Lec 12  Plant Defenses: Surface Protectants and Secondary Metabolites

I.    Plant Cutin, suberin and waxes

      Cutin, suberin and waxes help reduce transpiration and pathogen invasion.

II.   Secondary metabolites

1.    Secondary metabolites defend plants against herbivores and pathogens.

2.    Plant defenses evolved to maintain reproductive fitness.

(1). Terpenes serve as antiherbivore defense compounds in many plants.

(2). Phenolic compounds

a.    Release of simple phenolics affects the growth of other plants-allelopathy.

b.    Lignin and flavonoids

(3). Nitrogen-containing compounds

IV.      Plant defense against pathogens

1.         Antimicrobial compounds that are synthesized before pathogen attack.

2.         Antipathogen defense that are induced by infection.

3.         Interactions between plant and pathogen.

Lec 13  Plant Development

I.    Plant Growth, Differentiation, and Development

1.    Plant growth is defined as a permanent increase in size, weight and cell number.

2.    Differentiation and development

(1).  Differentiation: a process of cellular specialization (a two way street).

(2).  Development: the growth and differentiation of cells into tissues, organs and organisms.

(3). Determinate growth and indeterminate growth

3.    Totipotency

II.   Plant Growth and Development

1.    Control of plant growth and development

(1). Genetic control of development

(2). Hormonal regulation of development

(3). Environmental regulation of development

2.    Plant development

(1). Seed germination

(2). Shoot development

a.    Shoot meristem

b.   Internodes and phytomer

c.    Phyllotaxy

(3). Root development

a.    Root apical meristem

b.   Root cap and mucigel

c.    Quiescent zone

(4). Flower evocation and development

(5). Flower and fruit development

III.  How Do Cells Grow?

1.    The driving force for cell enlargement is water uptake.

2.   Kinetic analysis of growth: absolute growth rate and relative growth rate

 

Photomophogenesis

I.    Photomorphogenesis

1.    Photoreceptor

(1). Phytochrome: red light and far-red light

(2). Cryptochrome: blue and UV-A-absorbing

(3).  UV-B receptors

2.     Signal transduction chain

3.     Response

II.          Phytochrome

1.     The photochemical and biochemical properties of phytochrome

(1).  A blue protein pigment with a molecular mass of about 125 kDa.

a.     Phytochrome consists of a family of regulatory photoreceptor chromoproteins that control many aspects of plant growth and development through photo reversible conversions between a Pr and a Pfr form (660 nm, 735 nm).   Pr: nonfunctional; Pfr: active form.

b.    Photo stationary state and dark and thermal reversion.

(2).  Phytochrome is a dimmer composed of two polypeptides.

a.          Each subunit consists of a light-absorbing pigment (chromophore), the apoprotein, and a polypeptide chain (apoprotein), the holoprotein.

b.         The chromophore is a linear tetrapyrrole (phytochromobilin), it is attached to the apoprotein through a thioether linkage to a cysteine residue (C321).

c.         Phytochromobilin is synthesized in plastids from 5-aminolevulinic acid.

d.         Both the chromophore and the protein undergo conformational changes.

(3). PHY genes encode type I and type II phytochromes.

a.          PHYA gene (type I)

(a).  PHYA mRNA and polypeptide accumulation in response to light.

(b). Pfr form of phytochrome inhibits the expression of its own gene.

(c). PfrA is unstable

b.         PHYB-PHYE (type II)

(a). PHYB (PHYE) mRNA is not significantly changed by light.

(b). The PHYB-PHYE proteins are more stable in the Pfr form than PHYA is.

2.    Localization of phytochrome in tissues and cells.

3.         Phytochrome-induced whole-plant responses.

(1). Phytochrome responses can be distinguished by the amount of light.

a.          Very low fluence (VLF) responses

(a). 0.1 nmol m-2 to 50 nmol m-2

(b). VLFR converts less than 0.02% of the total phytochrome to Pfr.

(c). Far-red cannot reverse VLFRs.

b.         Low fluence (LF) responses

(a).  1 to 103 mmol m-2, phtoreversible responses.

(b). The promotion of lettuce seed germination: photoblastic.

(c). The regulation of leaf movement

(d). VLFR and LFR obey the law of reciprocity (a reciprocal relationship between the fluence rate and the irradiation time).

c.         High irradiance response (HIR)

(a).  10 mmol m-2, prolonged and continuous irradiation.

(b). Induction of flowering in Hyoscyamus.

(c). Inhibition of hypocotyl elongation in mustard, lettuce.

(d). Anthocyanin synthesis

(e).  Ethylene production in sorghum

(f).  Plumular hook opening in lettuce

(3).  The high irradiance responses (HIR) of etiolated seedlings to far-red light are mediated by PHYA, whereas the HIR of green seedlings to red light is mediated by PHYB.

(4). Phytochrome regulates certain daily rhythms.

a.          Blue light stimulates closed leaflets to open, and red light followed by darkness causes open leaflets to close.

b.   Gene expression and circadian rhythms (LHCB mRNA)

(5). Phytochrome B determines the response to continuous red or white light.

       Phytochrome B is responsible for regulating hypocotyls length in response to red light (LFR and HIR) and regulates photo reversible seed germination

(6). Phytochrome A is required for the response to continuous far-red light.

a.          PHYA-deficient mutants can grow normally under continuous red light, but not a chromophore-deficient mutant, which also lacks PHYB.

b.         PHYA is restricted to de-etiolating and far-red response.

c.         PHYA functions as photoreceptor for seed germination of Arabidopsis.

(7). Phytochrome interactions are important early in germination.

a.          Continuous red light absorbed by PHYB stimulates de-etiolating by maintaining high levels of PfrB.

b.         In open sunlight, which is enriched in red light compared with canopy shade, de-etiolating is mediated primarily by the PHYB system.

c.         A seedling emerging under canopy shade, enriching in far-red light, initiates de-etiolating primarily through the PHYA system.  The response is taken over by PHYB (in the mature plants) because PHYA is labile.

3.           Phytochrome structure and function

(1).  Overexpression of phytochrome causes a dwarfed, dark green phenotype.

(2).  Light-detecting part of phytochrome resides in the N-terminal end with the chromophore, the C-terminal end is required to transmit light signal.

4.           Cellular and molecular modes of action

(1).  Phytochrome regulates membrane potentials and ion fluxes.

a.           During phytochrome-mediated leaflet closure, the apoplastic pH of flexor cells decreased, while the apoplastic pH of the extensor cells increased.

b.          Red light caused the K+ channels of the flexor protoplasts to open, and far-red light reversed the effect of red light.

c.           Blue light caused K+ channels of extensors to open, and those of flexors to close.  Phytochrome regulates specifically the K+ channels of flexor cells.

(2). Phytochrome regulates gene expression

a.           SSU of Rubisco and LHCIIb

b.          Cis-acting elements-GT-1 (GGTTAA).

(3).  Phytochrome acts through multiple signal transduction pathways: Action of a heterotrimeric G protein and the calmodulin.

 

Blue-light Responses: Stomatal Movements and Morphogenesis

I.     The photo physiology of blue-light responses

1.           Blue light stimulates asymmetric growth and bending:    positive and negative phototropism.

2.           Blue light inhibits stem elongation in dark-grown lettuce and mustard.

3.           Blue light stimulates stomatal opening.

(1).  Stomata open as incident photon fluxes increase.

(2).  Stomata can be very sensitive to ambient CO2 concentration.

(3).  DCMU causes a partial inhibition of light-stimulated stomatal opening.

(4).  Increase in stomatal apertures above the level reached in the presence of saturating red light indicates a different photoreceptor system (blue light).

4.           Blue light activates a proton pump in the guard cell plasma membrane.

(1).  Fusicoccin stimulates stomatal opening, CCCP inhibits the stimulation.

(2).  Blue light-stimulated proton extrusion is nearly 15-fold more efficient, on photon flux basis, than ATP synthesis in the guard cell chloroplast.

(3).  The magnitude of the stomatal response to blue light increases with the fluence rate of background red light.

5.           Blue light regulates osmotic relations of guard cells.

(1).  Potassium concentration can increase several fold in open stomata (100 mM to 400 to 800 mM, counter ions malate or chloride).

(2).  Potassium and chloride fluxes depend on secondary transport driven by the H+-ATPase-generated proton motive force.  (D-50 mV, DpH 0.5 to 1).

6.           Sucrose is an osmotically active solute in guard cells:   Upon potassium efflux, sucrose becomes the dominant osmotically active solute, and the stomatal closing at the end of the day parallels a decrease in the sucrose content of guard cells.