Cytokinin-induced mRNAs in cultured soybean cells

The Cytokinins BAP and 2-iP Modulate Different Molecular Mechanisms on Shoot Proliferation and Root Development in Lemongrass (Cymbopogon citratus)

The known activities of cytokinins (CKs) are promoting shoot multiplication, root growth inhibition, and delaying senescence. 6-Benzylaminopurine (BAP) has been the most effective CK to induce shoot proliferation in cereal and grasses. Previously, we reported that in lemongrass (Cymbopogon citratus) micropropagation, BAP 10 µM induces high shoot proliferation, while the natural CK 6-(γ,γ-Dimethylallylamino)purine (2-iP) 10 µM shows less pronounced effects and developed rooting. To understand the molecular mechanisms involved, we perform a protein-protein interaction (PPI) network based on the genes of Brachypodium distachyon involved in shoot proliferation/repression, cell cycle, stem cell maintenance, auxin response factors, and CK signaling to analyze the molecular mechanisms in BAP versus 2-iP plants. A different pattern of gene expression was observed between BAP- versus 2-iP-treated plants. In shoots derived from BAP, we found upregulated genes that have already been demonstrated to be involved in de novo shoot proliferation development in several plant species; CK receptors (AHK3, ARR1), stem cell maintenance (STM, REV and CLV3), cell cycle regulation (CDKA-CYCD3 complex), as well as the auxin response factor (ARF5) and CK metabolism (CKX1). In contrast, in the 2-iP culture medium, there was an upregulation of genes involved in shoot repression (BRC1, MAX3), ARR4, a type A-response regulator (RR), and auxin metabolism (SHY2).

Transcriptomic and proteomic analyses of embryogenic tissues in Picea balfouriana treated with 6-benzylaminopurine

The cytokinin 6-benzylaminopurine (6-BAP) influences the embryogenic capacity of the tissues of Picea balfouriana during long subculture (after 3 months). Tissues that proliferate in 3.6 and 5 µM 6-BAP exhibit the highest and lowest embryogenic capacity, respectively, generating 113 ± 6 and 23 ± 3 mature embryos per 100 mg of tissue. In this study, a comparative transcriptomic and proteomic approach was applied to characterize the genes and proteins that are differentially expressed among tissues under the influence of different levels of 6-BAP. A total of 51 375 unigenes and 2617 proteins were obtained after quality filtering. There were 2770 transcripts for proteins found among these unigenes. Gene ontology (GO) analysis of the differentially expressed unigenes and proteins showed that they were involved in cell and binding activity and were enriched in ribosome and glutathione metabolism pathways. Ribosomal proteins, glutathione S-transferase proteins, germin-like proteins and calmodulin-independent protein kinases were up-regulated in the embryogenic tissues with the highest embryogenic ability (treated with 3.6 µM 6-BAP), which was validated via quantitative real-time polymerase chain reaction (qRT-PCR) analysis, and these proteins might serve as molecular markers of embryogenic ability. Data are available via Sequence Read Archive (SRA) and ProteomeXchange with identifier SRP042246 and PXD001022, respectively.

A soybean β-expansin gene GmEXPB2 intrinsically involved in root system architecture responses to abiotic stresses

Root system architecture responds plastically to some abiotic stresses, including phosphorus (P), iron (Fe) and water deficiency, but its response mechanism is still unclear. We cloned and characterized a vegetative β-expansin gene, GmEXPB2, from a Pi starvation-induced soybean cDNA library. Transient expression of 35S::GmEXPB2-GFP in onion epidermal cells verified that GmEXPB2 is a secretory protein located on the cell wall. GmEXPB2 was found to be primarily expressed in roots, and was highly induced by Pi starvation, and the induction pattern was confirmed by GUS staining in transgenic soybean hairy roots. Results from intact soybean composite plants either over-expressing GmEXPB2 or containing knockdown constructs, showed that GmEXPB2 is involved in hairy root elongation, and subsequently affects plant growth and P uptake, especially at low P levels. The results from a heterogeneous transformation system indicated that over-expressing GmEXPB2 in Arabidopsis increased root cell division and elongation, and enhanced plant growth and P uptake at both low and high P levels. Furthermore, we found that, in addition to Pi starvation, GmEXPB2 was also induced by Fe and mild water deficiencies. Taken together, our results suggest that GmEXPB2 is a critical root β-expansin gene that is intrinsically involved in root system architecture responses to some abiotic stresses, including P, Fe and water deficiency. In the case of Pi starvation responses, GmEXPB2 may enhance both P efficiency and P responsiveness by regulating adaptive changes of the root system architecture. This finding has great agricultural potential for improving crop P uptake on both low-P and P-fertilized soils.

Overexpression of a type-A response regulator alters rice morphology and cytokinin metabolism

Genome-wide analyses of rice (Oryza sativa L.) cytokinin (CK)-responsive genes using the Affymetrix GeneChip(R) rice genome array were conducted to define the spectrum of genes subject to regulation by CK in monocotyledonous plants. Application of trans-zeatin modulated the expression of a wide variety of genes including those involved in hormone signaling and metabolism, transcriptional regulation, macronutrient transport and protein synthesis. To understand further the function of CK in rice plants, we examined the effects of in planta manipulation of a putative CK signaling factor on morphology, CK metabolism and expression of CK-responsive genes. Overexpression of the CK-inducible type-A response regulator OsRR6 abolished shoot regeneration, suggesting that OsRR6 acts as a negative regulator of CK signaling. Transgenic lines overexpressing OsRR6 (OsRR6-ox) had dwarf phenotypes with poorly developed root systems and panicles. Increased content of trans-zeatin-type CKs in OsRR6-ox lines indicates that homeostatic control of CK levels is regulated by OsRR6 signaling. Expression of genes encoding CK oxidase/dehydrogenase decreased in OsRR6-ox plants, possibly accounting for elevated CK levels in transgenic lines. Expression of a number of stress response genes was also altered in OsRR6-ox plants.

A transcriptome-based characterization of habituation in plant tissue culture

For the last 50 years, scientists have recognized that varying ratios of the plant hormones cytokinin and auxin induce plant cells to form particular tissues: undifferentiated calli, shoot structures, root structures, or a whole plant. Proliferation of undifferentiated callus tissue, greening, and the formation of shoot structures are all cytokinin-dependent processes. Habituation refers to a naturally occurring phenomenon whereby callus cultures, upon continued passage, lose their requirement for cytokinin. Earlier studies of calli with a higher-than-normal cytokinin content indicate that overproduction of cytokinin by the culture tissues is a possible explanation for this acquired cytokinin independence. A transcriptome-based analysis of a well established habituated Arabidopsis (Arabidopsis thaliana) cell culture line was undertaken, to explore genome-wide expression changes underlying the phenomenon of habituation. Increased levels of expression of the cytokinin receptor CRE1, as well as altered levels of expression of several other genes involved in cytokinin signaling, indicated that naturally acquired deregulation of cytokinin-signaling components could play a previously unrecognized role in habituation. Up-regulation of several cytokinin oxidases, down-regulation of several known cytokinin-inducible genes, and a lack of regulation of the cytokinin synthases indicated that increases in hormone concentration may not be required for habituation. In addition, up-regulation of the homeodomain transcription factor FWA, transposon-related elements, and several DNA- and chromatin-modifying enzymes indicated that epigenetic changes contribute to the acquisition of cytokinin habituation.

Cycloheximide treatment of cotton ovules alters the abundance of specific classes of mRNAs and generates novel ESTs for microarray expression profiling

Fibres of cotton (Gossypium hirsutum L.) are single elongated epidermal cells that start to develop on the outer surface of cotton ovules on the day of anthesis. Little is known about the control of fibre initiation and development. As a first step towards discovering important genes involved in fibre initiation and development using a genomics approach, we report technical advances aimed at reducing redundancy and increasing coverage for anonymous cDNA microarrays in this study. Cotton ovule cDNA libraries (both normalised and un-normalised) from around the time of fibre initial formation have been prepared and partially characterised by sequencing. Re-association-based normalisation partially reduced library redundancy and increased representation of novel sequences. However, another library generated from in vitro cultured cotton ovules treated with the protein synthesis inhibitor, cycloheximide, showed a significantly altered gene representation including a greater proportion of protein phosphorylation genes, transport genes and transcription factors and a much reduced proportion of protein synthesis genes than were identified in the conventional types of libraries. Over 10,000 expressed sequence tag (EST) clones randomly selected from the three libraries were printed on microarray slides and used to assess gene expression in tissue cultured ovules with and without cycloheximide treatment. The microarray results showed that cycloheximide had a dramatic effect in modifying the pattern of the gene expression in cultured ovules, affecting the same types of genes identified in the preliminary analysis on relative EST abundance in the different ovule cDNA libraries. Cycloheximide clearly provided a simple and useful method for enriching novel gene sequences for genomic studies.

Brassinolides and IAA induce the transcription of four alpha-expansin genes related to development in Cicer arietinum

Four different cDNAs encoding alpha-expansins have been identified in Cicer arietinum (Ca-EXPA1, Ca-EXPA2, Ca-EXPA3 and Ca-EXPA4). The shared amino acid sequence similarity among the four alpha-expansin proteins ranged from 67 to 89%. All of them display common characteristics such as molecular mass (around 24 kDa), amino acid numbers, and also the presence of a signal peptide. The transcription pattern of chickpea alpha-expansin genes in seedlings and plants suggests a specific role for each of the four alpha-expansins in different phases of development or in different plant organs. High levels of Ca-EXPA2 transcripts coincide with maximum epicotyl and stem growth, indicating an important involvement of this particular alpha-expansin in elongating tissues. Ca-EXPA3 would be related to radicle development, while Ca-EXPA4 seems to be involved in pod development. A considerable increase in the level of all Ca-EXPA transcripts accompanied the indole acetic acid (IAA) plus brassinolide (BR)-induced elongation of excised epicotyl segments. This IAA + BR induction was seen even for the chickpea expansin genes whose transcription was not affected by IAA or BR alone.

Expression profiling of cytokinin action in Arabidopsis

The phytohormone cytokinin is an important regulator of plant growth and development; however, relatively few genes that mediate cytokinin responses have been identified. Genome-wide analyses of Arabidopsis seedlings using the approximately 8,300-element Affymetrix Arabidopsis GeneChips (Affymetrix, Santa Clara, CA) to examine cytokinin-responsive genes were conducted, revealing at least 30 genes whose steady-state level of mRNA was elevated and at least 40 that were down-regulated at multiple time points after application of cytokinin. The cytokinin up-regulated genes include the type-A Arabidopsis response regulators (ARRs), which had been shown previously to be cytokinin primary response genes, cytokinin oxidase, which encodes an enzyme that degrades cytokinins, and several transcription factors. Cytokinin down-regulated genes include several peroxidases and kinases and an E3 ubiquitin ligase. We identified a common sequence motif enriched in the upstream regions of the most consistently cytokinin up-regulated genes. This motif is highly similar to the optimal DNA-binding sites for ARR1/ARR2, type-B ARRs that have been implicated in the transcriptional elevation of the type-A ARRs. Additionally, genome-wide analyses of cytokinin receptor mutants (wol/cre1) revealed large-scale changes in gene expression, including down-regulation of the type-A ARRs and several meristem and cell cycle genes, such as CycD3. Mutations in CRE1 reduced but did not eliminate the effect of cytokinin on gene expression for a subset of cytokinin-responsive genes and had little or no effect on others, suggesting functional redundancy among the cytokinin receptors.

Severe oral allergy syndrome and anaphylactic reactions caused by a Bet v 1- related PR-10 protein in soybean, SAM22

BACKGROUND

Anaphylactic reactions to soy products have been attributed to stable class 1 food allergens.

OBJECTIVE

IgE- mediated reactions to a soy-containing dietary food product in patients allergic to birch pollen were investigated.

METHODS

Detailed case histories were taken from 20 patients. Their sera were analyzed for IgE (UniCAP) specific for birch, grass, mugwort, the recombinant birch allergens rBet v 1 and rBet v2, and soy protein. Extracts from birch pollen, soy isolate, rBet v 1, and the recombinant PR-10 soy protein rSAM22 were coupled to paper disks or nitrocellulose for IgE measurements (enzyme allergosorbent test) or Western blot analysis. Enzyme allergosorbent testing, Western blot inhibition, and histamine release studies were performed with the same allergens.

RESULTS

Most patients (17/20) experienced facial, oropharyngeal, and/or systemic allergic symptoms within 20 minutes after ingesting the soy product for the first time. Birch pollen allergy (16/20) was common, along with oral allergy syndrome to apple (12/20) or hazelnut (11/20). IgE levels to birch and Bet v 1 but not to other inhalants were high in 18 of 20 patients. Significant IgE binding to rSAM22 occurred in 17 of 20 patients. Blot experiments with the soy isolate revealed IgE-binding bands at 17 kd (15/20), 22 kd (1/20), and 35 to 38 kd (2/20); the former was inhibited by preincubation of the sera with rBet v 1 or rSAM22. Birch extract and soy isolate, rBet v 1, and rSAM22 induced dose-dependent histamine release in the nanomolar range.

CONCLUSION

Immediate-type allergic symptoms in patients with birch pollen allergy after ingestion of soy protein-containing food items can result from cross-reactivity of Bet v 1 -specific IgE to homologous pathogenesis-related proteins, particularly the PR-10 protein SAM22.

Expression and processing of a hormonally regulated beta-expansin from soybean

Expansin proteins are essential components of acid-induced cell wall loosening in plants. Beta-expansins, which constitute a subfamily of related expansin proteins, include the group I grass pollen allergens. To provide a better description of beta-expansin expression, we have characterized a cytokinin-inducible beta-expansin from soybean (Glycine max cv Mandarin) called Cim1. Our results demonstrate that the hormones cytokinin and auxin act synergistically to induce the accumulation and proteolytic processing of Cim1. Carboxyl terminal truncation of a 35-kD form of Cim1 is predicted to remove the putative cellulose binding domain from the amino terminal cysteine-rich domain, resulting in a 20-kD form of the protein. Furthermore, the identical amino termini of the 35- and 20-kD forms of Cim1 correspond to a position 11 amino acids downstream of the predicted signal sequence cleavage site, suggesting proteolysis of a short amino terminal propeptide after removal of the signal peptide. This propeptide fragment contains a consensus site for N-glycosylation and our data suggest that it is glycosylated by a tunicamycin-sensitive mechanism in cultured soybean cells. The onset of Cim1 expression correlates with increased growth of soybean cultures. Ultimately, Cim1 is rapidly and specifically proteolyzed as soybean cultures reach stationary phase. These findings are consistent with the hypothesis that beta-expansin proteins are extensively modified by post-translational N-glycosylation and proteolysis.

Interaction of DNA-binding proteins with the 5'-flanking region of a cytokinin-responsive cucumber hydroxypyruvate reductase gene

Transcription of the cucumber hpr-A gene is responsive to cytokinin and light. To investigate the molecular basis for transcriptional regulation by cytokinin, we have identified DNA sequences and proteins that may be involved in the regulation of hpr-A gene expression. Transient expression assays in etiolated cucumber cotyledons indicate that the 315 bp fragment (-382 to -67) contains sequences necessary for cytokinin responsiveness of the luciferase reporter gene. Band shift assays detected cytokinin-enhanced and -reduced protein binding sites in a 97 bp fragment (-382 to -285) upstream of the hpr-A gene. DNase I footprinting identified two protein-protected sites, a 15 bp sequence, 5'-AAATGACGAAAATGC-3', that contains an as-1 TGACG motif found in other plant promoters, and a 13 bp sequence, 5'-AAGATTGATTGAG-3', of unknown function. Two-dimensional band shift analysis of the cytokinin-responsive DNA protein complex revealed the presence of six DNA protein interactions. Band shift assays showed that cytokinin and light have different effects on the interaction of nuclear proteins to the 97 bp fragment of the hpr-A gene. These data suggest that cytokinin and light do not share identical signal transduction pathways in regulating hpr-A gene expression.

Cytokinin regulates the expression of a soybean beta-expansin gene by a post-transcriptional mechanism

The cytokinin-inducible soybean mRNA Cim1 accumulates 20-60-fold upon cytokinin addition to cytokinin-starved soybean suspension cultures. In this report, we demonstrate that cytokinin-induced stability of the Cim1 mRNA plays an important role in the accumulation of the message. We also present evidence that cytokinin-induced Cim1 stability is blocked by the addition of the protein phosphatase inhibitor okadaic acid. Thus, we suggest that protein phosphatase activity is required for the cytokinin-induced stability and subsequent accumulation of Cim1 in soybean cells. The deduced amino acid sequence of the Cim1 protein product is similar to the group I pollen allergens from various plants, which constitute a subfamily of expansin proteins. The relatedness between Cim1 and the expansins supports our hypothesis that the protein product of Cim1 is localized to the cell wall and suggests a role for Cim1 in cytokinin-regulated cell wall expansion. Thus, post-transcriptional regulation of Cim1 by cytokinin may represent a molecular link between cytokinin and changes in cell shape and size.

Posttranscriptional Regulation of the Sesbania rostrata Early Nodulin Gene SrEnod2 by Cytokinin

The mRNA from the Sesbania rostrata early nodulin gene SrEnod2 accumulates in response to cytokinin application. Nuclear run-on assays using isolated root nuclei have shown that this accumulation occurs posttranscriptionally, and northern blot analysis of nuclear and total RNA levels revealed that it occurs primarily in the cytoplasm and not in the nucleus. After cytokinin enhancement of SrEnod2 mRNA accumulation and the subsequent removal of cytokinin, the levels of SrEnod2 mRNA did not return to basal levels, but oscillated over a 36-h time course. Application of the translational inhibitor cycloheximide was found to inhibit the enhancement of SrEnod2 mRNA accumulation by cytokinin and to cause its rapid decay. Okadaic acid and staurosporine, inhibitors of protein phosphatases and kinases, respectively, also inhibited cytokinin enhancement of SrEnod2 mRNA accumulation. In addition, okadaic acid was found to cause a decrease in SrEnod2 mRNA levels. These results provide evidence for a posttranscriptional mechanism of cytokinin enhancement of SrEnod2 mRNA accumulation, which appears to require concurrent protein synthesis, to involve protein phosphatases and kinases, and to occur primarily in the cytoplasm of the plant cell.

Transcriptional regulation of hydroxypyruvate reductase gene expression by cytokinin in etiolated pumpkin cotyledons

To understand the mechanisms by which the expression of a specific gene is modulated by cytokinin, the regulation of hydroxypyruvate reductase (HPR) transcript levels by N6-benzyladenine (BA) in etiolated pumpkin (Cucurbita pepo L. cv. Halloween) cotyledons was investigated. A pumpkin HPR cDNA was generated by reverse transcriptase-polymerase chain reaction and its nucleotide sequence was determined. An antisense HPR RNA was prepared for RNase protection analysis of HPR-mRNA expression patterns in the cotyledons of dark-grown pumpkin seedlings. Treatment of the cotyledons with BA was shown to modulate HPR mRNA levels in a dose- and time-dependent manner. Similarly, nuclear run-on studies showed that the rate of transcription was also enhanced by BA treatment of the cotyledons. These results suggest that the enhancement of HPR mRNA by cytokinin is, at least in part, at the level of transcription.

A novel cytokinin-resistant mutant of Arabidopsis with abbreviated shoot development

Cytokinins influence several fundamental processes of plant growth and development, including cell division and organogenesis. To identify genes involved in cytokinin response, a screen was carried out for mutants of Arabidopsis thaliana (L.) Heynh. that are resistant to elevated levels of exogenous cytokinins. One such mutant was isolated and named cyr1. A cross to another Arabidopsis cytokinin-resistant mutant, ckr1 (Su and Howell 1992), indicated that the two mutants are not allelic. The recessive cyr1 mutation causes tenfold decreased sensitivity to benzyladenine in a root-elongation assay but does not confer resistance of roots to indole-3-acetic acid, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, or abscisic acid. The mutant has increased sensitivity to abscisic acid. This mutation is apparently pleiotropic, giving rise to shoot abnormalities. The phenotype of the cyr1 shoot includes abbreviated development with reduction in cotyledon and leaf expansion, limited leaf production, reduced chlorophyll accumulation, failure to accumulate anthocyanins in response to cytokinins, and the formation of a single infertile flower. These traits, as well as root resistance to cytokinin, are all consistent with a defect in cytokinin action, and are probably due to mutation of a single gene. The cyr1 gene is located on chromosome 5, between 76.2 and 77.6 cM.

A family of cyclin D homologs from plants differentially controlled by growth regulators and containing the conserved retinoblastoma protein interaction motif

A new family of three related cyclins has been identified in Arabidopsis by complementation of a yeast strain deficient in G1 cyclins. Individual members show tissue-specific expression and are conserved in other plant species. They form a distinctive group of plant cyclins, which we named delta-type cyclins to indicate their similarities with mammalian D-type cyclins. The sequence relationships between delta and D cyclins include the N-terminal sequence LXCXE. This motif was originally identified in certain viral oncoproteins and is strongly implicated in binding to the retinoblastoma protein pRb. By analogy to mammalian cyclin D, these plant homologs may mediate growth and phytohormonal signals into the plant cell cycle. In support of this hypothesis, we show that, on restimulation of suspension-cultured cells, cyclin delta 3 is rapidly induced by the plant growth regulator cytokinin and cyclin delta 2 is induced by carbon source.

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.

Light and nutritional regulation of transcripts encoding a wheat protein kinase homolog is mediated by cytokinins

Many metabolic processes in plants are regulated by phosphorylation of proteins by kinases, but little is known of the roles that specific protein kinase play in the various signal transduction pathways or the mechanisms by which these kinases themselves are regulated. We report here the isolation of a gene, wpk4, encoding a putative protein kinase from wheat that appears to belong to the SNF1 kinase subfamily and that shows increased transcript levels in response to multiple stimuli: light, nutrient deprivation, and cytokinin application. Although wpk4 mRNA is undetectable in etiolated seedlings, it rapidly accumulates within 1 hr of illumination. General nutrient deprivation also increases wpk4 mRNA levels, but only under light conditions. In addition, of the various phytohormones tested, cytokinin (N6-benzylaminopurine) specifically increases wpk4 mRNA levels regardless of the light conditions, whereas in the presence of a cytokinin antagonist the level of wpk4 mRNA is not increased by either light or nutrient deprivation. These results suggest that the light and nutrient signals that induce wpk4 mRNA accumulation may be mediated through cytokinins and provide a strong basis for examining the coordinated regulation of protein phosphorylation by light, cytokinins, and nutritional cues in a single transduction pathway.

Gene expression during leaf senescence

Leaf senescence is a hiphly-controlled sequence of events comprising the final stage of development. Cells remain viable during the process and new gene expression is required. There is some similarity between senescence in plants and programmed cell death in animals. In this review, different classes of senescence-related genes are defined and progress towards isolating such genes is reported. A range of internal and external factors which appear to cause leaf senescence is considered and various models for the mechanism of senescence- initiation are described. The current understanding of senescence at the wrganelle and molecular levels is presented. Finally, same ideas are mooted as to why senescence occurs and why it should be studied further. Contents Summary 419 I. Introduction 420 II. Internal factors that cause senescence 423 III. External factors that cause senescence 427 IV. What is the mechanism of senescence initiation? 428 V. Progress in the understanding of organelle senescence 431 VI. Progress in the understanding of senescence at the molecular level 434 VII. The control of senescence in animals and plants 440 VIII. Why is senescence necessary? 441 IX. Why study senescence? 441 References 442.

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.

Activation of anthocyanin synthesis genes by white light in eggplant hypocotyl tissues, and identification of an inducible P-450 cDNA

Eggplant seedlings (Solanum melongena) grown under red light irradiation showed a normal morphology with green, fully expanded cotyledons. When the seedlings grown under red light were irradiated with ultraviolet-containing white light, anthocyanin synthesis was induced in the hypocotyl tissues, especially when a UV light supplement was added. The accumulation of pigments was closely associated with the expression of genes involved in flavonoid synthesis. These genes include chalcone synthase (CHS) and dihydroflavonol 4-reductase (DFR). Using subtracted probes, which had been enriched for the accumulated mRNA, one white light-responsive cDNA was identified as being a P450 gene by comparison with database sequences. The maximal amino acid homology this cDNA had with other P450s was 36%. This was with CYP71 from avocado (Persea americana). Thus it represents a new P-450 family, which has been named CYP75. The mRNA of this gene was localized in the hypocotyl tissues of eggplant seedlings, which had been white light-irradiated. The transcript was accumulated by changing the light source, as in the case of other flavonoid biosynthesis genes. In delphinidin producing petunia plants, the mRNAs corresponding to the eggplant P-450 and flavonoid biosynthesis genes such as CHS and DFR were most abundant during the mid stage of flower bud development, but could not be detected in leaf tissues. These results suggest that this P-450 gene encodes a hydroxylating enzyme involved in flavonoid biosynthesis.

Inhibition of growth of cultured tobacco cells at low concentrations of lovastatin is reversed by cytokinin

De novo synthesis of mevalonic acid, which is catalyzed by 3-hydroxy-3-methylglutaryl coenzyme A reductase, is the first committed step in the formation of isoprenoid compounds. Various studies have shown that mevalonic acid-derived compounds are required for growth of plant and animal cells, a conclusion supported by the observation that cells treated with lovastatin (a potent inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase) cease growth. We show that Nicotiana tabacum BY-2 cells, which require exogenous auxin for growth in culture but do not require exogenous cytokinin, are growth inhibited by 1 mum lovastatin. However, these cells are capable of growing in the presence of 1 mum lovastatin if 8 mum zeatin is supplied in the medium. Furthermore, benzyladenine, kinetin, and thidiazuron effectively reverse the inhibition of growth of these cells at 1 mum lovastatin, whereas adenine and 6-methyladenine have no effect. These results demonstrate that restoration of growth to lovastatin-treated cells is cytokinin specific and is not caused by metabolism of cytokinin into other isoprenoid compounds. Cytokinin does not effectively reverse the effects of higher concentrations of lovastatin, but mevalonic acid does, consistent with the hypothesis that cytokinin biosynthesis is more sensitive to lovastatin than the biosynthesis of other essential isoprenoid compounds in tobacco cells. This observation suggests that lovastatin can be used to induce cytokinin dependence in cytokinin-autonomous tobacco cell cultures.

Expression of a ribosomal protein gene in axillary buds of pea seedlings

Axillary buds of intact pea seedlings (Pisum sativum L. cv Alaska) do not grow and are said to be dormant. Decapitation of the terminal bud promotes the growth of these axillary buds, which then develop in the same manner as terminal buds. We previously showed that unique sets of proteins are expressed in dormant and growing buds. Here we describe the cloning, sequencing, and expression of a cDNA clone (pGB8) that is homologous to ribosomal protein L27 from rat. RNA corresponding to this clone increases 13-fold 3 h after decapitation, reaches a maximum enhancement of about 35-fold after 12 h, and persists at slightly reduced levels at later times. Terminal buds, root apices, and elongating internodes also contain pGB8 mRNA but fully expanded leaflets and fully elongated internodes do not. In situ hybridization analysis demonstrates that pGB8 mRNA increases in all parts of the bud within 1 h of decapitation. Under appropriate conditions, growing buds can be made to stop growing and become dormant; these buds subsequently can grow again. Therefore, buds have the capacity to undergo multiple cycles of growth and dormancy. RNA gel blots show that pGB8 expression is reduced to dormancy levels as soon as buds stop growing. However, in situ hybridization experiments show that pGB8 expression continues at growing-bud levels in the apical meristem for 2 d after it is reduced in the rest of the bud. When cultured stems containing buds are treated with indoleacetic acid at concentrations >/=10 mum, bud growth and expression of pGB8 in the buds are inhibited.

Cytokinins and auxins control the expression of a gene in Nicotiana plumbaginifolia cells by feedback regulation

Both cytokinin (N6-benzyladenine [BA]) and auxin (2,4-dichlorophenoxyacetic acid [2,4-D]) stimulate the accumulation of an mRNA, represented by the cDNA pLS216, in Nicotiana plumbaginifolia suspension culture cells. The kinetics of RNA accumulation were different for the two hormones; however, the response to both was transient, and the magnitude of the response was dose dependent. Runoff transcription experiments demonstrated that the transient appearance of the RNA could be accounted for by feedback regulation of transcription and not by the induction of an RNA degradation system. The feedback mechanism appeared to desensitize the cells to further exposure of the hormone. In particular, cells became refractory to the subsequent addition of 2,4-D after the initial RNA accumulation response subsided. A very different response was observed when the second hormone was added to cells that had been desensitized to the first hormone. Under such conditions, BA produced a heightened response in cells desensitized to 2,4-D and vice versa. These findings support a model in which cytokinin further enhances the auxin response or prevents its feedback inhibition. The hormone-induced RNA accumulation was blocked by the protein kinase inhibitor staurosporin. On the other hand, the protein phosphatase inhibitor okadaic acid stimulated expression, and, in particular, okadaic acid was able to stimulate RNA accumulation in cells desensitized to auxin. This suggests that hormone activation involves phosphorylation of critical proteins on the hormone signaling pathway, whereas feedback inhibition may involve dephosphorylation of these proteins. The sequence of pLS216 is similar to genes in other plants that are stimulated by multiple agonists such as auxins, elicitors, and heavy metals, and to the gene encoding the stringent starvation protein in Escherichia coli. It is proposed that this gene family in various plants be called multiple stimulus response (msr) genes.

Characterization of a stress-induced, developmentally regulated gene family from soybean

We describe a family of stress-induced, developmentally regulated soybean genes for which cDNAs have been obtained from two different cultivars (Glycine max cv. Mandarin and Glycine max cv. Williams). The mRNAs corresponding to these cDNAs, called SAM22 and H4, respectively, accumulate predominantly in the roots of soybean seedlings but are present at high levels in the roots and leaves of mature plants. SAM22 accumulation is especially dramatic in senescent leaves. In addition, SAM22 accumulation can be induced on young leaves by wounding or by transpiration-mediated uptake of salicylic acid, methyl viologen, fungal elicitor, hydrogen peroxide or sodium phosphate (pH 6.9). Taken together, these data indicate that the genes corresponding to SAM22 and H4 are induced by various stresses and developmental cues. Southern blot analysis indicates that multiple copies of sequences related to SAM22 exist in the soybean genome. We also show that the nucleotide sequences of the cDNAs corresponding to SAM22 and H4 are 86% identical at the nucleotide level to each other and 70% identical at the amino acid level to the 'disease resistance response proteins' of Pisum sativum.

Induction of Specific mRNAs in Cultured Soybean Cells during Cytokinin or Auxin Starvation

We report the isolation of five cDNA clones whose corresponding mRNAs accumulate in cultured soybean cells (Glycine max cv Mandarin) during cytokinin or auxin starvation. The levels of three of these mRNAs decrease rapidly after addition of 5 micromolar zeatin to cytokinin-starved cells or after addition of 10 micromolar alpha-naphthaleneacetic acid to auxin-starved cells. These mRNAs also exhibit various patterns of accumulation in the tissues of intact soybean plants. Partial nucleotide sequence analysis demonstrates that one of the cDNAs in the collection, called SAM46, is 46% identical at the amino acid level to the iron superoxide dismutase gene of Escherichia coli. Expression of this cDNA in Escherichia coli cells results in detectable iron superoxide dismutase activity, confirming the identity of the cDNA.