Establishment of a Transient Transformation Protocol in Cinnamomum camphora

Cinnamomum camphora is an excellent evergreen broad-leaved tree species with strong stress tolerance, but its molecular character revelation as well as ecological and economic value improvement were limited due to the lack of a genetic transformation system. To establish a simple and efficient transient transformation system for uncovering the molecular mechanism of plant tolerating stresses and promoting the selective breeding of good varieties, the infection method, co-cultivation time, infection solution concentration, and growth density of Agrobacterium tumefaciens containing green fluorescent protein (GFP)-based calmodulin protein 3 gene (GCaMP3) were identified by monitoring the fluorescence emitted from GCaMP3 bound to Ca2+. Meanwhile, the transient transformation effects were evaluated via cytoplasmic Ca2+ concentration variations at high temperatures of 35 °C and 40 °C. When C. camphora leaves were infected with A. tumefaciens containing GCaMP3 via injection and soaking, no significant difference was detected in the fluorescence intensity over 48 h, indicating that the two infection methods had the same transient transformation efficiency. By prolonging the co-cultivation time, the fluorescence intensity gradually increased, reached its strongest at the 48th h, and then gradually declined. For the infection solution concentration, an OD600 of 0.7 led to the strongest fluorescence intensity, with an increase of 42.2%, 13.7%, 4.2%, and 14.2%, respectively, compared to that at OD600 of 0.5, 0.6, 0.8, and 0.9. When A. tumefaciens growth density OD600 was 0.5–0.7, the strongest fluorescence intensity was detected after transient transformation. Combining these optimum conditions, GCaMP3 was transferred into C. camphora, which indicated the variations in cytoplasmic Ca2+ concentration at high temperatures, with the fluorescence intensity at 35 °C and 40 °C increasing by 12.6% and 30.6%, respectively, in contrast to that at 28 °C. Therefore, it should be an efficient transient transformation system for C. camphora, with A. tumefaciens growth density OD600 of 0.5–0.7, infection solution concentration OD600 of 0.7, and co-cultivation time of 48 h by using both injection and soak infection methods, which is beneficial for uncovering the Ca2+ signal transduction in the plant tolerating stresses and promoting its molecular biology development and selective breeding of good varieties.