Bioengineering and Molecular Manipulation of Salicylic Acid Signaling System to Activate Plant Immune Responses for Crop Disease Management

Abstract

Plant innate immune system provides potential weapons to the plants for fighting against pathogens. However, specific signals are needed to activate the system. Salicylic acid (SA) is the most important endogenous signal molecule which triggers the plant defense system. Plants do not have much endogenous SA. Increased synthesis and accumulation of salicylic acid in plants result in increased expression of defense genes. It has been shown that by increasing the SA content, defense genes can be activated and diseases can be controlled. Several molecular technologies have been developed to increase the biosynthesis of SA by engineering genes encoding enzymes involved in SA biosynthesis. Isochorismate synthase (ICS) and isochorismate pyruvate lyase (IPL) are the key enzymes involved in biosynthesis of SA. The genes encoding ICS and IPL cloned from two different bacteria have been exploited to develop disease-resistant plants by triggering SA accumulation. Transgenic tobacco plants expressing both the ICS and IPL genes show high increase in SA accumulation and these plants show enhanced disease resistance against Tobacco mosaic virus and the powdery mildew pathogen Oidium lycopersici. The regulatory protein PAD4 is known to activate SID1 and SID2. SID2 is an isochorismate synthase that is involved in SA biosynthesis and SID1 encodes a protein, which transports precursors for SA biosynthesis. Transgenic wheat plants overexpressing the Arabidopsis PAD4 coding sequence have been developed and these transgenic plants show accumulation of SA and resistance against the Fusarium head blight (FHB) pathogen Fusarium graminearum. A RNA-binding protein (RBP) gene from Arabidopsis thaliana, AtRBP-DR1 has been exploited for developing disease-resistant plants by inducing SA biosynthesis. A camodulin binding protein, CBP60g, has been exploited to develop disease resistant plants by activating SA biosynthesis. Transgenic Arabidopsis plants overexpressing CBP60g gene have been developed and these plants show elevated SA accumulation and enhanced resistance against diseases. Several transcription factors are known to take part in the regulation of SA signaling pathway and genes encoding these transcription factors have been exploited to develop disease-resistant plants. Ubiquitin- and proteasome-mediated degradation of proteins plays an important role in plant defense signaling system. E3 ubiquitin ligases play a key role in the ubiquitin-proteasome system. Ubiquitin-proteasome pathway has been manipulated to trigger SA signaling system for crop disease management. NPR1 gene is a master regulator of the SA-mediated induction of systemic acquired resistance (SAR). NPR1 directly binds SA and activates SA signaling system. NPR1 gene cloned from Arabidopsis thaliana has been used to develop several transgenic crop plants including rice, tomato, citrus, carrot, and strawberry. NPR1-like genes isolated from rice, grapevine, apple and tobacco have also been utilized to develop disease-resistant transgenic plants. NPR1 gene expression can be enhanced by treatment with some synthetic chemicals. BTH (benzo[1,2,3]thiadiazole-7-carbothioic acid S-methyl ester) is the most successfully developed commercial compound to activate plant innate immune system by enhancing NPR1 gene expression. BTH treatment induces NPR1 mRNA accumulation by several-fold. BTH may also contribute to the establishment of SAR through an interaction with methyl salicylate esterase that is critical for the perception of defense-inducing signals in systemic tissues. Treatment of plants with BTH, which triggers SA signaling, causes the induction of a unique physiological state called “priming”. BTH induces histone modifications, which may be involved in the gene priming.The expression of the WRKY genes is enhanced in BTH-treated plants. BTH triggers NPR1-dependent chromatin modification on WRKY promoters to activate defense gene expression. BTH activates SA-dependent SAR in many crops and has been found to be useful in management of several crop diseases caused by oomyctes, fungi, bacteria, and viruses. N-cyanomethyl-2-chloroisonicotinamide (NCI) is another potential chemical that activates NPR1-dependent SA signaling system. NCI activates SAR by stimulating the site between SA and NPR1. 3-chloro-1-methyl-1H-pyrazole-5-carboxylic acid (CMPA) activates NPR1 in SA signaling pathway. CMPA acts downstream of SA accumulation and acts in the SA signaling pathway between SA production and NPR1 activity. It protects rice from infection by rice blast pathogen Magnaporthe oryzae and bacterial blight pathogen Xanthomonas oryzae pv. oryzae It enhances resistance of tobacco to Pseudomonas syringae pv. tabaci and Oidium sp. Tiadinil (3,4-dichloro-N-(2-cyanophenyl)-1,2-thiazole-5-carboxamide) is another potential chemical, which triggers SA signaling pathway by activating NPR1 gene expression. Tiadinil induces resistance against various fungal, bacterial, and viral diseases in tobacco and is practically used to control rice blast disease. SV-03 is a metabolite of Tiadinil. It stimulates SA signaling pathway downstream of SA production and triggers resistance against various viral, bacterial and fungal pathogens. Probenazole (3-allyloxy-1,2-benzisothiazole-1,1-dioxide) and its metabolite 1,2-benzisothiazole-3 (2H)-one 1,1-dioxide (BIT, saccharin) are potential plant defense activators and both of them are known to induce SA accumulation and activate SA signaling system. Probenazole/BIT intervenes in SA signaling system at SA accumulation stage as well as at NPR1 stage to trigger resistance against pathogens. The nonprotein amino acid β-aminobutyric acid (BABA) induces broad-spectrum resistance in a range of crops. BABA induces priming in the SAR induction pathway. The descendants of primed plants exhibit next-generation systemic acquired resistance. SA signaling system can also be activated using plant-derived products. Azelaic acid, a natural compound found in several plants is a signal molecule triggering plant defense responses. Azelaic acid does not directly induce defense responses, but confers on the plants the ability to mount a faster and stronger defense response if and when the plant is attacked again. It does this by increasing the production of SA. Azelaic acid stimulates the production of AZ11, a protein which helps prime the plant to build up its immunity by generating additional SA. 3-acetonyl-3-hydroxyoxindole (AHO), isolated from the extracts of Strobilanthes cusia is an activator of SA signaling system. When tobacco plants are treated with AHO, SA accumulates in the leaf tissues and induces disease resistance. An oligosaccharide product obtained from burdock (Arctium lappa) plant triggers production of methyl salicylate involved in SA signaling system and confers disease resistance. N-Acyl-L-homoserine lactones (AHLs)–producing bacteria, which induce SA-dependent systemic resistance, have been shown to be potential tools for management of crop diseases. Some of the rhizobacterial strains activate the plant innate immune system by triggering SA signaling system and they are widely used for management of crop diseases. SA signaling system can be activated by some MAMPs (for Microbe-associated molecular patterns) for effective crop disease management. The MAMP yeast elicitor treatment activates SA signaling system and induces resistance against oomycete, fungal, and bacterial pathogens in many crop plants.