ABA AND PHYTOCHROME
As mentioned above, the short day perception of
cold acclimation of trees was demonstrated over 35
years ago by several research groups (see review by
Weiser 1970). Recent studies have demonstrated
that overexpression of the oat (Avena sativa) phyA
gene in hybrid aspen (Populus tremula · P. tremuloides)
changes the critical day length and prevents
cold acclimation under short days (Olsen and others
1997). In contrast, reduction of PHYA results in
increased sensitivity to photoperiod (Eriksson
2000). Exposure of the lines overexpressing PHYA
to 4C resulted in cold acclimation comparable to
the wild type (Welling and others 2002). Similar
levels of ABA were measured in the line overexpressing
PHYA under both long days and short days
when grown at 18C, although ABA was twofold
lower than in the wild type. Exposure to 0.5C resulted
in a dramatic increase in ABA in both the
wild type and the line overexpressing PHYA. The
authors concluded that phytochrome A is involved
in the photoperiodic regulation of ABA, but low
temperature regulates ABA by a different mechanism.
Both short day and low temperature are
perceived by leaves of silver birch (Betula pendula),
which results in enhanced freezing tolerance. A
dehydrin was isolated from the leaves exposed to
low-temperature dehydration, salt, and exogenous
ABA (Puhakainen and others 2004). Short day
exposure resulted in a minimal increase in the
transcripts for this dehydrin; however, if the short
day treatment was followed by low temperatures,
the transcripts significantly increased. These results
suggest that the short photoperiod potentiates the
low temperature response. This supports the earlier
suggestion by Weiser (1970) that translocatable
hardiness promoters produced in leaves exposed to
short days sensitize cells to perceive the low temperature
cue. In birch, both short days and low
temperature increase endogenous ABA (Li and
others 2002) that could operate in concert with low
temperature to upregulate dehydrins. Dehydrins
have been closely associated with increase in
freezing tolerance of trees (Arora and Wisniewski
1994; Wisniewski and others 1996) and herbaceous
plants (Robertson and others 1994b). Balk and
others (2004) found that of the top 50 genes that
had the highest correlation with a functional shift in
cold hardiness, 18 were dehydrin genes.