ISOLATION AND CHARACTERIZATION OF SIX cDNAs INVOLVED IN CARBON METABOLISM IN LAMINARIA DIGITATA (PHAEOPHYCEAE)

An expressed sequence tag (EST) approach was used to retrieve cDNA clones involved in carbon metabolism in Laminaria digitata Lamouroux. Six partial open reading frames were identified, respectively encoding an α-type carbonic anhydrase (CA), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), phosphoglycerate kinase (PGK), glycolate oxidase (GLO), and GDP-4-keto-6-d-mannose-3,5-epimerase-4-reductase, also known as fucose synthase (FS). These enzymes were further characterized through Southern blot analyses, amino acid sequence comparisons, and patterns of expression. In contrast to the other genes, which are expressed in both the gametophytic and sporophytic generations of L. digitata, the α-type carbonic anhydrase messenger RNA was shown by RT-PCR and northern blot analyses to be present in the gametophytes only. The evolutionary relationships of these genes and their interest as molecular tools for investigating carbon fluxes in brown algae are discussed.     

SPOROGENESIS UNDER ULTRAVIOLET RADIATION IN LAMINARIA DIGITATA (PHAEOPHYCEAE) REVEALS PROTECTION OF PHOTOSENSITIVE MEIOSPORES WITHIN SORAL TISSUE: PHYSIOLOGICAL AND ANATOMICAL EVIDENCE1

To study the effect of different radiation conditions on sporogenesis of Laminaria digitata (Huds.) J. V. Lamour., excised disks were induced to form sporangia under PAR (P), PAR + ultraviolet‐A (UVA) (PA), and PAR + UVA + ultraviolet‐B (UVB) (PAB) conditions in the laboratory. Vitality of meiospores, released from sori induced under different radiation conditions in the laboratory and from sori of wild sporophytes acclimated to in situ solar radiation in the presence and absence of ultraviolet radiation (UVR), was measured in terms of their germination capacity. Sorus induction in disks of laboratory‐grown sporophytes was not hampered under light supplemented with UVR, and sorus area was not significantly different among P, PA, and PAB. Vitality and germination rate of meiospores released from sori induced under different radiation treatments was comparable. Likewise, screening of UVR of the natural solar radiation did not promote higher germination rates of meiospores released from wild sporophytes. Germination rates were, however, higher in meiospores released from laboratory‐induced sori compared to sori of wild sporophytes. Higher DNA damage (formation of cyclobutane pyrimidine dimers, CPDs) was observed in laboratory‐grown nonsorus compared to sorus tissue, while CPDs were nondetectable in both sorus and nonsorus tissue of wild sporophytes. To explain the apparent protection of developing meiospores and the unexpected UV resistance of soral tissue, concurrent anatomical investigations of sporogenic tissue were performed. We observed the previously unreported existence of two types of sterile paraphysis cells. One type of paraphysis cells, the most frequent type, contained several red‐fluorescing plastids. The other type, less frequently occurring, was completely filled with substances emitting blue fluorescence under violet excitation, presumably brown algal phenolic compounds (phlorotannins). Cells of this type were irregularly scattered within the sorus and did not contain red‐fluorescing plastids. Meiospore‐containing sporangia were positioned embedded between both types of paraphysis cells. In vegetative tissue, blue autofluorescence was observed only in injured parts of the blade. Results of our study suggest that the sorus structure with phlorotannins localized in the specialized paraphysis cells may be able to screen harmful UVR and protect UV‐sensitive meiospores inside the sporangia.     

IDENTIFICATION OF THE HIGH‐TEMPERATURE RESPONSE GENES FROM PORPHYRA SERIATA (RHODOPHYTA) EXPRESSION SEQUENCE TAGS AND ENHANCEMENT OF HEAT TOLERANCE OF CHLAMYDOMONAS (CHLOROPHYTA) BY EXPRESSION OF THE PORPHYRA HTR2 GENE1

Temperature is one of the major environmental factors that affect the distribution, growth rate, and life cycle of intertidal organisms, including red algae. In an effort to identify the genes involved in the high‐temperature tolerance of Porphyra, we generated 3,979 expression sequence tags (ESTs) from gametophyte thalli of P. seriata Kjellm. under normal growth conditions and high‐temperature conditions. A comparison of the ESTs from two cDNA libraries allowed us to identify the high temperature response (HTR) genes, which are induced or up‐regulated as the result of high‐temperature treatment. Among the HTRs, HTR2 encodes for a small polypeptide consisting of 144 amino acids, which is a noble nuclear protein. Chlamydomonas expressing the Porphyra HTR2 gene shows higher survival and growth rates than the wild‐type strain after high‐temperature treatment. These results suggest that HTR2 may be relevant to the tolerance of high‐temperature stress conditions, and this Porphyra EST data set will provide important genetic information for studies of the molecular basis of high‐temperature tolerance in marine algae, as well as in Porphyra.