Benzyladenine modulation of the expression of two genes for nuclear-encoded chloroplast proteins in Lemna gibba: Apparent post-transcriptional regulation

Involvement of cytokinins in STOP1-mediated resistance to proton toxicity

STOP1 (sensitive to proton rhizotoxicity1) is a master transcription factor that governs the expression of a set of regulatory and structural genes involved in resistance to aluminum and low pH (i.e., proton) stresses in Arabidopsis. However, the mechanisms and regulatory networks underlying STOP1-mediated resistance to proton stresses are largely unclear. Here, we report that low-pH stresses severely inhibited root growth of the stop1 plants by suppressing root meristem activities. Interestingly, the stop1 plants were less sensitive to exogenous cytokinins at normal and low pHs than the wild type. Significantly, low concentrations of cytokinins promoted root growth of the stop1 mutant under low-pH stresses. Moreover, lateral and adventitious root formation was stimulated in stop1 and by low-pH stresses but suppressed by cytokinins. Further studies of the expression patterns of a cytokinin signaling reporter suggest that both the loss-of-function mutation of STOP1 and low-pH stresses suppressed cytokinin signaling outputs in the root. Furthermore, the expression of critical genes involved in cytokinin biosynthesis, biodegradation, and signaling is altered in the stop1 mutant in response to low-pH stresses. In conclusion, our results reveal a complex network of resistance to low-pH stresses, which involves coordinated actions of STOP1, cytokinins, and an additional low-pH-resistant mechanism for controlling root meristem activities and root growth upon proton stresses.

Regulation of chloroplast development and function by cytokinin

A role of the plant hormone cytokinin in regulating the development and activity of chloroplasts was described soon after its discovery as a plant growth regulator more than 50 years ago. Its promoting action on chloroplast ultrastructure and chlorophyll synthesis has been reported repeatedly, especially during etioplast-to-chloroplast transition. Recently, a protective role of the hormone for the photosynthetic apparatus during high light stress was shown. Details about the molecular mechanisms of cytokinin action on plastids are accumulating from genetic and transcriptomic studies. The cytokinin receptors AHK2 and AHK3 are mainly responsible for the transduction of the cytokinin signal to B-type response regulators, in particular ARR1, ARR10, and ARR12, which are transcription factors of the two-component system mediating cytokinin functions. Additional transcription factors linking cytokinin and chloroplast development include CGA1, GNC, HY5, GLK2, and CRF2. In this review, we summarize early and more recent findings of the long-known relationship between the hormone and the organelle and describe crosstalk between cytokinin, light, and other hormones during chloroplast development.

Effect of thidiazuron and gibberellic acid on leaf yellowing of cut stock flowers

Plant hormones such as cytokinins and gibberellins are able to inhibit leaf yellowing in different species of cut flowers and potted plants. These hormones can be used alone or in combination among them for preserving chlorophyll in floriculture items. In the present study thidiazuron was tested alone or combined with GA3 for delaying leaf yellowing of cut stock flowers during vase life. Cut flowers were placed in a controlled environment and treated for 24 hours with the following solutions: distilled water (control) or solutions containing 5, 10 µM thidiazuron (TDZ), 0.5 mM gibberellic acid (GA3), or a combination of 0.5 mM GA3 with 5 µM TDZ. The effect of treatments was evaluated by measuring chlorophyll content, ethylene production, leaf gas exchanges and chlorophyll a fluorescence. Results showed that TDZ was able to delay leaf yellowing in light during whole experimental period (30 days). The effect of TDZ on dark stored flowers was less effective, and also delayed chlorophyll losses for 10–12 days. TDZ and GA3 combination did not show any synergistic nor beneficial effect. Gas exchange values such as net photosynthesis, vapour pressure deficit, stomatal conductance and water use efficiency were higher in the TDZ only treatment.

Structural-functional changes in detached cucumber leaves, and modelling these by hormone-treated leaf discs

Senescence and rejuvenation were investigated in detached cucumber (Cucumis sativus L.) leaves after cultivation in nutrient solution for one week or four weeks. Rooting of the petiole (visible generally from the 7th day) elicited a combination of different morphological, anatomical, and physiological changes in the lamina. Extensive growth in area and thickness, extreme regreening, changes of chloroplast structure and activity, as well as the pattern of Chl-protein complexes were observed and compared either to the corresponding parameters of young detached leaves or mature attached leaves. These responses could be provoked separately by treating excised leaf discs with kinetin, benzyladenine, or indolylacetic acid. The hormones showed mutuality in their effects, benzyladenine being responsible for the growth of cells, while indolylacetic acid and kinetin promoted an increase in chlorophyll content. However, none of the treatments resulted in the growth of the chloroplasts in the leaf discs, which was only prominent in the rooting leaves.

Rejuvenation of ageing bean leaves under the effect of low-dose stressors

The effect of low concentrations of some stress-inducing compounds like Cd, Pb, Ni, and Ti salts and DCMU on the senescence of chloroplasts was investigated in detached primary leaves of bean. After the petioles of ageing leaves had developed roots, these low-dose stressors stimulated chlorophyll synthesis and photosynthetic activity, as compared to the control, thus causing rejuvenation in treated leaves. The amount of photosystem I (lowest in DCMU-treated leaves) and light-harvesting complex II increased, while that of photosystem II decreased or remained unchanged. Fluorescence induction parameters indicated unchanged electron transport (except for DCMU treatment). CO2 fixation and, in some cases, starch accumulation was stimulated. In parallel, the occurrence of large plastoglobuli seemed to decrease in plastids of heavy metal-treated leaves. A cytokinin bioassay of leaf extracts confirmed the cytokinin-mediated effect of low-dose stressors, as the slopes of Chl and cytokinin curves were similar during the rejuvenation process. It is assumed that these stressors generate non-specific alarm reactions, which involve changes in the hormonal balance by increasing the synthesis of cytokinins.

Cytokinins Induce Photomorphogenic Development in Dark-grown Gametophytes of Ceratopteris richardii

The cytokinins benzylaminopurine, kinetin and isopentenyladenine induce photomorphogenesis in dark-grown gametophytes of the fern Ceratopteris richardii. At sub-nanomolar concentrations each altered the rate and pattern of cell division, elongation and differentiation, mimicking aspects of the light-mediated transition from filamentous to prothallial growth. Untreated dark-grown gametophytes grow as narrow, elongate, asexual filaments with an apical meristem. Cytokinin treatments as low as 10(-12) M reduced the length-to-width ratio through decreased cell elongation, increased periclinal cell division and induced the formation of rhizoid initials in the cells immediately below the apical meristem. Higher concentrations (10(-9)-10(-8) M) induced conversion of the meristem from apical to notch morphology. Cytokinins induced both red- and blue-light-mediated photomorphogenic events, suggesting stimulation of both phytochrome and cryptochrome signaling; however, cytokinin treatment only partially substituted for light in that it did not induce hermaphroditic sexual development or spore germination in the dark. Additionally, cytokinins did not increase chlorophyll synthesis in dark-grown gametophytes, which unlike angiosperms are able to produce mature chloroplasts in the dark. Cytokinin treatment had only slight effects on light-grown gametophytes. These results suggest evolutionary conservation between angiosperms and pteridophytes in the role of cytokinins in regulating photomorphogenesis.

Characterization of the stimulating effect of low-dose stressors in maize and bean seedlings

The effect of some more or less harmful compounds like Cd, Pb, Ni, Ti salts and DCMU at low concentrations on the development of chloroplasts in maize and bean seedlings was investigated. Chlorophyll content, chlorophyll a/b ratio, photosynthetic activity (14CO2 fixation), chlorophyll-protein composition of thylakoid membranes, fluorescence spectra of chloroplasts, fluorescence induction parameters of leaves and electron microscopic structure of maize and bean chloroplasts as well as growth parameters were studied. Stimulation of chlorophyll synthesis and photosynthetic activity was observed at different intervals during all of the treatments, while chlorophyll a/b ratios and fluorescence properties of leaves or chloroplasts did not change considerably except in DCMU treated plants. Heavy metal treatments increased the amount of photosystem I and light-harvesting complex II, while decreased amount of photosystem I and higher amount of light-harvesting complex II was found in DCMU treated thylakoids. Electron microscopy showed only sligth differences in the morphology of chloroplast lamellar system (mostly in DCMU treated plants), while the status of the plasmalemma and tonoplast seemed to be altered as a result of certain metal treatments. Results showed the expression of a cytokinin-like effect on the development of chloroplasts. It is assumed, that these low-dose stressors generate non-specific alarm reactions in plants, which may involve changes of the hormonal balance.

Low temperature regulates Arabidopsis Lhcb gene expression in a light-independent manner

Low temperature treatment of dark-grown seedlings of Arabidopsis thaliana results in a rapid increase in the amount of mRNAs encoding for the major polypeptides of the light-harvesting complex of photosystem II (Lhcb1 genes). This increase is transient and seems to be due mainly to the accumulation of Lhcb1*3 transcripts, indicating that low temperature differentially regulates the expression of the Arabidopsis Lhcb1 gene family in the dark. A 1.34 kb fragment of the Lhcb1*3 promoter is sufficient to confer low temperature regulation to a reporter gene in transgenic Arabidopsis etiolated seedlings, suggesting that the regulation is occurring at the transcriptional level. The cold-induced accumulation of Lhcb1*3 mRNA is not part of a general response to stressful conditions since no accumulation is detected in response to water stress, anaerobiosis or salt stress. The amount of Lhcb1*3 mRNA decrease in response to exogenous abscisic acid (ABA) suggesting that this phytohormone acts as a negative regulator. Moreover, the accumulation of Lhcb1*3 mRNAs in cold-treated ABA deficient etiolated seedlings is higher than that of wild-type and ABA insensitive etiolated seedlings, indicating that low temperature regulation of Lhcb1*3 is not mediated by ABA.

LIGHT CONTROL OF SEEDLING DEVELOPMENT

Light control of plant development is most dramatically illustrated by seedling development. Seedling development patterns under light (photomorphogenesis) are distinct from those in darkness (skotomorphogenesis or etiolation) with respect to gene expression, cellular and subcellular differentiation, and organ morphology. A complex network of molecular interactions couples the regulatory photoreceptors to developmental decisions. Rapid progress in defining the roles of individual photoreceptors and the downstream regulators mediating light control of seedling development has been achieved in recent years, predominantly because of molecular genetic studies in Arabidopsis thaliana and other species. This review summarizes those important recent advances and highlights the working models underlying the light control of cellular development. We focus mainly on seedling morphogenesis in Arabidopsis but include complementary findings from other species.

Increased endogenous cytokinin in the Arabidopsis amp1 mutant corresponds with de-etiolation responses

The high-cytokinin Arabidopsis thaliana (L.) Heynh. mutant amp1 of has been further characterised. We extend our previous work on the cytokinin level in the amp1 mutant and show that it contains high levels of endogenous cytokinins in both light- and dark-grown plants, and that amp1 can be phenocopied in the dark by growing wild-type plants on exogenous cytokinin. Previously we showed that dark-grown amp1 plants display de-etiolated characteristics such as short unhooked hypocotyls, opened cotyledons and formation of leaves. We further show that amp1 mutants display de-etiolated plastid morphology and increased levels of transcripts of light-regulated genes, indicating that in this mutant light-mediated processes are partially induced in the dark. The amp1 mutant also shows a reduced level of expression of several light-regulated genes compared with the wild type when grown in light, and has an altered dark-adaptation response when compared with the wild type. These results demonstrate an association between high cytokinin levels and de-etiolation, and we infer that cytokinin itself or a cytokinin-mediated process is involved in regulation of etiolation. The map location, phenotypes and de-etiolation responses in the amp1 mutant are different from those of previously described de-etiolated mutants such as det1, det2, cop1 and cop9. We propose a model in which cytokinin acts as a component of the induction of photomorphogenic processes via a signal transduction pathway which is independent of light.

A distinctly regulated protein repair L-isoaspartylmethyltransferase from Arabidopsis thaliana

Protein-L-isoaspartate (D-aspartate) O-methyltransferases (EC 2.1.1.77) that catalyze the transfer of methyl groups from S-adenosylmethionine to abnormal L-isoaspartyl and D-aspartyl residues in a variety of peptides and proteins are widely distributed in procaryotes and eucaryotes. These enzymes participate in the repair of spontaneous protein damage by facilitating the conversion of L-isoaspartyl and D-aspartyl residues to normal L-aspartyl residues. In this work, we have identified an L-isoaspartyl methyltransferase activity in Arabidopsis thaliana, a dicotyledonous plant of the mustard family. The highest levels of activity were detected in seeds. Using degenerate oligonucleotides corresponding to two highly conserved amino acid regions shared among the Escherichia coli, wheat, and human enzymes, we isolated and sequenced a full-length genomic clone encoding the A. thaliana methyltransferase. Several methyltransferase cDNAs were also characterized, including ones that would encode full-length polypeptides of 230 amino acid residues. Messenger RNAs for the A. thaliana enzyme were found in a variety of tissues that did not contain significant amounts of active enzyme suggesting the possibility of translational or posttranslational controls on methyltransferase levels. We have identified a putative abscisic acid-response element (ABRE) in the 5'-untranslated region of the A. thaliana L-isoaspartyl methyltransferase gene and have shown that the expression of the mRNA is responsive to exogenous abscisic acid (ABA), but not to the environmental stresses of salt or drought. The expression of the A. thaliana enzyme appears to be regulated in a distinct fashion from that seen in wheat or in animal tissues.

Signal-transduction pathways controlling light-regulated development in Arabidopsis

All metazoan cells are able to make decisions about cell division or cellular differentiation based, in part, on environmental cues. Accordingly, cells express receptor systems that allow them to detect the presence of hormones, growth factors and other signals that manipulate the regulatory processes of the cell. In plants, an unusual signal-light-is required for the induction and regulation of many developmental processes. Past physiological and molecular studies have revealed the variety and complexity of plant responses to light but until recently very little was known about the mechanisms of those responses. Two major breakthroughs have allowed the identification of some photoreceptor signalling intermediates: the identification of photoreceptor and signal transduction mutants in Arabidopsis, and the development of single-cell microinjection assays in which outcomes of photoreceptor signalling can be visualized. Here, we review recent genetic advances which support the notion that light responses are not simply endpoints of linear signal transduction pathways, but are the result of the integration of a variety of input signals through a complex network of interacting signalling components.

Light-induced biogenesis of the light-harvesting complexes of Photosystems I and II : Gene expression and protein accumulation

The light-harvesting complexes of Photosystems I and II contain multiple chlorophyll-carotenoid-binding proteins. The stoichiometry and topology of the LHCs is precisely defined to optimally funnel captured light energy to the reaction center. The manner in which this exact arrangement is accomplished is not known. As an initial means to understand the mechanisms involved in establishing a functional LHC, the influence of light on LHC gene expression and protein accumulation was studied during the light-induced greening of etiolated wild type and chlorophyll b-less mutant barley seedlings. Light, involving phytochrome, promotes the expression of all LHC genes with the same relative kinetics. LHC protein accumulation closely parallels the increases observed in transcript levels. Differential accumulation of LHC transcripts or protein was not evident in wild type seedlings. Post-translational factors are likely to be involved in fine tuning the position and stoichiometry of the individual LHCs around the reaction center.

Cytokinin regulation of a soybean pollen allergen gene

Cytokinin treatment of suspension-cultured soybean cells stimulated the accumulation of an mRNA, called cim 1, by a factor of ca. 20 within 4 h. Induction of cim 1 mRNA accumulation occurred at benzyladenine concentrations as low as 10(-8) M. Furthermore, cim 1 mRNA accumulation was stimulated in the absence of cytokinin by staurosporine (an inhibitor of protein kinases) and inhibited in the presence of cytokinin by okadaic acid (an inhibitor of protein phosphatases 1 and 2a), suggesting that cim 1 accumulation in response to cytokinin is dependent on cytokinin-induced dephosphorylation of one or more cellular proteins. The deduced amino acid sequence of the cim 1 protein product, derived from the complete nucleotide sequence of a cim 1 cDNA, was 40% identical to that of a perennial rye grass pollen allergen cDNA (Lol Pl). This sequence also indicated that the cim 1 protein product contains a putative signal peptide followed by predominantly hydrophilic residues, consistent with the hypothesis that it is exported to the apoplast.

Isolation of a cDNA clone for a cytokinin-repressed gene in excised cucumber cotyledons

Rapid changes in gene expression were studied during incubation of cucumber (Cucumis sativus L.) cotyledons with cytokinins in darkness. Complementary-DNA clones for mRNAs whose levels decreased within 4 h of treatment with N6-benzyladenine (BA) were isolated by differential hybridization. One of them (CR9) was sequenced. It is 588 bp long, and would encode a protein consisting of 137 amino-acid residues and having a molecular mass of 15 kDa. The sequence shows a high homology with a light-induced gene from rice. Northern blot analysis of the CR9 transcript showed the level of the mRNA (0.7 kb) to decrease tenfold within 4 h of BA treatment, i.e. well before BA-induced cotyledon expansion was observed. The repression became greater with increasing concentration of BA (10(-8)-10(-5) M). The expression of the CR9 gene was repressed specifically by cytokinins (BA, isopentenyladenine and t-zeatin), but not by adenine or 2,4-dichlorophenoxyacetic acid (auxin). The results are discussed in relation to the primary action of cytokinin.

Out of darkness: mutants reveal pathways controlling light-regulated development in plants

Plant growth and development are governed by complex interactions between environmental signals and endogenous developmental programs. Mutations that disrupt light signal perception or transduction have provided an important resource to begin dissection of the pathways that control light-regulated development in young seedlings.

Phenotypically normal transgenic T-cyt tobacco plants as a model for the investigation of plant gene expression in response to phytohormonal stress

The tumour-inducing T-DNA gene 4 (T-cyt gene) of the nopaline Ti plasmid pTiC58 was cloned and introduced into tobacco cells by leaf disc transformation using Agrobacterium plasmid vectors. Tobacco shoots exposed to elevated cytokinin levels were unable to develop roots and lacked apical dominance. Using exogenously applied phytohormone manipulations we were able to regenerate morphologically normal transgenic tobacco plants which differed in endogenous cytokinin levels from normal untransformed plants. Although T-cyt gene mRNA levels, as revealed by dot-blot hybridization data, in these rooting plants were only about half those in primary transformed shoots the total amount of cytokinins was much lower than in crown gall tissue or cytokinin-type transformed shoots as reported by others. Nevertheless the cytokinin content in T-cyt plants was about 3 times greater than in control tobacco plants. Elevated cytokinin levels have been shown to change the expression of several plant genes, including some nuclear genes encoding chloroplast proteins. Our results show that the mRNA levels of chloroplast rbcL gene increase in cytokinin-type transgenic tobacco plants as compared with untransformed plants. Data obtained suggest that T-cyt transgenic plants are a good model for studying plant gene activity in different parts of the plant under endogenous cytokinin stress.

Down-regulation of phytochrome mRNA abundance by red light and benzyladenine in etiolated cucumber cotyledons

Northern blot analysis revealed that a single 4.2 kb phytochrome mRNA species was detectable in cotyledons excised from five-day-old etiolated cucumber seedlings. Intact etiolated five-day-old cucumber seedlings were given a red light or benzyladenine treatment, and cotyledons were harvested at various times following treatment. The abundance of phytochrome mRNA in the cotyledons was quantitated using 32P-labeled RNA probes and slot blot analysis. By 2 h after irradiation the phytochrome mRNA level was reduced to 40% of the initial abundance and reaccumulation began by 3 h after irradiation. Reaccumulation of phytochrome mRNA to the time-zero dark control level was achieved by 10 h after treatment. A decrease in phytochrome mRNA abundance was evident by 2 h after benzyladenine treatment, and a maximal reduction to 45% of the time-zero dark control was attained by 4 h after treatment. No recovery of the phytochrome mRNA level was evident by 8 h after benzyladenine treatment. The abundance of actin mRNA was unaffected by benzyladenine treatment.

Temporal control of phytochrome-dependent gene expression during radish seedling development

The level of two nuclear-encoded transcripts (the small subunit of ribulose-1,5-bisphosphate carboxylase and the chlorophyll a/b-binding protein of photosystem II) and two chloroplastencoded mRNAs (the large subunit of ribulose-1,5-bisphosphate carboxylase and the chloroplast 32-kDa polypeptide) were analysed during the first 4 d of radish (Raphanus sativus L.) seedling development. A single 5-min red-light pulse increased the concentrations of all transcripts by various orders of magnitude, depending on the particular mRNA. For all transcripts the maximal effect of the red light was observed when the irradiation was applied 48 h following sowing. The red-light treatment did not lead to changes in the total RNA level or to alterations in the levels of nuclear and chloroplastic DNAs; the level of β-actin mRNA was also unaffected by the light treatment. It is concluded that during the first days of development, cells of radish cotyledons undergo light-independent molecular changes, occurring at the transcriptional level, conferring on them a transient competence towards the photoreceptor phytochrome.

Hormonal regulation of gene expression in barley aleurone layers : Induction and suppression of specific genes

As part of our investigation of the mode of action of plant hormones in barley (Hordeum vulgare L.) aleurone layers, we have studied the expression of five identified and three unidentified mRNA species in the presence of exogenous gibberellic acid (GA3) and abscisic acid. Three of the mRNAs are GA3-inducible, three are suppressed by GA3, and two are constitutive. The extent of the GA3 effect differs considerably for both inducible and suppressible mRNAs. For example, a ten-fold higher concentration of GA3 (10(-8) M) is required for full induction of the high-pl group α-amylase mRNA than is required for the low-pI α-amylase mRNA (10(-9) M). Temporal regulation of mRNA abundance also varies between the two α-amylase isoenzyme groups. The three GA3-suppressible mRNA species studied, alcohol dehydrogenase (ADH1), a probable amylase and protease inhibitor, and an unidentified barley mRNA species also varied in response to GA3. The ADH1 mRNA decreased drastically within 8 h of GA3 treatment, whereas the other two began to decrease in abundance only after 12-16 h of GA3 treatment. Abscisic-acid treatment counteracted the GA3 effects for both the inducible and suppressible mRNA species. Comparison of α-amylase-mRNA levels and α-amylase-synthesis rates showed a strong correlation between the two parameters, the only exception being a lack of α-amylase synthesis in the presence of α-amylase mRNA at low GA3 concentrations. Therefore, the expression of α-amylase seems to be regulated primarily by its mRNA levels.

Regulation and expression of the multigene family coding light-harvesting chlorophyll a/b-binding proteins of photosystem II

The current state of knowledge concerning the expression of the nuclear genes that code the light-harvesting chlorophyll a/b-binding polypeptides of photosystem II is presented. This review covers the structure of these genes, the complex multistep pathway involved in their expression, and the environmental and other factors which regulate their expression. Some of the effects of these factors are mediated, at least in part, at the level of transcription, but other effects can be explained only by the existence of multiple posttranscriptional regulatory steps.

Characterization of a negatively light-regulated mRNA from Lemna gibba

We have isolated a cDNA clone, called pLg106, which corresponds to a negatively light-regulated mRNA in the aquatic monocot Lemna gibba. Lg106 mRNA is more abundant in dark-treated plants than in plants grown under continuous white light. The levels of Lg106 mRNA reached a maximum 12-24 hours after plants had been placed in the dark. In vitro-labelled transcripts from nuclei isolated from white light-grown and darktreated Lemna hybridized equally to Lg106 DNA, indicating that transcription of the Lg106 gene was similar in both white light and darkness. Therefore, the higher level of Lg106 mRNA in the dark was not attributable to a change in transcription. We suggest that the stability of Lg106 mRNA is affected by changes in illumination. The steady-state level of Lg106 did not appear to be under phytochrome regulation, as the abundance of Lg106 mRNA from plants grown in intermittent red light or intermittent red followed immediately by far-red light was not significantly different from that of dark-treated plants. The size of the mRNA was estimated to be 650-700 nt. Genomic Southern blots indicated the presence of a single gene for Lg106. The Lg106 cDNA was sequenced and examined for similarities to nucleotide sequences in GenBank.

Changes in the tissue-specific prevalence of translatable mRNAs in transgenic tobacco shoots containing the T-DNA cytokinin gene

Two-dimensional gel electrophoresis of in vitro translation products was used to examine differences between the steady state RNA populations of an untransformed tobacco plant line and a non-rooting tobacco shoot line transformed with a T l -DNA segment from Agrobacterium tumefaciens carrying the cytokinin gene (T-cyt). The analysis comprised about 240 translation products representing the more abundant mRNAs. Approximately 8% of the translation products were found to have significantly different concentrations, due to both increases and decreases, when the shoot parts of the transformed and untransformed lines were compared. Only a few of these differences were specific for the comparison of transformed and untransformed shoots. Most of the differences were also observed when the shoot and root parts of the untransformed line were compared. This implies that the shoot or root prevalence of several mRNA species in normal plants is altered in transgenic T-cyt shoots. The observed changes in the mRNA population of transgenic T-cyt shoots are discussed in relation to the transformed phenotype and previously cloned mRNAs showing similar changes in tissue-specific prevalence.

Cytokinin modulation of LHCP mRNA levels: the involvement of post-transcriptional regulation

When white-light-grown Lemna gibba plants are placed in the dark, the levels of mRNAs for two nuclear-encoded chloroplast proteins, the small subunit of ribulose-1,5-bisphosphate carboxylase (SSU) and the major chlorophyll a/b-binding protein of light-harvesting complex II (LHCP), decline to a small fraction of their previous level. We have reported [4] that red light (R), acting through phytochrome, and benzyladenine (BA), a synthetic cytokinin, independently stimulate accumulation of both mRNAs in the dark. Here, we have analyzed the products of transcription in isolated nuclei to determine if cytokinins act primarily through stimulation of transcription or if post-transcriptional processes are involved. We find that BA pretreatment may slightly stimulate transcription of LHCP RNA either with or without a red-light treatment. However, the effects of BA on the LHCP RNA accumulation were much greater than on transcription. Two naturally occurring cytokinins are also effective in increasing the mRNA abundance. We therefore conclude that, in Lemna, post-transcriptional processes are important in regulation of the LHCP RNA by cytokinins.

The major light-harvesting complex of Photosystem II: aspects of its molecular and cell biology

The light-harvesting complex of photosystem II (LHC II) contains one major (LHC IIb) and at least three minor chlorophyll-protein components. The apoproteins of LHC IIb (LHCP) are encoded by nuclear genes and synthesized in the cytoplasm as a higher molecular weight precursor(s) (pLHCP). Several genes coding for pLHCP have been cloned from various higher plant species. The expression of these genes is dependent upon a variety of factors such as light, the developmental stage of the plastids and the plant. After its synthesis in the cytoplasm, pLHCP is imported into plastids, inserted into thylakoids, processed to its mature form, and assembled into LHC IIb. The pathway of assembly of LHC IIb in the thylakoid membranes is currently being investigated in several laboratories. We present a model that gives some details of the steps in the assembly process. Many of the steps involved in the synthesis and assembly are dependent on light and the stage of plastid development.

Level of messenger RNA encoding small subunit ribulose bisphosphate carboxylase is enhanced by cytokinins in tobacco cell suspension cultures

Chloroplast differentiation is induced by cytokinins in suspension cell cultures of Nicotiana tabacum, line 19M which is independent of the hormone supply for growth. Poly(A)RNA from cells cultured in basal medium or in kinetin-supplemented medium were analyzed by Northern blot and dot blot hybridizations to a 'RUBISCO' small subunit-encoding cDNA probe. It was found that the small subunit-encoding mRNA of cytokinin-supplemented cells was synthesized much earlier during the culture and in amounts one order of magnitude larger than in hormone-starved cells.

Control of plastid gene expression during development: the limited role of transcriptional regulation

We have analyzed the transcriptional regulation of plastid genes during chloroplast development in illuminated spinach cotyledons and during leaf formation. The RNAs encoded by plastid genes accumulate with different kinetics during the developmental transitions. Using a novel plastid run-on transcription assay we demonstrate that the transcriptional regulation of a large, diverse group of chloroplast genes is of relatively minor importance for the control of their expression. The general transcriptional activity of the plastid genome increases after illumination and decreases during leaf development. This modulation of general transcriptional activity affects most plastid genes simultaneously and is not correlated with adjustments of the plastid DNA copy number. There are no major changes in the relative transcriptional activities of different genes, although their steady-state mRNA levels change dramatically. The analysis of ten specific plastid genes shows that their relative transcriptional activities are largely maintained throughout the developmental program. This limited transcriptional regulation suggests that plastid gene expression in higher plants is effectively controlled at the posttranscriptional level.