Aequorin-based luminescence imaging reveals differential calcium signalling responses to salt and reactive oxygen species in rice roots

It is well established that both salt and reactive oxygen species (ROS) stresses are able to increase the concentration of cytosolic free Ca(2+) ([Ca(2+)](i)), which is caused by the flux of calcium (Ca(2+)). However, the differences between these two processes are largely unknown. Here, we introduced recombinant aequorin into rice (Oryza sativa) and examined the change in [Ca(2+)](i) in response to salt and ROS stresses. The transgenic rice harbouring aequorin showed strong luminescence in roots when treated with exogenous Ca(2+). Considering the histological differences in roots between rice and Arabidopsis, we reappraised the discharging solution, and suggested that the percentage of ethanol should be 25%. Different concentrations of NaCl induced immediate [Ca(2+)](i) spikes with the same durations and phases. In contrast, H(2)O(2) induced delayed [Ca(2+)](i) spikes with different peaks according to the concentrations of H(2)O(2). According to the Ca(2+) inhibitor research, we also showed that the sources of Ca(2+) induced by NaCl and H(2)O(2) are different. Furthermore, we evaluated the contribution of [Ca(2+)](i) responses in the NaCl- and H(2)O(2)-induced gene expressions respectively, and present a Ca(2+)- and H(2)O(2)-mediated molecular signalling model for the initial response to NaCl in rice.