Effects of Melatonin and Intense Light on Respiration and Mitochondrial Gene Expression in Leaves of Wild-Type and Brassinosteroid Mutants of Arabidopsis thaliana

The effect of photooxidative stress and melatonin on the activity of the alternative (AP) and cytochrome (CP) pathways of mitochondrial respiration and on the expression levels of the corresponding genes was studied in the leaves of wild-type (WT) Arabidopsis thaliana and mutants in the genes for metabolism (det2) and reception (bri1-6) of brassinosteroids (BS). Three-day incubation of control WT plants in a medium with melatonin (50 μM) increased the intensity of leaf respiration mainly due to the activation of CP (by 96%) and to a lesser extent (by 18%) due to AP. Under the conditions of this experiment, it was shown that exposure of control plants to intense light for 24 h, which caused photooxidative stress, always significantly increased leaf respiration, not only due to an increase in the AP power, but also in the CP activity. Increase in the AP capacity of respiration positively correlated with an increase in the content of transcripts of the AOX1a and AOX1d genes encoding AOX. At the same time, the expression of the studied CP genes (cob, cox1) was inhibited under stress conditions. The use of mutants in the genes of brassinosteroid synthesis and signaling revealed a direct dependence of the expression of the AOX1a gene and the activity of AP on the level of endogenous BS and exogenous melatonin, which suppresses this process. It was also found that changes in the BS level in mutants lead to significant changes in the expression of a number of genes. For example, the AOX1d gene, a characteristic feature of which in wild-type plants was stress-induced activation and melatonin-induced inhibition of its expression, acquired opposite properties with an increase in the BS level and a supposed switching of signaling pathways to receptors BRL1 and BRL3 in the bri1-6 mutant. Analysis of the obtained results and available literature data allows us to make a preliminary conclusion about the close interaction of phytomelatonin and BS under conditions of photooxidative stress to optimize the intensity of respiration and the activity of various mitochondrial oxidative pathways.