonas syringae and Peronospora parasitica, respectively, turnip crinkle virus (TCV) and cucumber mosaic virus (CMV) [10]; [86]; [116]; [20] and [38]. In contrast, JAsignaling has an important role in the basal resistance to the fungal pathogen Botrytis cinerea [13]. ET is a critical third player from the perspective of understanding how plants prioritize and tailor their responses to diverse attackers [1]. Also ET modulates SArelated plant defense signaling both positively and negatively.
At the end of this section, it is important to point out that a complex network of cross – talk between the SAand JApathways further fine tunes plant defense responses [10] and [11]. Most studies have identified antagonistic interactions between the SA – and JA – mediated signaling pathways [112]. But the relationship between SAand JA is not always antagonistic [101]. The following examples that are presented sequential help to clear this concept. The application of JAdepressed the activation of the genes for the acidic PRproteins, [Pathogenesis – related protein(s)] which are SAdependent [112]. In rice is demonstrated that JAsignalling positively regulates plant resistance to the biotrophic pathogen, Xanthomonas oryzae pv. oryzae (Xoo), [101] possibly due to a common defence system activated by both SAand JA [24].
Conclusion
As sessile organisms, plants are under frequent attack from abroad spectrum of microbial pathogens, including biotrophic and necrotrophic pathogens, namely biotrophs and necrotrophs respectively. SA is a crucial defense signal molecule against biotrophs. Also ethylene and jasmonate, playa major role in defense responses against necrotrophs. Although SA – dependent and JA/ethylene – dependent pathways induce expression of different sets of PRgenes and result in plant resistance to different pathogens, there appear to be considerable interactions between these two pathways in systemic acquired resistance.