HPCA1 and MSL3 are required for systemic cell-to-cell calcium signaling in response to light stress. A, Arabidopsis plants were subjected to a HL stress treatment applied to a single leaf (Local; indicated with a circle), and cytosolic calcium accumulation was imaged using Fluo-4-AM in whole plants (local and systemic tissues). Representative time-lapse images of whole-plant cytosolic calcium accumulation in WT, hpca1-1 and hpca1-2 plants are shown alongside bar graphs of combined data from all plants used for the analysis at the 0- and 30-min time points (local and systemic). B, Same as in (A), but for WT, msl3-1 and msl3-2 plants. Compared to WT, the msl3-1 mutant is also deficient in cell-to-cell calcium signaling in response to a local application of H2O2 (Supplementary Figure S1). All experiments were repeated at least three times with 10 plants of each genotype per experiment. Data are presented as box plot graphs; X is mean ± se, N = 30, *P < 0.05, **P < 0.01, Student t test. Scale bar, 1 cm. See Movie 1 for live imaging.