Agriculture Reference
In-Depth Information
Plants contain specific photoreceptors that perceive light ranging from
UV to far-red light. Natural radiation concurrently excites multiple pho-
toreceptors in higher plants. Specific receptors initiate distinct signaling
pathways, leading to wavelength-specific light responses. Photoreceptors
phototropins, cryptochromes, and phytochromes have been identified at
the molecular level (Ahmad and Cashmore 1993; Ahmad et al. 1998; Casal
2000; Lasceve et al. 1999; Lin et al. 1996; Quail 1997; Reymond et al. 1992;
Swartz et al. 2001).
Phototropin is a blue light (360−500 nm) flavoprotein photoreceptor re-
sponsible for phototropism, rapid inhibition of hypocotyls growth, stomata
opening, chloroplast movements, and leaf expansion. The phototropins,
such as phot1 and phot2, are a family of flavoproteins that function as
the primary photoreceptors in plant phototropism and in intracellular
chloroplast movements. Phot1, which is a plasma-membrane-associated
member of this family, has two 12-kD flavin mononucleotide (FMN) bind-
ingdomainsLOV1(light,oxygen,andvoltage)andLOV2withinitsN-
terminal region and a C-terminal serine/threonine protein kinase domain.
Phototropin, when activated by light, undergoes a conformational change.
Phot1 and phot2 bind FMN and undergo light-dependent autophosphoryla-
tion. Phot2 is localized in the plasma membrane. It regulates phototropism
and intracellular chloroplast movements. Phot1 and phot2 bind FMN and
undergo light-dependent autophosphorylation.
Phytochrome is a bluish protein photoreceptor which regulates many as-
pects of plant development (Quail 1997; Smith 2000). Phytochrome A and
phytochrome B perceive light as an environmental signal for the adap-
tation to fluctuating circumstances by essentially different manners in
terms of effective wavelengths, required fluence, and photoreversibility.
Phytochrome at its N-terminus contains a chromoprotein that can adopt
two spectroscopically distinct, but interconvertible forms Pr and Pfr. Phy-
tochrome is synthesized in the dark as Pr, the inactive form of the pigment.
Pr absorbs red light (600−700 nm) and it is converted to Pfr, the active form
of phytochrome. In turn, active Pfr absorbs far-red light (700−750 nm) and
isconvertedtoPr.Invivo,PrissynthesizedatnightandconvertedtoPfr,
the active form, during the day. Then, the following night the reverse can
occurandPfrcanbeconvertedbacktoPr.Hence,phytochromeenables
plants to measure and adapt to temporal changes in light conditions. It
also functions as a time-keeping mechanism. There are five forms of phy-
tochromes which have been identified: phy A-phy E. Plant phytochromes
are also light-modulated protein kinases that process dual ATP-dependent
autophosphorylation and protein phosphotransferase activities.
Cryptochromes (cry1 and cry2) are flavoproteins of the family of pho-
toreceptors responsible for photomorphogenesis (Cashmore et al. 1999).
They perceive (UV-A) light as well as blue light (360−500 nm). Although
 
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