Comparative transcriptome profiling of Kappaphycus alvarezii (Rhodophyta, Gigartinales) in response to two extreme temperature treatments: an RNA-seq-based resource for photosynthesis research

Kappaphycus alvarezii provides an important raw material for the extraction of kappa-carrageenan. Comparative transcriptome analysis of K. alvarezii after 24 h extreme temperature treatments (17 degrees C and 34 degrees C) was performed to uncover the molecular mechanisms controlling temperature-related photosynthesis regulation. Optimum temperature (28 degrees C) was employed as the control. After de novo assembly of these three transcriptome samples and searching the assembled transcripts by BLAST against the NCBI nr databases, 11,537 top-hit transcripts were assigned as unigenes. Through comparisons, 302 significantly differentially expressed genes were identified. Twenty-three of the differentially expressed unigenes encoded photosystem antenna protein subunits and a further 25 were associated more broadly with photosynthesis functions. Compared to the light-harvesting complex (LHC) protein, the expression of phycobiliproteins (PBPs) was stable after extreme low and high temperature (ELT/EHT) treatments. Both ELT and EHT were found to inhibit the expression of subunits of photosystem II (PsbA, PsbC, Psb27, PsbW, PsbS), photosystem I (PsaF, PsaG, PsaK) and photosynthetic electron transport (PetE). These results were confirmed through targeted analysis of the mRNA and protein abundance of individual proteins, and were congruent with the results of chlorophyll a fluorescence (OJIP) measurements performed with temperature-stressed lines. In conclusion, similarities and differences in the behaviours of photosynthesis-related genes were summarized to provide opportunities for understanding the responses of K. alvarezii after extreme temperature treatments.