The poplar bark storage protein gene (Bspa) promoter is responsive to photoperiod and nitrogen in transgenic poplar and active in floral tissues, immature seeds and germinating seeds of transgenic tobacco

In Populus, seasonal nitrogen storage involves the accumulation of a 32 kDa bark storage protein (BSP) in the inner bark parenchyma and xylem rays. Poplar BSPs are encoded by a multigene family and one member, bspA, has been cloned and sequenced. The regulation of bspA was investigated by transforming either hybrid poplar or tobacco with a chimeric gene consisting of the 2.8 kb bspA promoter fused to the coding region of beta -glucuronidase (uidA). In transformed poplar, the bspA 2.8 kb promoter conferred both short-day (SD) and nitrogen (N) inducibility to GUS and activity was localized to the bark (primary and secondary phloem, and cortex) and xylem rays. Night-break treatments inhibited SD induction of GUS. Deletion of the 1.6 kb distal DNA sequences from the bspA promoter eliminated SD induction of GUS while some N induction was retained. These results indicate that although poplar BSP is encoded by a multigene family, transcriptional activation of bspA per se can account for bsp expression in bark and xylem rays in response to either SD or N treatment. These results also show that the elements responsible for SD or N induction are separable. Because of the long generation intervals associated with trees, the developmental regulation of bspA in flowers, developing seeds, and germinating seeds was investigated by transforming the 2.8 kb bspA-promoter::uidA chimeric gene into tobacco. The bspA promoter was active in developing tobacco floral tissues and in seeds during early stages of embryogenesis, decreased progressively during seed maturation and regained activity upon seed germination. Although seed storage proteins of poplar share some similarities to poplar BSP, the observed developmental expression patterns in tobacco are consistent with a role for bspA in vegetative rather than seed storage protein storage.