1717
Photomorphogenesis: responding to light
Fig.Fig. 11 22 33 44 55 66 77 88
99 1010 1111 1212 1313 1414 1515 1616 1717
1818
Tab.Tab. 11 22 33 44
Light perception in plants
• Because plants do not enjoy the luxury of being able to change their environment or seek shelter from adverse conditions by changing their location, they must be more sensitive to changes in their surrounding so they can adapt accordingly.
• Plants can sense light gradients and detect subtle differences in spectral composition.
Photomorphogenesis
• Photomorphogenesis is referring to the response of plant to light, which is the central theme in plant development.
Photoreceptors
• Most photomorphogenic responses in higher plants appear to be under control of one (or more) of four classes of photoreceptors:1. Phytochromes (red and far-red)2. Cryptochrome (blue and UV-A): seedling development and flowering3. Phototropin (blue and UV-A): differential growth in a light gradient4. UV-B receptors: unknown
Chapter outline
• Red and far-red responses
• Blue and UV-A responses
• Interactions between photoreceptorsUV-B responses
Phytochromes
• Phytochromes are plants photoreceptors.
• Phytochromes are photochromic. They can absorb red (665nm) and far-red (730nm) light and they have two forms, red-absorbing form (Pr) and far red-absorbing form (Pfr).
Phytochrome is photoreversible
• Pr and Pfr forms of phytochrome can change to the other form when expose to red or far-red light, respectively.
Phytochrome is down regulated after activation
• The down regulation of phytochrome involved mRNA and protein degradation.
• Also, the expression of phytochrome will be down regulated at transcriptional level after activation.
Figure 17.5
declines because Pfr is declining.
is relatively unstable, with a half life (t1/2) of 1~1.5hr
Figure 17.7Five seconds of red light causes
mRNA level declines
15 minutes of lag period follows
mRNA drops 50% within the first hour
mRNA drops 95% within first two hours
Very Low Fluence Responses (VLFRs)0.1nmol/m2 ~ 50 nmol/m2
only converts less than 0.01% of total phytochrome to Pfr form
Because far-red light can only convert 97% Pfr to Pr, which is more than what needed to induce VLFRs, so VLFRs are not reversible
Very Low Fluence Responses (VLFRs)the principle evidence that VLFRs is mediated
by phytochrome is the similarity of its action spectrum to the absorption spectrum of Pr.
Most VLFRs are related to germination.It obeys the law of reciprocity.It peaks at red and blue.
Low Fluence Responses (LFRs)1~1000 mol/m2
Seed germinationSeedling developmentBioelectric potentials and ion distributionPhotoreversibleExhibit reciprocity between duration of
irradiation and fluence ratePeaks at red and far-redLFR is induced by poising the system with a
maximum level of Pfr for a very brief period of time.
LFRs in seed germinationpositively photoblastic – germination
stimulated by lightnegatively photoblastic – germination
inhibited by light
A one mm thickness of fine soil will block more than 99% of light. Only light with wavelength longer than 700nm will be able to pass.
Very little Pfr is required to stimulate germination.
LFRs in bioelectric potentials and ion distribution
phytochrome-induced changes in the surface potential of the dark-grown barley roots (T. Tanada)
red light root tip become positively charged
far-red light root tip restore its negative charge
Figure 18.11
Red light induces a depolarization of the membrane within 5-10s following a red light treatment.Subsequent far-red treatment causes a slow return to normal polarity or small hyperpolarization.
Pulvinus (bulbous zone) at the base of leaf/leaflet will drive leaf movement by altering its shape as a result of differential changes in the volume of cells on the upper and lower side of the organ.
Pulvinus is osmotically driven by rapid redistribution of K+, Cl- and malate.
H+ efflux K+ channels open
High Irradiance Responses (HIRs)-prolonged/continuous exposure to light (far-red or direct sunlight) of relatively high irradiance-Response is proportional to the irradiance within a certain range (That’s why they are called HIRs, not HFRs.)-Not photoreversible-Not obeying the law of reciprocity-Many of them are also LFRs
Example 1: anthocyanin synthesisExample 2: Inhibition of stem elongation
The initiation of anthocyanin accumulation is classical LFR, peaks at red region. However, when the duration of irradiation lengthens, peak shifts from R FR.
Example 1: Anthocyanin synthesis
Example 2: Inhibition of stem elongation in white mustard
Only dark-grown tissue respond to far-red. Green tissue is more responsive to red light.During de-etiolation, HIR peak shifts from far-red to red.
Light-grown
Dark-grown
Phytochrome under natural conditions
• Under natural conditions, phyA may just detect the presence/absence of light since it only accumulate under dark-grown conditions.
• Other phytochrome response observed under natural conditions is shade avoidance syndrome.
Shade avoidance is triggered by far-red light, which can be shown in end-of-day treatment
Far
-red
FR
+R
Phytochrome signal transduction
• Phytochrome is a protein kinase.• When activated, it will phosphorylate other
proteins and begin signal pathways.
Phytochrome regulates gene expression
• A lot of nuclear-encoded genes are regulated by phytochromes, including the small subunit of rubsico (RBCS) and the light-harvesting chlorophyll a/b binding proteins (CAB).
• Some proteins are positively regulated, like RBCS and CAB; others are negatively regulated, like phyA and NADPH-protochlorophyllide oxidoreductase.
Phytochrome also regulates other transcription factor’s activities
• PIF3 (phytochrome interacting factor 3) is a transcription activator.
• When phytochrome activates (Pr Pfr), the Pfr form binds to PIF3 and activates it. Then activated PIF3 will activate transcription of a large variety of proteins containing G-box motifs.
Cryptochromes
• Cryptochromes are blue/UV-A photoreceptors mediating seedling development/flowering responses in plants.
• In Arabidopsis, there are two cryptochromes, cry1 and cry2. The structure of cry2 is also similar to cry1 with two chromophores.
• Cry2 has a role in determining flowering time.
Phototropin
• Phototropin is also a flavoprotein with two flavin mononucleotide (FMN) chromophores.
• FMN chromophores binds to domain called LOV (light, oxygen and voltage) domain.
Hook straightening and cotyledon unfolding are controlled by all three photoreceptors
Cotyledon expansion is controlled by phyB and cry1
phyB controls hypocotyl elongation
Top Related