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Title: DOWN-REGULATION OF A RIPENING-RELATED BETA-GALACTOSIDASE GENE (TBG1) IN TRANSGENIC TOMATO FRUIT

Author
item SEYMOUR, GRAHAM - HORT RES INTL, U.K.
item CAREY, ANNETTE - HORT DEV COUN, U.K.
item Smith, David
item HARRISON, ELISABETH - HORT RES INTL, U.K.
item BIRD, COLIN - ZENECA PLANT SCI, U.K.
item Gross, Kenneth
item TUCKER, GREG - UNIV OF NOTTINGHAM, U.K.

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Changes in fruit texture have a major impact on quality and shelf-life of fresh fruits and vegetables. Thus, we have studied the mechanisms involved in the loss of firmness that occurs during tomato fruit ripening. A critical feature of fruit softening is the disassembly of the cell wall (the sugar envelope surrounding plant cells) that occurs during ripening. If this disassembly and loss of integrity can be slowed down, we should be able to create fruit that can be picked vine- ripe and still withstand current commercial handling practices. Previous research showed a dramatic loss of galactose (a sugar cell wall component) from the wall during ripening occurred, and we suspect this loss may play a significant role in softening The present study reports on the cloning of a gene, one of seven we have identified in tomato, that was possibly involved in degrading the fruit cell wall. However, molecular-genetic experiments which reduced the expression of this gene by 90 percent had no effect on the loss of cell wall galactose or fruit firmness. This information confirms our contention that another gene, which we have been focusing on is the major gene involved in tomato fruit softening and supports our continued emphasis on that gene. Eventually, such information should lead to our ability to allow tomato fruit to ripen normally while slowing down the softening process, leading to increased quality and shelf-life.

Technical Abstract: Exo-galactanase/B-galactosidase (E.C. 3.2.1.23) activity is thought to be responsible for the loss of galactosyl residues from the cell walls of ripening tomatoes. Transgenic tomato plants (Lycopersicon esculentum Mill cv. Ailsa Craig) with reduced exo-galactanase/B-galactosidase message were generated to test this hypothesis and to investigate the role of the enzyme in fruit softening. A previously identified tomato B-galactosidase cDNA clone, TBG1, was used in the experiments. Heterologous expression of the clone in yeast demonstrated that TBG1 could release galactosyl residues from tomato cell wall galactans. Northern blot analysis confirmed that the pattern of expression of TBG1 was ripening-related and mainly confined to the pericarp tissue. Transgenic plants were generated which showed a reduction in TBG1 message to 10 percent of normal levels in the ripening fruits. However, despite the reduction in message, total B-galactosidase and exo- galactanase activity was unaffected. Furthermore, there was no apparent effect on levels of cell wall galactosyl residues when compared with the control. It was concluded that during the ripening of tomato fruits a family of B-galactosidases capable of degrading cell wall galactans are active and down-regulation of TBG1 message to 10 percent was insufficient to alter the degree of galactan degradation.