Multicellular Plants Research Articles

Phylogenetic implications of patterns of cap development in selected species of Acetabularia/Polyphysa (Dasycladales, Chlorophyta)

Cap initiation and differentiation were studied by scanning electron microscopy in three species of the green algal family Acetabulariaceae (Dasycladales): Acetabularia acetabulum (Linnaeus) Silva, Acetabularia kilneri J. Agardh, and Polyphysa peniculus C. Agardh. The morphology of cap formation in all three species follows the same principal pattern. The phylogenetic significance of cap morphogenesis in Acetabularia/Polyphysa is discussed. Based on morphology, we propose that the few extant species that are presently grouped in the Acetabularia/Polyphysa complex, remnants of the once highly diverse family Acetabulariaceae, are derived from three or more ancestral genera. Morphogenesis in the unicellular plants of Acetabulariaceae resembles that of multicellular plants because primary patterning occurs in terminally located blastozones with the competence to form an organ or organ-like structure.

Rapid‐cycling CAM; an hypothetical variant of photosynthetic metabolism

The currently recognized forms of CAM photosynthesis do not represent all possible variants on the theme. It is predicted here that there may exist an undiscovered variant in which the CO2‐acquiring and CO2‐reducing phases of CAM alternate over time periods shorter (possibly much shorter) than the diel cycle. The process would occur entirely in the photoperiod and may be likened to a form of C4 photosynthesis in which the same photosynthetic cell alternates between performing the functions of a C4 mesophyll and C4 bundle sheath cell. Rapid‐cycling CAM, which would be unlikely to be detected by methods commonly used to measure photosynthesis, could provide a CO2‐concentrating mechanism in both unicellular and multicellular plants.

Identification of a novel delta 6-acyl-group desaturase by targeted gene disruption in Physcomitrella patens.

The moss Physcomitrella patens contains high levels of arachidonic acid. For its synthesis from linoleic acid by desaturation and elongation, novel delta 5- and delta 6-desaturases are required. To isolate one of these, PCR-based cloning was used, and resulted in the isolation of a full-length cDNA coding for a putatively new desaturase. The deduced amino acid sequence has three domains: a N-terminal segment of about 100 amino acids, with no similarity to any sequence in the data banks, followed by a cytochrome b5-related region and a C-terminal sequence with low similarity (27% identify) to acyl-lipid desaturases. To elucidate the function of this protein, we disrupted its gene by transforming P. patens with the corresponding linear genomic sequence, into which a positive selection marker had been inserted. The molecular analysis of five transformed lines showed that the selection cartridge had been inserted into the corresponding genomic locus of all five lines. The gene disruption resulted in a dramatic alteration of the fatty acid pattern in the knockout plants. The large increase in linoleic acid and the concomitant disappearance of gamma-linolenic and arachidonic acid in all knockout lines suggested that the new cDNA coded for a delta 6-desaturase. This was confirmed by expression of the cDNA in yeast and analysis of the resultant fatty acids by GC-MS. Only the transformed yeast cells were able to introduce a further double bond into the delta 6-position of unsaturated fatty acids. To our knowledge, this is the first report of a successful gene disruption in a multicellular plant resulting in a specific biochemical phenotype.

Role of the ZWILLE gene in the regulation of central shoot meristem cell fate during Arabidopsis embryogenesis.

Postembryonic development in higher plants is marked by repetitive organ formation via a self-perpetuating stem cell system, the shoot meristem. Organs are initiated at the shoot meristem periphery, while a central zone harbors the stem cells. Here we show by genetic and molecular analyses that the ZWILLE (ZLL) gene is specifically required to establish the central-peripheral organization of the embryo apex and that this step is critical for shoot meristem self-perpetuation. zll mutants correctly initiate expression of the shoot meristem-specific gene SHOOT MERISTEMLESS in early embryos, but fail to regulate its spatial expression pattern at later embryo stages and initiate differentiated structures in place of stem cells. We isolated the ZLL gene by map-based cloning. It encodes a novel protein, and related sequences are highly conserved in multicellular plants and animals but are absent from bacteria and yeast. We propose that ZLL relays positional information required to maintain stem cells of the developing shoot meristem in an undifferentiated state during the transition from embryonic development to repetitive postembryonic organ formation.

Kin Selection and Social Insects

Social insects so dominate many terrestrial habitats (Wilson 1990) that they can hardly escape the attention of biologists, but even if they were rare, they would still attract special interest because of the intricate cooperation within their societies. William Morton Wheeler (1911) described the social insect colony as an organism (or as a higher-level organism or superorganism) because of the degree to which individuals appear to operate as a unit that is dedicated to the perpetuation and reproduction of the colony as a whole. The reinvention of the organism at a higher level has occurred at a number of crucial junctures in the history of life (Maynard Smith and Szathmary 1995). For example, the eukaryotic cell arose from several prokaryotic ancestors (Margulis 1970), and multicellular plants, animals, and fungi arose from single-celled ancestors (Buss 1987). Because insect societies are macroscopic, and because they span the entire range from solitary individuals to essentially superorganismal colonies, they offer an accessible model for how such transitions can happen.

Slanted truths: essays on Gaia, symbiosis, and evolution

Lynn Margulis, and her son, the writer Dorion Sagan, here present a selection from their many essays that have been published. Margulis's proposal that eukaryotic cells (the cells of all multicellular animals and plants) are made up of symbiotic unions of more primitive cells was at first widely derided but is now mainstream science. She has described the previously unrecognized role microbial life plays in the maintenance of all life on earth. And she is, with James Lovelock, one of the founders of Gaia theory. In these essays, one can see how these apparently unrelated interests combine into a single, coherent scientific world-view about the natural tendency of living systems to form complex interactive communities.

Nuclear mutations that block group II RNA splicing in maize chloroplasts reveal several intron classes with distinct requirements for splicing factors.

To elucidate mechanisms that regulate chloroplast RNA splicing in multicellular plants, we sought nuclear mutations in maize that result in chloroplast splicing defects. Evidence is presented for two nuclear genes whose function is required for the splicing of group II introns in maize chloroplasts. A mutation in the crs1 (for chloroplast RNA splicing 1) gene blocks the splicing of only the atpF intron, whereas a mutation in the crs2 gene blocks the splicing of many chloroplast introns. In addition, a correlation was observed between the absence of plastid ribosomes and the failure to splice several chloroplast introns. Our results suggest that a chloroplast-encoded factor and a nuclear-encoded factor whose activity requires crs2 function facilitate the splicing of distinct sets of group II introns. These two genetically defined intron sets also differ with regard to intron structure: one set consists of only subgroup IIA introns and the other of only subgroup IIB introns. Therefore, it is likely that distinct splicing factors recognize subgroup-specific features of intron structure or facilitate subgroup-specific aspects of the splicing reaction. Of the 12 pre-mRNA introns in the maize chloroplast genome, only one is normally spliced in both crs2 mutants and in mutants lacking plastid ribosomes, indicating that few, if any, of the group II introns in the chloroplast genome undergo autocatalytic splicing in vivo.

CARBOHYDRATE-MODULATED GENE EXPRESSION IN PLANTS.

Plant gene responses to changing carbohydrate status can vary markedly. Some genes are induced, some are repressed, and others are minimally affected. As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability. However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs. Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization. Abundant sugar levels exert opposite effects through a combination of gene repression and induction. Long-term changes in metabolic activity, resource partitioning, and plant form result. Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation. The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes.

A nimA-like protein kinase transcript is highly expressed in meristems of Antirrhinum majus.

In plants, cell proliferation occurs mostly within meristems but a significant amount also occurs at other well-defined sites during specific stages of development. We have developed molecular markers to follow the location and progress of cell division within multicellular plant organs and thereby gain some insight into how cell division might be regulated during morphogenesis. As in other eukaryotes, cell division in plants is regulated by a highly conserved set of protein kinases and phosphatases. The molecular information available on these molecules from other eukaryotes has allowed the design of strategies by which plant homologues can be isolated. In this report we describe the identification of a nimA-like gene from Antirrhinum majus and describe the pattern in which its transcript is expressed. Comparison of the pattern of AmnimA gene expression with that of genes which are expressed in a cell cycle-dependent manner suggests that this gene is expressed in actively dividing tissues but expression is not specific to any particular phase of the cell cycle nor specific to any particular tissue type.

5451513 Method for stably transforming plastids of multicellular plants : Maliga Pal; Maliga Zora S East Brunswick, NJ, United States assigned to Rutgers The State University of New Jersey

5451513 Method for stably transforming plastids of multicellular plants : Maliga Pal; Maliga Zora S East Brunswick, NJ, United States assigned to Rutgers The State University of New Jersey

Molecular Genetics of Sexuality in Chlamydomonas

Recent molecular and cellular data on the sexual cycle of Chlamydomonas reinhardtii, a unicellular green alga, is considered in the context of current theories on the origins and evolution of eukaryotic sex. The mating-type locus of C. reinhardtii controls gamete recognition and fusion, organelle inheritance, and sporulation, the three traits that characterize most sexual cycles in lower eukaryotes. The mating-type locus comprises approximately one megabase of DNA on linkage group VI, is highly rearranged in its central region, and contains identified genes that govern both recognition/fusion and chloroplast inheritance. Sporulation, a diploid program designed to negotiate changing environments, is analogous if not homologous to the somatic differentiation program of multicellular plants and animals, and its expression requires the fusion of compatible gametes.

Establishment of the apical-basal axis in multicellular plant embryos

Apical-basal polarity of multicellular plant embryos is defined originally in the zygote before the first cell division and the apical and basal daughter cells follow distinct developmental programs. Mechanisms controlling these initial embryonic steps are still poorly understood but advances both in the flowering plant Arabidopsis and the alga Fucus have lead to various models and hypotheses. These point to the positional information as a major source for patterning the early embryo. Hypotheses concerning the possible location of this positional information are reviewed and discussed.

Pseudouridine and O2-methylated nucleosides. Significance of their selective occurrence in rRNA domains that function in ribosome-catalyzed synthesis of the peptide bonds in proteins

Pseudouridine (5-ribosyluracil, Ψ) was the first of a host of modified nucleoside constituents detected in cellular RNA and it remains the most abundant, being broadly distributed in the RNA of archaebacteria, eubacteria and eukaryotes. Like some other modifications, Ψ is particularly abundant in more complex organisms, reaching 2—3% of the total nucleoside constituents in tRNA, snRNA and rRNA of multicellular plants and animals. Like all other modified nucleosides, Ψ arises by site-specific, enzymically catalyzed modification of a nucleoside residue in an RNA molecule. Unlike all other modified nucleosides, Ψ arises by isomerisation (not substitution) of a classical nucleoside, uridine (1-ribosyluracil). There have been suggestions that key processes such as ribosome assembly and peptidyl transfer may rely, more than is generally appreciated, on RNA modifications such as O 2-methylation and pseudouridylation, respectively. However, a persuasive case for the view that secondary modifications are of primary importance in ribosome function has not been convincingly made. Accordingly, we think it is timely to broaden what is generally meant by the ‘catalytic properties of rRNA’, and to ask, to what extent do modifications contribute to in vivo rates of ribosome assembly and ribosomal peptide-bond synthesis? The first part of this article sets forth the evidence that there is a conspicuous association between modified nucleosides and cellular RNAs that participate in group-transfer reactions. The second part reviews evidence in support of the view that the functions of Ψ and other modified nucleosides are likely of central importance for understanding the dynamics and stereostructural modeling at functionally significant sites in the ribosome.

Molecular Genetics of Sexuality in Chlamydomonas

Recent molecular and cellular data on the sexual cycle of Chlamydomonas reinhardtii, a unicellular green alga, is considered in the context of current theories on the origins and evolution of eukaryotic sex. The mating-type locus of C. reinhardtii controls gamete recognition and fusion, organelle inheritance, and sporulation, the three traits that characterize most sexual cycles in lower eukaryotes. The mating-type locus comprises approximately one megabase of DNA on linkage group VI, is highly rearranged in its central region, and contains identified genes that govern both recognition/fusion and chloroplast inheritance. Sporulation, a diploid program designed to negotiate changing environments, is analogous if not homologous to the somatic differentiation program of multicellular plants and animals, and its expression requires the fusion of compatible gametes.

Cell differentiation and the cytoskeleton in Acetabularia.

In multicellular organisms, differentiation of individual cells is typically linked to the development of the whole organism. As cells acquire tissue-specific morphologies and become functionally specialized they lose in turn a number of other functions. A free living, single celled organism, however, maintains all such functions. Compartmentalization and intracellular communication are two basic principles by which expression of specialized features is achieved within a unicell. Both in turn depend on the structure and dynamics of the cytoskeleton. Giant algal unicells lend themselves as experimental models for the study of the cytoskeleton, because the cytoskeletal arrays inside these cells become equally enormous in size. Some of these organisms are large enough to be mistaken for multicellular plants, equipped with holdfast, stem and assimilatory organ. The marine green alga Acetabularia is one of these giant cells, which has already been well known to phycologists and cell biologists for several decades. The current review discusses recent progress in the study of the cytoskeleton in Acetabularia and examines classic concepts of cell morphogenesis from the perspective of cytoskeletal function. Contents Summary 369 I. Introduction 369 II. Morphogenetic stages 371 III. Post-transcriptional control of morphogenesis 687 IV. Apparent plasticity of morphogenesis 389 V. Prospects of using molecular approaches 391.

Detection and quantification of melatonin in a dinoflagellate, Gonyaulax polyedra: Solutions to the problem of methoxyindole destruction in non‐vertebrate material

Preservative procedures are described for the extraction and quantification of melatonin in cell material from the dinoflagellate Gonyaulax polyedra, an organism in which this indoleamine is rapidly degraded due to interaction with free oxygen radicals and photooxidation. Cells were shock-frozen in liquid nitrogen and pulverized. Various extraction methods were applied to the powder, and rates of recovery were compared. For the determination of melatonin by high performance liquid chromatography (HPLC), extractions with acetone or perchloric acid were more suitable than the use of other solvents. For purposes of radioimmunoassay (RIA), extraction with acetone gave the best results. Several other inorganic solvents, which are often applied in melatonin research, such as chloroform, dichloromethane, and diethyl ether, led to considerable losses of the indoleamine. The procedures developed for HPLC with either electrochemical or fluorescence detection also allow the quantification of other indolic compounds, in particular, tryptophan and 5-methoxytryptamine. The methods described may be of value in the further search for melatonin and related indoleamines in non-vertebrate material, especially, from unicells, multicellular plants, and invertebrates.

Cell Fate Determination by the Cell Wall in Early Fucus Development

In multicellular plants, development starts with an asymmetric division of the zygote into two differentiated cells. The nature and distribution of fate-determining factors operating during embryogenesis remain largely obscure. Laser microsurgery was used here to dissect two-celled embryos of the alga Fucus spiralis. Removal of protoplasts from the cell wall induced dedifferentiation. However, isolated cells within the walls followed their restricted fate. Moreover, contact of one cell type with the isolated cell wall of the other cell type caused its fate to be switched. The cell wall thus appears to maintain the differentiated state and to direct cell fate in plant development.

Size-Dependent Variations in Plant Growth Rates and the "¾ Power Rule"

Size-dependent variations in morphological and physiological variables adduced to influence growth rate (e.g., cell surface area and volume, chlorophyll a concentration per cell) were determined by reevaluating published data from unicellular and multicellular plants and animals. With respect to cell volume, reduced major axis regression of the available data indicated that cell surface area decreases roughly as the 0.69-power, the concentration of Chl a decreased roughly as the 0.80-power, and cell mass decreased as the 0.77-power. Computer simulations indicated that the scaling exponent for cell surface area was the consequence of size-dependent variations in cell geometry and aspect ratio (i.e., cell length/width) rather than the result of geometric similitude among cells differing in size. The anisometric relation between cell mass and volume indicated that bulk cell density declines with increasing cell volume. Reanalyses of published data showed that growth rate and weight-specific growth rate scale as the ¾- and negative ¼-power, respectively, with respect to the body mass of unicellular and multicellular plants and animals. It is speculated that the anisometric relation between the growth rate and mass of unicellular plants is attributable to a “dilution” of metabolically active cellular constituents with increasing cell size in combination with the scaling of surface area with respect to volume (and therefore cell mass). It is further speculated that similar biological scalings may account for the ¾-power rule obtained for taxonomically and ecologically diverse multicellular plants and animals.

Size‐dependent variations in plant growth rates and the “¾‐power rule”

Size‐dependent variations in morphological and physiological variables adduced to influence growth rate (e.g., cell surface area and volume, chlorophyll a concentration per cell) were determined by reevaluating published data from unicellular and multicellular plants and animals. With respect to cell volume, reduced major axis regression of the available data indicated that cell surface area decreases roughly as the 0.69‐power, the concentration of Chl a decreased roughly as the 0.80‐power, and cell mass decreased as the 0.77‐power. Computer simulations indicated that the scaling exponent for cell surface area was the consequence of size‐dependent variations in cell geometry and aspect ratio (i.e., cell length/width) rather than the result of geometric similitude among cells differing in size. The anisometric relation between cell mass and volume indicated that bulk cell density declines with increasing cell volume. Reanalyses of published data showed that growth rate and weight‐specific growth rate scale as the ¾‐ and negative ¼‐power, respectively, with respect to the body mass of unicellular and multicellular plants and animals. It is speculated that the anisometric relation between the growth rate and mass of unicellular plants is attributable to a “dilution” of metabolically active cellular constituents with increasing cell size in combination with the scaling of surface area with respect to volume (and therefore cell mass). It is further speculated that similar biological scalings may account for the ¾‐power rule obtained for taxonomically and ecologically diverse multicellular plants and animals.

The senescence syndrome in plants: an overview of phytogerontology

SynopsisThe deteriorative processes leading to the death of a population, individual or part of an individual can be genetically programmed or induced by environmental perturbations, physical damage, pests and diseases. Senescence in multicellular plants is typically a phenomenon resulting from cell differentiation and loss of totipotency. Recycling of nutrients released from senescent cells, abscission layer formation, containment of pathogens and dispersal of progeny are crucial aspects of senescence management. Senescence-related autocatalytic changes induced by substances generally thought to regulate senescence may not mirror the sequence of changes occurring naturally through correlative processes in the intact plant. The chloroplast has a key role in reversing senescence-related degradation of other organelles. Conventional symptoms of senescence used in plant sciences have obscured common theories of senescence regulation for all types of organism.