Brassinosteroids (BRs) play important roles in many aspects of plant growth and development. BR-induced AtBEE3 (brassinosteroid enhanced expression 3) is required for a proper BR response in Arabidopsis. Here, we identified a poplar (Populus alba x P. glandulosa) BEE3 homolog encoding a putative basic helix-loop-helix (bHLH)-type transcription factor through microarray analysis. Transcripts of PagBEE3 were mainly detected in stems, with the internode having a low level of the transcripts and the node having a relatively higher level. The function of the PagBEE3 gene was investigated through the phenotypic analyses with PagBEE3-overexpressing (ox) transgenic lines. This work mainly focused on a potential role of PagBEE3 in stem growth and development of polar. The PagBEE3-ox poplar showed thicker and longer stems than wild-type plants. The xylem cells from the stems of PagBEE3-ox plants revealed remarkably enhanced proliferation, resulting in an earlier thickening growth than wild-type plants. Microarray analysis revealed that the expression of many genes involved in xylem cell proliferation and development was altered in the PagBEE3-ox plants. Therefore, this work suggests that xylem development of poplar is accelerated in PagBEE3-ox plants and PagBEE3 plays a role in the stem growth by increasing the proliferation of xylem cells to promote the initial thickening growth of poplar stems.

The role of an expansin gene (IbEXP1) in the formation of the storage root (SR) was investigated by expression pattern analysis and characterization of IbEXP1-antisense sweetpotato (Ipomoea batatas cv.Yulmi) plants in an attempt to elucidate the molecular mechanism underlying SR development in sweetpotato. Transcript level of IbEXP1 was high in the fibrous root (FR) and petiole at the FR stage, but decreased significantly at the young storage root (YSR) stage. IbEXP1-antisense plants cultured in vitro produced FRs which were both thicker and shorter than those of wild-type (WT) plants. Elongation growth of the epidermal cells was significantly reduced, and metaxylem and cambium cell proliferation was markedly enhanced in the FRs of IbEXP1-antisenseplants, resulting in an earlier thickening growth in these plants relative to WT plants. There was a marked reduction in the lignification of the central stele of the FRs of the IbEXP1-antisense plants, suggesting that the FRs of the mutant plants possessed a higher potential than those of WT plants to develop into SRs. IbEXP1-antisense plants cultured in soil produced a larger number of SRs and, consequently, total SR weight per IbEXP1-antisense plant was greater than that per WT plant. These results demonstrate that SR development was accelerated in IbEXP1-antisense plants and suggest that IbEXP1 plays a negative role in the formation of SR by suppressing the proliferation of metaxylem and cambium cells to inhibit the initial thickening growth of SRs.