Photomorphogenesis: Responding to light

Photomorphogenesis: Growthresponses to light•Light is an important environmental signalthat plants use to regulate their growth anddevelopment.•Both the quality and quantity of light areimportant.•The regulation of plant development bylight is called photomorphogenesis.

Photoreceptors inphotomorphogenesis•There are four classes of plant photoreceptors.–Phytochromes absorb red (660 nm) and far red light (735nm).–Cryptochromes detect blue (400-450 nm) and UV-A light(320-400 nm).–Phototropins also detect blue and UV-A light.–A newly-identified class of photoreceptors that responds tolow levels of UV-B light.•The first three photoreceptors are chromoproteins,meaning that they are a complex (holoprotein)consisting of a pigment (chromophore) and acatalytic protein (apoprotein).

Phytochromes

Phytochromes•The photoreversibility of phytochrome isa unique aspect of phytochrome.•Plant responses to red light can benullified with an immediate treatment withfar-red light.

Phytochromes

Phytochromes•Photoreversibility occurs because the two formsof phytochrome are in a photoequilibriumcreated by light.

Phytochromes•The dynamic relationship between the Prand Pfr forms can be seen clearly inetiolated (dark-grown) plants.

Phytochromes

Phytochromes•The loss of phytochrome following a redirradiation can be explained by two factors:

Phytochromes•The photoequilibrium (is describedmathematically by:= PPfrTOTWhere P TOT is the sum of the Pr and Pfr forms•Light conditions dictate the specific ratio.–Under pure red light, = 0.8 (80% pFr).–Under pure far-red light, = 0.03 (3% pFr).–In sunlight, the value of will change through the day.

Phytochromes•The proportion of Pfr will depend on atleast three factors:-the relative proportions of red and far-redwavelengths in the light source-the forward and reverse rates ofphotoconversion between Pr and Pfr-the rate of thermal reversion of Pfr to Pr

Phytochromes•Phytochrome responses are groupedbased upon their fluence requirement.–Low fluence responses (LFRs) arestimulated by light doses in the 1 –1,000mole light m -2 and are FR-reversible.–Very low fluence responses (VLFRs) areirreversible responses to light levels of 10 -6 to10 -3 mole light m -2 and are notphotoreversible.–High irradiance reactions (HIRs) are thosethat respond to sustained exposures to light.

Cryptochromes•Cryptochromes mediate the growth responsesto blue and UV-A light, including aspects of deetiolationsuch as the inhibition of hypocotylelongation and stimulation of cotyledonexpansion, gene expression, flowering time…...

Cryptochromes•Examples of blue light photoreceptors include:–Cryptochrome 1 (cry1), a receptor which bindsflavins such as FAD (flavin adenine dinucleotide).–The Arabidopsis cry1 mutant had lost the capacityto respond to blue light–Cryptochrome 2 (cry2) mediates blue-lightsuppression of hypocotyl elongation, cotyledonexpansion, and anthocyanin production.–Cry2 has a role in determining flowering time

Developmental responses mediated byphytochrome and cryptochrome•Seed germination is influenced by light.

Developmental responses mediated byphytochrome and cryptochrome•Specific changes occur when dark-grownplants are exposed to light (de-etiolation).

Developmental responses mediated byphytochrome and cryptochrome

Developmental responses mediated byphytochrome and cryptochrome•Shading triggers changes in plant growth.

Developmental responses mediated byphytochrome and cryptochrome

Developmental responses mediated byphytochrome and cryptochrome

Developmental responses mediated byphytochrome and cryptochrome•Phytochrome is the receptor that helpsplants detects end-of-day signals.

Developmental responses mediated byphytochrome and cryptochrome•Biosynthesis of the red-blue anthocyaninpigments are mediated by light.

Developmental responses mediated byphytochrome and cryptochrome•Some phytochrome responses occurwithin a matter of seconds to minutes andtypically involve membrane-basedchanges in bioelectric potential or ion flux.

Developmental responses mediated byphytochrome and cryptochrome•The different forms of phytochrome likelyhave different fundamental roles in theperception of light.

Mode of action of phytochrome

Mode of action of phytochrome•The apoprotein for phytochrome is a smallpeptide.–The peptide has a photosensory domain onthe N-terminus and a regulatory domain atthe C-terminus.–The regulatory domain contains a sub-domainwith characteristics of a histidine kinase.–Phytochrome, in a sense, acts as a twocomponentregulatory system like thoseinvolved in hormone sensing.

Mode of action of phytochrome

Mode of action of phytochrome•The mechanism by which phytochromemediates plant development involves threeimportant factors.

Mode of action of phytochrome•Phosphorylation is key to the signaling roleof phytochromes.

Mode of action of phytochrome•The migration of phytochrome to thenucleus also provides control of geneexpression.

Mode of action of phytochrome•Figure 22.11

Mode of action of phytochrome•The migration of phytochrome to thenucleus also provides control of geneexpression.

Mode of action of phytochrome•Figure 22.12

Mode of action of cryptochrome•The structure of cryptochromes is similarto that of DNA repair enzymes.

Mode of action of cryptochrome

Mode of action of cryptochrome

Mode of action of cryptochrome•The mechanism by which cryptochromeelicits developmental responses has notbeen fully characterized.

Mode of action of cryptochrome•The mechanism by which cryptochrome elicitsdevelopmental responses has not been fullycharacterized.

Plant responses to UV-B light•Examples of UV-B responsive process inplants include:–An increase in anthocyanin production–An increase in flavonoid biosynthesis,triggered by phytochrome, cryptochrome, andanother UV-B receptor.

Interactions between phytochromeand cryptochrome•De-etiolation responses in Arabidopsisinvolve an interaction between phyA, phyB,and cry1.–If phyB is defective, hypocotyl elongation issuppressed.–If phyA is also deficient, the hypocotyls areeven longer than in etiolated plants.–In a triple mutant which includes a defect incry1, the hypocotyl hook does not straightenand the cotyledons do not unfold.

Interactions between phytochrome•Figure 22.15and cryptochrome