Effect of induced changes in membrane permeability on the defence response of Chlorella vulgaris to infection by Acholeplasma laidlawii

The defence response of photosynthesising cells to the stress of infection by mycoplasma was investigated using an algae model of Chlorella vulgaris exposed to the Mollicute, Acholeplasma laidlawii. The first line of the defence is at the interface between the pathogen and its host is at the cell wall and its subjacent plasma membrane. Thus, one of the important non-specific changes in cells on pathogen stress is the change of membrane permeability that in turn disturbs ion homeostasis. For a more complete understanding of the mechanisms of the cell response to the infection, three known effectors of ion permeability in the plant cell plasma membrane were used, namely Gd3+, RNase, and ATP. Gd3+ is a Ca2+-channel inhibitor and possibly affects other ion channels in the plasma membrane. It is thought that the action of RNase may be through low molecular fragments formed by the hydrolysis of high molecular RNA degradation. ATP in the living cell not only has an energetic role but also takes part directly, for instance through cyclic AMP, and/or indirectly in intracellular metabolic signalling, resulting in the activation of the reactions involved in phosphorylation, catalyzed by the membrane kinases. It is considered that changes in the distribution of ions between the cytoplasm and the external environment alter the energy metabolism of the cells because ion homeostasis is an active energy-requiring process, which is tightly related to the intensity of its defence reactions. The results showed that the changes in the energy metabolism of the cells depend on the requirements of them for coping with changes of the environment. Thus, the separate application of Gd3+, RNase and ATP had an insignificant effect on the rates of heat production and oxygen consumption in the dark, photosynthesis measured as oxygen evolution, generation of superoxide on the Chlorella cells, grown under the optimal condition. On the other hand, the rate of the energetic processes significantly increased when infected Chlorella were challenged separately by the three compounds. It was concluded that an additional expenditure of metabolic energy necessary for the defence of microalgal cells in stress conditions