Manipulation of phosphoribulokinase and phosphate translocator activities in transgenic tobacco plants

Abstract

Abstract Transgenic tobacco plants containing altered amounts of phosphoribulokinase (PRK) and the chloroplast phosphate translocator (PT) have been produced by transformation with tobacco cDNAs in the sense or antisense orientation expressed from a tobacco rbcS promoter. Plants expressing antisense PRK RNA contained a range of PRK activities from wild type to less than 5% of wild type. CO2 assimilation rate was not inhibited until more than 85% of PRK activity had been removed. With reduction in PRK activity between 85% and 95%, assimilation rates were reduced by up to half compared to wild type. It is estimated that changes in amounts of Ru5P, RuBP, ATP, ADP, and 3-PGA would have altered the fine control of PRK in vivo, to realize the assimilation rates observed. Amounts of hexoses and starch, in particular, were reduced in plants expressing the lowest PRK activities; amounts of sucrose were little affected. Transgenic plants expressing sense or antisense PT cDNAs showed a 15-fold range of PT activity, from 20% to 300% of wild-type activity, but there were no consistent differences in photo-synthetic rates between transformants. Starch content was increased by 20% and total soluble sugars decreased by 20% in leaves of antisense transformants compared to sense transformants. The 40% change in the ratio of starch to soluble sugars in these plants indicates that partitioning of assimilate between starch and sugars had been altered.