Leaf-Specific Antisense Inhibition of Starch Biosynthesis in Transgenic Potato Plants Leads to an Increase in Photoassimilate Export from Source Leaves during the Light Period

In an attempt to study the importance of starch synthesis in leaves with respect to sink-source interactions, we investigated daily turnover of carbohydrates in leaves of transgenic potato plants inhibited for ADP-glucose pyrophosphorylase (AGPase). Down-regulation of AGPase has been performed using two different promoters: the near-constitutive CaMV 35S promoter, and the STLSI promoter which is active in photosynthetic cells only. Residual AGPase activity in leaves was between 6 and 30% in individual transformants as compared to wild-type potato plants. We found that: (i) photosynthesis is not significantly altered relative to wild-type plants; (ii) levels of starch are markedly reduced in leaves of transgenic plants; (iii) levels of soluble sugars and malate are largely unaffected by the inhibition of AGPase; (iv) the reduction of starch synthesis leads to a higher portion of assimilated carbon being transported from leaves to sink tissues during the light period; (v) altered leaf export characteristics do not change tuber yield under greenhouse conditions. Collectively, these data demonstrate a striking flexibility of the potato plant with respect to day/night rhythms of carbon export from leaves and utilization by the major storage sinks, i.e. developing tubers.