Transgenic aequorin monitors cytosolic calcium transients in soybean cellschallenged with beta-glucan or chitin elicitors

Transgenic soybean (Glycine max L.) cells expressing aequorin were used to monitor changes in cytosolic Ca2+ concentrations in response to treatment w ith fungal elicitors. After an apparent lag phase of about 60 s, both chiti n fragments and beta-glucan elicitors caused a rapid increase in cytosolic Ca concentration, which peaked within 2-2.5 min of treatment. The Ca2+ conc entration then decreased and reached the basal level after about 5 min in t he case of the treatment with chitin fragments, while a second rise in the Ca2+ concentration with a maximum occurring after about 7-8 min was observe d in the case of beta-glucan treatment. Calibration of the signals showed t hat the elicitors enhanced the cytosolic Ca2+ concentration from resting co ncentrations as low as 0.1 mu M to highest levels of about 2 mu M. Dose-res ponse experiments showed that the concentration of elicitors giving a Ca2response at the 50% level was 0.4 nM for the chitin fragment and 28 mu M an d 72 mu M, respectively, for a synthetic hepta-beta-glucoside and a fungal beta-glucan fraction. The beta-glucan- or N,N'.N ",N"'-tetraacetyl chitotet ratose (CH4)-induced Ca2+ signals were inhibited by both the Ca2+ chelator 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) and by t he Ca2+-channel inhibitor La3+. Neomycin, whose target in plant cells has n ot yet been clearly identified, reduced predominantly the expression of the second peak of the biphasic Ca2+ curve following beta-glucan treatment. Ba cterial cyclic beta-glucans known to elicitor-sensing mechanism of soybean cells, and is probably connected with the subsequent activation of defence responses.