Characteristic and differential calcium signals from cell structures of the root cap detected by energy-filtering electron microscopy (EELS/ESI).

Characteristic calcium signals were analyzed in structures of three cell types of the root cap of cress: statocytes, meristematic cells and secretion cells. Twenty-four hour-old roots were fixed with glutaraldehyde (postfixed with osmiate/potassium bichromate) or with potassium permanganate. No visible precipitates were formed, but calcium signals typical for different cell structures could be detected by means of energy-filtering transmission electron microscopy (EELS/ESI). In statocytes, calcium signals were recorded from the plasma membrane, the membranes of the endoplasmic reticulum, the amyloplast envelope and the mitochondrial membranes. In contrast to the excitable statocytes, the two other, non-excitable cell types, meristematic and secretion cells, exhibited much lower intensities of the calcium signals recorded from the same membrane systems. The revealed characteristic calcium-related properties of the different membrane systems may be related to the special function of statocytes, namely transduction of the gravity stimulus. In all three cell types, additional calcium signals were recorded from cell structures with well known calcium contents, i.e., mitochondrial granules, starch grains and cell walls. For the first time, clear calcium signals were detected from the lipid bodies which are mobilized during the developmental stage of the examined roots. It is supposed that free fatty acids and lipases are the binding sites for calcium. The reliability of the applied method is especially proven by comparison of the electron microscopic images from lipid bodies according to the two different fixation methods. After glutaraldehyde fixation followed by osmiate/potassium bichromate postfixation, the lipid bodies were well fixed and appeared homogeneously grey with homogeneous calcium signals. However, due to potassium permanganate fixation the lipid bodies were only partly fixed; they had an electron-lucid core, from which the lipids are lost during the dehydration procedure, without calcium signals and an electron-dense border zone, which is a reaction product of potassium permanganate with triacylglycerols, with calcium signals.