Ectopic Expression of a Transmembrane Protein KaCyt b ₆ from a Red Seaweed <i>Kappaphycus alvarezii</i> in Transgenic Tobacco Augmented the Photosynthesis and Growth

Abstract

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, <i>Cytochrome b₆</i> (<i>KaCyt b₆</i>) gene from <i>Kappaphycus alvarezii</i> (a red seaweed) was overexpressed in tobacco. A 935 base pair (bp) long <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> 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 <i>KaCyt b₆</i> gene from <i>K. alvarezii</i> for enhanced photosynthetic efficiency and growth in tobacco.