Abstract
Host disease resistance is the most desirable strategy for control of citrus canker, a disease caused by a gram-negative bacterium Xanthomonas citri subsp. citri. However, no resistant commercial citrus cultivar has been identified. Cybridization, a somatic hybridization approach that combines the organelle and nuclear genomes from different species, was used to create cybrids between citrus canker resistant ‘Meiwa’ kumquat (Fortunella crassifolia Swingle snym. Citrus japonica Thunb.) and susceptible grapefruit (Citrus paradisi Macfad) cultivars. From these fusions, cybrids with grapefruit nucleus, kumquat mitochondria and kumquat chloroplasts and cybrids with grapefruit nucleus, kumquat mitochondria and grapefruit chloroplasts were generated. These cybrids showed a range of citrus canker response, but all cybrids with kumquat chloroplasts had a significantly lower number of lesions and lower Xanthomonas citri subsp. citri populations than the grapefruit controls. Cybrids with grapefruit chloroplasts had a significantly higher number of lesions than those with kumquat chloroplasts. To understand the role of chloroplasts in the cybrid disease defense, quantitative PCR was performed on both cybrid types and their parents to examine changes in gene expression during Xanthomonas citri subsp. citri infection. The results revealed chloroplast influences on nuclear gene expression, since isonuclear cybrids and ‘Marsh’ grapefruit had different gene expression profiles. In addition, only genotypes with kumquat chloroplasts showed an early up-regulation of reactive oxygen species genes upon Xanthomonas citri subsp. citri infection. These cybrids have the potential to enhance citrus canker resistance in commercial grapefruit orchards. They also serve as models for understanding the contribution of chloroplasts to plant disease response and raise the question of whether other alien chloroplast genotypes would condition similar results.
Highlights
Citrus canker, caused by Xanthomonas citri subsp. citri, is a serious disease in terms of economic losses to the citrus industry and is present in most citrus-producing areas of the wet subtropics worldwide (Graham et al, 2004; Pruvost et al, 2014)
Citrus canker symptoms appeared on grapefruit cultivars about 7 days post inoculation with a suspension of Xanthomonas citri subsp. citri at 104 cfu mL−1
The hypothesis of this study is that kumquat chloroplasts and/or mitochondria, when combined with the grapefruit nucleus, may enhance grapefruit resistance to citrus canker
Summary
Citrus canker, caused by Xanthomonas citri subsp. citri, is a serious disease in terms of economic losses to the citrus industry and is present in most citrus-producing areas of the wet subtropics worldwide (Graham et al, 2004; Pruvost et al, 2014). Chloroplast and Citrus Canker Response disseminated, and once the bacterium is established in an orchard, the disease can cause defoliation, blemished fruit, premature fruit drop and general tree decline (Gottwald et al, 2002; Graham et al, 2004). In Florida, citrus plants and fruits moving into and out of the state need to be inspected and certified by the USDA (Dewdney and Graham, 2012). Besides these regulatory obstacles, control of citrus canker is costly for growers. In areas where citrus canker is endemic, use of windbreaks, protection of fruits and leaves with copper bactericidal sprays, control of leaf miner and use of tolerant citrus cultivars are recommended (Graham et al, 2004; Dewdney and Graham, 2012)
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