Cotton is an economically important crop worldwide that suffers severe losses due to a wide range of fungal/bacterial pathogens and nematodes. Given its susceptibility to various pathogens, it is important to obtain a broad-spectrum resistance in cotton. Resistance to several fungal and bacterial diseases has been obtained by overexpressing the Non-expressor of Pathogenesis-Related genes-1 (NPR1) in various plant species with apparently minimal or no pleiotropic effects. We examined the efficacy of this approach in cotton by constitutive expression of the Arabidopsis (Arabidopsis thaliana) NPR1 gene. The results show that NPR1-expressing lines exhibited significant resistance to Verticillium dahliae isolate TS2, Fusarium oxysporum f. sp. vasinfectum, Rhizoctonia solani, and Alternaria alternata. Interestingly, the transformants also showed significant resistance to reniform nematodes. Analysis of defense-related, biochemical and molecular responses suggest that when challenged with pathogens or certain systemic acquired resistance-inducing chemicals, the transgenic lines respond to a greater degree compared to the wild-type plants. Importantly, the basal activities of the defense-related genes and enzymes in uninduced transformants were no different than those in their non-transgenic counterparts. The results provide additional evidence supporting the role of NPR1 as an important part of the plant defense system and suggest a means to achieve broad-spectrum resistance to pathogens via genetic engineering.

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There are many reports on obtaining disease-resistance trait in plants by overexpressing genes from diverse organisms that encode chitinolytic enzymes. Current study represents an attempt to dissect the mechanism underlying the resistance to Rhizoctonia solani in cotton plants expressing an endochitinase gene from Trichoderma virens. Several assays were developed that provided a powerful demonstration of the disease protection obtained in the transgenic cotton plants. Transgene-dependent endochitinase activity was confirmed in various tissues and in the medium surrounding the roots of transformants. Biochemical and molecular analyses conducted on the transgenic plants showed rapid/greater induction of ROS, expression of several defense-related genes, and activation of some PR enzymes and the terpenoid pathway. Interestingly, even in the absence of a challenge from the pathogen, the basal activities of some of the defense-related genes and enzymes were higher in the endochitinase-expressing cotton plants. This elevated defensive state of the transformants may act synergistically with the potent, transgene-encoded endochitinase activity to confer a strong resistance to R. solani infection.

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Verticillium wilt, caused by Verticillium dahliae Kleb., causes severe yield and quality losses in most cotton growing areas of the world. Only moderate resistance has been achieved by traditional breeding. Therefore, transgenic approaches offer a possible alternative to obtain resistance against this disease. Overexpression of a homologous or heterologous NPR1 gene has been effective in conferring broad-spectrum resistance to diverse pathogens in a variety of plants. We investigated the resistance of cotton plants, expressing the Arabidopsis NPR1 (AtNPR1) gene, to defoliating and non-defoliating pathotypes of V. dahliae. The transgenic cotton plants showed significant resistance against two non-defoliating V. dahliae isolates. Both visual symptoms and pathogen colonization were reduced, indicating that disease progression was curtailed in the AtNPR1-transformants. In contrast, the same transgenic lines showed little, if any, resistance to two defoliating isolates. The NPR1-mediated activation of cottonÕs defences apparently is not sufficient to counter the disease mechanism(s) utilized by the defoliating pathotype of V. dahliae.