Effect of simultaneous Overexpression of Polygalacturonase and Expansin on rin Tomato Softening

ARIEL VICENTE, CARL GREVE, ANN POWELL, ALAN BENNETT, JOHN LABAVITCH

California USA

Conferencia; American Society of Plant Biologists Western Section Annual Winter Meeting; 2007

American Society of Plant Biologists

     Softening is one of the main problems limiting the postharvest life of fruits. Textural changes in plant tissues have been associated with modifications in cell wall structure and composition caused by several proteins including polygalacturonases (PGs) and expansins (Exp). Although our understanding of the role of these proteins has markedly increased in the last years, still little is known about the ways they interact to disassemble the cell wall pectic and cellulosic-hemicellulosic (Cel-Hem) matrices. The objective of the present work was to evaluate the cell wall degradation and softening in rin tomato with increased expression of expansin and PG alone or simultaneously. Transgenic rin tomato plants over-expressing PG and Exp alone or in combination, driven by a fruit specific and ethylene inducible promoter (E8) were generated. Fruit was harvested and stored in ethylene or air and changes in firmness and cell wall composition and structure were analyzed. The results showed that Exp1 is a limiting factor in cell wall degradation of rin tomato being its effect on softening more dramatic that the effect of PG. The double transgenic fruit over-expressing PG and Exp1 showed the highest softening rate. However, these fruit still softened less that wild type tomato showing that Exp1 and PG are not sufficient to recover normal softening. Changes in firmness in fruit over-expressing PG and Exp were mainly associated with higher solubilization, depolymerization and increased changes in the composition of covalently linked pectins and hemicelluloses. Interestingly the over-expression of Exp affected pectic matrix disassembly while over-expression of PG also affected Cel-Hem matrix degradation, showing a clear in vivo evidence of functional interaction between the Cel-Hem and pectic matrices.