Journal
MOLECULAR PLANT PATHOLOGY
Volume 19, Issue 5, Pages 1210-1221Publisher
WILEY
DOI: 10.1111/mpp.12611
Keywords
bacterial plant pathogenesis; Clavibacter michiganensis; expansin; horizontal gene transfer; Ralstonia solanacearum
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Funding
- National Science Foundation (NSF)-Graduate Research Fellowships Program [DGE-1144153]
- New York State Agricultural Experiment Station
- University of Wisconsin-Madison College of Agricultural and Life Sciences
- National Institutes of Health (NIH) [T32 GM07215]
- Agriculture and Food Research Initiative Competitive Grant from the US Department of Agriculture, National Institute of Food and Agriculture (USDA NIFA) [2015-67011-22799]
- NSF [IOS1456636]
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Expansin proteins, which loosen plant cell walls, play critical roles in normal plant growth and development. The horizontal acquisition of functional plant-like expansin genes in numerous xylem-colonizing phytopathogenic bacteria suggests that bacterial expansins may also contribute to virulence. To investigate the role of bacterial expansins in plant diseases, we mutated the non-chimeric expansin genes (CmEXLX2 and RsEXLX) of two xylem-inhabiting bacterial pathogens, the Actinobacterium Clavibacter michiganensis ssp. michiganensis (Cmm) and the beta-proteobacterium Ralstonia solanacearum (Rs), respectively. The Cmm Delta CmEXLX2 mutant caused increased symptom development on tomato, which was characterized by more rapid wilting, greater vascular necrosis and abundant atypical lesions on distant petioles. This increased disease severity correlated with larger inplanta populations of the Delta CmEXLX2 mutant, even though the strains grew as well as the wild-type invitro. Similarly, when inoculated onto tomato fruit, Delta CmEXLX2 caused significantly larger lesions with larger necrotic centres. In contrast, the Rs Delta RsEXLX mutant showed reduced virulence on tomato following root inoculation, but not following direct petiole inoculation, suggesting that the RsEXLX expansin contributes to early virulence at the root infection stage. Consistent with this finding, Delta RsEXLX attached to tomato seedling roots better than the wild-type Rs, which may prevent mutants from invading the plant's vasculature. These contrasting results demonstrate the diverse roles of non-chimeric bacterial expansins and highlight their importance in plant-bacterial interactions.
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