Molecular changes associated with the setting up of secondary growth in aspen.

Vascular secondary growth results from the activity of the vascular cambium, which produces secondary phloem and secondary xylem. By means of cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis along aspen stems, several potential regulatory genes involved in the progressive transition from primary to secondary growth were identified. A total of 83 unique transcript-derived fragments (TDFs) was found to be differentiated between the top and the bottom of the stem. An independent RT-PCR expression analysis validated the cDNA-AFLP profiles for 19 of the TDFs. Among these, seven correspond to new genes encoding putative regulatory proteins. Emphasis was laid upon two genes encoding, respectively, an AP2/ERF-like transcription factor (PtaERF1) and a RING finger protein (PtaRHE1); their differential expression was further confirmed by reverse northern analysis. In situ RT-PCR revealed that PtaERF1 was expressed in phloem tissue and that PtaRHE1 had a pronounced expression in ray initials and their derivatives within the cambial zone. These results suggest that these genes have a potential role in vascular tissue development and/or functioning.