Development of Tomato (s. Lycopersicum) Lines with Resistance to Xanthomonas Spp. and Use of Genetic Resources to Characterize Infection and Diversity in Pathogen Populations PDF Download

Author: Eduardo Bernal (Plant pathologist)
Publisher:
ISBN:
Category : Bacterial diseases of plants
Languages : en
Pages : 148

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Book Description
Bacterial spot disease of tomato is caused by Xanthomonas euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri. The disease causes black necrotic spots on leaves and sunken lesions on fruit, which affect the yield and marketability of the crop. I utilized a DNA-based marker assisted background genome selection strategy to develop near-isogenic tomato lines (NILs) with resistance and then challenged this germplasm with multiple species of Xanthomonas. The NILs were released to genetic stock centers and the seed industry. Subsequently I used these genetic resources to characterize infection by X. gardneri on tomato leaves and fruit in time-course experiments using in vivo imaging technology (IVIS) and scanning electron microscopy (SEM). I used a bioluminescent X. gardneri strain, engineered with the luxCDABE operon, to visualize infection. IVIS provided a method to quantify and distinguish differences between resistant NILs and susceptible tomato lines. In parallel, bioluminescence signals from IVIS illuminated infection processes on leaf surfaces and fruit. X. gardneri colonization occurred at hydathodes on leaf margins. I determined that bacterial spot symptoms are more numerous when small immature fruit between 0.5 - 1.5 cm are infected. IVIS technology allowed us to determine that bacteria can colonize fruit affected by Blossom End Rot. This observation is important to the seed industry as a likely source of infected seed. Plant genetic variation effects lesion development on fruit, and tomato germplasm with the highest density of multi-cellular trichomes had the fewest number of lesions, suggesting a genetic approach to reduce fruit infection. Finally, bacterial populations continue to shift over space and time. To assess which Xanthomonas species are contributing to epidemics I characterized populations in the Midwest between 2017-2019. I observed that X. gardneri has been displaced by X. perforans as the dominant species causing bacterial spot of tomato in Ohio and Indiana, and that race T4 has displaced race T3. Whole genome sequencing was utilized to characterize strains and a phylogenetic approach demonstrated that all Midwestern strains were nearly identical, grouping under the same clade as X. perforans race T4 isolates from Florida collected in 2012. All T4 isolates contained the same previously described mutation in the bacterial effector avrXv3. In addition they contained the effector avrBsT, which is associated with increased fitness. These findings suggest a role for migration in changing pathogen populations. This updated understanding of Xanthomonas populations in Midwestern states will influence the deployment of resistant tomato germplasm and the continued hunt for further sources of resistance to bacterial spot.