Ultrastructural changes in in vitro callus plant cells producing secondary metabolites

The objective of our research was to analyze the ultrastructure of the callus cells of Leontopodium alpinum (Edelweiss), Fragaria x ananassa (strawberry) and Cotinus coggygria (smokebush) producing large amounts of secondary metabolites. Unlike plants, the in vitro callus has the peculiarity of proliferating (meristem-like) and at the same time synthesizing tissue-specific compounds with physiological significance in different stages of development. The cell population of the callus consists of two types of cells: those which divide and those which are producing the secondary metabolites, in different ratios, depending on the signals from the culture environment. Our TEM observations showed that in the metabolic active callus cells a complex network of membrane-bound organelles are involved in the synthesis, sorting, transport, degradation, storing or exocytosis of secondary metabolites such as anthocyanin pigments, flavonoids, polyphenols, tannins, oils, polysaccharides, alkaloids, many of them with antioxidant activity and with potential biotechnological value. Our results indicate that in the experimental conditions of the in vitro culture, callus cells have developed a complex endomembrane system that includes endoplasmic reticulum, Golgi apparatus, trans-Golgi network, multivesicular bodies, multilamellar bodies, membranous vesicles and vacuoles (vacuom) as an end point of the catabolic pathway. In the biosynthetic active callus cells, the entire cellular machinery is subordinated to the production of secondary metabolites, evidencing phenomena of autophagy, through which different useful cellular components are recycled. Finally, the secondary metabolites are irreversibly stored in large amounts in the vacuoles, at the tonoplast level, some of them into the cytoplasm, into the modified plastids or on the external face of the cell wall. In the intercellular space we noticed excreted vesicles, or most often, a reticulated extracellular matrix, probably involved in cell protection, recognition and integration.