Sensor based recording of cell-specific calcium changes
To monitor [Ca2+]cyto during the whole double fertilization process, we expressed the Ca2+ sensor CerTN-L15 (ref. 31) under control of cell-type-specific promoters in synergid cells (MYB98-promoter; AT4G18770 (ref. 33)), egg cell (EC1.1 promoter; AT1G76750 (ref. 12)) and the central cell (DD65-promoter; AT3G10890 (ref. 34)) as indicated (Fig. 1a). This sensor is based on the Ca2+−binding domain of troponin C and two spectrally overlapping fluorescent molecules cerulean and citrine that act as donor and acceptor for FRET, respectively. As the CerTN-L15 sensor has not been used in plants before, we first validated its functionality in roots expressing the sensor constitutively under control of a 35S promoter, using a microfluidic device that allows imaging under controlled perfusion of roots35. Repeated pulsed treatments with a hyperosmotic solution resulted in [Ca2+]cyto signatures with a minor increase in FRET ratio during the treatment-induced root shrinkage and a steep incline of the FRET ratio after wash-out and return to original root length, followed by a gradual return of the FRET ratio to the baseline (Supplementary Fig. 1). This characteristic signature has been observed before using Yellow Cameleon sensors29, 36 indicating that CerTN-L15 is functional in plants.