Phytochrome-Mediated Photoperiod Perception, Shoot Growth, Glutamine, Calcium, and Protein Phosphorylation Influence the Activity of the Poplar Bark Storage Protein Gene Promoter (bspA)

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Abstract
In poplars (Populus), bspA encodes a 32-kD bark storage protein that accumulates in the inner bark of plants exposed to either short-day (SD) photoperiods or elevated levels of nitrogen. In this study, poplars transformed with a chimeric gene consisting of the bspA promoter fused to β-glucuronidase (uidA) were used to investigate the transcriptional regulation of the bspA promoter. Photoperiodic activation of the bspA promoter was shown to involve perception by phytochrome and likely involves both a low fluence response and a parallel very low fluence response pathway. Activity of the bspA promoter was also influenced by shoot growth. High levels of bspA expression usually occur in the bark of plants during SD but not long day orSD with a night break. When growth was inhibited under growth permissive photoperiods (SD with night break) levels of bark β-glucuronidase (GUS) activity increased. Stimulating shoot growth in plants treated with SD inhibited SD-induced increases in bark GUS activity. Because changes in photoperiod and growth also alter carbon and nitrogen partitioning, the role of carbon and nitrogen metabolites in modulating the activity of thebspA promoter were investigated by treating excised stems with amino acids or NH4NO3 with or without sucrose. Treatment with either glutamine or NH4NO3 resulted in increased stem GUS activity. The addition of sucrose with either glutamine or NH4NO3 resulted in synergistic induction of GUS, whereas sucrose alone had no effect. Glutamine plus sucrose induction of GUS activity was inhibited by EGTA, okadaic acid, or K-252A. Inhibition by EGTA was partially relieved by the addition of Ca2+. The Ca2+ ionophore, ionomycin, also induced GUS activity in excised shoots. These results indicate that transcriptional activation of bspA is complex. It is likely that SD activation of bspA involves perception 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 metabolites, Ca2+, and protein phosphorylation/dephosphorylation.