Prior exposure to lipopolysaccharide potentiates expression of plant defenses in response to bacteria

Lipopolysaccharide (LPS) is a ubiquitous component of Gram-negative bacteria which has a number of diverse biological effects on eukaryotic cells. In contrast to the large body of work in mammalian and insect cells, the effects of LPS on plant cells have received little attention. LPS can induce defense-related responses in plants, but in many cases these direct effects are weak. Here we have examined the effects of prior inoculation of LPS on the induction of plant defense-related responses by phytopathogenic xanthomonads in leaves of pepper (Capsicum annuum). The resistance of pepper to incompatible strains of Xanthomonas axonopodis pv. vesicatoria or to X. campestris pv. campestris is associated with increased synthesis of the hydroxycinnamoyl-tyramine conjugates, feruloyl-tyramine (FT) and coumaroyl-tyramine (CT). FT and CT are produced only in trace amounts in response to compatible strains of X axonopodis pv. vesicatoria. Treatment of leaves with LPS from a number of bacteria did not induce the synthesis of FT and CT but altered the kinetics of induction upon subsequent bacterial inoculation. In incompatible interactions FT and CT synthesis was accelerated, whereas in compatible interactions synthesis was also considerably enhanced. The ability of the tissue to respond more rapidly was induced within 4 h of LPS treatment and the potentiated state was maintained for at least 38 h. Earlier treatment with LPS also potentiated the expression of other defense responses such as transcription of genes encoding acidic beta-1,3-glucanase. Our findings indicate a wider role for LPS in plant-bacterial interactions beyond its limited activity as a direct inducer of plant defenses.