The induction and modulation of plant defense responses by bacterial lipopolysaccharides

Lipopolysaccharides (LPSs) are ubiquitous, indispensable components of the cell surface of Gram-negative bacteria that apparently have diverse roles i n bacterial pathogenesis of plants. As an outer membrane component, LPS may contribute to the exclusion of plant-derived antimicrobial compounds promo ting the ability of a bacterial plant pathogen to infect plants. In contras t, LPS can be recognized by plants to directly trigger some plant defense-r elated responses. LPS can also alter the response of plants to subsequent b acterial inoculation; these delayed effects include alterations in the expr ession patterns of genes coding for some pathogenesis-related (PR) proteins , promotion of the synthesis of antimicrobial hydroxycinnamoyl-tyramine con jugates, and prevention of the hypersensitive reaction caused by avirulent bacteria. Prevention of the response may allow expression of resistance in the absence of catastrophic tissue damage. Recognition of LPS (and other no nspecific determinants) may initiate responses in plants that restrict the growth of nonpathogenic bacteria, whereas plant pathogens may possess hrp g ene-dependent mechanisms to suppress such responses.