The energy-dissipating systems (EDS) of a photosynthetic cell — the violaxanthin cycle (VXC) in chloroplasts and the alternative respiratory pathway (AP) in mitochondria — are involved in protecting against excess light energy. The mechanisms of functional interaction of EDS are poorly understood. The aim was to study the effect of high light on respiration and AP activity in plants with suppression of encoding the VXC enzyme — violaxanthine de-epoxidase. Four-week-old plants of line and Columbia-0 wild type line (Col-0) grown at 90 mmol/(m s) were exposed to high light, 400 mmol/(m s), for 8 hours. Under stress conditions, the line showed significantly lower values of non-photochemical quenching of chlorophyll fluorescence and the level of de-epoxidation compared to the wild-type line (Col-0), which indicated the absence of zeaxanthin-dependent protection of the photosynthetic apparatus. The plants of the mutant line reacted to high light by increasing respiration due to activation of both alternative and cytochrome pathways. At the same time, the part of AP from total respiration in the line was consistently high and amounted to about 50%, regardless of the light conditions. Activation of AP and accumulation of the alternative oxidase (AOX) protein were obviously facilitated by increased expression of most AOX genes, the level of transcripts of which was higher under control conditions (0 h), but decreased by the end of the experiment. At the same time, the amount of mRNA of the most stress-inducible AOX1a gene was the lowest among all AOX genes. It is assumed that in the line the signaling pathway supported by the transcriptional factor of MYB4, a negative regulator of phenylpropanoid synthesis, is weakened. This could be the reason for the low expression of AOX1a, which contains a large number of MYB4-binding sites in the promoter, and an increased content of anthocyanins in leaves compared to Col-0. Unlike Col-0 plants, the line was characterized by half the activity of superoxide dismutase (SOD), with a predominance of Fe-SOD localized mainly in chloroplasts. However, based on the content of superoxide anion radical and hydrogen peroxide, plants of the line showed a higher level of oxidative reactions in high-light conditions compared with the wild type line. The data obtained showed a reduced ability of plants with suppression of the genes of two key components of energy-dissipating systems ( and ) to withstand stress. The results indicate the mutual regulation of the EDS of mitochondria and chloroplasts to protect against photooxidation, the importance of AOX in the modulation of respiratory function, and the existence of fast adaptive metabolic rearrangements that provide plant viability under stressful conditions.
E.V. Garmash (2025) studied this question.