Uzoma E. Ihemere¹ and Richard T. Sayre^{1, 2}

Departments of ¹Horticulture and Crop Science, and ^1,2 Plant Biology

The Ohio State University, Columbus, OH 43210.

Our objective is to investigate the role of sucrose hydrolysis in determining the sink strength in transgenic cassava root transformed with two vectors: pV2 (patatin/cellwall invertase, ivr2) and pV1 (patatin/cytosolic invertase, ivr1) The invertases, both originating from maize, are driven by the patatin, a tuber-specific promoter. Based on transient expression assays we have demonstrated that this promoter also drives gene expression in cassava roots. Currently, we have obtained putative transformed cassava plantlets. We have analyzed the plants by PCR using primers to screen for the promoter/gene border fusions and confirmed that the plants are transformed. We presently are carrying out invertase enzyme assays, Southern and northern blotting for definitive conformation of transformation. This is part of a broad experiment aimed also at over-expressing sucrose phosphate synthase (with the leaf-specific CAB1 promoter) and a modified glgC gene (with the root-specific patatin promoter). It is hypothesized that the expression of this set of genes will increase starch production in cassava. Our overall objective is to increase sucrose production in the leaf with sucrose phosphate synthase and increase the breakdown of sucrose in the sink through invertase. This will create a gradient that will draw sucrose from the leaves to the sink. Substrates for starch production therefore will be made readily available for AGPase, the enzyme encoded by the glgC gene and the rate limiting step in starch synthesis. The end result will be a cassava plant with increased starch production in the root.