Nod factors and chitooligomers elicit an increase in cytosolic calcium in aequorin-expressing soybean cells

Rhizobial Nod factors (NFs) function as nodulation signals that trigger symbiotic responses of leguminous host plants. NFs consist of a chitin oligomer backbone carrying a fatty acid at the non-reducing end. Depending on the rhizobial strain, NFs carry additional substituents, which may determine host specificity. Transgenic suspension-cultured soybean (Glycine max [L.] Merr.) cells expressing aequorin have been used to record cytosolic [Ca2+] changes upon treatment with purified NFs and chitin fragments. Both compounds elicited an increase of cytosolic [Ca2+] at nanomolar concentrations. The shape and amplitude of cytosolic [Ca2+] changes was similar to the response elicited by un-derivatized chitin oligomers. Cells challenged first with NFs did not respond to a subsequent treatment with chitin oligomers and vice versa. Dose-response experiments showed that un-derivatized chitin oligomers were more active compared with NFs. The capacity of NFs to elicit the calcium response depended on their structure. The presence of reducing end substituents in methylfucosylated NFs from Rhizobium sp. NGR234 and the O-acetyl group at the non-reducing end in NFs from Sinorhizobium meliloti attenuated the activity to cause the calcium changes. The sulfate group in NFs from Rhizobium tropici did not affect the elicitor activity. Pentameric S. meliloti NFs were more active than tetrameric molecules, whereas trimeric or dimeric degradation products were inactive. Substituents in NFs may have the function to avoid stimulation of defense reactions mediated by the perception system for chitin oligomers.