Simultaneous down-regulation of caffeic/5-hydroxy ferulic acid-O-methyltransferase I and cinnamoyl-coenzyme a reductase in the progeny from a cross between tobacco lines homozygous for each transgene. Consequences for plant development and lignin synthesis

Inhibition of specific lignin biosynthetic steps by antisense strategy has previously been shown to alter lignin content and/or structure. In this wor k, homozygous tobacco (Nicotiana tabacum) lines transformed with cinnamoyl- coenzyme A reductase (CCR) or caffeic acid/5-hydroxy ferulic acid-O-methylt ransferase I (COMT I) antisense sequences have been crossed and enzyme acti vities, lignin synthesis, and cell wall structure of the progeny have been analyzed. In single transformed parents, CCR inhibition did not affect COMT I expression, whereas marked increases in CCR activity were observed in CO MT I antisense plants, suggesting potential cross talk between some genes o f the pathway. In the progeny, both CCR and COMT I activities were shown to be markedly decreased due to the simultaneous repression of the two genes. In these double transformants, the lignin profiles were dependent on the r elative extent of down-regulation of each individual enzyme. For the siblin gs issued from a strongly repressed antisense CCR parent, the lignin patter ns mimicked the patterns obtained in single transformants with a reduced CC R activity. In contrast, the specific lignin profile of COMT I repression c ould not be detected in double transformed siblings. By transmission electr on microscopy some cell wall loosening was detected in the antisense CCR pa rent but not in the antisense COMT I parent. In double transformants, immun olabeling of non-condensed guaiacyl-syringyl units was weaker and revealed changes in epitope distribution that specifically affected vessels. Our res ults more widely highlight the impact of culture conditions on phenotypes a nd gene expression of transformed plants.