Phytochrome-mediated photoperiod perception, shoot growth, glutamine, calcium, and protein phosphorylation influence the activity of the poplar bark storage protein gene promoter (bspA)

In poplars (Populus), bspA encodes a 32-kD bark storage protein that accumu lates in the inner bark of plants exposed to either short-day (SD) photoper iods or elevated levels of nitrogen. In this study, poplars transformed wit h a chimeric gene consisting of the bspA promoter fused to beta -glucuronid ase (uidA) were used to investigate the transcriptional regulation of the b spA promoter. Photoperiodic activation of the bspA promoter was shown to in volve perception by phytochrome and likely involves both a low fluence resp onse and a parallel very low fluence response pathway. Activity of the bspA promoter was also-influenced by shoot growth. High levels of bspA expressi on usually occur in the bark of plants during SD but not long day or SD wit h a night break. When growth was inhibited under growth permissive photoper iods (SD with night break) levels of bark beta -glucuronidase (GUS) activit y increased. Stimulating shoot growth in plants treated with SD inhibited S D-induced increases in bark GUS activity. Because changes in photoperiod an d growth also alter carbon and nitrogen partitioning, the role of carbon an d nitrogen metabolites in modulating the activity of the bspA promoter were investigated by treating excised stems with amino acids or NH4NO3 with or without sucrose. Treatment with either glutamine or NH4NO3 resulted in incr eased stem GUS activity. The addition of sucrose with either glutamine or N H4NO3 resulted in synergistic induction of GUS, whereas sucrose alone had n o effect. Glutamine plus sucrose induction of GUS activity was inhibited by EGTA, okadaic acid, or K-252A. Inhibition by EGTA was partially relieved b y the addition of Ca2+. The Ca2+ ionophore, ionomycin, also induced GUS act ivity in excised shoots. These results indicate that transcriptional activa tion of bspA is complex. It is likely that SD activation of bspA involves p erception by phytochrome coupled to changes in growth. These growth changes may then alter carbon and nitrogen partitioning that somehow signals bspA induction by a yet undefined mechanism that involves carbon and nitrogen me tabolites, Ca2+, and protein phosphorylation/dephosyhorylation.