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.

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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.

Abbreviations: Fv/Fm -maximum quantum yield of PSII; HL -high light; MS -Murashige and Skoog medium; NEP -single-subunit nuclear-encoded RNA polymerase of the phage-type; NPQ -nonphotochemical quenching; PAM -pulse amplitude modulation; PEP -bacterial type plastid-encoded RNA polymerase; RT-qPCR -quantitative Real-Time PCR; WT -wild type; ФPSII -effective quantum yield of PSII.

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