Role of the Cytokinin-Activated Type-B Response Regulators in Hormone Crosstalk.

Cytokinin is an important phytohormone that employs a multistep phosphorelay to transduce the signal from receptors to the nucleus, culminating in activation of type-B response regulators which function as transcription factors. Recent chromatin immunoprecipitation-sequencing (ChIP-seq) studies have identified targets of type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) and integrated these into the cytokinin-activated transcriptional network. Primary targets of the type-B ARRs are enriched for genes involved in hormonal regulation, emphasizing the extensive crosstalk that can occur between cytokinin, auxin, abscisic acid, brassinosteroids, gibberellic acid, ethylene, jasmonic acid, and salicylic acid. Examination of hormone-related targets reveals multiple regulatory points including biosynthesis, degradation/inactivation, transport, and signal transduction. Here, we consider this early response to cytokinin in terms of the hormones involved, points of regulatory crosstalk, and physiological significance.

Coordination of Chloroplast Development through the Action of the GNC and GLK Transcription Factor Families.

Fundamental questions regarding how chloroplasts develop from proplastids remain poorly understood despite their central importance to plant life. Two families of nuclear transcription factors, the GATA NITRATE-INDUCIBLE CARBON-METABOLISM-INVOLVED (GNC) and GOLDEN TWO-LIKE (GLK) families, have been implicated in directly and positively regulating chloroplast development. Here, we determined the degree of functional overlap between the two transcription factor families in Arabidopsis (Arabidopsis thaliana), characterizing their ability to regulate chloroplast biogenesis both alone and in concert. We determined the DNA-binding motifs for GNC and GLK2 using protein-binding microarrays; the enrichment of these motifs in transcriptome datasets indicates that GNC and GLK2 are repressors and activators of gene expression, respectively. ChIP-seq analysis of GNC identified PHYTOCHROME INTERACTING FACTOR and brassinosteroid activity genes as targets whose repression by GNC facilitates chloroplast biogenesis. In addition, GNC targets and represses genes involved in ERECTA signaling and thereby facilitates stomatal development. Our results define key regulatory features of the GNC and GLK transcription factor families that contribute to the control of chloroplast biogenesis and photosynthetic activity, including areas of independence and cross talk.

Cytokinin induces genome-wide binding of the type-B response regulator ARR10 to regulate growth and development in Arabidopsis.

The plant hormone cytokinin affects a diverse array of growth and development processes and responses to the environment. How a signaling molecule mediates such a diverse array of outputs and how these response pathways are integrated with other inputs remain fundamental questions in plant biology. To this end, we characterized the transcriptional network initiated by the type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) that mediate the cytokinin primary response, making use of chromatin immunoprecipitation sequencing (ChIP-seq), protein-binding microarrays, and transcriptomic approaches. By ectopic overexpression of ARR10, Arabidopsis lines hypersensitive to cytokinin were generated and used to clarify the role of cytokinin in regulation of various physiological responses. ChIP-seq was used to identify the cytokinin-dependent targets for ARR10, thereby defining a crucial link between the cytokinin primary-response pathway and the transcriptional changes that mediate physiological responses to this phytohormone. Binding of ARR10 was induced by cytokinin with binding sites enriched toward the transcriptional start sites for both induced and repressed genes. Three type-B ARR DNA-binding motifs, determined by use of protein-binding microarrays, were enriched at ARR10 binding sites, confirming their physiological relevance. WUSCHEL was identified as a direct target of ARR10, with its cytokinin-enhanced expression resulting in enhanced shooting in tissue culture. Results from our analyses shed light on the physiological role of the type-B ARRs in regulating the cytokinin response, mechanism of type-B ARR activation, and basis by which cytokinin regulates diverse aspects of growth and development as well as responses to biotic and abiotic factors.

Abscisic acid affects transcription of chloroplast genes via protein phosphatase 2C-dependent activation of nuclear genes: repression by guanosine-3'-5'-bisdiphosphate and activation by sigma factor 5.

Abscisic acid (ABA) represses the transcriptional activity of chloroplast genes (determined by run-on assays), with the exception of psbD and a few other genes in wild-type Arabidopsis seedlings and mature rosette leaves. Abscisic acid does not influence chloroplast transcription in the mutant lines abi1-1 and abi2-1 with constitutive protein phosphatase 2C (PP2C) activity, suggesting that ABA affects chloroplast gene activity by binding to the pyrabactin resistance (PYR)/PYR1-like or regulatory component of ABA receptor protein family (PYR/PYL/RCAR) and signaling via PP2Cs and sucrose non-fermenting protein-related kinases 2 (SnRK2s). Further we show by quantitative PCR that ABA enhances the transcript levels of RSH2, RSH3, PTF1 and SIG5. RelA/SpoT homolog 2 (RSH2) and RSH3 are known to synthesize guanosine-3'-5'-bisdiphosphate (ppGpp), an inhibitor of the plastid-gene-encoded chloroplast RNA polymerase. We propose, therefore, that ABA leads to an inhibition of chloroplast gene expression via stimulation of ppGpp synthesis. On the other hand, sigma factor 5 (SIG5) and plastid transcription factor 1 (PTF1) are known to be necessary for the transcription of psbD from a specific light- and stress-induced promoter (the blue light responsive promoter, BLRP). We demonstrate that ABA activates the psbD gene by stimulation of transcription initiation at BLRP. Taken together, our data suggest that ABA affects the transcription of chloroplast genes by a PP2C-dependent activation of nuclear genes encoding proteins involved in chloroplast transcription.

Chloroplast RNA polymerases: Role in chloroplast biogenesis.

Plastid genes are transcribed by two types of RNA polymerase in angiosperms: the bacterial type plastid-encoded RNA polymerase (PEP) and one (RPOTp in monocots) or two (RPOTp and RPOTmp in dicots) nuclear-encoded RNA polymerase(s) (NEP). PEP is a bacterial-type multisubunit enzyme composed of core subunits (coded for by the plastid rpoA, rpoB, rpoC1 and rpoC2 genes) and additional protein factors (sigma factors and polymerase associated protein, PAPs) encoded in the nuclear genome. Sigma factors are required by PEP for promoter recognition. Six different sigma factors are used by PEP in Arabidopsis plastids. NEP activity is represented by phage-type RNA polymerases. Only one NEP subunit has been identified, which bears the catalytic activity. NEP and PEP use different promoters. Many plastid genes have both PEP and NEP promoters. PEP dominates in the transcription of photosynthesis genes. Intriguingly, rpoB belongs to the few genes transcribed exclusively by NEP. Both NEP and PEP are active in non-green plastids and in chloroplasts at all stages of development. The transcriptional activity of NEP and PEP is affected by endogenous and exogenous factors. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Inhibition of the electron transport strongly affects transcription and transcript levels in Arabidopsis mitochondria.

Mitochondrial transcription rate and RNA steady-state levels were examined in shoots of Arabidopsis seedlings. The shoots were treated with inhibitors of complex III and IV of the cytochrome pathway (CP) and with an inhibitor of the alternative oxidase (AOX) of the mitochondrial electron transport chain. The inhibition of AOX and CP complexes III and IV affected transcription and transcript levels in different ways. CP and AOX inhibitors had opposite effects. Our data support the idea that the redox state of the electron transport chain is involved in the regulation of mitochondrial gene expression at transcriptional and post-transcriptional levels.

Abscisic acid represses the transcription of chloroplast genes.

Numerous studies have shown effects of abscisic acid (ABA) on nuclear genes encoding chloroplast-localized proteins. ABA effects on the transcription of chloroplast genes, however, have not been investigated yet thoroughly. This work, therefore, studied the effects of ABA (75 µM) on transcription and steady-state levels of transcripts in chloroplasts of basal and apical segments of primary leaves of barley (Hordeum vulgare L.). Basal segments consist of young cells with developing chloroplasts, while apical segments contain the oldest cells with mature chloroplasts. Exogenous ABA reduced the chlorophyll content and caused changes of the endogenous concentrations not only of ABA but also of cytokinins to different extents in the basal and apical segments. It repressed transcription by the chloroplast phage-type and bacteria-type RNA polymerases and lowered transcript levels of most investigated chloroplast genes drastically. ABA did not repress the transcription of psbD and a few other genes and even increased psbD mRNA levels under certain conditions. The ABA effects on chloroplast transcription were more pronounced in basal vs. apical leaf segments and enhanced by light. Simultaneous application of cytokinin (22 µM 6-benzyladenine) minimized the ABA effects on chloroplast gene expression. These data demonstrate that ABA affects the expression of chloroplast genes differentially and points to a role of ABA in the regulation and coordination of the activities of nuclear and chloroplast genes coding for proteins with functions in photosynthesis.

Mutation of the pentatricopeptide repeat-SMR protein SVR7 impairs accumulation and translation of chloroplast ATP synthase subunits in Arabidopsis thaliana.

RNA processing, RNA editing, RNA splicing and translational activation of RNAs are essential post-transcriptional steps in chloroplast gene expression. Typically, the factors mediating those processes are nuclear encoded and post-translationally imported into the chloroplasts. In land plants, members of the large pentatricopeptide repeat (PPR) protein family are required for individual steps in chloroplast RNA processing. Interestingly, a subgroup of PPR proteins carries a C-terminal small MutS related (SMR) domain. Here we analyzed the consequences of mutations in the SVR7 gene, which encodes a PPR-SMR protein, in Arabidopsis thaliana. We demonstrate that SVR7 mutations lead to a specific reduction in chloroplast ATP synthase levels. Furthermore, we found aberrant transcript patterns for ATP synthase coding mRNAs in svr7 mutants. Finally, a reduced ribosome association of atpB/E and rbcL mRNAs in svr7 mutants suggests the involvement of the PPR-SMR protein SVR7 in translational activation of these mRNAs. We describe that the function of SVR7 in translation has expanded relative to its maize ortholog ATP4. The results provide evidence for a relaxed functional conservation of this PPR-SMR protein in eudicotyledonous and monocotyledonous plants, thus adding to the knowledge about the function and evolution of PPR-SMR proteins.

Functional characterization of the GATA transcription factors GNC and CGA1 reveals their key role in chloroplast development, growth, and division in Arabidopsis.

Chloroplasts develop from proplastids in a process that requires the interplay of nuclear and chloroplast genomes, but key steps in this developmental process have yet to be elucidated. Here, we show that the nucleus-localized transcription factors GATA NITRATE-INDUCIBLE CARBON-METABOLISM-INVOLVED (GNC) and CYTOKININ-RESPONSIVE GATA1 (CGA1) regulate chloroplast development, growth, and division in Arabidopsis (Arabidopsis thaliana). GNC and CGA1 are highly expressed in green tissues, and the phytohormone cytokinin regulates their expression. A gnc cga1 mutant exhibits a reduction in overall chlorophyll levels as well as in chloroplast size in the hypocotyl. Ectopic overexpression of either GNC or CGA1 promotes chloroplast biogenesis in hypocotyl cortex and root pericycle cells, based on increases in the number and size of the chloroplasts, and also results in expanded zones of chloroplast production into the epidermis of hypocotyls and cotyledons and into the cortex of roots. Ectopic overexpression also promotes the development of etioplasts from proplastids in dark-grown seedlings, subsequently enhancing the deetiolation process. Inducible expression of GNC demonstrates that GNC-mediated chloroplast biogenesis can be regulated postembryonically, notably so for chloroplast production in cotyledon epidermal cells. Analysis of the gnc cga1 loss-of-function and overexpression lines supports a role for these transcription factors in regulating the effects of cytokinin on chloroplast division. These data support a model in which GNC and CGA1 serve as two of the master transcriptional regulators of chloroplast biogenesis, acting downstream of cytokinin and mediating the development of chloroplasts from proplastids and enhancing chloroplast growth and division in specific tissues.

Reverse protection assay: a tool to analyze transcriptional rates from individual promoters.

Transcriptional activity of entire genes in chloroplasts is usually assayed by run-on analyses. To determine not only the overall intensity of transcription of a gene, but also the rate of transcription from a particular promoter, we created the Reverse RNase Protection Assay (RePro): in-organello run-on transcription coupled to RNase protection to define distinct transcript ends during transcription. We demonstrate successful application of RePro in plastid promoter analysis and transcript 3' end processing.

Measurement of transcription rates in Arabidopsis chloroplasts.

The regulation of gene expression is still one of the major issues in modern plant molecular biology. The amount of RNA in a cell is regulated by both transcriptional and posttranscriptional events. Methods to determine these steady-state levels of RNAs, such as Northern analysis, ribonuclease protection assay (RPA), and quantitative real-time PCR, do not discriminate between regulation by de novo RNA synthesis and the influence by degradation or stabilization. To assess the rate of transcription of individual genes, run-on transcription is utilized. To this end, isolated chloroplasts are used in brief in vitro transcription reactions in the presence of radiolabeled nucleotides, with a subsequent hybridization of the isolated RNA with DNA fragments spotted on membranes. Here, we describe a protocol for run-on transcription in chloroplasts isolated from Arabidopsis leaves and present data on the transcriptional activity of several plastid genes in detached leaves of different Arabidopsis ecotypes.

Methyl jasmonate, gibberellic acid, and auxin affect transcription and transcript accumulation of chloroplast genes in barley.

Phytohormones control growth and development of plants. Their effects on the expression of nuclear genes are well investigated. Although they influence plastid-related processes, it is largely unknown whether phytohormones exert their control also by regulating the expression of plastid/chloroplast genes. We have therefore studied the effects of methyl jasmonate (MeJA), gibberellic acid (GA(3)), an auxin (indole-3-acetic acid, IAA), a brassinosteroid (24-epibrassinolide, BR) and a cytokinin (6-benzyladenine) on transcription (run-on assays) and transcript levels (RNA blot hybridization) of chloroplast genes after incubation of detached barley leaves in hormone solutions. BR was the only hormone without significant influence on chloroplast transcription. It showed, however, a weak reducing effect on transcript accumulation. MeJA, IAA and GA(3) repressed both transcription and transcript accumulation, while BA counteracted the effects of the other hormones. Effects of phytohormones on transcription differed in several cases from their influence on transcript levels suggesting that hormones may act via separate signaling pathways on transcription and transcript accumulation in chloroplasts. We observed striking differences in the response of chloroplast gene expression on phytohormones between the lower (young cells) and the upper segments (oldest cells) of barley leaves. Quantity and quality of the hormone effects on chloroplast gene expression seem to depend therefore on the age and/or developmental stage of the cells. As the individual chloroplast genes responded in different ways on phytohormone treatment, gene- and transcript-specific factors should be involved. Our data suggest that phytohormones adjust gene expression in the nucleo-cytoplasmic compartment and in plastids/chloroplasts in response to internal and external cues.

Cytokinin stimulates chloroplast transcription in detached barley leaves.

Chloroplasts are among the main targets of cytokinin action in the plant cell. We report here on the activation of transcription by cytokinin as detected by run-on assays with chloroplasts isolated from apical parts of first leaves detached from 9-d-old barley (Hordeum vulgare) seedlings and incubated for 3 h on a 2.2 x 10(-5) m solution of benzyladenine (BA). Northern-blot analysis also detected a BA-induced increase in the accumulation of chloroplast mRNAs. A prerequisite for BA activation of chloroplast transcription was preincubation of leaves for 24 h on water in the light, resulting in a decreased chloroplast transcription and a drastic accumulation of abscisic acid. Cytokinin enhanced the transcription of several chloroplast genes above the initial level measured before BA treatment, and in the case of rrn16 and petD even before preincubation. Cytokinin effects on basal (youngest), middle, and apical (oldest) segments of primary leaves detached from plants of different ages revealed an age dependence of chloroplast gene response to BA. BA-induced stimulation of transcription of rrn16, rrn23, rps4, rps16, rbcL, atpB, and ndhC required light during the period of preincubation and was further enhanced by light during the incubation on BA, whereas activation of transcription of trnEY, rps14, rpl16, matK, petD, and petLG depended on light during both periods. Our data reveal positive and differential effects of cytokinin on the transcription of chloroplast genes that were dependent on light and on the age (developmental stage) of cells and leaves.