SUPPRESSION OF O-METHYLTRANSFERASE GENE BY HOMOLOGOUS SENSE TRANSGENEIN QUAKING ASPEN CAUSES RED-BROWN WOOD PHENOTYPES

Homologous sense suppression of a gene encoding lignin pathway caffeic acid O-methyltransferase (CAOMT) in the xylem of quaking aspen (Popul us tremuloides Michx.) resulted in transgenic plants exhibiting novel phenotypes with either mottled or complete red-brown coloration in the ir woody stems. These phenotypes appeared in all independent transgeni c lines regenerated with a sense CAOMT construct but were absent from all plants produced with antisense CAOMT. The CAOMT sense transgene ex pression was undetectable, and the endogenous CAOMT transcript levels and enzyme activity were reduced in the xylem of some transgenic lines . in contrast, the sense transgene conferred overexpression of CAOMT a nd significant CAOMT activity in all of the transgenic plants' leaves and sclerenchyma, where normally the expression of the endogenous CAOM T gene is negligible. Thus, our results support the notion that the oc currence of sense cosuppression depends on the degree of sequence homo logy and endogene expression. Furthermore, the suppression of CAOMT in the xylem resulted in the incorporation of a higher amount of conifer yl ardehyde residues into the lignin in the wood of the sense plants. Characterization of the lignins isolated from these transgenic plants revealed that a high amount of coniferyl aldehyde is the origin of the red-brown coloration-a phenotype correlated with CAOMT-deficient maiz e (Zea mays L.) brown-midrib mutants.