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Pyropia seriata, a marine red alga belonging to the order Bangiales (Rhodophyta), is one of the most valuable and widely cultivated Pyropia species. Water temperature and salinity are major factors affecting the growth and yield of Pyropia cultivation. Currently, there is limited data regarding the regulatory pathways and molecular mechanisms of the abiotic stress responses in red algae. In this study, we generated 1,403,321 expression sequence tags (ESTs) using 454 sequencing technology from the gametophyte thalli under control and high temperature conditions to identify the abiotic stress response genes. De novo assembly of the transcriptome reads generated 11,218 contigs, whereas 135,292 sequences remained as unassembled reads. Approximately 61.9 % of the contigs shared significant homology with known genes in our database, including the NCBI RefSeq database, plus Pyropia and Porphyra sequences. Sequence analyses demonstrated that the Pyropia transcriptome has a relatively high guaninecytosine (GC) content with predominant trinucleotide repeats (93.9 %). GGC was the most common motif in identified simple sequence repeats (SSRs). We selected 754 differentially expressed genes (DEGs) response to abiotic stress based on reads per kilobase per million reads (RPKM) values, many of which have no significant homology with known sequences in public database. These transcriptome sequences will facilitate future studies to understand the common processes and novel mechanisms involved in abiotic stress tolerance in red algae.

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.Abstract Water temperature is one of the major factors that impacts the growth and life cycle of Pyropia tenera, one of the most valuable and cultivated marine red algae belonging to Bangiales (Rhodophytes). We analyzed transcriptome from gametophyte of P. tenera under normal and high temperature conditions, and identified four small heat shock proteins (sHSPs). They have no significant amino acid sequence homology with known proteins in public databases except PhsHSP22 from Pyropia haitanensis. PtsHSP19.3 gene responded to high temperature but slightly or not to desiccation, freezing or high salt condition. When the PtsHSP19.3 gene was overexpressed in Chlamydomonas reinhardtii, transformed Chlamydomonas lines revealed much higher growth rate than that of control cells under heat stress condition. Transformed cells also grew well in those of the control cell onto the medium containing high salt or H 2 O 2 . When the PtsHSP19.3 was fused to GFP and introduced into tobacco protoplast, fluorescence was detected at several spots. Results indicate that PtsHSP19.3 may form super-molecular assembles and be involved in tolerance to heat stress.

Pyropia tenera (Kjellman) are marine red algae that grow in the intertidal zone and lose more than 90% of water during hibernal low tides every day. In order to identify the desiccation response gene (DRG) in P. tenera, we generated 1,444,210 transcriptome sequences using the 454-FLX platform from the gametophyte under control and desiccation conditions. De novo assembly of the transcriptome reads generated 13,170 contigs, covering about 12 Mbp. We selected 1160 differentially expressed genes (DEGs) in response to desiccation stress based on reads per kilobase per million reads (RPKM) expression values. As shown in green higher plants, DEGs under desiccation are composed of two groups of genes for gene regulation networks and functional proteins for carbohydrate metabolism, membrane perturbation, compatible solutes, and specific proteins similar to higher plants. DEGs that show no significant homology with known sequences in public databases were selected as DRGs in P. tenera. PtDRG2 encodes a novel polypeptide of 159 amino acid residues locating chloroplast. When PtDRG2 was overexpressed in Chlamydomonas, the PtDRG2 confer mannitol and salt tolerance in transgenic cells. These results suggest that Pyropia may possess novel genes that differ from green plants, although the desiccation tolerance mechanism in red algae is similar to those of higher green plants. These transcriptome sequences will facilitate future studies to understand the common processes and novel mechanisms involved in desiccation stress tolerance in red algae.

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