EMF, an Arabidopsis Gene Required for Vegetative Shoot Development

In higher plants, the transition from the vegetative to the; reproductive state in the shoot meristem initiates flowering. To study this floral transition, Constitutively flowering mutant of Arabidopsis thaliana ecotype columbia, embryonic flower (emf), was characterized. No vegetative shoots were produced from emf embryos or calli; the shoot apical meristems (SAMs) in the emf embryos were altered compared to wild-type SAMs. The mutant SAMs enlarged precociously and produced inflorescence meritems upon germination. These results suggest that the dominant, wild-type allele EMF is required for the vegetative state of the SAM. In the absence of EMF function, the mutant embryo assumes the reproductive state.

Embryonic proteins in somatic embryos of carrot

The translational profile of cultured carrot cells (callus) was compared with that of the somatic embryos derived from them. The two tissues synthesize almost the same number and kinds of polypeptides except for two "embryonic" proteins. These were found in the somatic embryos but were nearly undetectable in the callus. Both embryo development and the production of embryonic proteins were induced by the same trigger (transfer of the callus to fresh medium) and were suppressed by the same factor (2,4-dichlorophenoxyacetic acid). But the appearance and disappearance of the proteins occurred several days prior to embryo formation and to the conversion of embryo to callus, respectively. Carrot cell lines incapable of embryogenesis could not synthesize the embryonic proteins. These findings indicate that the embryonic proteins play a key role in the process of embryo development. The function of these proteins is presently unknown; however, they can serve as early developmental markers for studying the mechanisms underlying somatic embryogeny in plants.

Developmental regulation of embryonic genes in plants

Somatic embryogenesis from cultured carrot cells progresses through successive morphogenetic stages termed globular, heart, and torpedo. To understand the molecular mechanisms underlying plant embryogenesis, we isolated two genes differentially expressed during embryo development. The expression of these two genes is associated with heart-stage embryogenesis. By altering the culture conditions and examining their expressions in a developmental variant cell line, we found that these genes were controlled by the developmental program of embryogenesis and were not directly regulated by 2,4-dichlorophenoxyacetic acid, the growth regulator that promotes unorganized growth of cultured cells and suppresses embryo morphogenesis. These genes are also expressed in carrot zygotic embryos but not in seedlings or mature plants.

Coordinate gene expression during somatic embryogenesis in carrots

There are several biochemical differences between the callus and the embryos of carrot culture. Callus tissue produces callus-specific proteins and a conditioning factor that is necessary for the synthesis of callus-specific proteins. By contrast, embryos produce embryo-specific proteins [Sung, Z. R. & Okimoto, R. (1981) Proc. Natl. Acad. Sci. USA 78, 3683-3687] and develop the capability to inactivate cycloheximide [Sung, Z. R., Lazar, G. J. & Dudits, D. (1981) Plant Physiol. 68, 261-264]. A mutant, WCH105, that can inactivate cycloheximide in the callus as well as in the embryos produces the embryo-specific proteins instead of the callus-specific proteins and fails to produce the conditioning factor by the callus tissue. Callus tissues also produce a conditioning factor for callus growth. This factor is not the same as the conditioning factor for the synthesis of the callus-specific proteins, as WCH105 can grow as callus. The existence of WCH105 demonstrates that the callus-specific and embryo-specific traits are coordinately regulated, but in an opposite manner. A common mechanism apparently activates one set and inactivates the other set of functions. WCH105 seems to be impaired in this mechanism.

Cloning of genes developmentally regulated during plant embryogenesis

Genes specifically induced during somatic embryogenesis may play key roles in plant embryo development. An antiserum against an extract of carrot somatic embryos revealed a few rare antigens induced at the onset of embryogenesis. Through differential immunoadsorption techniques, we purified antibodies against the embryo-specific antigens and probed a phage lambda gt11 library of cDNA from carrot somatic embryos. This paper describes three distinct cDNA clones that hybridize to embryo-specific RNAs. Monospecific antibodies, purified by affinity to the recombinant phage fusion proteins, confirm that the cloned cDNAs encode unique embryo-specific peptide antigens. One 50-kDa protein correlates with embryogenic ability in cultures of other plant species, including cereals.

Regulation of sulfate uptake in carrot cells: Properties of a hypercontrolled variant

Sulfate uptake in haploid carrot cultures can be experimentally controlled by the sulfur source provided for growth. The rate of sulfate uptake is low in cells grown on cystine or sulfate and high in sulfur-starved cells. A selenate-resistant variant cell line has been isolated from a haploid carrot line. The variant shows hypersuppression of sulfate uptake by cystine and essentially normal control by the other treatments. While both lines efflux intracellular sulfate in the presence of external sulfate, the rate of efflux from the variant is 4-6 times higher at comparable levels of initial intracellular sulfate. Further, properties of the efflux and uptake in both lines suggest that they are mediated by the same system. We propose that the variant possesses an altered uptake-efflux system that is more readily reversed and more subject to control by some metabolite derived from cystine.

Mutagenesis of cultured plant cells

Experiments were designed to study the effectiveness of the chemical mutagens ethylmethane sulfonate and nitrosoguanidine on plant cells growing in liquid suspensions. Mutation frequency was defined as the number of colonies appearing on selective plates divided by the number of colonies growing on non-selective plates. The compounds tested usually increased mutation frequency by one order of magnitude over the spontaneously occurring rate, although the increase ranged from one to 140-fold. Cell killing was found to be directly correlated with mutation frequency.

EMBRYONIC FLOWER2, a novel polycomb group protein homolog, mediates shoot development and flowering in Arabidopsis

In higher plants, developmental phase changes are regulated by a complex gene network. Loss-of-function mutations in the EMBRYONIC FLOWER genes (EMF1 and EMF2) cause Arabidopsis to flower directly, bypassing vegetative shoot growth. This phenotype suggests that the EMF genes play a major role in repression of the reproductive program. Positional cloning of EMF2 revealed that it encodes a zinc finger protein similar to FERTILIZATION-INDEPENDENT SEED2 and VERNALIZATION2 of Arabidopsis. These genes are characterized as structural homologs of Suppressor of zeste 12 [Su(z)12], a novel Polycomb group gene currently identified in Drosophila. In situ hybridization studies have demonstrated that EMF2 RNA is found in developing embryos, in both the vegetative and the reproductive shoot meristems, and in lateral organ primordia. Transgenic suppression of EMF2 produced a spectrum of early-flowering phenotypes, including emf2 mutant-like phenotype. This result confirms the role of EMF2 in phase transitions by repressing reproductive development.

EMF1, a novel protein involved in the control of shoot architecture and flowering in Arabidopsis

Shoot architecture and flowering time in angiosperms depend on the balanced expression of a large number of flowering time and flower meristem identity genes. Loss-of-function mutations in the Arabidopsis EMBRYONIC FLOWER (EMF) genes cause Arabidopsis to eliminate rosette shoot growth and transform the apical meristem from indeterminate to determinate growth by producing a single terminal flower on all nodes. We have identified the EMF1 gene by positional cloning. The deduced polypeptide has no homology with any protein of known function except a putative protein in the rice genome with which EMF1 shares common motifs that include nuclear localization signals, P-loop, and LXXLL elements. Alteration of EMF1 expression in transgenic plants caused progressive changes in flowering time, shoot determinacy, and inflorescence architecture. EMF1 and its related sequence may belong to a new class of proteins that function as transcriptional regulators of phase transition during shoot development.

EMF1, a novel protein involved in the control of shoot architecture and flowering in Arabidopsis

Shoot architecture and flowering time in angiosperms depend on the balanced expression of a large number of flowering time and flower meristem identity genes. Loss-of-function mutations in the Arabidopsis EMBRYONIC FLOWER (EMF) genes cause Arabidopsis to eliminate rosette shoot growth and transform the apical meristem from indeterminate to determinate growth by producing a single terminal flower on all nodes. We have identified the EMF1 gene by positional cloning. The deduced polypeptide has no homology with any protein of known function except a putative protein in the rice genome with which EMF1 shares common motifs that include nuclear localization signals, P-loop, and LXXLL elements. Alteration of EMF1 expression in transgenic plants caused progressive changes in flowering time, shoot determinacy, and inflorescence architecture. EMF1 and its related sequence may belong to a new class of proteins that function as transcriptional regulators of phase transition during shoot development.

Mechanisms of plant embryo development

1. Evolution in plants has favored both a simpler body plan with fewer cell types and the epigenetic flexibility to regenerate, via growth, dedifferentiation, and redifferentiation, to recover from environmental insults. It has become increasingly apparent that a plant cell uses external signals to differentiate and to maintain or to change the differentiated state. A cell-cell signaling and positional information strategy seems to be the predominant mechanism employed in plant development. 2. An axis can be initiated by physical/chemical forces such as light and ion current, requiring no new gene action. Random chemical fluctuations and physicochemical forces could explain the initiation of differences among cells of equal developmental potential. Amplification of chemical polarizing events may lead to biochemical differences, new gene expression, and finally shoot/root axis establishment. 3. Radial and axial patterning may be governed by a mechanism involving polar auxin transport. 4. Because the meristems and the three fundamental tissues formed during embryogenesis are renewed and extended throughout the life of the plant, with some exceptions, most genes expressed in the embryo are also expressed during postgermination development. 5. Embryogenic competence is acquired during reproductive development. While the zygote is determined for embryogenesis, the developing embryo and often the seedling remain embryogenic. Embryogenic potential declines during vegetative development. The embryogenic strength of a tissue is correlated with its developmental distance from the zygote.

The role of the Arabidopsis ELD1 gene in cell development and photomorphogenesis in darkness

Because cell growth and differentiation are regulated by complex interactions among different signaling pathways, a growth defect affects subsequent differentiation. We report on a growth-defective mutant of Arabidopsis, called eld1 (elongation defective 1). Cell elongation was impaired in every organ examined. Later characteristics of the eld1 phenotype include defective vascular tissue differentiation, the inability to grow in soil, ectopic deposition of suberin around twisted vascular bundles, the de-etiolation phenotype, and continuation of shoot development and flowering in the dark. The dwarf phenotype of eld1 could not be rescued by treatment with exogenous growth regulators. Because defective cell elongation is the earliest and most universal feature detected in eld1 mutants, control of or activity in cell elongation may be the primary function of the ELD1 gene. The impaired cell growth results in pleiotropic effects on cell proliferation and differentiation, and the retardation in hypocotyl elongation enables growth and development in darkness.

The ROOT MERISTEMLESS1/CADMIUM SENSITIVE2 gene defines a glutathione-dependent pathway involved in initiation and maintenance of cell division during postembryonic root development

Activation of cell division in the root apical meristem after germination is essential for postembryonic root development. Arabidopsis plants homozygous for a mutation in the ROOT MERISTEMLESS1 (RML1) gene are unable to establish an active postembryonic meristem in the root apex. This mutation abolishes cell division in the root but not in the shoot. We report the molecular cloning of the RML1 gene, which encodes the first enzyme of glutathione (GSH) biosynthesis, gamma-glutamylcysteine synthetase, and which is allelic to CADMIUM SENSITIVE2. The phenotype of the rml1 mutant, which was also evident in the roots of wild-type Arabidopsis and tobacco treated with an inhibitor of GSH biosynthesis, could be relieved by applying GSH to rml1 seedlings. By using a synchronized tobacco cell suspension culture, we showed that the G(1)-to-S phase transition requires an adequate level of GSH. These observations suggest the existence of a GSH-dependent developmental pathway essential for initiation and maintenance of cell division during postembryonic root development.

The ROOT MERISTEMLESS1/CADMIUM SENSITIVE2 gene defines a glutathione-dependent pathway involved in initiation and maintenance of cell division during postembryonic root development

Activation of cell division in the root apical meristem after germination is essential for postembryonic root development. Arabidopsis plants homozygous for a mutation in the ROOT MERISTEMLESS1 (RML1) gene are unable to establish an active postembryonic meristem in the root apex. This mutation abolishes cell division in the root but not in the shoot. We report the molecular cloning of the RML1 gene, which encodes the first enzyme of glutathione (GSH) biosynthesis, gamma-glutamylcysteine synthetase, and which is allelic to CADMIUM SENSITIVE2. The phenotype of the rml1 mutant, which was also evident in the roots of wild-type Arabidopsis and tobacco treated with an inhibitor of GSH biosynthesis, could be relieved by applying GSH to rml1 seedlings. By using a synchronized tobacco cell suspension culture, we showed that the G(1)-to-S phase transition requires an adequate level of GSH. These observations suggest the existence of a GSH-dependent developmental pathway essential for initiation and maintenance of cell division during postembryonic root development.

Responses of plant vascular systems to auxin transport inhibition

To assess the role of auxin flows in plant vascular patterning, the development of vascular systems under conditions of inhibited auxin transport was analyzed. In Arabidopsis, nearly identical responses evoked by three auxin transport inhibitor substances revealed an enormous plasticity of the vascular pattern and suggest an involvement of auxin flows in determining the sites of vascular differentiation and in promoting vascular tissue continuity. Organs formed under conditions of reduced auxin transport contained increased numbers of vascular strands and cells within those strands were improperly aligned. In leaves, vascular tissues became progressively confined towards the leaf margin as the concentration of auxin transport inhibitor was increased, suggesting that the leaf vascular system depends on inductive signals from the margin of the leaf. Staged application of auxin transport inhibitor demonstrated that primary, secondary and tertiary veins became unresponsive to further modulations of auxin transport at successive stages of early leaf development. Correlation of these stages to anatomical features in early leaf primordia indicated that the pattern of primary and secondary strands becomes fixed at the onset of lamina expansion. Similar alterations in the leaf vascular responses of alyssum, snapdragon and tobacco plants suggest common functions of auxin flows in vascular patterning in dicots, while two types of vascular pattern alterations in Arabidopsis auxin transport mutants suggest that at least two distinct primary defects can result in impaired auxin flow. We discuss these observations with regard to the relative contributions of auxin transport, auxin sensitivity and the cellular organisation of the developing organ on the vascular pattern.

EMF genes regulate Arabidopsis inflorescence development

Mutations in EMBRYONIC FLOWER (EMF) genes EMF1 and EMF2 abolish rosette development, and the mutants produce either a much reduced inflorescence or a transformed flower. These mutant characteristics suggest a repressive effect of EMF activities on reproductive development. To investigate the role of EMF genes in regulating reproductive development, we studied the relationship between EMF genes and the genes regulating inflorescence and flower development. We found that APETALA1 and AGAMOUS promoters were activated in germinating emf seedlings, suggesting that these genes may normally be suppressed in wild-type seedlings in which EMF activities are high. The phenotype of double mutants combining emf1-2 and apetala1, apetala2, leafy1, apetala1 cauliflower, and terminal flower1 showed that emf1-2 is epistatic in all cases, suggesting that EMF genes act downstream from these genes in mediating the inflorescence-to-flower transition. Constitutive expression of LEAFY in weak emf1, but not emf2, mutants increased the severity of the emf phenotype, indicating an inhibition of EMF activity by LEAFY, as was deduced from double mutant analysis. These results suggest that a mechanism involving a reciprocal negative regulation between the EMF genes and the floral genes regulates Arabidopsis inflorescence development.

Cellular differentiation regulated by gibberellin in the Arabidopsis thaliana pickle mutant

The plant growth regulator gibberellin (GA) has a profound effect on shoot development and promotes developmental transitions such as flowering. Little is known about any analogous effect GA might have on root development. In a screen for mutants, Arabidopsis plants carrying a mutation designated pickle (pkl) were isolated in which the primary root meristem retained characteristics of embryonic tissue. Expression of this aberrant differentiation state was suppressed by GA. Root tissue from plants carrying the pkl mutation spontaneously regenerated new embryos and plants.

Organization of cortical microtubules at the plasma membrane in Arabidopsis

To understand the role of microtubules in the regulation of cell elongation, we characterized microtubule patterns in fass, a cell shape mutant of Arabidopsis thaliana (L.) Heynh. Examining microtubule patterns via immunocytochemistry, we found that fass cells were able to organize their microtubules into mitotic spindles and phragmoplasts. During interphase or preprophase, fass cells had cortical microtubules, verified by transmission electron microscopy, but these microtubules were not organized into the cortical array or preprophase band. Using chromatin condensation and tubulin localization on the nuclear envelope as preprophase stage markers, we found that although fass cells lacked the preprophase band and cortical array, their cell division cycle appeared normal. To pinpoint the defect in fass cells, we delineated the sequential events leading to cortical array formation in Arabidopsis cells and found that fass cells initiated and recolonized cortical microtubules in the same manner as wild-type cells, but failed to order them into the cortical array. Taken together, these results suggest fass cells are impaired in a component of the microtubule organizing center(s) required for the proper ordering of cortical microtubules at the plasma membrane.

Expression of DC8 is associated with, but not dependent on embryogenesis

DC8 is a late embryogenesis-abundant (LEA) protein gene isolated from carrot (Daucus carota). Deletion analysis of the DC8 promoter was performed to determine the sequences required for ABA and seed-specific regulation of DC8 transcription. To investigate the mechanism of DC8 expression during seed development, chimeric gene constructs containing DC8 promoter fragments fused to a promoterless beta-glucuronidase gene (DC8:GUS) were introduced into carrot, tobacco (Nicotiana tobacum) and Arabidopsis thaliana plants. Seed-specific DC8 expression patterns was conserved among the three plant species. However, differences among the species in the patterns of DC8 expression in the embryo and endosperm that correlated with differences in the rates of embryo and endosperm growth were found. Lack of correspondence between DC8 activation and embryo development among the seeds of the three species suggests that DC8 expression, which is associated with seed maturation, is not coupled to the embryo development program. The presence of DC8 activity in carrot callus and endosperm is consistent with the notion that DC8 expression is independent of embryo morphogenesis. A similar DC8 activity time-course during callus induction and seed development suggests that explantation and 2,4-D treatment initiates a course of events similar to that in the carrot ovule. After fertilization, two pathways one leading to embryo development and another to seed maturation are initiated, but they are not closely linked. As a result we find DC8, part of the maturation program, being activated at different embryonic stages in different plant species.

Studies on the role of the Arabidopsis gene MONOPTEROS in vascular development and plant cell axialization

In the embryo of Arabidopsis thaliana (L.) Heynh., formation of the hypocotyl/root axis is initiated at the early-globular stage, recognizable as oriented expansion of formerly isodiametric cells. The process depends on the activity of the gene MONOPTEROS (MP); mp mutant embryos fail to produce hypocotyl and radicle. We have analyzed the morphology and anatomy of mp mutant plants throughout the Arabidopsis life cycle. Mutants form largely normal rosettes and root systems, but inflorescences either fail to form lateral flowers or these flowers are greatly reduced. Furthermore, the auxin transport capacity of inflorescence axes is impaired and the vascular strands in all analyzed organs are distorted. These features of the mutant phenotype suggest that the MP gene promotes cell axialization and cell file formation at multiple stages of plant development.

Genetic regulation of shoot development in Arabidopsis: role of the EMF genes

To investigate the genetic mechanism regulating Arabidopsis shoot maturation and development, we characterized eight emf mutants that bypassed the vegetative phase of the life cycle. Genetic complementation studies identified two EMF loci; both mapped to chromosome five. Double mutant analysis showed that the early- and late-flowering mutants, co, fb, elf1, elf2, and elf3, could not rescue vegetative development in the emf mutants, confirming the need for both EMF gene activities for rosette development. A series of phenotypes involving successive loss of reproductive organs was also observed in emf single mutants, in emf1-1/emf1-2 transheterozygotes, and in emf1 emf2 double mutants, suggesting that the EMF genes not only specify the rosette (vegetative) but also are involved in inflorescence and flower (reproductive) development. Phenotypic analysis of double mutants between emf and tfl1, lfy, and ag indicated interactions between EMF and genes regulating inflorescence meristem development and floral organ identity. A model depicting the role of the EMF genes in regulating shoot maturation and their interaction with genes that affect phase transitions is presented.

RML1 and RML2, Arabidopsis genes required for cell proliferation at the root tip

New cells are produced from the meristematic tissues located at the shoot and root tip throughout the life of higher plants. To investigate the genetic mechanism regulating meristematic activity, we isolated and characterized four single-gene, recessive mutants in Arabidopsis thaliana called root meristemless (rml). Complementation tests identified two RML loci; RML1 maps to chromosome IV and RML2 maps to chromosome III. These mutants produce normal embryonic roots that either did not undergo or experienced limited cell division following germination, resulting in primary roots of less than 2.0 mm in length. Mutants can produce lateral and adventitious roots, which can grow to a length comparable to the embryonic root and arrest, indicating that the growth arrest is unrelated to the embryonic dormancy process. Neither the addition of growth regulators to the media nor the removal of shoots can rescue mutant roots from growth arrest, indicating that the mutant phenotype is not caused by a shortage of known growth regulators or by a transmissible shoot inhibitor. Normal cell division ability in mutant embryo, shoot, and callus cells indicates that the RML gene functions are not part of the general cell division processes; rather, they are involved specifically in activating the cell division cycle in the root apical cells.

Abscisic Acid Regulation of DC8, A Carrot Embryonic Gene

DC8 encodes a hydrophylic 66 kilodalton protein located in the cytoplasm and cell walls of carrot (Daucus carota) embryo and endosperm. During somatic embryogenesis, the levels of DC8 mRNA and protein begin to increase 5 days after removal of auxin. To study the role of abscisic acid (ABA) in the regulation of DC8 gene, fluridone, 1-methyl-3-phenyl,-5(3-trifluoro-methyl-phenyl)-4(1H)-pyridinone, was used to inhibit the endogenous ABA content of the embryos. Fluridone, 50 micrograms per milliliter, effectively inhibits the accumulation of ABA in globular-tage enbryos. Western and Northern analysis show that when fluridone is added to the culture medium DC8 protein and mRNA decrease to very low levels. ABA added to fluridone supplemented culture media restores the DC8 protein and mRNA to control levels. Globular-stage embryos contain 0.9 to 1.4 x 10(-7) molar ABA while 10(-6) molar exogenously supplied ABA is the optimal concentration for restoration of DC8 protein accumulation in fluridone-treated embryos. The mRNA level is increased after 15 minutes of ABA addition and reaches maximal levels by 60 minutes. Evidence is presented that, unlike other ABA-regulated genes, DC8 is not induced in nonembryonic tissues via desiccation nor addition of ABA.

Molecular and genetic analysis of an embryonic gene, DC 8, from Daucus carota L

To understand the morphogenetic and physiological processes occurring during plant embryogenesis, we isolated cDNA clones homologous to genes preferentially expressed during somatic embryogenesis. One of these cDNA clones detected an embryo-specific mRNA species with a corresponding protein of 66 kDa. The expression pattern of the mRNA is similar between somatic and zygotic embryos of carrots. To characterize the gene encoding this mRNA, we isolated the corresponding genomic clones. Molecular analysis of the DNA from several haploid and diploid carrots showed that the mRNA was encoded by a single copy gene, named DC 8. DNA sequence analysis showed that the gene consisted of three exons and coded for a hydrophilic protein with a central region composed of 17 repeats. At the NH2-terminus no typical signal sequence was found. Immunocytochemical analysis localized the protein primarily in the vacuoles and protein bodies of zygotic embryos; the cytoplasm showed some antibody staining. The protein was also found in cell walls of endosperm tissue. The amount of DC 8 protein was too low for it to be categorized as a seed storage protein; its role in embryo-genesis remains to be determined.

Common amino acid sequence domains among the LEA proteins of higher plants

LEA proteins are late embryogenesis abundant in the seeds of many higher plants and are probably universal in occurrence in plant seeds. LEA mRNAs and proteins can be induced to appear at other stages in the plant's life by desiccation stress and/or treatment with the plant hormone abscisic acid (ABA). A role in protecting plant structures during water loss is likely for these proteins, with ABA functioning in the stress transduction process. Presented here are conserved tracts of amino acid sequence among LEA proteins from several species that may represent domains functionally important in desiccation protection. Curiously, an 11 amino acid sequence motif is found tandemly repeated in a group of LEA proteins of vastly different sizes. Analysis of this motif suggests that it exists as an amphiphilic α helix which may serve as the basis for higher order structure.

A nuclear protein associated with cell divisions in plants

A nuclear protein, present in carrot meristems and rapidly proliferating cultured cells of carrot (Daucus carota L.) has been identified by the use of a monoclonal antibody (MAb 21D7). By combining the techniques of two-dimensional polyacrylamide gel analysis and blotting separated proteins onto nitrocellulose sheets, it was shown that the antibody detected a single polypeptide of apparent molecular mass (M r) of 45000 and an isoelectric focusing point (pI) of 6.7. This protein was found by subcellular fractionation and immunofluorescence to be highly concentrated in the nucleoli of somatic and zygotic embryos of a wide range of plants. It was not detectable in logarthmically growing cells ofEscherichia coli, yeast, embryos ofDrosophila melanogaster or cultured C3H mouse cells. These data indicate that this protein is a highly conserved non-histone protein associated with nuclei of rapidly dividing plant cells.

Isolation and Characterization of ABA-Insensitive Cell Lines of Carrot

While abscisic acid (ABA) exerts multiple effects on somatic embryogenesis, the most pronounced of these effects is the arrestment of torpedo-stage embryos, preventing them from developing into plantlets. In order to understand the mechanism of ABA inhibition of plantlet formation, we have isolated seven ABA-insensitive cell lines capable of developing into plantlets in the presence of ABA. These ABA-insensitive cell lines, whose frequency of appearance is 7 x 10(-6), have been isolated from a haploid cell line of Daucus carota L. var Juwarot. Surprisingly, all seven cell lines exhibit auxin insensitivity as evidenced by their ability to produce heart-stage embryos in various auxins including 2,4-dichlorophenoxyacetic acid (2,4-D), naphthalene acetic acid, and indolacetic acid. Three of the cell lines, ABA 1, ABA 15, and ABA 17, have been further characterized. We found that all three showed lower levels of ABA uptake which may be the cause of ABA insensitivity. However, the uptake of 2,4-D is higher in the three cell lines than in the wild type. The basis of the interaction between ABA and 2,4-D responses is discussed.

Effect of 2,4-dichlorophenoxyacetic Acid on the expression of embryogenic program in carrot

Embryogenesis in a wild carrot cell line, W001C, can start and progress up to the first morphogenetic stage (the globular-stage embryo) in 2,4-dichloropenoxyacetic acid (2,4-D). To clarify the quantitative effect of 2,4-D on this cell line, morphological and biochemical criteria have been used to monitor embryogenesis in the presence of increasing concentrations of 2,4-D. The biochemical criteria are the ability to inactivate cycloheximide and the expression of an embryogenic polypeptide E(1). The results show that 2,4-D can affect embryogenesis in a quantitative manner but never fully suppresses embryogenesis unless it is coupled with high cell density.

Developmental regulation of polyamine metabolism in growth and differentiation of carrot culture

Polyamine levels and the activities of two polyamine biosynthetic enzymes, arginine decarboxylase (EC 4.1.1.19) and S-adenosylmethionine decarboxylase (EC 4.1.1.50), were determined during somatic embryogenesis of carrot (Daucus carota L.) cell cultures. Embryogenic cultures showed severalfold increases in polyamine levels over nondifferentiating controls. A mutant cell line that failed to form embryos but grew at the same rate as the wild-type line also failed to show increases in polyamine levels, thus providing evidence that this increased polyamine content was in fact associated with the development of embryos. Furthermore, inhibition of these increases in polyamines caused by drugs inhibited embryogenesis and the effect was reversible with spermidine. The activities of arginine decarboxylase and Sadenosylmethionine decarboxylase were found to be suppressed by auxin; however, the specific effects differed between exogenous 2,4-dichlorophenoxyacetic acid and endogenous indole-3-acetic acid. The results indicate that increased polyamine levels are required for cellular differentiation and development occurring during somatic embryogenesis in carrot cell cultures.

Characterization of a selenocystine-resistant carrot cell line : alterations in cystine and sulfate uptake

A selenocystine-resistant carrot cell line, C-1, was isolated from a haploid carrot (Daucus carota) cell culture, HA. The C-1 variant takes up cystine, but not cysteine, more slowly than does HA. The selenocystine resistance is maintained in culture in the absence of selection and is expressed in regenerated plants. Results based on chromatographic separation of sulfur metabolites from cells fed with [(35)S]cystine suggest a block either in the uptake or reduction of cystine in the variant. Both lines can grow on cystine as sole sulfur source. Growth of the HA line on cystine suppressed the development of sulfate uptake capacity (Furner, Sung 1982 Proc Natl Acad Sci USA 79: 1149-1153), while cystine-grown C-1 cells have high levels of sulfate uptake capacity.We suggest that the C-1 line, grown on cystine, accumulates an insufficient quantity of some sulfur metabolite, which is involved in the control of sulfate uptake, to suppress the uptake. C-1 grown on cystine is more sensitive than HA to growth inhibition by the sulfate analog selenate.

Cycloheximide resistance in carrot culture: a differentiated function

Cultured carrot cells grow as unorganized callus tissue in medium containing auxin. Upon removal of the auxin from the medium, they grow in an organized manner and differentiate into embryos. In the normal cell line, W001C, the callus growth can be inhibited by cycloheximide, but the embryonic growth cannot. A variant cell line, WCH105, whose callus growth is resistant to cycloheximide, was isolated. The mechanism of cycloheximide resistance in embryos of both lines and in WCH105 callus was found to be cycloheximide inactivation. In addition to auxin, bromodeoxyuridine can also promote callus growth in carrot culture. Callus cultures maintained by bromodeoxyuridine behave the same as do those maintained by auxin. WCH105 callus is resistant, whereas W001C callus is sensitive to cycloheximide inhibition. Except for the onset of embryogenesis, cycloheximide inactivation is expressed throughout the embryo developmental stages up to the plantlets. These results suggest that cycloheximide inactivation is a function expressed in the differentiated, but not in the undifferentiated, tissues.

Expression of cycloheximide resistance in carrot somatic hybrids and their segregants

Cycloheximide resistance (CH(r)) was shown to be a function expressed in differentiated plant tissues, but not in unorganized callus tissues. A variant, WCH105, expressing CH(r) in the callus, as well as in regenerated plantlets, was isolated from a cell line derived from a wild carrot plant. The plantlets regenerated from WCH105 are green, but do not produce normal, dissected leaves. Protoplasts of WCH105 were fused with that of a cycloheximidesensitive (CH(s)) cell line derived from an albino, domesticated carrot. Hybrid selection was based on (1) irreversible growth inhibition of WCH105 protoplasts by iodacetamide, and (2) restoration of green plants producing dissected leaves.--Analysis of the CH(r) trait as an unselected marker in the callus cells of the somatic hybrids indicated that it behaved as a recessive. The combined recessive and resistant phenotype of this trait allowed the recovery of CH(r) segregants from CH(s) hybrids at a frequency of 10(-4), 1000 times higher than the spontaneous frequency of CH(r). The recovery of CH(r) somatic segregants confirmed the recessiveness of the CH(r) trait.

Regulation of pyrimidine and arginine biosynthesis investigated by the use of phaseolotoxin and 5-Fluorouracil

Purified phaseolotoxin inhibits the growth of carrot cells. Such inhibitions can be reversed completely by citrulline but not by arginine. This toxin inhibits ornithine transcarbamylase activity in vitro, which leads to an accumulation of ornithine and a decrease in arginine levels intracellularly. In carrot cells, 5-fluorouracil (5-FU) toxicity can be reduced by the addition of purified toxin and citrulline, or ornithine. The toxin also decreases the incorporation of [(14)C]uracil and [(14)C]5-FU into trichloroacetic acid precipitable material by 50%. Finally, a 5-FU-resistant line, F5 (Sung ZR, Jacques S 1980 Planta 148: 389-396), was found to be more sensitive to the toxin than were 5-FU-sensitive cells. One millimolar 5-FU roughly doubled the ability of F5 to tolerate phaseolotoxin. These results demonstrate a close regulation between the pyrimidine and arginine path-ways in carrots.

5-Fluorouracil resistance in carrot cell cultures : Its use in studying the interaction of the pyrimidine and arginine pathways

Physiological studies of 5-fluorouracil (5-FU)-resistant cell line of wild carrot (Daucus carota L.), F5, showed that this variant is also resistant to 5-fluorouridine, but is as sensitive to 6-azauracil as the 5-FU-sensitive parent line, WOO1C. High levels of exogenous uracil, uridine, and thymine are slightly toxic to F5, but not to WOO1C. 5-FU sensitivity in WOO1C cannot be reversed by bases and nucleosides; bases like uracil and thymine even increase 5-FU toxicity. No substantial differences were found in the uptake, incorporation and degradation of WOO1C and F5. Carrot cultures seem to take up 5-FU by rapid diffusion, the kinetics being characteristic of non-saturable uptake, with infinite Km and zero Vmax. The rapid uptake of 5-FU and extensive degradation of bases and nucleosides are probably responsible for the inability of uracil and uridine to reverse the growth inhibition caused by 5-FU in carrot cells while, as shown earlier, phaseolotoxin ((N-phosphosulfamyl)ornithinylalanylhomoarginine), an inhibitor of the arginine biosynthetic enzyme, ornithine transcarbamylase was capable of reducing 5-FU toxicity. F5 callus contained less histidine and arginine than WOO1C. 5-FU increased the endogenous levels of arginine, histidine and aspartate in both lines. The aspartate transcarbamylase of F5 appears to be normal; it is as sensitive to uridine-monophosphate inhibition as that of WOO1C. The 5-FU resistance of F5 was stable in undifferentiated cells, but only 8 out of 50 calli reinitiated from the regenerated plantlets remained resistant to 5-FU. F5 is an aneuploid culture. Five 5-FU-sensitive reinitiated calli that were examined were all diploid whereas of the eight 5-FU-resistant reinitiated calli two became diploid and six remained as aneuploid.

Quantitative studies of embryogenesis in normal and 5-methyltryptophan-resistant cell lines of wild carrot : The effects of growth regulators

The frequency of embryo formation was determined in normal and 5-methyltryptophan-resistant (5-MT(r)) cell lines of wild carrot (Daucus carota L.) grown in the presence or absence of 2-isopentenyladenine (2-ip) and 2,4-dichlorophenoxyacetic acid (2,4-D). 2-ip stimulated the intitation of embryo formation and also accelerated embryo development. 2.4-D inhibited embryo differentiation at several stages: at 0.1 mg/l, it stopped regeneration at the earliest stage, resulting in callus growth instead of embryo formation; at 0.04 mg/l 2,4-D, some globular embryos were produced, but they did not develop into more advanced embryos. Variant cell lines with higher levels of auxin (indole-3-acetic acid, IAA) were used to study the effect of an elevated endogenous concentration of auxin on embryogenesis. IAA at these concentrations suppressed regeneration in the same manner as the exogenous auxin, 2,4-D, did. This result confirms the hypothesis that high levels of IAA are responsible for the suppression of regeneration in the 5-MT(r) cell lines.

Relationship of indole-3-acetic acid and tryptophan concentrations in normal and 5-methyltryptophan-resistant cell lines of wild carrots

A 5-methyltryptophan(5-MT)-resistant cell line of wild carrot (Daucus carota L.), W001, that exhibited auxin-independent callus growth, was found to accumulate indole-3-acetic acid (IAA) and tryptophan (trp). Anthranilate-synthetase activity in W001 cell extract was less sensitive to feedback inhibition by trp than in the original 5-MT-sensitive cell lines. It is hypothesized that the resistant enzyme allowed more trp synthesis and accumulation which, in turn, affected the IAA concentration in the cell. Since carrot cultures cannot regenerate in the presence of exogenous auxin, the elevated IAA concentration in W001 may be responsible for its drastically reduced capacity to regenerate. The relationship between trp and IAA levels was further investigated by examining the effect of 2,4-dichlorophenoxy acetic acid (2,4-D) on the endogenous concentration of trp and IAA. In general, the IAA level was reduced but the trp concentration was elevated when 2,4-D was present in the culture medium.

Turbidimetric measurement of plant cell culture growth

Turbidimetric measurement of cultures grown in sidearm flasks was used to measure the growth of plant cells. The turbidity was shown to vary proportionally with cell number and dry weight over time. The effect of different freezing conditions on the growth of the culture was presented to demonstrate the application of the sidearm-turbidity method.