Callose deposition was monitored by potassium hydroxide-aniline blue staining. Briefly, the materials were vacuum infiltrated in 1 M potassium hydroxide for 2 h at room temperature, followed by staining with 0.05% (w/v) aniline blue in water. The stained materials were placed in glass slides and examined by means of epifluorescence microscopy (Leica DMRBE, Switzerland).