Ihemere¹ UE and Sayre², RT.

Departments of Horticulture and Crop Science¹, and Plant Biology², The Ohio State University, Columbus, OH 43210.

We are studying the role of sucrose loading and uptake in determining the sink strength (starch) of transgenic cassava roots. We have transformed cassava somatic embryos using two vectors: pV2 (patatin/cellwall invertase, ivr2) and pV1 (patatin/cytosolic invertase, ivr1). Both vectors contain an invertase (ivr) gene originating from maize that is driven by the patatin (tuber-specific) promoter. We previously have demonstrated that the patatin promoter drives gene expression in cassava roots. The patatin promoter was chosen because the sink for starch accumulation is the root. Presently, we have obtained putative transformed cassava plantlets from cultivars TMS 71173 and Mcol 2215. Biochemical studies will follow soon. These assays will include analysis of root invertase activity and starch content. The manipulation of invertase activity is part of our overall objective aimed at increasing carbohydrate export from leaves and to roots and its conversion to starch. To facilitate carbohydrate export we are over-expressing sucrose phosphate synthase in leaves using the leaf-specific, CAB1 promoter. In addition, we propose to increase starch synthesis in roots via root-specific expression of the E. coli glgC gene encoding ADP glucopyrophosphorylase, the rate-limiting enzyme in starch production. The overall objective is to produce a cassava plant with increased root starch production so as to increase yields and/or reduce the growing season.

Uzoma E. Ihemere, Diana I. Arias-Garzon and Richard T. Sayre

Departments of Horticulture and Crop Science, and Plant Biology

The Ohio State University, Columbus, OH 43210.

Attempts are made to transform tomato via Agrobacterium-mediated transformation with two vectors pCAT (CAB1/TP glgC) and pCA (CAB1/glgC). The two vectors contain modified AGPase gene driven by heterologous CAB1 promoter. By contrast, pCAT contains a transit peptide while pCA does not. We are investigating the impact of over-expressing a mutated glgC gene from E. coli. driven by CAB1 promoter on increasing tomato fruit starch content. This effort will also determine the location of AGPase activity (chloroplast or cytosol) in tomato. Tomato fruits are used in making source and paste for the cuisinary industry, but, unfortunately,it comprises about 80% water, whereas the desirable ingredient in making source and paste is starch. This makes it cumbersome to transport from the farms to the processing centers. Also, during ripening, most of the starch is hydrolysed and transported out of the fruit as hexoses by glucose transporters. Our objective is to increase the starch content of the fruit and later on, block the transport of starch through its breakdown products, by antisense expression of glucose transporter. This way, we will produce tomato fruits with increase in starch content. At the moment, hypocotyl and cotyledon explants were used in transformation of the tomato cultivar Roma VF with pCA and pCAT constructs. Plantlets have been regenerated in both hypocotyl and cotyledon explants on a regenerating medium supplemented with 75 mg/l kanamycin. Putative transformants have been confirmed through fluorescence quenching which determines whether or not plants under selection are under stress or growing normally. Biochemical and molecular analysis will be carried out when plantlets produce enough materials for analysis.