Aquorin Bioluminescence-Based Ca2+ Imaging Reveals Differential Calcium Signaling Responses to Abiotic Stresses in Physcomitrella patens.

Calcium ions (Ca2+) are an important secondary messenger in plant signal transduction networks. The cytosolic free Ca2+ concentration ([Ca2+]i) of plants changes rapidly when they are subjected to different abiotic stresses, which drives calcium signaling. Although this process has been extensively studied in spermatophytes, the details of calcium signaling in bryophytes remains largely unknown. In our study, we reconstituted aequorin in the bryophyte Physcomitrella patens, optimized the percentage of ethanol in the Ca2+ discharging solution, and measured the [Ca2+]i changes induced by different stresses. In addition, we observed that the sources of Ca2+ accessed following exposure to cold, drought, salt, and oxidative stress were different. Furthermore, we showed that long-term saline environments could suppress the basal [Ca2+]i of P. patens, and the peak value of [Ca2+]i induced by different stresses was lower than that of plants growing in non-stressed environments. This is the first systematic study of calcium signaling in bryophytes, and we provided an efficient and convenient tool to study calcium signaling in response to different abiotic stresses in bryophytes.