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

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 ␤-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 or SD with a night break. When growth was inhibited under growth permissive photoperiods (SD with night break) levels of bark ␤-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 the bspA promoter were investigated by treating excised stems with amino acids or NH 4 NO 3 with or without sucrose. Treatment with either glutamine or NH 4 NO 3 resulted in increased stem GUS activity. The addition of sucrose with either glutamine or NH 4 NO 3 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 Ca 2ϩ . The Ca 2ϩ 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, Ca 2ϩ , and protein phosphorylation/dephosphorylation.

Improved growth in a field trial of transgenic hybrid poplar overexpressing glutamine synthetase

“…According to its pattern of accumulation and to its abundance in bark, differences between plants in their VSP content were only observed during the first months of the development of the tree. Interestingly, it has been reported that expression of VSP in poplar is regulated by glutamine (Zhu & Coleman, 2001) and this amino acid is the main form of N transported in poplar (Sauter & van Cleve, 1992). Therefore faster accumulation of the 32-kDa VSP in transgenics could be a consequence of the differences in the GS content detected in transgenic and control plants, and increased availability of organic N in the form of glutamine for tree growth and development.…”

Changes of carbon and nitrogen metabolites in white spruce (Picea glauca [Moench] Voss) of contrasted growth phenotypes

“…bark proteome induced by short days (8 h) were associated with growth cessation and terminal bud set. Interestingly, poplar (Populus deltoı¨des) bark storage proteins (bsp) started to accumulate after a photoperiod of 14.1 h (Coleman et al 1991), and bsp gene regulation appeared to be not only phytochromemediated, but also influenced by primary metabolites like sucrose and glutamine (Zhu and Coleman 2001). Even though an earlier study by Wetzel and Greenwood (1989) did not reveal the presence of storage protein in bark of Picea spp., it is likely, based on our results, that proteins responsive to photoperiod accumulate in stems and roots of white spruce.…”

The Control of Autumn Senescence in European Aspen

“…The cited studies established (among other things) that autumn senescence starts in a given aspen tree every year at around the same date, suggesting that the main trigger is the reduction in the photoperiod (Keskitalo et al, 2005). Interestingly, the expression of bark storage protein genes during autumn is also regulated by the photoperiod, through the action of phytochrome (Zhu and Coleman, 2001). These findings provide strong support for the hypothesis that the start of autumn senescence in aspen is controlled by photoperiod.…”