Vegetative storage protein expression during terminal bud formation in poplar

Trees recycle nitrogen (N) to conserve this valuable nutrient. The processes that regulate N recycling within trees are poorly understood at the molecular level. Because vegetative storage proteins (VSPs) are thought to play important roles in within-plant N cycling, we are investigating the expression of VSP genes to gain insights into how seasonally controlled N cycling is regulated in trees. We compared steady-state mRNA levels of three different VSP homologs during short day induced terminal bud formation in hybrid poplar (Populus trichocarpa Torr. & Gray × Populus deltoides Bartr. ex Marsh.) - WIN4 (wound-inducible protein 4), BSP (bark storage protein), and pni288 (poplar nitrogen-regulated cDNA 288, a newly identified sequence). We determined that win4 and pni288 transcripts decrease, while bsp transcripts increase, as the terminal bud is formed. Immunolocalization analysis indicated that, during apical bud formation, BSP accumulates in the ground meristem and in parenchyma cells adjacent to xylem and proximal to the apical dome. Based on messenger RNA and protein expression analysis, we conclude that different VSPs play distinct roles in the poplar shoot apex, with BSP accumulating as a reserve near the shoot apex during terminal bud formation.