Starch self-processing in transgenic sweet potato roots expressing a hyperthermophilic α-amylase

Sweet potato is a major crop in the southeastern United States, which requires few inputs and grows well on marginal land. It accumulates large quantities of starch in the storage roots and has been shown to give comparable or superior ethanol yields to corn per cultivated acre in the southeast. Starch conversion to fermentable sugars (i.e., for ethanol production) is carried out at high temperatures and requires the action of thermostable and thermoactive amylolytic enzymes. These enzymes are added to the starch mixture impacting overall process economics. To address this shortcoming, the gene encoding a hyperthermophilic α-amylase from Thermotoga maritima was cloned and expressed in transgenic sweet potato, generated by Agrobacterium tumefaciens-mediated transformation, to create a plant with the ability to self-process starch. No significant enzyme activity could be detected below 40°C, but starch in the transgenic sweet potato storage roots was readily hydrolyzed at 80°C. The transgene did not affect normal storage root formation. The results presented here demonstrate that engineering plants with hyperthermophilic glycoside hydrolases can facilitate cost effective starch conversion to fermentable sugars. Furthermore, the use of sweet potato as an alternative near-term energy crop should be considered.