The influence of transgenic modification of gene expression during ripening on physicochemical characteristics of processed tomato products

Accumulation of ripening related polygalacturonase (PG) and expansin (Exp1) was modified in two lines of tomato (Lycopersicon esculentum Mill.): a processing hybrid and the fresh market variety Ailsa Craig by introducing transgenes either to silence expression of the PG and Exp1 genes, or over-express Exp1. Rheological performance of tomato juice concentrates prepared from lines suppressed in either gene or in both and a line over-expressing Exp1 was compared to azygous controls. Relative to unmodified control lines, diluted concentrates produced from fruit suppressed in either PG or Exp1 had enhanced consistency. Fruit in which both PG and Exp1 expression were suppressed produced concentrates with slightly higher recoverable viscosities than control fruit or the single suppressed lines. Juices and pastes produced by a line overexpressing Exp1 showed the highest viscosity increase. Particle size determinations by laser diffraction showed changes in size distribution in juices and concentrates from the transgenic lines including an increase in particle diameters in the Exp1 over-expressing line. 1H NMR relaxometry revealed differences in water properties in the serum phase and in the particles among transgenic and control lines consistent with the viscosity changes. Size exclusion chromatography of cell wall polysaccharides in sequential extracts indicated only slight changes in the size distributions of individual classes of cell wall polymers. Results indicated that suppressing PG abundance influenced viscosity by modifying serum properties whereas suppressing or over-expressing Exp1 altered particle properties. Hence altering the activity of these cell wall enzymes may be a route to the selection of improved processing tomato varieties. When results from the Ailsa Craig transgenic lines were compared to those obtained from a processing hybrid variety with the same genetic modifications, similar trends were observed in rheological properties but of different magnitude. Differences were present mainly in cell wall polymer extractability, size distribution and neutral sugar composition of sequential cell wall extracts between the two lines, making Ailsa Craig a moderately successful model to predict the effect of ripening related modifications on the rheological properties of processed juices and concentrates. Attempts were made to express Exp1 in a baculovirus-insect cell system and in tomato callus cultures.