Live Cell Imaging with R-GECO1 Sheds Light on flg22-and Chitin-Induced Transient [Ca2+]cyt Patterns in Arabidopsis

Intracellular Ca2+ transients are an integral part of the signaling cascade during pathogen-associated molecular pattern (PAMP)-triggered immunity in plants. Yet, our knowledge about the spatial distribution of PAMP-induced Ca2+ signals is limited. Investigation of cell-and tissue-specific properties of Ca2+ dependent signaling processes requires versatile Ca2+ reporters that are able to extract spatial information from cellular and subcellular structures, as well as from whole tissues over time periods from seconds to hours. Fluorescence-based reporters cover both a broad spatial and temporal range, which makes them ideally suited to study Ca2+ signaling in living cells. In this study, we compared two fluorescence-based Ca2+ sensors: the Forster resonance energy transfer (FRET)-based reporter yellow cameleon NES-YC3.6 and the intensity-based sensor R-GECO1. We demonstrate that R-GECO1 exhibits a significantly increased signal change compared with ratiometric NES-YC3.6 in response to several stimuli. Due to its superior sensitivity, R-GECO1 is able to report flg22- and chitin-induced Ca2+ signals on a cellular scale, which allowed identification of defined [Ca2+](cyt) oscillations in epidermal and guard cells in response to the fungal elicitor chitin. Moreover, we discovered that flg22- and chitin-induced Ca2+ signals in the root initiate from the elongation zone.