Lignin characteristics and ecological interactions of PtCOMT-modified silver birch

The 35Sand UbB1-PtCOMT genes were transferred by biolistic bombardment into silver birch, Betula pendula Roth, and the effects on syringyl (S) lignin unit synthesis were investigated. The transgenes were stably integrated into the B. pendula genomes, and their variable expression was observed. In the 35S-PtCOMT lines, a reduced syringyl/guaiacyl (S/G) ratio and incorporation of abnormal 5-OH-G units into lignin were found. This was apparently due to the RNA interference (RNAi)-based suppression of the COMT gene, leading to reduced S lignin content in stems, leaves and roots. This work supports the essential role of COMT for S unit synthesis and the current view on lignin biosynthesis in woody angiosperms. The unchanged morphology and growth characteristics of the 35S-PtCOMT modified B. pendula lines also indicate that plants are able to tolerate a large variation in the lignin S/G ratio. PtCOMT-promoter-GUS (β-glucuronidase) modified B. pendula lines were produced in order to investigate the expression pattern of the PtCOMT gene. The main activity during the growing season was present in the new xylem and lignified phloem fibres. Our results also suggest that COMT plays a role in tension wood formation but not in the response to wounding. In in vitro studies focusing on the potential ecological interactions between PtCOMTmodified B. pendula lines and insect herbivores and ectomycorrhizal fungi (ECM), the role played by birch clone or transgenic line was found to be important. The preferential leaf quality to insect herbivores was not directly related to the PtCOMT-modified lignin, but resulted potentially from indirect factors such as a changed growth rhythm. All PtCOMT lines were able to form ECM with P. involutus, but the specific ECM characteristics, e.g. formation of a Hartig net (HN), differed from those of the control. The extent to which the different ECM characteristics of specific PtCOMT lines are related to lignin modification or unintended effects of transgenes remains to be solved.