Programmed Cell Death in Plants: Ultrastructural Changes in Pea Guard Cells

Abstract

Treatment with cyanide of epidermal peels isolated from pea leaves resulted in destruction of nuclei in the guard cells of stomata, which is visible with a light microscope. The process was accelerated by illumination. Electron microscopy revealed significant CN⁻ induced changes in the ultrastructure of guard cells, which increased with time. Margination of chromatin, which is one of the first signs of apoptosis, was observed in the guard cells even after 1 h incubation of the isolated epidermis with CN⁻. Subsequent chromatin condensation, swelling of the endoplasmic reticulum with formation of large tanks covered with ribosomes, changes in the structure of dictyosomes, and a slight swelling of mitochondria were observed after 3 h of the epidermis incubation with CN⁻. After 6 h of incubation with CN⁻, the bulk volume of the guard cells was filled with vacuoles, the cytoplasm occupied the thin marginal layer, the nucleus was in the center similarly to the control experiment, but it was polylobal, extended in narrow cytoplasmic bands, and, despite the loss of the nuclear envelope integrity, appeared to be a self-dependent structure. In the envelope-free open regions of the nucleus, mitochondria and chloroplasts directly contacted with chromatin. Much like the cell nucleus, chloroplasts lost the integrity of the membrane, but did not swell and retained the stroma and integrity of the thylakoid system. An antioxidant di-tert-butyl-4 hydroxytoluene prevented ultrastructural changes in the cells observed after 6 h of incubation with CN⁻. Thus, the CN⁻ induced death of the guard cells of stomata occurs through the mechanism of apoptosis.