Suppression of an elicitor-induced oxidative burst reaction in Medicago sativa cell cultures by Sinorhizobium meliloti lipopolysaccharides

The biological activity of lipopolysaccharides (LPS) from the symbiotic soi l bacterium Sinorhizobium meliloti was analysed in cell cultures of the hos t plant Medicago sativa (alfalfa) and the nonhost plant Nicotiana tabacum ( tobacco). LPS of S. meliloti were purified and chemically characterized. Alfalfa and tobacco suspension cell cultures responded to yeast elicitors with an alkal inization of the culture medium and the induction of an oxidative burst. Th is assay was used to study the biological activity of isolated LPS. In alfalfa cell cultures the simultaneous addition of purified LPS of S. me liloti suppressed the elicitor induced alkalinization and oxidative burst r eaction. Cell cultures of the nonhost tobacco reacted differently to the ap plication of S. meliloti LIPS. In these cell cultures, the S. meliloti LIPS itself caused an alkalinization of the culture medium and an oxidative bur st reaction. S. meliloti LPS released from the bacterial surface might function as a spe cific signal molecule, promoting the symbiotic interaction and suppressing a pathogenic response in the host plant, alfalfa. (C) New Phytologist (2001 ).