Previously, effect of A1 ions on calcium signaling was assessed in tobacco cells expressing a Ca2+ -monitoring luminescent protein, aequorin and a newly isolated putative plant Ca2+ channel protein from Arabidopsis thaliana. AtTPC1 (two-pore channel 1). TPC1 channels were shown to be the only channel known to be sensitive to At and they are responsive to reactive oxygen species and cryptogein, a fungal elicitor protein. Thus, involvement of TPC1 channels in calcium signaling leading to development of plant defense mechanism has been suggested. Then, the use of At as a specific inhibitor of TPC1-type plant calcium channels has been proposed. Here, using transgenic tobacco BY-2 cells expressing aequorin, we report on the evidence in support of the involvement of A1-sensitive signaling pathway requiring TPC1-type channel-dependent Ca2+ influx in response to salicylic acid, a key plant defense-inducing agent, but not to an elicitor prepared from the cell wall of rice blast disease fungus Magnaporthe grisea. In addition, involvement of A1-sensitive Ca2+ channels in response to cold shock was also tested. The data suggested that the elicitor used here induces the Ca2+ influx via A1-insensitive path, while salicylic acid and cold-shock-stimulate the influx of Ca2+ via At-sensitive mechanism. (c) 2005 Elsevier Inc. All rights reserved.