INTRODUCTION

Phytohormones play an important role in the regulation of chloroplast biogenesis and their adaptation to changing environmental conditions at different stages of ontogenesis. However, the pathways of perceiving the hormonal signal in these organelles are still poorly understood and, apparently, can be duplicated by several mechanisms. Along with the direct binding to cis-elements in the promoter zones of plastid genes of hormone-regulated trans-factors, the action of the hormones can be mediated through nuclear genes that determine the expression of plastome genes. Of the approximately 3000–4000 chloroplast proteins, about 98% are encoded by the nuclear genome, and it is they that determine the type of plastids and their metabolic activity, the stages of plastid genome expression, and the system of protein import into chloroplasts. This suggests that phytohormones, modulating the nuclear signal, affect chloroplast biogenesis. The possible validity of this statement is indirectly confirmed by the fact that anterograde signaling is the leading one in the regulation of chloroplast biogenesis. In addition, not a single receptor for any phytohormone has been found so far in chloroplasts despite the tremendous advances in the study of the mechanisms of phytohormone action.

It is well known that two phytohormones, cytokinins and ABA, have an opposite effect on chloroplast biogenesis. We found that they affect the expression of plastid genes both at the transcriptional [1, 2] and at the posttranscriptional levels [3, 4]. To study the possible involvement of these phytohormones in anterograde signaling, we chose two nuclear genes of the plastid transcription apparatus, RPOTp and RPOTmp, encoding phage-type single-subunit RNA polymerases (NEP—nuclear-encoded RNA polymerase). In addition to RPOTp and RPOTmp, the Arabidopsis genome contains the phage-type RNA polymerase RPOTm, which transcribes mitochondrial genes. The RPOTp and RPOTmp genes are believed to originate as a duplication of the RPOTm gene and subsequent acquisition by polymerases of a transit peptide that directs them to plastids (RPOTp) or to plastids and mitochondria (RPOTmp), in dicotyledonous plants) [5]. In contrast to mitochondria, plastid genes are also transcribed by multisubunit RNA polymerase of the prokaryotic type, encoded by a plastome (PEP—plastid-encoded RNA polymerase).

The specificity of the action of plastid polymerases has been the subject of detailed analysis by a number of researchers. PEP polymerase is shown to be primarily responsible for the transcription of photosynthetic genes, while NEP enzymes are active at early stages of ontogenesis and transcribe housekeeping genes, including the four core PEP subunits. However, upon suppression of PEP-dependent transcription due to genetic defects or unfavorable physiological conditions, NEPs are capable of transcribing even photosynthetic genes from NEP-dependent promoters [6, 7].

NEP enzymes transcribe plastid genes by binding to three different types of promoters similar to those of bacteriophage and mitochondrial genes. The presence of various types of promoters suggests the existence of specific trans-factors that would help RNA polymerases to determine the sites of transcription initiation. Nevertheless, no trans-factor has been identified for NEP to date, which is possibly explained by the low concentration that does not allow them to be isolated by modern methods.

The question of hormonal regulation of NEP activity also remains open. Therefore, the objective of our study was to analyze the peculiarities of hormonal regulation of chloroplast genes, as well as genes for metabolism and signaling of cytokinins (CKs) and ABA, in wild-type plants and mutants with defective RPOTp and RPOTmp genes.

MATERIALS AND METHODS

The studies were performed with 3-week-old wild-type plants of Arabidopsis thaliana (L.) Heynh. (Columbia-0), and homozygous T-DNA insertion knockout mutants scabra3-2 (sca3-2) and rpotmp-2, selected from NASC stock lines N593884 (Salk 0938840) and N6328420 (Salk 132842), respectively. Plants were grown on agar half-strength MS medium in a climatic chamber at an illumination intensity of 70 μE/(m2 s), a temperature of 23°C, and a 16/8 h photoperiod.

Measurement of parameters of variable chlorophyll fluorescence using a PAM-100 fluorimeter (Heinz Walz, Germany) and determination of the chlorophyll content were made in accordance with the protocol [8].

To study the effect of hormones, plants were treated with solutions of ABA (50 μM) and trans-zeatin (5 μM) for 3 or 24 h, after which they were fixed in liquid nitrogen. The optimal concentrations of phytohormones were determined in preliminary experiments. The relative level of transcripts of the target genes was determined by quantitative real-time PCR after reverse transcription (RT-PCR) using a LigthCyclerR96 amplifier (Roche, Switzerland) according to the method described in [9]. The sequences of the primers used are given in Supplementary (Table S1).

Changes in the content of chloroplast proteins under the effect of phytohormones were assessed by Western blotting using specific antibodies against RbcL (AS03 037), PsbD (AS06 146), AtpB (AS05 085), and Lhcb2 (AS01 003) proteins (Agrisera, Sweden). Total plant protein was isolated by homogenizing rosette leaves in a buffer containing 50 mM HEPES-KOH, pH 7.5, 330 mM sorbitol, 2 mM EDTA, 1 mM MgCl2, followed by filtration of the homogenate through two layers of a Miracloth filter (Calbiochem, United States). The quantitative determination of total protein in the obtained extracts was carried out using the BCA™ Protein Assay Kit (Thermo Fisher Scientific, United States). Samples containing 20 μg of total protein were separated in 10% SDS-PAGE, and then proteins were transferred to a PVDF membrane by semi-dry transfer using a semi-dry device (Bio-Rad, United States). The interaction of specific antibodies with the studied samples of proteins immobilized on the membrane was carried out under the conditions recommended by the antibody manufacturer (Agrisera). The immunoreaction was visualized using an ECL kit (Bio-Rad) and the chemiluminescent signal was recorded with a digital scanner C-Digit (Li-Сor, United States).

The experiments were carried out in at least three biological replicates. The significance of differences was checked using the ANOVA method followed by the Tukey test.

RESULTS

Disruption of RPOTp and RPOTmp Genes Promoted Changes in Physiological Characteristics and Expression of Genes for Signaling and Metabolism of CKs and ABA

Convincing evidence showing that NEPs are directly involved in hormonal responses may be provided by analysis of the corresponding knockout mutants. Scabra3-2 (sca3-2) transgenic plants with the mutant RPOTp gene were used in our experiments. The mutant gene contains a T-DNA insert at nucleotide position 1894 (from the translation initiation codon) and encodes a polypeptide lacking 604 amino acids and nine of 11 domains, including the C-motif. This motif is part of the catalytic site of T7 RNA polymerases, and its absence causes dysfunction of the RpoTp enzyme [10]. The scabra3-2 plants showed significant changes in physiological parameters, including pigmentation and fluorescence indicators (Table 1). In particular, the plants were characterized by approximately ten times lower chlorophyll content and more than five times lower content of carotenoids compared to the wild type. The decrease in the amount of chlorophyll completely correlated with the indices of the base fluorescence (Fs), which probably indicates a reduced number of reaction centers. However, the calculated indicators of the potential maximum quantum yield (Fv/Fm), quantum yield (Y(II)), and nonphotochemical quenching (NPQ), which characterize the efficiency of photosystem II, were only slightly lower than those of the wild type. This, apparently, indicates the preservation and relatively normal functioning of all elements of the photosystem.

Table 1. Content of pigments and parameters of photosynthetic activity of photosystem II in 3-week-old wild-type plants (Columbia-0) and insertional knockout mutants for the RPOTp gene (sca3-2) and for the RPOTm gene (rpotmp-2)
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Unlike sca3-2, RPOTmp transcripts were completely absent in the rpotmp-2 mutant, which was defective in the RPOTmp gene. This mutant was also characterized by delayed development, which was especially noticeable in juvenile seedlings. However, it did not differ from wild-type plants or even slightly exceeded them in terms of the chlorophyll content and indicators of the activity of the photosynthetic apparatus of adult leaves (Table 1). We also did not find any differences in the protein content of the main complexes of photosynthetic membranes in the rpotmp-2 mutant (Fig. 1).

Fig. 1.
figure 1

Chloroplast protein levels in 3-week-old wild-type plants (Columbia-0) and mutants for the RPOTp (sca3-2) and RPOTmp (rpotmp-2) genes. (a) Plants were treated with ABA (50 μM) or trans-zeatin (5 μM) for 24 h. The proteins of the total extract were analyzed by Western blotting using specific antibodies to RbcL, AtpB, PsbD, and Lhcb2. (b) The same loading of tracks (20 μg) was checked by staining with Ponceau S. (1) without treatment; (2) treatment with trans-zeatin; (3) treatment with ABA.

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To confirm the effectiveness of hormonal treatment, the accumulation of transcripts of the ARR5 (Response Regulator 5) and RD29A (Responsive-to-Dehydration 29A) genes, used as markers of cytokinin and ABA action, respectively, was analyzed. Both genes were up-regulated by the hormones in all studied samples, and the scabra3-2 mutant was particularly susceptible to the action of ABA (Table 2, Supplementary Table S2). Recently, the hypersensitivity of this mutant to ABA during seed germination and seedling formation was reported by Lidón-Soto et al. [11]. To assess the reasons for the increased sensitivity of this mutant line to ABA, we investigated the effect of an exogenous hormone on the expression of some genes for ABA metabolism and signaling. The steady state expression of genes of the ABA biosynthesis pathway ABA1 and ABA2 was reduced in sca3-2, but the mutant plants did not differ from wild-type plants by the response of these genes to exogenous ABA (Table 2). Among the genes of the ABA signaling chain—ABI1, ABI2, and HAB1—which are up-regulated by ABA [12, 13], sca3-2 showed an increased steady state level of HAB1 expression. However, the levels of induction of these genes by the hormone hardly differed in the mutant and Col-0 as well as the levels of multiplied suppression of ABA receptor gene transcripts, PYR1, and PYL5 (Table 2). Thus, disruption of the RPOTp gene promoted changes in the steady state levels of some genes for ABA metabolism and signaling and, as a consequence, could enhance the reaction of ABA marker genes but did not lead to a significant change in the expression of signaling genes in response to exogenous ABA.

Table 2. Changes in the expression of genes for signaling and metabolism of cytokinins and ABA in 3-week-old wild-type plants (Columbia-0) and an insertion knockout mutant for the RPOTp gene (sca3-2)
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Trans-zeatin had no significant effect on the expression of genes for CK receptors AHK2 and AHK3 and type B response regulators ARR1, ARR10, and ARR12 but strongly induced the expression of the genes for AHK4 receptor and CK-regulated trans-factor GNL in all studied samples (Table 2, Supplementary Table S2). However, the sca3-2 was characterized by a reduced up-regulation of the AHK4 and GNL genes, as well as the marker gene ARR5, compared to the wild type CK-activated expression (Table 2). The reduced induction of the last three genes may also have been a consequence of the altered hormonal status of this mutant. Indeed, sca3-2 showed a threefold increase in the steady state expression of the genes for the synthesis of CKs, IPT3 and IPT5, and the CKX5 gene for the catabolism of the hormone. The level of the steady state expression of the CKX3 gene did not differ from that of wild-type plants. Thus, the altered sensitivity of the sca3-2 mutant could be a consequence of the increased content of endogenous CKs. It should be noted that the rpotmp-2 mutant also exhibited an increased accumulation of IPT3, IPT5, and CKX5 transcripts although it was not as significant as in sca3-2 (Supplementary Table S2).

Differentiated Effects of CKs and ABA on the Expression of Plastid Genes and Genes for the Transcriptional Apparatus of rpotmp and rpotp Mutants

The responses of wild-type plants and rpotmp-2 mutants were practically the same when the plants were treated with trans-zeatin and ABA. Cytokinin activated the accumulation of transcripts of genes encoding the chloroplast transcription apparatus (RPOTp, RPOTm, SIG2, SIG6, and CKA4) by 2–4 times compared with the initial values as well as the genes encoding the PEP-associated proteins (PAP1–12). Meanwhile, ABA only weakly down-regulated their expression or had no significant effect (Supplementary Table S3). At the same time, ABA promoted the accumulation of mRNA of the SIG5 gene in both samples. This reaction is not accidental. Under stress and increased ABA levels, this trans-factor in combination with PEP provides transcription of a number of chloroplast genes from BLRP promoters while inhibiting the expression of the majority of plastome genes [14].

A somewhat different picture was obtained when analyzing the CK-dependent gene expression of the transcription apparatus in the scabra3-2 mutant. CKs hardly regulated the accumulation of transcripts of the RPOTp gene fragment and the PAP2 and SCA4 genes, and, to an insignificant extent, activated the accumulation of transcripts of other genes of the transcription apparatus (Table 3). These results indicate a decrease in the up-regulating effect of CKs. At the same time, the reaction to ABA treatment in the sca3-2 mutant, on the contrary, was more pronounced: the expression of the majority of the studied genes after 3 h of exposure to the hormone was inhibited by 2–3 times compared with the initial levels. Thus, the genetic environment directly influenced the nature and magnitude of the positive or negative effect of hormones on the expression of genes of the plastid transcription apparatus.

Table 3. Changes in the expression of genes for the transcription apparatus in 3-week-old wild-type plants (Columbia-0) and an insertion knockout mutant for the RPOTp gene (sca3-2)
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This, in turn, suggests a possible change in the expression of at least two classes of genes encoded by the plastid genome: class II (NEP and PEP-dependent genes) and class III (NEP-transcribed genes). Indeed, the rpoB (class III), atpB, and clpP (class II) genes were not up-regulated by trans-zeatin in sca3-2, in contrast to the rpotmp line or wild-type plants, where trans-zeatin stimulated an increased content of the transcripts of these genes (Table 4, Supplementary Table S4). Thus, the absence of an up-regulating effect of CKs on the expression of the genes encoding the transcription apparatus in this mutant correlated with the retention of the steady state level of transcripts of NEP-dependent plastome genes. The quantity of templates of the PEP-dependent psbD gene (class I) was also not reliably regulated by CKs, in contrast to the rpotmp-2 mutant or wild-type plants, where the expression of this gene was up-regulated by the hormone. ABA had no significant effect on the expression of plastome genes in wild-type plants and in the rpotmp-2 mutant under the experimental design, but inhibited the accumulation of plastid gene transcripts in sca3-2. From the data obtained it follows that the increased sensitivity to ABA as demonstrated in sca3-2 by the marker ABA-inducible gene (RD29a) also extends to the expression of plastome genes.

Table 4. Changes in the expression of genes for the transcription apparatus in 3-week-old wild-type plants (Columbia-0) and an insertion knockout mutant for the RPOTp gene (sca3-2)
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Effect of CKs and ABA on the Posttranscriptional Regulation of Chloroplast Gene Expression in NEP Mutants

The posttranscriptional level of regulation is of fundamental importance for the expression of plastid genes. As we have shown earlier, the accumulation of plastome gene transcripts does not always correlate with the level of chloroplast proteins encoded by them [3]. In this regard, it is of great interest to assess the effect of hormones on the content of plastid proteins in order to identify the mechanisms of the implementation of genetic information in chloroplasts.

According to the results of Western blot analysis, the levels of chloroplast proteins in Arabidopsis lines with mutant RPOTp and RPOTmp genes only partially correlated with the accumulation of templates of their encoding genes during hormonal treatment. Protein accumulation was hardly induced by trans-zeatin in the rpotmp-2 mutant, and the protein content in the sca3-2 line did not decrease in response to ABA or CK treatment (Figs. 1, 2). At the same time, modulations in the protein content in wild-type plants corresponded to the hormone-dependent response observed at the RNA level. That is, a slight increase in the accumulation of the chloroplast proteins PsbD, AtpB, and Lhcb2 was recorded in the presence of CKs, while their control levels were retained during ABA treatment. Obviously, the content of transcripts can hardly be regarded as the only factor determining the rate of synthesis and degradation of chloroplast proteins in NEP mutants in response to treatment with exogenous hormones. However, it should be noted that in quantitative terms, the content of chloroplast proteins in the sca3-2 line was significantly inferior to their content in wild-type plants or in the rpotmp mutant, which correlated with the accumulation of transcripts and fluorescence parameters.

Fig. 2.
figure 2

Changes in the expression of plastid coded genes and the nuclear coded gene (LHCB2) in 3-week-old wild-type plants (Columbia-0) and mutants for RPOTp (sca3-2) and RPOTmp (rpotmp-2) genes. Experimental plants were treated with ABA (50 μM) and trans-zeatin (5 μM) for 24 h. Designation of experimental variants: (1) wild type without treatment; (2) wild type treated with trans-zeatin; (3) wild type treated with ABA; (4) sca3-2 without treatment; (5) sca3-2 treated with trans-zeatin; (6) sca3-2 treated with ABA; (7) rpotmp-2 without processing; (8) rpotmp-2 treated with trans-zeatin; (9) rpotmp-2 treated with ABA. Values ​​are normalized relative to control (wild type without hormone treatment). Data represent mean values ± standard errors (n ≥ 3). Letters indicate statistically significant differences at P < 0.05 (ANOVA followed by Tukey’s test).

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DISCUSSION

The results of the study have shown that the changes in the expression of NEP genes under the influence of hormones are transformed into the activation or suppression of the expression of chloroplast genes of nuclear or plastome coding, thus ensuring the regulation of chloroplast functions through the mechanisms of anterograde signaling. Along with that, plastid dysfunction associated with NEP damage can trigger a reverse flow of information from chloroplasts to the nucleus and cause a compensatory change in the expression of genes encoding the transcription apparatus and rearrangement the functioning of the hormonal system.

According to our data, disruption of genes for nuclear-encoded plastid RNA polymerases changed the profiles of hormone-dependent expression of genes for plastid transcriptional complex and plastid encoded genes. The greatest deviations in the degree and specificity of regulation were found for the sca3-2 line. The response of the rpotmp-2 mutant to hormones did not differ from that of wild-type plants. Nevertheless, the content and the frequency of putative cis-elements that bind the ABA and CK-regulated trans-factors in the promoters of both genes and are responsible for hormone inducibility varied slightly. These results are consistent with the concept of the dominant role of RPOTp in the division of functions between RPOTp and RPOTmp in plastids.

RPOTmp is known to transcribe the rRNA operon from the PC promoter and from a number of other plastid promoters at the stage of seed soaking [15–17]. However, the main function of this enzyme during germination is shifted to mitochondria. According to Tarasenko et al. [18], RPOTmp in Arabidopsis plays a more important role not in chloroplasts but in mitochondria, where this enzyme is required for the expression of a number of genes encoding subunits of complexes I and IV of the respiratory chain (nad6, cox1, cob, and nad3). Interestingly, the level of mitochondrial gene templates transcribed by RPOTmp did not significantly change in response to CK or ABA according to the transcriptional analysis data presented in http://bar.utoronto.ca/efp/cgi-bin/efpWeb.cgi. We obtained a similar result for the cox1 gene transcribed by RPOTmp. According to the published data, mitochondrial polymerases require additional trans-factors that can stabilize promoter complexes or facilitate the release of the enzyme after the end of RNA synthesis for efficient in vivo transcription [19]. It can be assumed that these trans-factors are not regulated by hormones, in contrast to the trans-factors of chloroplasts. Recently the mechanism of cytokinin activation of chloroplast trans-factors was demonstrated for bacterial RNA polymerase σ-factors SIG2 and SIG6 [20], whose promoter elements, containing GATA sites, were bound by the cytokinin dependent GNL trans-factor.

Although the overall phenotypic effects of RPOTmp gene were significantly weaker than in sca3-2, the steady state levels of the transcripts of σ-factor and PAP genes the PEP-A enzyme complex were twice as high in rpotmp as well as the expression values of the genes for PEP subunits. This reaction is probably associated with a compensatory mechanism that allows the mutant to enhance PEP-dependent transcription in the absence of the RPOTmp enzyme. In addition, despite the delayed development, the rpotmp mutant had increased steady state levels of chloroplast gene transcripts and fluorescence parameters. Similarly, the sca3-2 mutant had a doubled steady state expression value of the mitochondrial cox1 gene transcribed by RPOTmp, as compared to the wild type. This proves that the functions of NEP and PEP are not only specific but also interdependent.

We found a decrease or even absence of up-regulation for some studied genes in the scabra3-2 mutant upon treatment with CKs as compared to wild-type plants. Along with plastid genes, these were some nuclear genes encoding proteins associated with PEP. This tendency was expressed both at the level of transcript content and at the level of the corresponding proteins for photosynthetic genes (Figs. 1, 2). Thus, cytokinins and ABA can act unidirectionally upon inactivation of the RPOTp gene, exerting a repressive effect on the expression of chloroplast genes. The reasons for this phenomenon require additional research. It is possible that such a biological response for nuclear genes was caused by the mechanisms of retrograde signaling, which contributed to a change in the expression of nuclear genes encoding PEP complex proteins in response to chloroplast dysfunction in scabra3-2. In this case, the repressive signal sent by defective chloroplasts in response to the action of CKs or, conversely, the absence of a positive signal was perceived selectively only by a part of the genes of the transcription apparatus. Interestingly, a recent study by Lidón-Soto et al. [11] showed that retrograde signaling from damaged organelles under conditions of salt stress activates NEP expression. However, the mechanism of such induction was not associated with the action of ABA, since the NEP genes in Col-0 plants were reliably suppressed under conditions of prolonged exposure to this hormone.

Dysfunction of chloroplasts, apparently, could lead to a change in the hormonal status of the mutant and, as a consequence, a lower sensitivity CKs, which was possibly associated with an increased content of the hormone. Indirect evidence is provided by the data on the expression of the ARR5 gene and IPT3, IPT5, and CKX5 genes for CK metabolism since the accumulation of their transcripts significantly exceeded the wild-type levels.

The decreased response to the action of CKs could also be caused by a change in the balance between CKs and ABA. According to microarray data of Hricova et al. [10], the expression of genes encoding proteins of the isoprenoid biosynthesis pathway was reduced including the ABA1 gene encoding zeaxanthine dehydrogenase, the key enzyme of ABA biosynthesis. For the ABA1 gene, the product of which is localized mainly in the stroma and thylakoid membranes of chloroplasts [21], we confirmed this result by RT-PCR (Table 2). In addition, the steady state expression of the ABA metabolism gene (ABA2) in scabra3-2 was decreased, and the level of HAB1 signaling gene transcripts was increased. Thus, the pleiotropic effect of the mutation affected not only the expression of the chloroplast genome but also influenced the regulation of the plant hormonal system. The possible consequences include a disruption in the synthesis of carotenoids, which are necessary for photosynthetic tissues, where they play a photoprotective role, as well as for the normal processing of scotomorphogenesis and the subsequent development of chloroplasts during the transition to photomorphogenesis [22].

A change in the expression of NEP genes upon direct binding of hormone-regulated trans-factors to the promoter region of the NEP genes could be indisputable evidence in favor of their direct involvement in the hormonal regulation of chloroplast genes. In silico analysis of the promoter regions of NEP polymerase genes showed that they contain putative cis-elements within a sequence of 500 bp downstream ATG start codon for a variety of ABA and CK-regulated trans-factors, including RAV1, DREB/CBF, RAP2.6, MYB52, ATB12, ARR11, ARR14, and GLK1. However, none of these trans-factors showed changes in the expression level in sca3-2, as follows from the microarray data [10]. Nevertheless, inactivation of the RPOTp gene promoted signal changes for a number of trans-factors, some of which (MYB47 and NF-YB2) were strongly induced by ABA. Thus, hormone-dependent responses regulated by these trans-factors may be indirect, although identifying specific participants in this process and all its components remains a challenge for future research.

The study of the biological role of CKs and ABA in the functioning of the chloroplast transcription apparatus would be incomplete without taking into account their effect on the level of chloroplast proteins. Treatment with exogenous ABA did not change the content of these proteins in any of the studied samples, which is consistent with the data on transcriptional analysis for wild-type plants and the rpotmp mutant. However, maintenance in the scabra3-2 mutant line of the levels of chloroplast proteins upon suppression of transcript levels of their coding genes following CK or ABA treatment could be associated with the peculiarities of their posttranscriptional regulation in an altered genetic background. It is possible that such an effect was caused by an impaired degradation of chloroplast proteins in the sca3-2 mutant. This is evidenced by a 1.5–2-fold decrease in the expression of the protease genes FtsH8, FtsH9, and DEG1, encoding proteases, which are localized in chloroplasts and participate in the cleavage of plastid proteins [10]. In addition, the transcriptome analysis showed that the sca3-2 mutant had a 2–3-fold decrease in the expression of four genes for E3-ubiquitin ligases (At5g01880, At5g01520, At1g57820, and At1g66050) associated with proteasomal degradation of proteins [10]. Interestingly, ubiquitin ligase encoded by the AtAIRP2 gene (Arabidopsis ABA-insensitive RING protein 2) is involved in the regulation of the ABA signal, in particular, in the ubiquitination of key components of the SNF1 protein kinase signaling chain (SnRK) [23], which, along with changes in gene expression metabolism, could contribute to a change in the sensitivity of the mutant to the effect of this hormone.

Summing up, we can state that the genes for the plastid transcription apparatus differentially respond to the action of exogenous CKs and ABA. The up- or down-regulation of expression is determined not only by the specificity of the effect of a particular hormone but also by the normal functioning of chloroplasts controlled by the genetic environment. Disruption of the genes for nuclear-coding plastid RNA polymerases changes the profiles of hormone-dependent expression of the functioning genes for the components of the transcription apparatus and chloroplast genes. In this case, regulation is observed both at the level of the content of transcripts and at the level of the corresponding proteins.