3 years ago

Plant-like bacterial expansins play contrasting roles in two tomato vascular pathogens

Tuan M. Tran, Tiffany M. Lowe-Power, Matthew A. Tancos, Christine D. Smart, Caitilyn Allen, F. Christopher Peritore-Galve
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 subsp. michiganensis (Cmm) and the β-proteobacterium Ralstonia solanacearum (Rs), respectively. The Cmm ΔCmEXLX2 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 in planta populations of the ΔCmEXLX2 mutant even though the strains grew equally well as wildtype in vitro. Similarly, when inoculated onto tomato fruit, ΔCmEXLX2 caused significantly larger lesions with larger necrotic centers. In contrast, the Rs ΔRsEXLX mutant had 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, ΔRsEXLX attached to tomato seedling roots better than wildtype 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. This article is protected by copyright. All rights reserved.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/mpp.12611

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