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Abstract
Bacterial fruit blotch (BFB) is an economically important seed-borne and seed-transmitted disease of cucurbitaceous plant species, including watermelon and melon crops. Acidovorax citrulli, the causal agent of BFB, can be divided into two major groups, I and II. The natural association of group I and II A. citrulli strains with different cucurbit species strongly suggests host preference. Despite the economic importance of BFB, little is known about fundamental aspects of A. citrulli-host interactions. The overall goal of this study was to gain a better understanding of host preference and virulence of the two major groups of A. citrulli. We set up field plots with four cucurbit species and introduced group I and II A. citrulli strains as the inoculum sources. The results from the field trials confirmed that A. citrulli strains exhibited a preference for different cucurbit species, which was more pronounced in fruit tissues. Group I and II A. citrulli strains were inoculated on detached melon fruits by syringe- and swab- inoculation, and also on attached melon fruits by swab-inoculation. From these
artificial inoculations, we found that physical penetration, colonization, and symptom development did not account for the host preference of A. citrulli on melon fruits. Additionally, we did not find detectable effects of XopJ homologs on A. citrulli virulence on watermelon and melon cotyledons. The deletion mutant of XopJ homologs was less virulent on attached melon fruits compared to the wild- type, while their population levels on melon fruits were variable across experiments. However, the HopAF homolog may be important for A. citrulli colonization and symptom development on watermelon and melon cotyledons. Additionally, we developed an immature watermelon fruit assay to distinguish group I and II A. citrulli strains and used it to identify virulence determinants of A. citrulli. Only group II strains induced water-soaked lesions on immature watermelon fruit tissues. By screening an A. citrulli AAC00-1 transposon mutant library, we found six mutants were non-pathogenic on watermelon fruits. Three of these mutants were disrupted in type 3 secretion system associated genes. Lastly, we investigated the effect of temperature on the colonization of germinating watermelon seeds/seedlings by A. citrulli. Mean BFB incidence was significantly lower for seeds that were sown at 28°C and transferred to 40°C at 3 days after sowing (das), compared to seeds incubated constantly at 28°C. This means that A. citrulli cells associated with germinating watermelon seeds are more sensitive to elevated temperature during the first 3 das relative to the later days.
artificial inoculations, we found that physical penetration, colonization, and symptom development did not account for the host preference of A. citrulli on melon fruits. Additionally, we did not find detectable effects of XopJ homologs on A. citrulli virulence on watermelon and melon cotyledons. The deletion mutant of XopJ homologs was less virulent on attached melon fruits compared to the wild- type, while their population levels on melon fruits were variable across experiments. However, the HopAF homolog may be important for A. citrulli colonization and symptom development on watermelon and melon cotyledons. Additionally, we developed an immature watermelon fruit assay to distinguish group I and II A. citrulli strains and used it to identify virulence determinants of A. citrulli. Only group II strains induced water-soaked lesions on immature watermelon fruit tissues. By screening an A. citrulli AAC00-1 transposon mutant library, we found six mutants were non-pathogenic on watermelon fruits. Three of these mutants were disrupted in type 3 secretion system associated genes. Lastly, we investigated the effect of temperature on the colonization of germinating watermelon seeds/seedlings by A. citrulli. Mean BFB incidence was significantly lower for seeds that were sown at 28°C and transferred to 40°C at 3 days after sowing (das), compared to seeds incubated constantly at 28°C. This means that A. citrulli cells associated with germinating watermelon seeds are more sensitive to elevated temperature during the first 3 das relative to the later days.