Plant Molecular Biology 46: 383–394, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
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
and Gary D. Coleman
Department of Natural Resource Sciences and Landscape Architecture and Program in Molecular and Cell
Biology, 2101 Plant Science Building, University of Maryland, College Park, MD 20742, USA (
present address: USDA, ARS, U.S. National Arboretum, Floral and Nursery Plants Research
Unit, Beltsville, MD 20705, USA
Received 20 December 1999; accepted in revised form 11 October 2000
Key words: bark storage protein, gene regulation, nitrogen, photoperiod, Populus, vegetative storage proteins
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 β-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.
During the annual growth cycle of temperate decid-
uous trees, leaf N is translocated to perennial tissues
during the fall and remobilized during spring shoot
growth (Taylor and May, 1967; Ryan and Bormann,
1982). From 40% to 80% of leaf N is resorbed from
fall senescing leaves and is associated with leaf pro-
tein hydrolysis and amino acid translocation (Chapin
and Kedrowski, 1983; Boerner, 1984). Chapin and Ke-
droswki (1983) estimated that leaf protein hydrolysis
and amino acid transport could account for up to 90%
of leaf nitrogen removal. The majority of resorbed leaf
nitrogen is stored as protein (Taylor and May, 1967;
Chapin and Kedrowski, 1983). Major protein storage
sites include shoot and root bark and xylem ray cells
(Kang and Titus, 1980; Sauter et al., 1989; Wetzel
et al., 1989). Seasonal nitrogen resorption and storage
provides a mechanism for retaining and conserving
nitrogen and potentially contributes to nitrogen use
efﬁciency (Vitousek, 1982; Chapin and Kedrowski,
1983; Boerner, 1984; Aerts, 1990).