Specialization of independently acquired flagellar FliC proteins in plant-associated
Russ D, Saha C, Paul K, Zheng Z, Law TF
Summary
7.4/10Plant-associated bacteria have evolved two distinct flagellar proteins: one for swimming that avoids plant immune detection, and one for colonization that triggers immunity. Plants can only recognize the protein needed for infection, which limits bacterial entry into internal tissues.
Key Findings
Sphingomonas use two independently acquired flagellin genes (FliC-H immunogenic and FliC-L nonimmunogenic) that partition motility and colonization functions
FliC-L is necessary for directional swimming while FliC-H is sufficient for attachment and tissue colonization
Plant immune receptor FLS2 recognizes the colonization variant (FliC-H) rather than the motility variant, potentially restricting bacterial penetration into internal leaf and root tissues
Original Abstract
Plants monitor their environment for microbial invaders using pattern-recognition receptors that detect microbe-associated molecular patterns (MAMPs). Flagellin, the main component of bacterial flagellum, contains the flg22 epitope recognized by the plant immune receptor FLS2. Immune recognition can create an evolutionary conflict, requiring bacteria to balance flagellar function and immune evasion. Here, we show that plant-associated Sphingomonas resolve this constraint by partitioning two flagellar functions, motility and colonization, across two divergent and independently acquired flagellin genes. Comparative genomics revealed widespread coexistence of FliC proteins expressing either an immunogenic variant (FliC-H) or a nonimmunogenic variant (FliC-L). The nonimmunogenic FliC-L is necessary and sufficient for full directional swimming, whereas FliC-H is dispensable for swimming, but sufficient for full attachment and colonization. Flagellin expression patterns mirror these functions. Thus, FLS2 recognizes the flagellar variant required for colonization rather than motility, potentially restricting colonizing bacteria from entering internal leaf and root tissues.