Dissected effect of a transit peptide of the ADP-glucose pyrophosphorylase gene from sweetpotato (ibAGP2) in increasing foreign protein accumulation

The transit peptide sequence of ibAGP2 (TP2) was found to be capable of targeting protein into the chloroplast in the Arabidopsis protoplasts. TP2 was fused to a β-glucuronidase (GUS) reporter gene and expressed in Arabidopsis under the control of the ibAGP2 promoter with the aim of dissecting the effect of the transit peptide in elevating foreign protein accumulation in the transgenic plant. β-glucuronidase protein levels were determined at three different developmental stages and in assorted tissues. TP2 dramatically elevated GUS protein accumulation regardless of developmental stage, but the level of the enhancing effect was developmental stage-dependent. This enhancing effect was strongest at the seedling stage (16-fold) and relatively moderate at the vegetative (tenfold) and reproductive (11-fold) stages. TP2 also elevated GUS protein accumulation to varying degrees (4 to 19-fold) in assorted tissues, with the effect being highest in the primary inflorescence stem and petiole (19-fold) and weakest in the root (fourfold). Although TP2 also increased GUS mRNA levels, the increased levels were not large enough to account for the elevated GUS protein levels, suggesting that the enhancing effect of TP2 does not solely result from increased levels of transcripts. Taken together, our results reveal that the TP2 significantly increased the levels of protein accumulation and that its effectiveness was developmental stage- and tissue-type-dependent in transgenic Arabidopsis. Possible differential targeting efficiencies of different transit peptides are discussed.