Rice Leaf Transcriptional Profiling Suggests a Functional Interplay Between Xanthomonas oryzae pv. oryzae Lipopolysaccharide and Extracellular Polysaccharide in Modulation of Defense Responses During Infection

Treatment of rice leaves with isolated Xanthomonas oryzae pv. oryzae lipopolysaccharide (LPS) induces the production of callose deposits, reactive oxygen species, and enhanced resistance against subsequent bacterial infection. Expression profiling of X. oryzae pv. oryzae LPS-treated rice (Oryza sativa subsp. indica) leaves showed that genes involved in the biosynthetic pathways for lig-nins, phenylpropanoids, chorismate, phenylalanine, salicylic acid, and ethylene, as well as a number of pathogenesis-related pro-teins are up-regulated. Gene ontology categories like cell-wall organization, defense response, stress response, and protein phosphorylation/kinases were found to be upregulated, while genes involved in photosynthesis were down-regulated. Coinfil-tration with xanthan gum, the xanthomonas extracellular poly-saccharide (EPS), suppressed LPS-induced callose deposition. Gene expression analysis of rice leaves that are treated with an EPS-deficient mutant of X. oiyzae pv. oryzae indicated that a number of defense-regulated functions are up-regulated during infection. These transcriptional responses are attenuated in rice leaves treated with an EPS-deficient mutant that is also deficient in the 0-antigen component of LPS. Overall, these results suggest that the 0-antigen component of X. oryzae pv. oryzae LPS induces rice defense responses during infection and that these are sup-pressed by bacterial EPS.