Maullinia ectocarpii gen. et sp. nov. (Plasmodiophorea), an intracellular parasite in Ectocarpus siliculosus (Ectocarpales, Phaeophyceae) and other filamentous brown algae

An obligate intracellular parasite infecting Ectocarpus spp. and other filamentous marine brown algae is described. The pathogen forms an unwalled multinucleate syncytium (plasmodium) within the host cell cytoplasm and causes hypertrophy. Cruciform nuclear divisions occur during early development. Mature plasmodia become transformed into single sporangia, filling the host cell completely, and then cleave into several hundred spores. The spores are motile with two unequal, whiplash-type flagella inserted subapically and also show amoeboid movement. Upon settlement, cysts with chitinous walls are formed. Infection of host cells is accomplished by means of an adhesorium and a stachel apparatus penetrating the host cell wall, and injection of the cyst content into the host cell cytoplasm. The parasite is characterized by features specific for the plasmodiophorids and is described as a new genus and species, Maullinia ectocarpii.     

Ecological significance of phosphomonoesters and phosphomonoesterase activity in a small Mediterranean river and its estuary

In 1998, the concentration of phosphomonoesters, a biologically available fraction of the dissolved organic phosphorus pool, was measured along Palmones River (a small Mediterranean river in Southern Spain) and its estuary. Due to the extremely low phosphomonoester concentration in the river (usually under detection limits by analytical procedures), a method using chromatographic cartridges to concentrate this compound was used. Phosphomonoester concentration was usually in the nanomolar range, although values up to 6 m were measured in a sampling station near an effluent of domestic sewage and a cellulose pulp plant. Concentrations were always lower than the dissolved reactive phosphorus (mostly orthophosphate). Phosphomonoesters represented a variable percentage of the dissolved organic phosphorus, from negligible percentages up to 56%. Besides, a highly significant correlation between these two phosphorus fractions was found.In addition, phosphomonoesterase (alkaline phosphatase) activity was measured in the water and macrophytobenthos (both algae and aquatic phanerogams) along the river and the estuary. In water, most of the phosphatase activity upstream was regarded as soluble. This was confirmed by the difficulty of performing kinetic studies in the unfiltered samples. On the contrary, the phosphomonoesterase activity in the estuary was attributed to different size fractions, suggesting that phosphomonoesters are used actively as a phosphate source by bacteria and phytoplankton, with relative contributions depending on the sampling period.     

Analysis of dielectric spectra of eukaryotic cells by computer modeling

An analysis of dielectric spectra, obtained by computer modeling, of spherical eukaryotic cells (lymphocytes in particular) is presented. The number of fitting parameters required to describe these spectra is determined. The influence of parameter variation on the spectral shape is illustrated. Received: 27 May 1999 / Revised version: 24 January 2000 / Accepted: 26 January 2000     

Variation in the carbon isotope composition of Semaprochilodus insignis, a detritivorous fish associated with oligotrophic and eutrophic Amazonian rivers

The Amazonian freshwater fish, Semaprochilodus insignis , varies in carbon isotopic composition during residence in different habitats within its home range. The results indicate that carbon produced in black-water systems contributes to fish stocks harvested from white-water systems, and S. insignis populations in the Central Amazonia are more integrated than previously thought.     

PIGMENT COMPOSITION AND rbcL SEQUENCE DATA FROM THE SILICOFLAGELLATE DICTYOCHA SPECULUM: A HETEROKONT ALGA WITH PIGMENTS SIMILAR TO SOME HAPTOPH Y TES

Monophyly of plastids in the morphologically diverse heterokont algae has rarely been questioned. However, HPLC analysis revealed that the pigment composition of the silicoflagellate Dictyocha speculum Ehrenberg is similar to that observed in a group of haptophytes (“type 4”sensu Jeffrey and Wright 1994 . Dictyocha speculum and type 4 haptophytes possess acylfucoxanthins (19′-butanoyloxy- and 19′-hexanoyloxyfucoxanthin) in addition to fuco-, diadino-, and diatoxanthin and chl a, c, and c₃. The pigment composition of two pedinellids (Apedinella radians[Lohmann] Campbell and Mesopedinella arctica Daugbjerg), a sister group to D. speculum, deviates from D. speculum by lack of chl c ₃ and acylfucoxanthins. The distinct pigment composition suggested that plastid evolution in D. speculum differs from that of other heterokont algae. This prompted determination of the plastid-encoded rbcL gene from D. speculum to gain further insight into the evolutionary history of plastids in heterokont algae and haptophytes. A phylogenetic inference based on parsimony, maximum likelihood, and LogDet transformation methods included 35 heterokonts, 19 haptophytes, 8 red algae, and 1 cryptomonad. Three proteobacteria possessing type I RUBISCO were used to root the tree. In phylogenetic analyses, D. speculum was closely related to Rhizochromulina sp. and pedinellids, despite the latter possessing a different pigment composition. Surprisingly, the Dictyochophyceae clustered outside the lineage of heterokont algae but not within the haptophytes. Hence, analyses deduced from rbcL sequences indicated that the plastids in heterokont algae might have a more complex evolutionary history and that the shared pigment composition in D. speculum and type 4 haptophytes could be explained by convergent evolution or gene transfer. The pigment composition in D. speculum may have implications for pigment-based characterization of phytoplankton community structure in natural samples.     

Compositional Properties of Green-Plant Plastid Genomes

We studied variation of GC contents among plastid (Pt) genomes of green plants. In the green plants, the GC contents of the whole Pt genomes range from 42.14 to 28.81%. These values are similar to those observed in the mitochondrial (Mt) genomes of the green plants, however, the GC contents in the Pt genomes are not related to those in the Mt genomes or the nuclear (Nc) genomes. In addition, some compositional properties of the three types of genomes are different. Thus, it is suggested that the GC contents of the Pt genomes are maintained independently of the other genomes within a cell. We found that the compositional bias toward AT is strong at the third codon position and in intergenic spacer (IGS) regions in the Pt genomes, and the GC contents (GC3 and GCIGS) at these sites are generally similar within each genome. Additionally, the GC3 and GCIGS are strongly related to the whole-genome GC content. Therefore, the interspecific variation of the GC contents in the Pt genomes is suggested to be mainly caused by the variation of the GC3 and GCIGS, both of which are considered to be under weak selective constraints. Using a maximum likelihood approach, we estimated equilibrium GC3 (eqGC₃) of 12 genes in the land-plant Pt genomes. We found an increase in eqGC₃ after the divergence of liverworts. These results suggest that genome-wide factors such as GC mutational bias are important for the biased base composition in the Pt genomes.Reviewing Editor: Dr. Brian Morton     

Plant genome size research: a field in focus

This Special Issue contains 18 papers arising from presentations at the Second Plant Genome Size Workshop and Discussion Meeting (hosted by the Royal Botanic Gardens, Kew, 8-12 September, 2003). This preface provides an overview of these papers, setting their key contents in the broad framework of this highly active field. It also highlights a few overarching issues with wide biological impact or interest, including (1) the need to unify terminology relating to C-value and genome size, (2) the ongoing quest for accurate gold standards for accurate plant genome size estimation, (3) how knowledge of species' DNA amounts has increased in recent years, (4) the existence, causes and significance of intraspecific variation, (5) recent progress in understanding the mechanisms and evolutionary patterns of genome size change, and (6) the impact of genome size knowledge on related biological activities such as genetic fingerprinting and quantitative genetics. The paper offers a vision of how increased knowledge and understanding of genome size will contribute to holisitic genomic studies in both plants and animals in the next decade.     

Translation initiation factors eIF4E and eIFiso4E are required for polysome formation and regulate plant growth in tobacco

Eukaryotic initiation factor eIF4E plays a pivotal role in translation initiation. As a component of the ternary eIF4F complex, eIF4E interacts with the mRNA cap structure to facilitate recruitment of the 40S ribosomal subunit onto mRNA. Plants contain two distinct cap-binding proteins, eIF4E and eIFiso4E, that assemble into different eIF4F complexes. To study the functional roles of eIF4E and eIFiso4E in tobacco, we isolated two corresponding cDNAs, NteIF4E1 and NteIFiso4E1, and used these to deplete cap-binding protein levels in planta by antisense downregulation. Antibodies raised against recombinant NteIF4E1 detected three distinct cap-binding proteins in tobacco leaf extracts; NteIF4E and two isoforms of NteIFiso4E. The three cap-binding proteins were immuno-detected in all tissues analysed and were coordinately regulated, with peak expression in anthers and pollen. Transgenic tobacco plants showing significant depletion of either NteIF4E or the two NteIFiso4E isoforms displayed normal vegetative development and were fully fertile. Interestingly, NteIFiso4E depletion resulted in a compensatory increase in NteIF4E levels, whereas the down-regulation of NteIF4E did not trigger a reciprocal increase in NteIFiso4E levels. The antisense depletion of both NteIF4E and NteIFiso4E resulted in plants with a semi-dwarf phenotype and an overall reduction in polyribosome loading, demonstrating that both eIF4E and eIFiso4E support translation initiation in planta, which suggests their potential role in the regulation of plant growth.     

Role of SulP, a nuclear-encoded chloroplast sulfate permease, in sulfate transport and H2evolution in Chlamydomonas reinhardtii

Antisense technology was applied to the green alga Chlamydomonas reinhardtiito probe the function of a novel nuclear gene encoding a chloroplast-envelope localized sulfate permease (SulP; GenBank Accession Numbers AF467891 and AF481828). Analysis showed that antiSulP transformants are impaired in sulfate uptake, a consequence of repression in the SulP gene expression. Antisense antiSulP transformants exhibited a sulfur-deprivation phenotype, strong induction of arylsulfatase activity, and global induction of sulfate assimilation gene expression. In sealed cultures, opposite to the wild-type control, antiSulP strains photo-evolved H₂, underlining the notion of sulfate uptake limitation by the chloroplast, a slow-down in the rate of oxygen evolution, establishment of anaerobiosis due to internal respiration and spontaneous expression of the [Fe]-hydrogenase in these strains. It is concluded that antiSulP strains are promising as tools to limit the supply of sulfates to the chloroplast, leading to a down-regulation of H₂O-oxidation and O₂-evolution activity, to the constitutive expression of the [Fe]-hydrogenase and continuous H₂-photoproduction in Chlamydomonas reinhardtii.Thus, antisulPstrains might permit a study of the biochemistry of H₂ metabolism in this green alga under constitutive anaerobic oxygenic photosynthesis conditions.