Nodule-inducing activity of synthetic Sinorhizobium meliloti nodulation factors and related lipo-chitooligosaccharides on alfalfa. Importance of the acyl chain structure

Sinorhizobium meliloti nodulation factors (NFs) elicit a number of symbioti c responses in alfalfa (Medicago sativa) roots. Using a semiquantitative no dulation assay, we have shown that chemically synthesized NFs trigger nodul e formation in the same range of concentrations (down to 10(-10) M) as natu ral NFs. The absence of O-sulfate or O-acetate substitutions resulted in a decrease in morphogenic activity of more than 100-fold and approximately 10 -fold, respectively. To address the question of the influence of the struct ure of the N-acyl chain, we synthesized a series of sulfated tetrameric lip o-chitooligosaccharides (LCOs) having fatty acids of different lengths and with unsaturations either conjugated to the carbonyl group (2E) or located in the middle of the chain (9Z). A nonacylated, sulfated chitin tetramer wa s unable to elicit nodule formation. Acylation with short (C8) chains rende red the LCO active at 10(-7) M. The optimal chain length was C16, with the C16-LCO being more than 10-fold more active than the C12- and C18-LCOs. Uns aturations were important, and the diunsaturated 2E,9Z LCO was more active than the monounsaturated LCOs. We discuss different hypotheses for the role of the acyl chain in NF perception.