Signalling events elicited in plants by defined oligosaccharide structures.

Developments during the past year have confirmed that several classes of oligosaccharides are able to activate the plant cell machinery, leading either to defence reactions or to plant developmental processes. Both fungal and plant cell walls contain molecules that elicit plant defence reactions; however, most of the studies focus on the activities induced by lipochito-oligomers (LCOs, or Nod factors) produced by bacteria which trigger plant infection and nodule formation (organogenesis). LCOs can be described as growth regulators of plants in general as they also induce protoplast cell divisions of tobacco plant (a nonlegume) at femtomolar concentrations. Recognition of the appropriate symbiotic bacteria by legumes is mediated by the structure of Nod factors, but the structural determinants involved in this recognition process are not always clearly understood. Although specific substitutions of the oligochitin backbone by several chemical groups are involved, it seems that some host range variations of the bacteria can only be explained by small variations in the hydrophobic balance between both ends of the molecule.