Transgenic citrus expressing the antimicrobial gene Attacin E (attE) reduces the susceptibility of \'Duncan\' grapefruit to the citrus scab caused by Elsinoe fawcettii

Citrus scab, caused by ElsinoA << fawcettii (anamorph Sphaceloma fawcettii), is a common foliar fungal disease affecting many citrus cultivars, including grapefruit. No commercial grapefruit cultivar is resistant to scab, and the disease results in severely blemished fruit which reduces its marketability. Transgenic 'Duncan' grapefruit trees expressing the antimicrobial attE gene were produced via Agrobacterium-mediated transformation. In in vitro leaf and greenhouse assays, several transgenic-lines had significantly lower susceptibility to E. fawcettii compared to the non-transformed control (P < 0.0001). In the greenhouse studies, sporulation on all transgenic lines except 1 was significantly reduced (P < 0.0001) but the level of sporulation over time did not correspond to disease severity ratings. Lesion size was also significantly reduced on transgenic lines compared to the non-transformed control (P < 0.0001) and the least susceptible line A-23 had the smallest lesions, but in general there was no correlation between lesion size and disease susceptibility. The level of attE mRNA was inversely related to the number of copies detected by Southern blot. The least susceptible line had a single inserted copy of the attE transgene whereas more susceptible lines had multiple copies. Since the attacin mode of action was thought to be specific to Gram-negative bacteria, it was unexpected to find that there was a significant activity against E. fawcettii.