Plant Physiology -- 04/20/11

Lecture 4-5

Announcements

• Quiz in lab
• measure ga plants
• signal transduction lecture/acting in lab
• set up gravitropism/phototropism experiment

Last time

• phytochrome PPT (will not duplicate in lecture--important stuff in PPT)
• phytochrome effects

Today in class

• more phytochrome, other light effects

How did anybody figure this day length/night length thing out?

• People noticed that plants flowered at different times of year
• They didn't actually understand what was happening
• figuring it out was a semi-accident, in a lit greenhouse out of season
• unusual kind of tobacco, Maryland mammoth
• pattern of flowering with respect to day length (handout)

What is the pattern?

• many plants need a particular daylength for flowering
• we call necessary light duration the critical period
• the critical period is the number of hours in the light in a 24 hour day
• some plants flower if light period is more than critical period: LDP
• some plants flower if light period is less than critical period: SDP
• remember that some plants are day neutral

Examples of plants and their critical periods (the time they are waiting for)

• short day plants
  • rice (one variety) flowers in days <12 h light in 24 h
  • cocklebur -- flowers <15.7 h light in 24 h
  • mums, soybeans, Maryland mammoth tobacco
  • note that critical period of SDP can be less or greater than 12 h
  • SDPs wait until day gets short enough
• long day plants
  • spinach -- flowers >11 h light in 24 h
  • dill -- flowers >13 h light (what happens at the equator)
  • spinach, most tobacco
  • note that critical period of LDP can be less or greater than 12 h
  • LDPs wait until day gets long enough
• day neutral examples
  • most rice varieties (equatorial)
  • sunflower
  • cucumber (some varieties)
  • many vegetable crops

Why is one plant a LDP, another a SDP, another DN?

• genetics (interbreeding or genetic engineering can change)
• response controlled by few genes (can breed)
• varies with variety, not just species
  • uses phytochrome system
  • just as: some lettuce germination light sensitive, some not
  • many crop plants: sensitivity bred out so can grow anywhere (most varieties of lettuce)

So how does the plant measure the length of the light period?

• oops--it doesn’t
• plant measures the length of the dark period
• on earth, with 24 h "day", you can’t tell the difference
• in growth chambers (or on Mars), we can see
• "critical period" means number of continuous light hours in 24 day
• what really counts is dark period, but we don’t have a special term
• I call this critical night period
• dark must be continuous

How can we figure out problems?

• remember the dark period that counts, not the light period
  • short day plant is really long night plant (plant waiting for night long enough)
  • long day plant is really short night plant (plant waiting for night short enough)
  • critical period is light period, based on day length (24 h days)
• plain in natural 24-h dark/light period: can use SDP, LDP terminology
  • determine if day is short enough (SDP)
  • determine if day is long enough (LDP)
  • a SPD might have CP =14 and a LDP might have CP = 11
• odd light (Mars, growth chamber): must use dark period to do problems
  • If "critical period" is 10 h, what is critical night period? (14)
  • if "critical period" is 18 h, what is critical night period? (6)
  • sdp might have a critical period longer than a ldp (example)

To solve a particular problem

• if you have simple, continuous light-dark cycles, pretend the plant measures the length of the light
  • examine the critical period (=continuous light in 24-h day)
  • does the SDP have a short enough light period? If so, it flowers
  • does the LDP have a long enough light period? If so, it flowers
• if not a simple light situation, first convert SDP/LDP to LNP/SNP
• convert "critical period" (means light in 24h) to "critical night period"
• state what the plant needs "day shorter than...night longer than..."
• figure out if you’ve got what the plant needs

Examples with real plants:

• dill is LDP with 13 h critical period
  • requires how many hour days on Earth? 13
  • would 14 h days cause flowering?
  • on Earth, what is night for days of 13 or more h? (11h or less)
  • LDP is really SNP
  • needs night shorter than critical night period
  • term "critical period" means light in 24 h days (rest dark)
• chrysanthemum (SDP) with 10 h CP
  • what kind of light period does it need on Earth? (10h or less)
  • what kind of night is that? (14 h or more)

OK, so how does phytochrome work to determine flowering?

• like other environmental responses: three steps
• perception of signal
• transduction
• differentiation which leads to flowering

What do we know about light perception?

• grafting experiments (transplants easy--no immune system) show leaves are essential
• pigment is phytochrome
• sitting in membranes
• causing action in cytoplasm

What do we know about how the light signal cause a change in gene expression?

• change from one type of signal to another
• have many steps in the chains of events within the cell
• calcium may be on of they key compounds (no surprise)

How does the differentiation lead to flowering and other differentiation?

• early: regulation of gene transcription
• (nuclear genes are involved for chloroplast proteins)
• hormones are produced
• middle: can see differentiation of meristem (growing point) to make flowers
• much later we see them bloom (anthesis)

What does phytochrome look like?

• broken open porphyrin ring
• like a lot of other pigments (chlorophyll, hemoglobin, esp. bilirubin)

What light does phytochrome absorb?

• two interconvertible forms of phytochrome, like 2 sides of coin
• both have two absorption peaks
• P_r is the red-absorbing form (also the shorter blue)
• P_fr is the far red-absorbing form (also the longer blue)
• P_fr is the active form (promotes or inhibits)

Reminder: phytochrome molecules can be in one of two forms, interconvertable

How do the two forms interconvert?

• P_r absorbs red light, and converts to the P_fr form
• P_fr absorbs far red light, and converts to the P_r form
• very fast reactions
• in pure red light....almost all will convert to P_fr
• in pure far red light...almost all will convert to P_r
• in mixed light....flipping back and forth, with some proportion of each

What happens in the dark?

• phytochrome actually measures dark, not light
• starts dark period with mostly P_fr (from mostly red light in sunlight)
• P_fr --> P_r slowly (decay = interconversion)
• if the night is long, the P_fr is all gone
• this way plants can measure length of dark period
• for some plants, 6 h is enough; others might need 16

Does it work this way with all plants, and all the time?

• speeds up with high temperature
• SPD need short days, which is really long nights to get rid of the inhibiting Pfr (SPD = LNP)
• LDP need long days, which is really short nights so they have some of the promoting Pfr (LDP = SNP) left
• same Pfr, just doing different jobs

What happens if we give some light in the middle of the night?

• LDP plant needs short night (night short enough)
  • light in middle makes 2 shorter nights
  • nights will still be short enough – OK
• SDP needs long night (night long enough)
  • light in middle makes 2 shorter nights
  • nights will be too short

Is this all there is for phytochrome and its responses? NO

• phytochromes are a family of related chemicals
• only some of them have the P_r/P_fr system (best known)
• we can tell if this system by P_fr reversal of P_r effects
• many different developmental events

So we have looked at how plants can use R and FR light to tell them about their light environment. With this system they can detect an overstory, neighbors, or the length of the day, or tell if they are near the surface of the soil, and act appropriately.

There is another major light detection system, which uses blue light but does not respond to red/far red. We call this the non-phytochrome blue system.

What things are controlled by blue light?

• some types of movement
• leaf expansion
• stomatal opening
• chlorophyll synthesis
• carotenoid synthesis
• many blue light activated genes\
• not always obvious what we're looking at

What absorbs blue light in the cell?

• phytochrome (red/far red plus blue [weakly])
• chlorophyll (red plus blue)
• another blue receptor (cryptochrome, absorbs blue)
• phototropin, yet another blue light receptor (won't cover)
• carotenoids
• xanthophylls
• sometimes other pigments

Phytochrome and the cryptochrome (true blue) receptors are used just for information.

How do we know that a blue response is due to the true blue receptor, and not phytochrome?

• red and far red don’t work (work for phytochrome)
• far red won’t reverse
• characteristic action spectrum
• called: cryptochrome response, or non-phytochrome blue light response
• often confused earlier

What are some growth & movement responses to blue light?

• Chuck Darwin worked on these
• bending of seedlings and other plant parts toward light
  • normally a growth (not reversible)
  • only works on still-growing tissue
  • plant in window, sunflower flowers
  • from redistribution of hormones
  • fungi, too
• leaves tipping toward or away from light
  • directed by the location of the light (tropism)
  • leaves follow sun, especially legumes
  • caused by swelling & shrinking of tissue (reversible)
• inhibition of hypocotyl elongation
  • hypocotyl is "stem" below seed
  • speed of reaction says blue light response (phyto slow)
  • membrane is depolarized right when blue light comes on
  • plants grown in the dark get very long (no blue)
  • note that a light signal can inhibit, not just promote

How does blue light relate to photosynthesis?

• blue light provides lots of energy for photosynthesis
• many photosynthetic genes are light-induced
• both phytochrome and non-phytochrome blue (cryptochrome) responses
• stomatal opening controlled by blue light

Is there a single blue light receptor?

• No, there is a whole family of them
• each contains a chromophore (absorbs light) with a flavin
• each contains an apoprotein (lov domain: used for light, redox, voltage)
• protein hooked to signal transduction pathway on inside
• also have in our eyes--control pupil opening

End of the section on light as information.