PHYTOCHROME

(It has already been seen that a brief exposure with red light during critical dark period
inhibits flowering in short-day plants and this inhibitory effect can be reversed by a subse-
quent exposure with far-red light. Similarly, the prolongation of the critical light period or
the interruption of the dark period stimulates flowering in long-day plants. This inhibition of

in short-day
flowering of plants and the stimulation of in
flowering long-day
plants involves the
operation a proteinaceous pigment called as phytochrome.
The pigment phytochrome exists in two different
forms, () red light absorbing from
which designated
is as
P, and (i) far-red light absorbing form which is designated
These forms
as
P
two
of the pigment photochemically interconvertible.
are
When Pa form of the pigment absorbs red light (660-665mu), it is converted into
PR form.
When PR form of the pigment absorbs far-red light (730-735mu), it is converted
into P form.
The P form of the pigment gradually changes into P form in dark.
660-665mp
PFR
730-735mp
.
Darkness
It is considered that during the day the P form of the
plant which is inhibitory to pigments is accumulated in the
flowering short-day plants but is stimulatory in
in
During critical dark period in short-day plants, this form gradually long-day plants.
ing in flowering. A brief changes
exposure with red light will convert this form
into P form result
result-
inhibiting flowering. Reversal of the inhibitory effect of red again into P.. form thus
SDPby subsequent far-red light exposure is because the light during critical dark period in
50-73Smu) will again be converted back into P, form. PRform after
absorbing far-red light
Prolongation of the critical light period or the interruption of the dark
n long-day plants will result in further
ating flowering in accumulation of the PR form of the period by red-light
Sucgn long-day plants. pigment, thüs stimu
Phytochrome and Enzymes

Seedling photomorphogenesis is associated with the appearance of enzymes necessary for

changes in red and far-red exposure. An enzyme. which has been extensively
activity in response to
studied, is phenylalanine ammonia lyase (PAL). It is the enzyme that catalyzes the conversion of
 NH
CH,CHCO0H CH CHCOOH Coumarins

Phenylalanine
ammonia lyase Lignins
(PAL)
Phenylalanine Clinnamic acid Flavonods
+NH,
Fig. 14.4: Action of PAL enzyme producing secondary metabolites

phenylalanine to coumaric acid and thus initiates the synthesis of compounds like coumarin, lignin
and flavonoids including anthocyanin pigments belonging to the class of
secondary metabolites(Fig
14.4). This is
enzyme present very low concentration in the dark but can be greatly increased on
in
exposure to far-red light. Another enzyme, ascorbic acid oxidase, has been shown to increase by Ptr
action.

It is not certain whether Pfr stimulates the synthesis of these enzymes or leads to an activation
of already existing enzymes.)

Some Pfr Responses are Mediated by Calcium and Calmodulin

Calmodulin is a
calcium-binding protein and calcium-calmodulin (Ca2*-CaM) complex mav
regulate plant responses which include several enzymes like plasma membrane-localized Ca~
pump
(Ca2-ATPase), NAD kinase and enzymes (kinases and
phosphatases) that cause phosphorylation
and activation of other enzymes. Several lines of evidence
indicate that Ca can mediate
phytochrome
responses. Calcium uptake into the cells is increased by Pfr and some Pfr-stimulated
enzymes (described
earlier) are also stimulated by calmodulin (CaM). By using Ca2 -ionophore, a chemical agent that
promotes Ca4 uptake into cells, some phytochrome responses can be induced in darkness. It is
suggested that chemically induced uptake of Ca2* into cell can mimic Pfr responses and acts as a
substitute for red light. It is tempting to conclude that calmodulin is
the agent that transduces Ca**
entry into physiological responses.