FIELD-EVALUATION OF TRANSGENIC POTATO GIANTS EXPRESSING AN ANTISENSE GRANULE-BOUND STARCH SYNTHASE GENE - INCREASE OF THE ANTISENSE EFFECT DURING TUBER GROWTH

Transgenic plants of a tetraploid potato cultivar were obtained in whi ch the amylose content of tuber starch was reduced via antisense RNA-m ediated inhibition of the expression of the gene encoding granule-boun d starch synthase (GBSS). GBSS is one of the key enzymes in the biosyn thesis of starch and catalyses the formation of amylose. The antisense GBSS genes, based on the full-length GBSS cDNA driven by the 35 S CaM V promoter or the potato GBSS promoter, were introduced into the potat o genome by Agrobacterium tumefaciens-mediated transformation. Express ion of each of these genes resulted in the complete inhibition of GBSS gene expression, and thus in the production of amylose-free tuber sta rch, in mature field-grown plants originating from rooted in vitro pla ntlets of 4 out of 66 transgenic clones. Clones in which the GBSS gene expression was incompletely inhibited showed an increase of the exten t of inhibition during tuber growth. This is likely to be due to the i ncrease of starch granule size during tuber growth and the specific di stribution pattern of starch components in granules of clones with red uced GBSS activity. Expression of the antisense GBSS gene from the GBS S promoter resulted in a higher stability of inhibition in tubers of f ield-grown plants as compared to expression from the 35 S CaMV promote r. Field analysis of the transgenic clones indicated that inhibition o f GBSS gene expression could be achieved without significantly affecti ng the starch and sugar content of transgenic tubers, the expression l evel of other genes involved in starch and tuber metabolism and agrono mic characteristics such as yield and dry matter content.