Fine-tuned interactions between melatonin (MT) and hormones affected by environmental inputs are crucial for plant growth. Under high light (HL) conditions, melatonin reduced photodamage in Arabidopsis thaliana and contributed to the restoration of the expression of the cytokinin (CK) synthesis genes IPT3, IPT5 and LOG7 and genes for CK signal transduction AHK2,3 and ARR 1, 4, 5 and 12 which were downregulated by stress. However, CK signaling mutants displayed no significant changes in the expression of CK genes following HL + MT treatment, implying that a fully functional cytokinin signaling pathway is a prerequisite for MT–CK interactions. In turn, cytokinin treatment increased the expression of the key melatonin synthesis gene ASMT under both moderate and HL in wild-type plants. This upregulation was further accentuated in the ipt3,5,7 mutant which is highly sensitive to CK. In this mutant, in addition to ASMT, the melatonin synthesis genes SNAT and COMT, as well as the putative signaling genes CAND2 and GPA1, displayed elevated transcript levels. The results of the study suggest that melatonin acts synergistically with CK to cope with HL stress through melatonin-associated activation or repression of the respective hormonal genes.

Cytokinins (CKs) are known to regulate the biogenesis of chloroplasts under changing environmental conditions and at different stages of plant ontogenesis. However, the underlying mechanisms are still poorly understood. Apparently, the mechanisms can be duplicated in several ways, including the influence of nuclear genes that determine the expression of plastome through the two-component CK regulatory circuit. In this study, we evaluated the role of cytokinins and CK signaling pathway on the expression of nuclear genes for plastid RNA polymerase-associated proteins (PAPs). Cytokinin induced the expression of all twelve Arabidopsis thalianaPAP genes irrespective of their functions via canonical CK signaling pathway but this regulation might be indirect taking into consideration their different functions and versatile structure of promoter regions. The disruption of PAP genes contributed to the abolishment of positive CK effect on the accumulation of the chloroplast gene transcripts and transcripts of the nuclear genes for plastid transcription machinery as can be judged from the analysis of pap1 and pap6 mutants. However, the CK regulatory circuit in the mutants remained practically unperturbed. Knock-out of PAP genes resulted in cytokinin overproduction as a consequence of the strong up-regulation of the genes for CK synthesis.

Melatonin is a signaling molecule that mediates multiple stress-dependent reactions. Under photooxidative stress conditions generating intensive ROS production, exogenous melatonin (50 µM) contributed to maintaining the expression of mitochondrial encoded genes and up-regulation of RNA-polymerase genes RPOTm and RPOTmp, operating through the CAND2 receptor and α-subunit of the heterotrimeric G protein GPA1 coupled with CAND2. Unlike wild-type plants, mutants with defective CAND2 and GPA1 genes exhibited no decrease in the alternative pathway of leaf respiration, as well as the activity of an alternative oxidase, and the expression of the AOX1a gene. At the same time, the protective effect of exogenous melatonin on some physiological indicators did not depend on the receptor and was associated with the direct antioxidant function of the regulator. Thus, melatonin under photooxidative stress conditions can act as an antioxidant and as a hormone capable of regulating the expression of nuclear and organelle genes through the components of melatonin signal perception.

The physiological and molecular responses of Arabidopsis thaliana plants to cold stress were studied. Exposure to a low non-freezing temperature (4°C, 5 days) caused a decrease in the physiological functions and activity of a number of photosynthetic genes and elevation in expression of the cold stress gene COR15a, the product of which protects chloroplasts. It was shown for the first time that in parallel to a general inhibition of physiological functions under hypothermia, an increase in the expression of most genes for the chloroplast transcription apparatus was observed. This is obviously one of the compensatory mechanisms of adaptation aimed to maintain cellular homeostasis and physiological functions under hypothermia.

In plants, abscisic acid (ABA) and melatonin (MT) are conventionally treated as molecules mitigating stress responses. To understand the mechanisms of ABA–MT interplay, we examined the effects of ABA and MT treatment in ABA and MT loss-of-function mutants of Arabidopsis thaliana exposed to high light (HL) stress. ABA constantly suppressed ASMT encoding N-acetylserotonin methyltransferase in the context of differential responses of other MT biosynthesis genes in both the wild type (WT) and mutants. However, this response was absent in the mutant with the disrupted ABI4. Given that the ASMT promoter region contains several potential ABI4-binding elements, these data suggest that ASMT can be a potential target gene for ABI4. A role for ABI4 in the interactions between ABA and MT is supported by the finding that ABI4 is constitutively derepressed in the MT signaling mutants cand2 and gpa1, which exhibited elevated steady state levels of ABI4 transcripts and were not regulated by either stress or melatonin. In addition, the abi4 mutant showed increased modulations in the expression of the MT catabolic genes M2H and M3H in response to ABA treatment, inferring that this transcription factor is a negative regulator of ABA-dependent changes in MT content. Furthermore, all tested mutants with impaired ABA synthesis or signaling displayed elevated steady state MT levels compared to WT, while MT treatment contributed to the downregulation of key ABA synthesis and signaling genes. Collectively, our results suggest that ABA and melatonin act antagonistically, modulating the expression of ABA and MT signaling and metabolism genes. To understand the mechanisms of ABA–MT interactions, we studied the effects of ABA and MT treatment in ABA and MT loss-of-function mutants of Arabidopsis thaliana exposed to severe light stress (SLS).