Transgenic Citrus Expressing an Arabidopsis NPR1 Gene Exhibit Enhanced Resistance against Huanglongbing (HLB; Citrus Greening)

Abstract

Commercial sweet orange cultivars lack resistance to Huanglongbing (HLB), a serious phloem limited bacterial disease that is usually fatal. In order to develop sustained disease resistance to HLB, transgenic sweet orange cultivars ‘Hamlin’ and ‘Valencia’ expressing an Arabidopsis thaliana NPR1 gene under the control of a constitutive CaMV 35S promoter or a phloem specific Arabidopsis SUC2 (AtSUC2) promoter were produced. Overexpression of AtNPR1 resulted in trees with normal phenotypes that exhibited enhanced resistance to HLB. Phloem specific expression of NPR1 was equally effective for enhancing disease resistance. Transgenic trees exhibited reduced diseased severity and a few lines remained disease-free even after 36 months of planting in a high-disease pressure field site. Expression of the NPR1 gene induced expression of several native genes involved in the plant defense signaling pathways. The AtNPR1 gene being plant derived can serve as a component for the development of an all plant T-DNA derived consumer friendly GM tree.

Response of AtNPR1-expressing cotton plants to Fusarium oxysporum f. sp. vasinfectum isolates

“…Similar limitations were observed in the AtNPR1 transformants against Verticillium wilt, where protection was observed only against the non-defoliating isolates of V. dahliae and not the defoliating isolates (Parkhi et al 2010b). Nevertheless, there are a large number of reports that show that NPR1 overexpression confers a strong protection against a variety of diseases (Lin et al 2004;Makandar et al 2006;Malnoy et al 2007;Wally et al 2009;Zhang et al 2010;Dutt et al 2015;Silva et al 2015). Thus, controlled expression of NPR1 gene may still prove to be a useful means to confer tolerance to some pathogens in the field.…”

“…Citri (Xcc). A recent study showed enhanced resistance against Huanglongbing disease in citrus plants expressing the AtNPR1 gene (Dutt et al 2015). In our earlier investigations, we found that expression of AtNPR1 in cotton conferred protection against two necrotrophic fungal pathogens, R. solani, (a cause of seedling damping-off) and A. alternata (a cause of foliar disease), two wilt diseases, Fusarium wilt caused by Fov and Verticillium wilt caused by nondefoliating pathotype of Verticillium dahliae, black root rot caused by T. basicola, and reniform nematodes (Parkhi et al 2010a;Parkhi et al 2010b;Kumar et al 2013).…”

Genetic Engineering and Sustainable Crop Disease Management: Opportunities for Case-by-Case Decision-Making

“…Such defenses include generation of reactive oxygen species, callose deposition, synthesis of pathogenesis-related (PR) proteins, and increased activation of systemic acquired resistance (SAR) [23,61]. As with the previously described strategies, this strategy takes advantage of the plant's own natural immune system and does not introduce new metabolic pathways.…”

“…As with the previously described strategies, this strategy takes advantage of the plant's own natural immune system and does not introduce new metabolic pathways. This approach has been successful against bacterial pathogens attacking several host species [62][63][64], and it offers promising results for enhancing resistance to citrus greening [23], a disease of urgency for the citrus industry. Upregulation of defense pathways was also successful against destructive fungal pathogens, including Rhizoctonia solani (the cause of many diseases) and Magnaporthe oryzae (the cause of rice blast) [61,65].…”

“…However, when conventional breeding and other management options are inadequate, or when linkage drag limits the usefulness of conventionally derived traits, GE offers alternatives. As examples of diseases for which GE presently appears to represent the only acceptable disease management in culturally or economically important crops, consider papaya ring spot in Hawaii [21], cassava brown streak disease in Africa [22], and citrus greening in Florida [23][24][25]. The loss of important crops to infectious diseases is completely contrary to the principles of sustainability.…”