Abstract
SummaryBacterial fruit blotch (BFB) caused by Acidovorax citrulli is one of the most important bacterial diseases of cucurbits worldwide. However, the mechanisms associated with A. citrulli pathogenicity and genetics of host resistance have not been extensively investigated. We idenitfied Nicotiana benthamiana and Nicotiana tabacum as surrogate hosts for studying A. citrulli pathogenicity and non‐host resistance triggered by type III secreted (T3S) effectors. Two A. citrulli strains, M6 and AAC00‐1, that represent the two major groups amongst A. citrulli populations, induced disease symptoms on N. benthamiana, but triggered a hypersensitive response (HR) on N. tabacum plants. Transient expression of 19 T3S effectors from A. citrulli in N. benthamiana leaves revealed that three effectors, Aave_1548, Aave_2708, and Aave_2166, trigger water‐soaking‐like cell death in N. benthamiana. Aave_1548 knockout mutants of M6 and AAC00‐1 displayed reduced virulence on N. benthamiana and melon (Cucumis melo L.). Transient expression of Aave_1548 and Aave_2166 effectors triggered a non‐host HR in N. tabacum, which was dependent on the functionality of the immune signalling component, NtSGT1. Hence, employing Nicotiana species as surrogate hosts for studying A. citrulli pathogenicity may help characterize the function of A. citrulli T3S effectors and facilitate the development of new strategies for BFB management.
Highlights
Bacterial fruit blotch (BFB) of cucurbits is caused by the seedborne Gram-negative bacterium Acidovorax citrulli (Schaad et al, 2008)
N. benthamiana was susceptible to A. citrulli infection while N. tabacum was resistant
To determine if A. citrulli strains are pathogenic on N. benthamiana, we inoculated 4-week-old plants by tissue infiltration and spray-inoculation methods
Summary
Bacterial fruit blotch (BFB) of cucurbits is caused by the seedborne Gram-negative bacterium Acidovorax citrulli (formerly Acidovorax avenae subsp. citrulli ) (Schaad et al , 2008). Bacterial fruit blotch (BFB) of cucurbits is caused by the seedborne Gram-negative bacterium Acidovorax citrulli A. citrulli gained recognition after severe outbreaks occurred in watermelon fields in several USA states in the late 1980s. Several research groups have screened plant germplasm for resistance to BFB. While some cucurbit germplasm lines have been reported to be partially tolerant to A. citrulli (Bahar et al , 2009a, Wechter et al , 2011), to date, no cucurbit lines have been identified with complete resistance. All commercial cultivars of watermelon (Citrullus lanatus Thunb.) and melon (Cucumis melo L.) are susceptible to A. citrulli. Despite the economic importance of BFB, little is known about the molecular basis of A. citrulli -cucurbit interactions (Bahar and Burdman, 2010; Burdman and Walcott, 2012)
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