EST analysis of functional genes associated with cell wall biosynthesis and modification in the secondary xylem of the yellow poplar (Liriodendron tulipifera) stem during early stage of tension wood formation

A cDNA library was constructed from secondary xylem in the stem of a 2-year-old yellow poplar after being bent for 6 h with a 45° configuration to isolate genes related to cell wall modification during the early stages of tension wood formation. A total of 6,141 ESTs were sequenced to generate a database of 5,982 high-quality expressed sequence tags (ESTs). These sequences were clustered into 1,733 unigenes, including 822 contigs and 911 singletons. Homologs of the genes regulate many aspects of secondary xylem development, including those for primary and secondary metabolism, plant growth hormones, transcription factors, cell wall biosynthesis and modification, and stress responses. Although there were only 1,733 annotated ESTs (28. 9%), the annotated ESTs obtained in this study provided sequences for a broad array of transcripts expressed in the stem upon mechanical bending, and the majority of them were the first representatives of their respective gene families in Liriodendron tulipifera. In the case of lignin, xylem-specific COMTs were identified and their expressions were significantly downregulated in the tension wood-forming tissues. Additionally, the majority of the auxin- and BR-related genes were downregulated significantly in response to mechanical bending treatment. Despite the small number of ESTs sequenced in this study, many genes that are relevant to cell wall biosynthesis and modification have been isolated. Expression analysis of selected genes allow us to identify the regulatory genes that may perform essential functions during the early stages of tension wood formation and associated cell wall modification.