Ectopic Expression of a Transmembrane Protein KaCyt b6 from a Red Seaweed Kappaphycus alvarezii in Transgenic Tobacco Augmented the Photosynthesis and Growth.

Cytochrome b₆f complex is a thylakoid membrane-localized protein and catalyses the transfer of electrons from plastoquinol to plastocyanin in photosynthetic electron transport chain. In the present study, Cytochrome b₆ (KaCyt b₆) gene from Kappaphycus alvarezii (a red seaweed) was overexpressed in tobacco. A 935 base pair (bp) long KaCyt b₆ cDNA contained an open reading frame of 648 bp encoding a protein of 215 amino acids with an expected isoelectric point of 8.67 and a molecular mass of 24.37 kDa. The KaCyt b₆ gene was overexpressed in tobacco under control of CaMV35S promoter. The transgenic tobacco had higher electron transfer rate and photosynthetic yield over wild-type and vector control tobacco. The KaCyt b₆ tobacco also exhibited significantly higher photosynthetic gas exchange (P_N) and improved water use efficiency. The transgenic plants had higher ratio of P_N and intercellular CO₂. The KaCyt b₆ transgenic tobacco showed higher estimates of photosystem II quantum yield, higher activity of the water-splitting complex, PSII photochemistry, and photochemical quenching. The basal quantum yield of nonphotochemical processes in PSII was recorded lower in KaCyt b₆ tobacco. Transgenic tobacco contained higher contents of carotenoids and total chlorophyll and also had better ratios of chlorophyll a and b, and carotenoids and total chlorophyll contents hence improved photosynthetic efficiency and production of sugar and starch. The KaCyt b₆ transgenic plants performed superior under control and greenhouse conditions. To the best of our knowledge through literature survey, this is the first report on characterization of KaCyt b₆ gene from K. alvarezii for enhanced photosynthetic efficiency and growth in tobacco.