Cytokinin membrane receptors of the Arabidopsis thaliana AHK2 and AHK3 play opposite roles in the expression of plastid genes and genes for the plastid transcriptional machinery during leaf senescence Loss-of-function mutants of Arabidopsis thaliana were used to study the role of cytokinin receptors in the expression of chloroplast genes during leaf senescence. Accumulation of transcripts of several plastid-encoded genes is dependent on the АНК2/АНК3 receptor combination. АНК2 is particularly important at the final stage of plant development and, unlike АНК3, a positive regulator of leaf senescence. Cytokinin-dependent up-regulation of the nuclear encoded genes for chloroplast RNA polymerases RPOTp and RPOTmp suggests that the hormone controls plastid gene expression, at least in part, via the expression of nuclear genes for the plastid transcription machinery. This is further supported by cytokinin dependent regulation of genes for the nuclear encoded plastid σ-factors, SIG1-6, which code for components of the transcriptional apparatus in chloroplasts.
De-etiolation or transition from etiolated growth (skotomorphogenesis) to photomorphogenesis is one of the most intriguing and intricate stages of plant ontogenesis. It comprises reprogramming of plant cell metabolism, reorganizing the operation of the hormonal system, and altering plant morphology. Dark growth in the soil mainly depends on phytohormones with gibberellins and brassinosteroids playing the leading role; on the soil surface, light as a major exogenous agent starts operating. It inhibits activity of the main repressor of photomorphogenesis (COP1) and regulators of transcription, which govern realization of gibberellin (DELLA) and brassinosteroid (BZR1/BES1) signals and activates trans-factors initiating transition to autotrophic nutrition (for instance, HY5). The strategy of etiolated growth consists in achieving a quick exposure to sunlight at the expense of active elongation of the stem. For transition to autotrophic nutrition, a plant must form a photosynthetic apparatus and protect itself from possible light injury. This review deals with the role of the main regulatory components ensuring etiolated growth and transition to photomorphogenic development.
As multifunctional regulators of physiological processes, phytohormones play an important role in the regulation of expression of the plastid genome and chloroplast biogenesis. Hormones can directly regulate the expression of genes localized in the chloroplast genome. However, many components of the plastid transcription apparatus are encoded by nuclear rather than plastid genes. It remains obscure whether these nuclear genes are subject to hormonal regulation. This is the first study to show that phytohormones exert differential effects on the expression of nuclear genes of the transcription machinery of the Arabidopsis thaliana plastome. RT-PCR analysis showed that the level of transcripts of the majority of studied genes was activated by trans-zeatin but decreased under the influence of ABA, methyl jasmonate, and salicylic acid, whereas ethylene had no significant effect, and the effects of brassinolide depended on the illumination conditions. The results of this study indicate that the hormonal regulation of the plastome expression can be mediated by differential regulation of the nuclear genes encoding plastid transcription machinery components.
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