BOT/TPSS 470
Photomorphogenesis
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Source: Taiz & Zeiger 2002, Chapters 17 and 18
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I am going to try and summarize
my lecture in my own words so that you can recognize the main points and concepts. You can look at the Glossary if you want to see the
“official” definitions for the terms
below.
Photomorphogenesis = Non-photosynthetic light regulation of plant growth, development and metabolism.
In general there are two regions of the visible spectrum that control Photomorphogenesis processes. These are the Red-Near Far Red and the Blue portions of the spectrum.
Phytochrome
Phytochrome is the photoreceptor that is responsible for “Red” light Photomorphogenesis.
Various photoreceptors regulate Blue Light responses.
Phytochrome Basics
Phytochrome consists of an Apportion and a Chromophore. The Chromophore absorbs light. This brings about a change in its structure which causes some change in the Apoprotein which leads to further events that lead to some sort of response.
The Holoprotein consists of the Apoprotein and the Chromophore. Only the Holoprotein can function properly in vivo.
The absorption spectrum for Phytochrome has peaks in Red-Far Red and the Blue spectral regions. Chlorophyll absorbs in these regions and Etiolated Plants are often studied because they have no Chlorophyll.
Small seeds often require light to germinate. With lettuce, red light stimulated germination and far-red light had the opposite effect. When alternating light treatments were given one after the other, the last treatment was decisive. The photoreceptor for this was called “Phytochrome”.
When Phytochrome was isolated it became clear that there was only one photoreceptor for these processes but Phytochrome had two forms that could cycle from one to the other depending on the spectral region employed.
Pr Phytochrome absorbs Red light which converts most of it into Pfr.
Pfr has an absorption maximum in the Far-Red spectral region, BUT it has a broad tail in the Red.
Pr has a sharp absorption spectrum BUT it absorbs slightly in the Far-Red spectral region.
Consequently the two forms of Phytochrome are Photoreversible. Many Phytochrome responses, like lettuce seed germination are Photoreversible. This is one of the main features of Phytochrome and it is one way to see if Phytochrome is the Photoreceptor for a Photomorphogenic event.
If Pr phytochrome is illuminated with red light most of it is converted to Pfr BUT Pfr also absorbs red light SO some Pfr is converted to Pr under red light. This sets up a Photostationary state in which the proportion of Pr to Pfr is determined by the spectral region employed.
Sun light has a broad spectrum. Consequently, Phytochrome is exposed to Red and Far-Red light in full sun light. Thus there will always be some Pr and Pfr in broad day light.
Far-Red light predominates in deep shade. Consequently, plants in the shade will have a different Photostationary concentration of Pr and Pfr than plants exposed to sunlight.
GOT THAT!!!! Yes Drill Sergeant!!!!!!!!!!!!!!!!!!
Pr absorbs some Far-Red Light. Consequently, illumination with Far Red Light will convert most of the Phytochrome into Pr BUT a small amount of Pr will be converted to Pfr.
It is possible to set up specific Photostationary States by using discrete wavelengths to illuminate plants.
This is a difficult but important concept! I was afraid that he was going to say that!
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In general, Pfr is considered the “Active Form” of Phytochrome.
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In many cases, the first measurable response to light is the stimulation of a Hydrogen Ion Membrane Pump which generates a membrane potential gradient between two sides of the membrane. This can lead to other membrane phenomena depending on the specific composition of the membrane in question. This is true for Blue Light responses as well.
Potassium Ions have a variety of membrane proteins that regulate their movement from one side of a membrane to another and K ions play a key role in some Photomorphogenic responses.
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There are two basic types of Phytochrome called Type I and Type II.
Type I predominates in Etiolated tissues.
Type I Phytochrome is the gene product of PHYA.
Type I is unstable in the light.
Type II phytochrome is produced by PHYB-E.
Type II is stable in the light.
Type I phytochrome responses are typical for Etiolated plants while Type II phytochrome is responsible for most other responses.
Phytochrome is most concentrated in meristematic and recently meristematic tissues.
Phytochrome responses can be characterized by the amount of light required to elicit a response.
VLFRs require minute amounts of light for short time periods. Arabidopsis seed germination is a VLFr. These are often Photoreversible but some require so little Pfr that they are not reversible.
LFRs require considerably more light than VLFRs but they occur at low fluences and with relatively short exposure times. Minutes of illumination may be sufficient. These are usually Photoreversible.
Most of the Red ß à Far-Red reversible responses like Lettuce Seed Germination are LFRs.
HIRs require high fluences over long time-spans (hours-days) to elicit a response. Inhibition of Lettuce Hypocotyl Growth is an HIR response.
HIR responses are due to the Photostationary state of Pr ß à Pfr.
HIRs are not Photoreversible
Unfortunately, some Photomorphogenic evens can be VLFR, LFR
and HIR responses. I interpret this to mean that plants have a lot of redundancy
so that a certain response will occur under more than one circumstance. Let’s
not worry too much about this right now! Concentrate on the main traits of these three phytochrome
responses.
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The ratio of Red : Far-Red Light varies according to the time of day and the Overstory. Leaves absorb most of the available Red light while Far-Red light passes through. Seeds and plants on the forest floor are exposed to a lot of Far-Red Light . Most of the Phytochrome is converted to Pr and the ratio of Pfr to Ptotal decreases. “Sun Plants” exhibit a “Shade Avoidance Response”. Their stems elongate rapidly and leaf development is minimal. They are “trying” to grow through the canopy to get sunlight. This is an HIR Response!
“Shade Plants” do not respond to alterations in the
Pfr/Ptotal ratio.
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The “sleep movements” of leaves are controlled by Phytochrome and a blue light photoreceptor.
Phytochrome controls leaf closure at the end of the day. This is red ß à far-red reversible.
Leaf movements are driven by Motor Cells that can rapidly change their volume in response to changes in Turgor Pressure. The influx or efflux of Potassium and Chloride Ions play a major role in regulating this.
There are two sets of Motor Cells (Dorsal & Ventral). The leaves close when K & Cl are transported into the Dorsal Motor Cells and exit the Ventral Motor Cells. The reverse processes lead to leaf opening.
The lag time for these responses is about 5 minutes which is not enough time for gene activation and protein synthesis. The first measurable response is the activation or deactivation of a Hydrogen Ion Proton Pump which alters membrane potentials such that K and Cl ions move into or out of the Motor Cells.
Blue Light Responses
Blue Light responses are not replaced by Red Light and are Not Red Far-Red Reversible.
They tend to have shorter lag times compared to Phytochrome responses.
Blue Light responses have a characteristic, “three fingered” action spectrum in the blue region of the spectrum. Neither Phytochrome nor Chlorophyll has this characteristic.
Phototropism is a classic Blue Light response. More growth occurs on the shaded side of the plant. This causes the stem to grow towards the directional light source.
Hypocotyl Elongation is inhibited by Blue Light and has the 3-fingered action spectrum. Phytochrome also inhibits Hypocotyl Elongation. Blue light inhibition has a shorter lag time compared to Phytochrome.
Dual Illumination experiments showed that Blue Light Inhibition is independent from Phytochrome. The activation of Ion Channels appears to be an early response to blue light.
Stomatal Opening and Closing involves Blue Light. Guard Cells have functional Chloroplasts which can contain Starch. Stomata Open when transferred from darkness to light. Photosynthesis is activated by light and it was hard to separate the Blue Light component. Dual Beam Experiments which saturated photosynthesis with red light demonstrated that Blue Light elicits a response that is independent from Photosynthesis. However, Photosynthesis is involved in this process.
Guard Cell
Protoplasts responded to light by swelling
and shrinking as expected for a turgor pressure-driven response.
Blue Light
activates a Hydrogen Ion Membrane Pump. Sound Familiar????????????
The lag time for stomatal movements and inhibition of hypocotyl elongation are similar.
The Potassium Ion concentration of open Guard Cells may be 8 times the normal level. When K Ions enter the cells, Cl ions also enter to balance the positive charge of K ions. Malate ions may also increase for the same reason. Consequently, a high concentration of osmotoically active ions is concentrated in the Guard Cells. All of this causes an increased Osmotic Pressure which leads to water uptake and an increased Turgor Pressure which causes the Opening of the Stomatal Pore.
(Malate is
produced from starch inside the Guard Cells while K and Cl ions move into and out of the Guard
Cells.)
Careful studies show that K concentrations increase rapidly in Guard Cells at dawn but
start to decline before
Sucrose comes from Starch Hydrolysis, Guard Cell Photosynthesis and Mesophyll Photosynthesis.
Blue Light Photoreceptors
Zeaxanthin: There is a lot of evidence that the Carotenoid Zeaxanthin is the Chromophore for the Blue Light effect on Stomata opening and closing. The Apoprotein for this is unknown.
Cryptochrome: Proteins called Cryptochrome is thought to be the Apoprotein involved in the Blue Light Inhibition of Hypocotyl Elongation. The Chromophore is uncertain, however.
Phototropin: Another group of proteins appear to be involved in Phototropism. These have been called Phototropins. As with Cryptochrome, the Chromophore is uncertain.