27: Plant Hormones

Plant Hormones

Objectives

  • Understand the importance of hormones in plants.
  • Understand the role of Gibberellin in Green Revolution crops.
  • Understand the role of Ethylene in fruit ripening.
  • Understand the role of ABA in drought stress.
  • Understand the role of Salicylic acid in disease resistance.
  • Understand the role of Auxin & cytokinin in cell growth & differentiation.

What is a Hormone?

  • A signaling molecule that exerts its effects far from its site of production.
  • Numerous kinds of molecules can be classified as hormones.
  • Unlike in animals, each plant cell is capable of producing hormones.
  • In plants, hormones modulate almost all aspects of development, including:
    • Embryogenesis
    • Organ size
    • Pathogen defense
    • Stress tolerance
    • Reproductive development

The 'Green Revolution'

  • From 1961, cereal production and yields rose strongly, and prices fell.
  • The Rht dwarfing genes of 'Green Revolution' wheat:
    • Bread wheat has the genotype AABBDD.
    • Rht-B1 and Rht-D1 are loci of the B and D genomes of hexaploid wheat.
    • Increasingly severe mutant alleles exist (Rht-1, Rht-B1d, Rht-B1b, Rht-D1b, Rht-D1d, Rht-B1e, Rht-B1c, Rht-D1c).
    • Semi-dwarf varieties are best for crops, while extreme dwarf varieties are too severe.

What do the Rht genes encode?

  • Gibberellins (Gibberellic Acid - 'GAs') are natural growth-regulators.
    • Increasing [GA3][GA3] (left to right) applied to rice seedlings affects growth.
    • Extreme dwarf (Rht3) wheats are GA-unresponsive.
    • No extra growth when GA is applied. rht1 (tall), Rht3 (dwarf), Rht3 +GA3

Arabidopsis thaliana

  • Short life-cycle → mutants
  • Small DNA content
  • 1st plant genome sequence (2000)

Arabidopsis thaliana GA-unresponsive dwarf mutant gai

  • Molecular genetics used to isolate wild-type GAI and mutant gai genes
  • Sequence deletion in mutant: gai dwarf mutant
  • Arabidopsis sequence used to isolate wheat Rht genes by homology
  • Mutations in Rht genes from semi-dwarf lines:

Rht product: growth repressor normally inactivated by GA

  • Mutant in dwarf wheat is GA- unresponsive ® growth repressed

Parallel ‘Green Revolution’ in Asian rice

  • 1966: semi-dwarf ‘IR8’ rice bred at International Rice Research Institute (‘IRRI’) in Philippines
  • Philippines rice production 3.7® 7.7 million tons in 2 decades

‘Green Revolution’ rice genes affect GA biosynthesis

  • Semi-dwarf ‘miracle rice’ originated in China, bred in Taiwan & international centre in Philippines
  • Semi-dwarf rice gene sd-1 encodes an enzyme in GA biosynthesis - plants are GA-deficient
  • Enzyme lost in sd-1 mutants needed for all reactions

Mendel’s classic pea genes

  • One of Mendel’s classic pea genes is GA-biosynthesis mutant
    • Seed shape
    • Seed color
    • Flower color
    • Pod shape
    • Pod color
    • Tall vs Dwarf
    • Axial flowers vs terminal flower

Rht product: growth repressor normally inactivated by GA

  • Mutant in dwarf wheat is GA- unresponsive, resulting in growth repressed.

Small grain crop (Teff)

  • Small grain crop commonly grown in Eritrea and Ethiopia.
  • Problem – very tall and prone to lodging.
  • Solution – target GA synthesis gene SD-1 to generate semi-dwarf phenotype.
  • Using gene editing tools, researchers quickly generated semi-dwarf lines.

Ethylene

  • Earliest plant hormone evidence: leaf-drop on street trees near St Petersburg gas-pipe leak (1901)
  • Gas-pipe crosses street
  • Nearest trees lost leaves

Ethylene & fruit ripening

  • Ethylene produced by ripening fruits accelerates ripening.
  • Used to ripen supermarket fruit.
  • Keeping fruit in bags accelerates ripening.

Ethylene & Delayed Ripening Technology

  • Suppressing ethylene production can slow the ripening process.
  • Premature ripening results in significant losses for both farmers and consumers.
  • Targeting the ACC1 oxidase gene, involved in the production of ethylene, slows ripening
  • A = WT plants, B & C = genetically modified

ABA in drought stressed plants

  • Water provided: ‘ABA’ & events in drought stressed plants
  • [ABA][ABA] increases
  • Water withheld:
    • Stomata close
    • Stomata re-open
  • [ABA][ABA] returns to normal

ABA

  • Drought-sensitive tomato mutant notabilis lacks ABA-biosynthesis gene ‘NCED1’
  • Genetics → proof of ABA’s role in drought stress
  • Over-expression of NCED1 in ‘transgenic’ Petunia → tolerate 14 days of drought

ABA Uses

  • Reduces water loss
  • Promotes cold hardiness
  • Promotes fruit ripening
  • Seed treatment for hybrid seed production
  • Increases shelf life and improves plant tolerance for transplanting seedlings

Salicylic acid

  • Another stress-induced hormone: salicylic acid increases in infected plants

Genetics → proof of salicylic acid’s role in pathogen defence

  • Gene nahG from Pseudomonas encodes salicylic acid degradation
  • Arabidopsis plants with nahG transgene cannot accumulate salicylic acid → much more susceptible to pathogen
  • nahG plants vs Wild-type
  • Infection with fungus (Peronospora parasitica)

Salicylic acid signalling

  • Immune Response
  • ER stress
  • Thermotolerance
  • Stomatal aperture
  • Nodulation
  • Transgenerational Memory
  • DNA damage repair
  • Respiration
  • Fruit yield
  • Seed germination
  • Leaf senescence
  • Growth and development

Salicylic Acid - other uses

  • Salicylic Acid is used to produce Acetylsalicylic acid (aspirin).
  • Salicylic Acid is used in skin care products.

Auxin & Shoot Growth

  • 1920s: diffusible growth- promotor in oat seedlings
  • Coleoptile - pointed protective sheath covering the emerging
  • ‘Auxin’ from Greek auxein ‘to grow’

Auxin ‘loosens’ cell wall ® cell expansion

  1. Auxin stimulates ‘proton pump’ ATPases in plasma membrane ® acidification of wall
  2. Lower pH in wall causes ‘expansin’ proteins to ‘loosen’ wall polysaccharides ® cell expansion

Auxin Herbicides

  • Earliest herbicide ‘2,4-D’: synthetic chemical with auxin effects
  • Disrupts cell growth functions in treated broad leaved weeds (dicots) but grass less sensitive
  • Widely used in lawn weedkillers

Auxin herbicides in Vietnam War

  • 2,4-D & 2,4,5-T (another synthetic auxin) were components of ‘Agent Orange’ – also contained trace amounts of a toxic dioxin
  • 19 million gallons defoliated Vietnam forests in late 60s

Auxin & Phototropism

  • Phototropism - the ability of a plant to grow directionally in response to a light source
  • Auxin accumulates on the shaded side to stimulate cell wall expansion
  • Concentration gradient achieved via the action of PIN proteins

Hormones in ‘plant tissue culture’

  • Key technique in plant biotechnology
  • Tissues on nutrient medium (with sugar & minerals)
  • Auxin & cytokinin control differentiation
    • High auxin: low cytokinin ® roots
    • Mid auxin: mid cytokinin ® ‘callus’
    • Low auxin: high cytokinin ® shoots

Hormones in Callus & Tumours

  • Mid auxin: mid cytokinin ® ‘callus’
  • ‘Crown gall’ tumor on tree
  • ‘Callus’ can form when trees wounded or suffer from tumors
  • Undifferentiated mass of plant cells

Hormones in ‘crown gall disease’

  • Infected plant cells incorporate auxin & cytokinin biosynthesis genes of Agrobacterium ‘Ti-plasmid’
  • Plant cells produce opines – unique nutrient source
  • Only known natural prokaryote-to-eukaryote DNA transfer

Agrobacterium in ‘genetic modification’

  • Hormone genes cloned out
  • New gene for plant
  • New T-DNA
  • New Ti plasmid cloned in Agrobacterium
  • Infection of plant cells
  • Transgenic plants regenerated

Hormones in Horticulture

  • High auxin: low cytokinin → roots
  • Promotion of roots by 'rooting' auxin garden products

Summary

  • Plant hormones
    • → growth: gibberellins in Green Revolution semi-dwarf crops
    • → stress responses: ABA in drought; salicylic acid in pathogen infection
    • → differentiation: ethylene in fruit ripening; auxin & cytokinin in root, shoot & callus formation