add GUS histochemical staining assay

assays/GUSHistochemicalStaining/protocols/GUSHistochemicalStainingProtocol.md

+## GUS histochemical staining
+
+Three-week-old plants were carefully removed from ArtSoil and rinsed with 70% ethanol. Each plant was inserted into one well in a 12-well plate before 1 mL of GUS staining solution (10 mM EDTA, 50 mM phosphate buffer, 0.1% Triton X-100, 1 mM potassium ferrocyanide, 1 mM potassium ferricyanide, 2 mM X-Gluc, 20% methanol) was added into each well. The plate was subsequently incubated in the dark at 37 °C for 1 h (SWEET1, 2, 17) or 4 h (SWEET4, 7, 8, 11, 12, 13). Prior to imaging, plants were removed from GUS staining buffer and rinsed with 70% ethanol.
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