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 sym biotic responses of leguminous host plants. NFs consist of a chitin oligome r backbone carrying a fatty acid at the non-reducing end. Depending on the rhizobial strain, NFs carry additional substituents, which may determine ho st specificity. Transgenic suspension-cultured soybean (Glycine max [L.] Me rr.) cells expressing aequorin have been used to record cytosolic [Ca2+] ch anges 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 N Fs did not respond to a subsequent treatment with chitin oligomers and vice versa. Dose-response experiments showed that un-derivatized chitin oligome rs were more active compared with NFs. The capacity of NFs to elicit the ca lcium response depended on their structure. The presence of reducing end su bstituents in methylfucosylated NFs from Rhizobium sp. NGR234 and the O-ace tyl group at the non-reducing end in NFs from Sinorhizobium meliloti attenu ated the activity to cause the calcium changes. The sulfate group in NFs fr om Rhizobium tropici did not affect the elicitor activity. Pentameric S. me liloti NFs were more active than tetrameric molecules, whereas trimeric or dimeric degradation products were inactive. Substituents in NFs may have th e function to avoid stimulation of defense reactions mediated by the percep tion system for chitin oligomers.