Abstract
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 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 inplanta populations of the ΔCmEXLX2 mutant, even though the strains grew as well as the wild-type invitro. Similarly, when inoculated onto tomato fruit, ΔCmEXLX2 caused significantly larger lesions with larger necrotic centres. In contrast, the Rs ΔRsEXLX 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, ΔRsEXLX 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.
Highlights
Plant primary cell walls are highly dynamic structures that fluctuate between rigid and relaxed states, enabling basic biological processes, such as growth, enlargement and cell division (Cosgrove, 1993, 2005)
We used a genetic approach to investigate the roles of nonchimeric plant-like bacterial expansins in the pathogenesis of two vascular plant-pathogenic bacteria
Gene nomenclature for bacterial expansins was based on the current standard for microbial expansins (Georgelis et al, 2015; Kende et al, 2004)
Summary
Plant primary cell walls are highly dynamic structures that fluctuate between rigid and relaxed states, enabling basic biological processes, such as growth, enlargement and cell division (Cosgrove, 1993, 2005). Plant expansins loosen the rigid carbohydrate matrix of the cell wall through an uncharacterized non-lytic slippage mechanism (Cosgrove, 2000). This key biological function may explain why expansins are conserved in all vascular plants, with isoforms regulating the cell wall changes required for cellular growth, vascular differentiation, fruit ripening, seed germination, abscission and leaf development (Cho and Cosgrove, 2000; Im et al, 2000; Kende et al, 2004; Rose et al, 1997).
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