Decreased Cell Wall Digestibility in Canola Transformed with Chimeric Tyrosine Decarboxylase Genes from Opium Poppy¹

Peter J. Facchini^*, Min Yu, and Catherine Penzes-Yost

Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4

Tyrosine decarboxylase (TYDC) is a common plant enzyme involved in the biosynthesis of numerous secondary metabolites, including hydroxycinnamic acid amides. Although a definite function has not yet been determined, amides have been proposed to form a physical barrier against pathogens because they are usually found as integral cell wall components. Canola (Brassica napus) was independently transformed with chimeric genes (35S::TYDC1 and 35S::TYDC2) under the transcriptional control of the cauliflower mosaic virus 35S promoter, and encoding two TYDC isoforms from opium poppy (Papaver somniferum). All T₀ plants displayed a suppressed level of wild-type TYDC activity, and transgene mRNAs were not detected. Silencing of 35S::TYDC1 was overcome in the T₁ progeny of self-pollinated T₀ plants, since high levels of TYDC1 mRNAs were detected, and TYDC activity increased up to 4-fold compared with wild-type levels. However, TYDC1 mRNA levels decreased in T₂ plants and were not detected in the T₃ progeny. TYDC activity also gradually declined in T₂ and T₃ plants to nearly wild-type levels. In contrast, silencing of 35S::TYDC2 was maintained through four consecutive generations. T₁ plants with a 3- to 4-fold increase in wild-type TYDC activity showed a 30% decrease in cellular tyrosine pools and a 2-fold increase in cell wall-bound tyramine compared with wild-type plants. An increase in cell wall-bound aromatic compounds was also detected in these T₁ plants by ultraviolet autofluorescence microscopy. The relative digestibility of cell walls measured by protoplast release efficiency was inversely related to the level of TYDC activity.

Plant Physiol. (1999) 120: 653-664
Copyright Clearance Center:   0032-0889/99/120//12
© 1999 American Society of Plant Physiologists

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