Identification of stress gene transcripts in Laminaria digitata (Phaeophyceae) protoplast cultures by expressed sequence tag analysis

To characterize stress and defense-induced genes in the brown alga Laminaria digitata (Hudson) J.V. Lamouroux, 1985 expressed sequence tags (ESTs) were generated from L. digitata protoplasts. Comparison of the ESTs with public databases allowed putative functions to be assigned to 45% of the sequences. Comparison with ESTs from L. digitata sporophytes showed that protoplasts expressed more stress genes than intact thalli. Several transcripts in the stress gene class coded for proteins involved in cell protection against oxygen radicals, including thioredoxins (six ESTs), thioredoxin peroxidases (two ESTs), and glutathione-S-transferase (GST) (41 ESTs). The GSTs appear to be part of the sigma class, making them the first GST sigma identified in a photosynthetic organism. Other stress genes included a new type of vanadium-dependent bromoperoxidases (vBPO) showing 71% similarity with vBPOs previously identified in the sporophytic-thalli phase of L. digitata. The ESTs coding for 22 different mannuronan-C5-epimerases were identified among the cell wall biosynthesis genes, and several ESTs showed similarity with the genome of the Ectocarpus siliculosus virus.