Gene cloning and expression regulation in the pathway of agar and floridean starch synthesis of Gracilariopsis lemaneiformis (Rhodophyta)

To investigate the mechanism of substrate competition of synthesis pathways of floridean starch and agar in Gracilariopsis lemaneiformis, the cDNA sequences of genes encoding mannose-6-phosphate isomerase (glpmi), mannose-1-phosphate guanylyltransferase (glmpg), GDP-mannose-3′,5′-epimerase (glgme), and starch synthase (glfss) of Gp. lemaneiformis were cloned using reverse transcription PCR and rapid amplification cDNA end techniques. Real-time quantitative PCR (qPCR) was applied to evaluate correlations between gene expression (glpmi, glmpg, glgme, and glfss) and agar and starch contents. The qPCR results suggested no correlation between glgme, glmpg, and glpmi expression and agar content for low salinity cultivation of 2 weeks, and there was no significant change in glfss expression and starch content. For N-limitation, P-limitation, and low salinity cultivation of 3 weeks, agar content was clearly positively correlated with glgme expression, had some degree of positive correlation with glmpg expression, and no correlation with glpmi expression; starch content was clearly positively correlated with glfss expression. In the hypothetical 3,6-anhydro-L-galactose synthesis pathway of this red alga, the closer the enzyme genes were to the terminal of agar synthesis, the more their expression increased (glgme > glmpg > glpmi), thus providing suggestive evidence for the hypothetical carbon metabolic pathways of G. lemaneiformis. For 4 weeks of N-limitation, P-limitation, and low salinity cultivation, expression of genes in the 3,6-anhydro-L-galactose and starch synthesis pathways decreased. Promoter analysis indicated that illumination intensity and methyl jasmonate were novel transcription promotion factors for expression of genes in the 3,6-anhydro-L-galactose and starch synthesis pathways, and they clearly promoted expression of glgme, glmpg, glpmi, and glfss. Salicylic acid only promoted expression of glgme.