Resistance against various fungal pathogens and reniform nematode in transgenic cotton plants expressing Arabidopsis NPR1

Vitis vinifera VvNPR1.1 is the functional ortholog of AtNPR1 and its overexpression in grapevine triggers constitutive activation of PR genes and enhanced resistance to powdery mildew

Studying grapevine (Vitis vinifera) innate defense mechanisms is a prerequisite to the development of new protection strategies, based on the stimulation of plant signaling pathways to trigger pathogen resistance. Two transcriptional coactivators (VvNPR1.1 and VvNPR1.2) with similarity to Arabidopsis thaliana NPR1 (Non-Expressor of PR genes 1), a well-characterized and key signaling element of the salicylic acid (SA) pathway, were recently isolated in Vitis vinifera. In this study, functional characterization of VvNPR1.1 and VvNPR1.2, including complementation of the Arabidopsis npr1 mutant, revealed that VvNPR1.1 is a functional ortholog of AtNPR1, whereas VvNPR1.2 likely has a different function. Ectopic overexpression of VvNPR1.1 in the Arabidopsis npr1-2 mutant restored plant growth at a high SA concentration, Pathogenesis Related 1 (PR1) gene expression after treatment with SA or bacterial inoculation, and resistance to virulent Pseudomonas syringae pv. maculicola bacteria. Moreover, stable overexpression of VvNPR1.1-GFP in V. vinifera resulted in constitutive nuclear localization of the fusion protein and enhanced PR gene expression in uninfected plants. Furthermore, grapevine plants overexpressing VvNPR1.1-GFP exhibited an enhanced resistance to powdery mildew infection. This work highlights the importance of the conserved SA/NPR1 signaling pathway for resistance to biotrophic pathogens in V. vinifera.

Transgenic tobacco plants constitutively expressing Arabidopsis NPR1 show enhanced resistance to root-knot nematode, Meloidogyne incognita

In Arabidopsis, non-expressor of pathogenesis related genes-1, NPR1 has been shown to be a positive regulator of the salicylic acid controlled systemic acquired resistance pathway and modulates the cross talk between SA and JA signaling. Transgenic plants expressing AtNPR1 constitutively exhibited resistance against pathogens as well as herbivory. In the present study, tobacco transgenic plants expressing AtNPR1 were studied further for their response to infection by the sedentary endoparasitic root knot nematode, Meloidogyne incognita. Transgenic plants showed enhanced resistance against the root-knot nematode infection. Prominent differences in the shoot and root weights of wild type and transgenic plants were observed post-inoculation with M. incognita. This was associated with a decrease in the number of root galls and egg masses in transgenic plants compared to WT. The transgenic plants also showed constitutive and induced expression of some PR protein genes, when challenged with M. incognita.

Salicylic acid and its function in plant immunity

The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant immune response. Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network. Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network. Full understanding of the mechanism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques. However, elucidating genetic components involved in these pathways by conventional genetics, biochemistry, and molecular biology approaches will continue to be a major task of the community. High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling.

Mungbean plants expressing BjNPR1 exhibit enhanced resistance against the seedling rot pathogen, Rhizoctonia solani

Mungbean, Vigna radiata (L.) Wilczek is an important pulse crop that is widely cultivated in semi- arid tropics. The crop is attacked by various soil-borne pathogens like Rhizoctonia solani, which causes dry rot disease and seriously affects its productivity. Earlier we characterized the non-expressor of pathogenesis related gene-1(BjNPR1) of mustard, Brassica juncea, the counterpart of AtNPR1 of Arabidopsis thaliana. Here, we transformed mungbean with BjNPR1 via Agrobacterium tumefaciens. Because of the recalcitrant nature of mungbean, the effect of some factors like Agrobacterium tumefaciens strains (GV2260 and LBA4404), pH, L: -cysteine and tobacco leaf extract was tested in transformation. The transgenic status of 15 plants was confirmed by PCR using primers for nptII. The independent integration of T-DNA in transgenic plants was analyzed by Southern hybridization with an nptII probe and the expression of BjNPR1 was confirmed by RT-PCR. Some of the T(0) plants were selected for detached leaf anti-fungal bioassay using the fungus Rhizoctonia solani, which showed moderate to high level of resistance depending on the level of expression of BjNPR1. The seedling bioassay of transgenic T(2) plants indicated resistance against dry rot disease caused by R. solani.

Screening for resistance against Pseudomonas syringae in rice-FOX Arabidopsis lines identified a putative receptor-like cytoplasmic kinase gene that confers resistance to major bacterial and fungal pathogens in Arabidopsis and rice

Approximately 20,000 of the rice-FOX Arabidopsis transgenic lines, which overexpress 13,000 rice full-length cDNAs at random in Arabidopsis, were screened for bacterial disease resistance by dip inoculation with Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). The identities of the overexpressed genes were determined in 72 lines that showed consistent resistance after three independent screens. Pst DC3000 resistance was verified for 19 genes by characterizing other independent Arabidopsis lines for the same genes in the original rice-FOX hunting population or obtained by reintroducing the genes into ecotype Columbia by floral dip transformation. Thirteen lines of these 72 selections were also resistant to the fungal pathogen Colletotrichum higginsianum. Eight genes that conferred resistance to Pst DC3000 in Arabidopsis have been introduced into rice for overexpression, and transformants were evaluated for resistance to the rice bacterial pathogen, Xanthomonas oryzae pv. oryzae. One of the transgenic rice lines was highly resistant to Xanthomonas oryzae pv. oryzae. Interestingly, this line also showed remarkably high resistance to Magnaporthe grisea, the fungal pathogen causing rice blast, which is the most devastating rice disease in many countries. The causal rice gene, encoding a putative receptor-like cytoplasmic kinase, was therefore designated as BROAD-SPECTRUM RESISTANCE 1. Our results demonstrate the utility of the rice-FOX Arabidopsis lines as a tool for the identification of genes involved in plant defence and suggest the presence of a defence mechanism common between monocots and dicots.

Silencing GhNDR1 and GhMKK2 compromises cotton resistance to Verticillium wilt

Cotton is an important cash crop worldwide, and is a significant source of fiber, feed, foodstuff, oil and biofuel products. Considerable effort has been expended to increase sustainable yield and quality through molecular breeding and genetic engineering of new cotton cultivars. Given the recent availability of the whole-genome sequence of cotton, it is necessary to develop molecular tools and resources for large-scale analysis of gene functions at the genome-wide level. We have successfully developed an Agrobacterium-mediated virus-induced gene silencing (VIGS) assay in several cotton cultivars with various genetic backgrounds. The genes of interest were potently and readily silenced within 2 weeks after inoculation at the seedling stage. Importantly, we showed that silencing GhNDR1 and GhMKK2 compromised cotton resistance to the infection by Verticillium dahliae, a fungal pathogen causing Verticillium wilt. Furthermore, we developed a cotton protoplast system for transient gene expression to study gene functions by a gain-of-function approach. The viable protoplasts were isolated from green cotyledons, etiolated cotyledons and true leaves, and responded to a wide range of pathogen elicitors and phytohormones. Remarkably, cotton plants possess conserved, but also distinct, MAP kinase activation with Arabidopsis upon bacterial elicitor flagellin perception. Thus, using gene silencing assays, we have shown that GhNDR1 and GhMKK2 are required for Verticillium resistance in cotton, and have developed high throughput loss-of-function and gain-of-function assays for functional genomic studies in cotton.