Characterization of tomato fruit expressing an antisense gene for pectin methylesterase

Denise Marie Tieman, Purdue University

Abstract

Pectin methylesterase (PME, EC 3.1.1.11) demethoxylates pectins and is believed to be involved in degradation of pectic cell wall components by polygalacturonase in ripening tomato fruit. We have introduced antisense and sense chimeric PME genes into tomato to elucidate the role of PME in fruit development and ripening. Fruits from transgenic plants expressing high levels of antisense PME RNA showed $<$10% of wild-type PME enzyme activity and undetectable levels of PME protein and mRNA. Lower PME enzyme activity in fruits from transgenic plants was associated with an increased molecular weight and methylesterification of pectins and decreased levels of total and chelator soluble polyuronides in cell walls. The fruits of transgenic plants also contained higher levels of soluble solids than did wild-type fruits. Reduction of PME activity in transgenic fruits causes an almost complete loss of tissue integrity during fruit senescence, but shows little effect on fruit firmness during ripening. Low PME activity in the transgenic fruit pericarp selectively impairs accumulation of $\rm Mg\sp{2+}$ over other major cations. Lowered PME activity is associated with a 30 to 70% decrease in bound $\rm Ca\sp{2+}$ and $\rm Mg\sp{2+}$ in transgenic fruit pericarp as compared to wild-type pericarp. Levels of soluble $\rm Ca\sp{2+}$ increase 10 to 60% in transgenic pericarp, while levels of soluble $\rm Mg\sp{2+}$ and $\rm Na\sp+$ are reduced by 20 to 60% with lowered PME activity. Field performance of transgenic tomato plants containing the antisense gene for pectin methylesterase was evaluated during the summers of 1992 and 1993. Field grown transgenic tomato plants had vegetative and fruit yields similar to control lines during both years. Transgenic plants showed an increase in both fruit number and yield in 1992 compared to both wild-type and azygous controls, but no differences were observed among the three genotypes in 1993. Average fruit weight did not show significant differences among the three genotypes in 1992, but was lower in azygous and transgenic plants than in wild-type Rutgers plants in 1993. Transgenic fruits had higher soluble and total solids and higher pH than control fruits, suggesting that transgenic fruits may have better processing quality.

Degree

Ph.D.

Advisors

Handa, Purdue University.

Subject Area

Botany|Molecular biology

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