THE AMMYB308 AND AMMYB330 TRANSCRIPTION FACTORS FROM ANTIRRHINUM REGULATE PHENYLPROPANOID AND LIGNIN BIOSYNTHESIS IN TRANSGENIC TOBACCO

MYB-related transcription factors are known to regulate different bran ches of flavonoid metabolism in plants and are believed to play wider roles in the regulation of phenylpropanoid metabolism in general. Here , we demonstrate that overexpression of two MYB genes from Antirrhinum represses phenolic acid metabolism and lignin biosynthesis in transge nic tobacco plants. The inhibition of this branch of phenylpropanoid m etabolism appears to be specific to AmMYB308 and AmMYB330, suggesting that they recognize their normal target genes in these transgenic plan ts. Experiments with yeast indicate that AmMYB308 can act as a very we ak transcriptional activator so that overexpression may competitively inhibit the activity of stronger activators recognizing the same targe t motifs. The effects of the transcription factors on inhibition of ph enolic acid metabolism resulted in complex modifications of the growth and development of the transgenic plants. The inhibition of monoligno l production resulted in plants with at least 17% less lignin in their vascular tissue. This reduction is of importance when designing strat egies for the genetic modification of woody crops.