Review for Exam 3 | Plant Physiology | PLB 415, Study notes of Plant Taxonomy and Evolution

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Characteristics of circadian rhythms

(biological clocks)

•

Period, phase, amplitude

• Entrainment (resetting of the clock)

• Temperature compensation (keeps time

regardless of temperature)

• Evolutionary function: to anticipate changes

in the environment (e.g., dawn, dusk)

•

Many genes in plants are at least partially

controlled by the circadian clock

review 3 PLB 415 2011

Photoperiodic Control of

Flowering

•

It’s not the DAY LENGTH, it’s the

NIGHT LENGTH that’s important.

• (^) Short Day Plants (SDPs) need a

long night; LDPs need a short

night.

• (^) SDP typically flower in the fall; LDP

typically flower in the Spring

What is the photoreceptor for the

“light break”?

1. In SDPs, red light is the most effective wavelength to inhibit flowering when given as a short pulse of light in the middle of a long night. This effect is reversible by FR, a hallmark of phytochrome involvement.
2. In some LDPs, RL is effective. But in Arabidopsis (a LDP), FR (phytochrome A) and blue (cryptochrome) are more effective than RL.

Where is the light signal perceived?

•

In the leaves for both LDP and SDP.

• (^) Therefore, there must be a signal that

moves from leaves to the meristem.

•

Known to be transported in the phloem.

• (^) The floral stimulus is called “florigen”.

The concidence model of regulation of flowering by circadian rhythm and photoperiod

• (^) CO mRNA peaks at ~18 hr.
• (^) CO protein is made when mRNA is high, but in the dark the protein is degraded.
• (^) CO protein is stabilized by light (destabilized by dark).
• (^) When CO gene is expressed and the CO protein is allowed to accumulate, the plant is triggered to flower.
• (^) That is, CO gene expression and light must coincide. Fig. 25.

Florigen summary

• Under photoperiodic inducing conditions,

signal (florigen = FT protein) moves to the

apex.

• FT protein interacts with FD (a transcription

factor) and activates “floral identity genes”.

• FT also induces its own expression in the

meristem (positive feedback loop).

• FT is also “florigen” in SDP and day neutral

plants (DNP).

Vernalization

• (^) Promotion of flowering by a cold treatment (a

form of acquired competence )

• (^) Another definition: “the acquisition or

acceleration of the ability to flower by a chilling

treatment”

• (^) Occurs to an imbibed seed or growing plant
• (^) Examples: winter wheat (“winter annual”),

biennials

• (^) Freezing (0ºC) to 10ºC for several weeks is

required.

• (^) Process occurs in the shoot meristem

Role of Epigenetic regulation in

Vernalization

• (^) FLC gene is an inhibitor of flowering. It inhibits

the floral identity gene LFY.

• (^) FLC encodes a MADS box transcription factor.
• (^) FLC gene is turned off by cold treatment, but is

turned back on in the next generation (an

example of epigenetic regulation)

• (^) Epigenetic because it is inherited mitotically, but

not meiotically.

Parts of a flower

Figure 25.5 Mutations in the

floral organ identity genes

25.6 The ABC model for the acquisition of floral organ identity

• (^) CHOPKNS CaFe Mg MoB CuMnZn (and Si and Cl and Na and Ni)
• (^) Macronutrients in standard fertilizer: “NPK” (nitrogen, phosphorus, potassium)
• (^) Micronutrients: Fe, Mg, Mo, Cu, Mn, Zn, etc.
• (^) Without them, plants are sick and show characteristic symptoms
• (^) Too much can also make plants sick.

Essential elements

Three other elements of

interest:

Aluminum :

• (^) plants often contain moderately high levels
• (^) apparently not necessary for plant health
• (^) is mobilized and can become toxic at low soil pH's (Al3+)
• (^) some plants have adaptations to tolerate high soil Al (e.g., secretion of Al chelators such as organic acids) Selenium :
• (^) some plants accumulate high levels (causing animal toxicity)
• (^) examples: Astragalus (locoweed), Xylorhiza , Stanleya
• (^) trace amounts needed by animals Cobalt :
• (^) vitamin B12, a component of several enzymes in N 2 -fixing microbes
• (^) required for N 2 fixation
• (^) not required by plants that do not fix N 2

5.3 Chelators: small molecules that bind the ions of heavy metals such as Fe, Al, etc. chelator DPTA – keeps insoluble elements such as Fe 3+ in solution and therefore accessible to plants Another common chelator is EDTA

Mycorrhizae

•

A fungal symbiosis with plants

• Two types are vesicular arbuscular (VAM)

and ectomycorrhizae

•

Important for mineral uptake (P, N) by

plants

• (^) without them plants are often unhealthy
• (^) fungi get carbohydrate in return
• (^) 83% of dicots, 79% of monocots, and all

gymnosperms are mycorrhizal

Phytoremediation

• (^) A branch of the field of “bioremediation”, which means using organisms to clean up the environment
• (^) Some plants accumulate heavy (toxic) metals from the soil. These are called “hyperaccumulators”.
• (^) In principle, these can be grown to remove the soil contaminants, and then harvested and hauled away to be disposed of safely (or volatilized).
• (^) Plants aren’t killed because they sequester the metals in vacuoles, or they chelate them with organic acids.

Physics of membrane transport

• (^) Passive vs. active
  • (^) passive transport requires no energy, e.g. osmosis
  • (^) active requires input of energy directly or indirectly
• (^) Chemical potential: all the forces that can drive a molecule to move from one place to another
• μ j = μ j * + RTln(c j ) + z j

FE + V

j

P

• where μj = chemical potential for solute j, R = gas constant, T = temperature, c = concentration, z = charge, F = Faraday’s constant, E = electrical field strength, V = partial molal volume, P = pressure
• (^) for neutral solutes (glucose), one can ignore electrical term
• (^) Δμ (delta mu) is called the electrochemical potential
• (^) remember: water potential ( w

= 

s

+ 

p

+ 

g ) Has units of pressure. Pressure = energy per unit of volume: (Joules/cm^3 )

• (^) Review: auxin transport depends on the plasma

membrane ATPase to acidify the cell wall.

Fig. 19.

• (^) Phloem is composed of sieve elements and

companion cells.

• (^) Both cell types are alive (unlike xylem).
• (^) Together, they are specialized for transport.
• (^) Sieve cells:
  • (^) lack nuclei, tonoplast, vacuoles, Golgi, ribosomes
  • (^) retain mitochondria, plastids, smooth ER
  • (^) very delicate – the slightest disturbance causes them to “seal off” and stop working
• (^) Companion cells
  • (^) provide ATP and often are responsible for transfer of materials
  • (^) come in several different types (ordinary, transfer cells, intermediary)

What do phloem sieve cells transport? Also hormones, protein, RNA, viruses

Figure 10.16 ATP-dependent sucrose transport in apoplastic sieve-element loading

The mechanism of transport in the phloem:

Münch pressure-flow model

•

Material moves in the phloem by bulk flow ( not

diffusion).

• (^) Velocities ~100 cm/hr (much faster than

diffusion).

• (^) Phloem loading occurs at one end (the

source ), and phloem unloading occurs at the

other (the sink ).

•

“Passive” process: energy is needed only for

maintenance of structures and

loading/unloading.

Pressure-flow model for phloem transport

• (^) An osmotically driven pressure gradient between source and sink
• (^) Ψ w = Ψp + Ψs
• (^) Phloem is loaded with sucrose at the source (such as leaves). This makes Ψs more negative, so water enters the cells (from xylem and apoplast), creating positive pressure Ψp.
• (^) At the sink end, removing sucrose makes Ψ s less negative, water leaves, pressure drops.
• (^) The gradient of pressure from source to sink results in bulk flow from source to sink (such as fruits and tubers).
• (^) "mass flow can occur from area of lower water potential to area of higher water potential" (no contradition because osmosis is not involved, therefore solute conc. not directly involved). For bulk flow, only the pressure difference is important.

SA and JA

New hypothesis: JA is the signal that moves from lower (wounded) leaves to upper leaves to induce PI-II proteinase inhibitor wounding---> systemin---->JA---->PI-II induction local response systemin----> JA ----> PI-II induction LOWER LEAVES (stock) UPPER LEAVES (scion) systemic response transport spr-2 spr-2 jai-1 jai-1 (Prediction: SPR-2 is necessary in the stock, but not in the scion)