Lecture18

1. Lecture18 Modification of food, taste and appearance

2. Modification of food plant taste and appearance • Preventing Discoloration • Sweetness • Starch content

3. Preventing Discoloration • enzymes responsible - polyphenol oxidases • nucleus-encoded enzymes • localized in chloroplast and mitochondrial membranes • inhibition of the enzyme polyphenol oxidase decrease discoloration • Vectors constructed with either the full-length or partial potato polyphenol oxidase cDNA in either the sense or antisense orientation • under the control of the cauliflower mosaic virus 35S promoter, the granule-bound starch synthase promoter, or the patatin type I promoter

4. Preventing discoloration • The sense constructs all synthesized increased amounts of polyphenol oxidase showed larger amounts of black spot than non transformed control plants. • Similar anitsense constructs reduce enzymatic discoloration in wide range of commercially important plants

5. Fig 1.Sense and antisense polyphenol oxidase gene constructs.Transcription of either the sense or antisense gene is separately under the control of the cauliflower mosaic virus 35S promoter (P35S), the granule-bound starch synthase promoter (PGBSS) or the patatin type I promoter (Ppatatin) with the nopaline synthase transcription terminator region (tNOS). The abbreviations RB and LB represent the right and left borders of the T-DNA.

6. Sweetness • Monellin , dimer with an A chain of 45 amino acid residues and a B chain of 50 residues • the chains held together by weak noncovalent bonds. • monellin composed of 2 separate polypeptide chains ,readily dissociated (denatured) • consequently loses its sweetness when exposed to heat or acid (e.g., lemon juice or vinegar)

7. Sweetness • Synthesis of a monellin gene encoding both chains as a single peptide • The fusion protein produced in transgenic tomato and lettuce plants • Two different promoters were used to express the monellin fusion protein gene • In the experiment with tomatoes, expression directed by the tomato fruit-specific promoter E8 • The construct for the lettuce under the control of the CaMV35S promoter • Monellin gene introduced into plant cells by Ti plasmid co integrate vector system • Monellin detected in ripe and partially ripe tomatoes and in lettuce leaves.

8. Sweetness • Fructans are naturally occurring polymers of fructose • used as a natural low-calorie sweetener instead of sucrose. • sugar beet plants engineered to convert the sucrose • by transforming sugar beets with a genetic construct that contained the 1-sucrose: sucrose fructosyl transferase cDNA from jerusalem artichokes • transgenic sugar beets accumulated fructan up to 40% of the taproot dry weight

9. Starch • starch in most crop plants consists : • 20 to 30% (straight-chain) amylose • 70 to 80% (branched-chain) amylopectin Biosynthesis of starch

10. Construction of an α-amylase-glucose isomerase fusion gene

11. Increased starch content • Construction of a bifunctional enzyme containing the essential regions of α-amylase and glucose isomerase • synthesized in E. coli, both α-amylase and glucose isomerase activities were found • chimeric gene was introduced into potato plants under control of the granule bound starch synthase promoter • biochemical analysis revealed considerable increase in glucose and fructose concentration