hormones Flashcards
what is a hormone
= a signalling molecule that exerts its effect far from its site of production
- numerous kinds of molecules can be classified as hormones
- unlike animals, each plant cell is capable of producing hormones
- hormones modulate almost all aspects of development in plants including embryogenesis, organ size, pathogen defence, stress tolerance, reproductive development
what do the Rht genes encode
- Gibberellins (gibberellic acid, GAs)
- natural growth regulators, promote stem elongation
- growth repressor inactivated by GA
- extreme dwarf (Rht3) wheats are GA-unresponsive, no extra growth when GA is applied
Arabidopsis thaliana, GA unresponsive mutant
- dwarf mutant gai
- molecular genetics used to isolate wild type GAI and mutant gai genes
- sequence deletion in mutant (no GA receptor), GA-unresponsive
- Arabidopsis sequence used to isolate wheat Rht genes by homology
semi-dwarf ‘miracle’ rice, GA-deficient mutant
- IR8, originated in China, bred in Taiwan and international centre in Philippines
- mutation in gene sd-1, encodes an enzyme in GA biosynthesis
- plants are GA-deficient
semi-dwarf lines of tef
- small grain crop commonly grown in Eritrea and Ethiopia
- very tall and prone to lodging (bending over)
- researchers used gene editing tools (CRISPR/Cas9) to generate semi-dwarf lines by targeting GA synthesis gene SD-1
earliest plant hormone evidence
leaf drop on trees that line St Petersburg street near gas-pipe leak, 1901, Ethylene
Ethylene and fruit ripening
- ethylene produced by ripening fruits accelerates ripening
- used to ripen supermarket fruit un pressurised ripening rooms with an ethylene generator
- keeping fruit in bags increases ripening
ethylene and 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
ABA
- abscisic acid
- produced under drought stress, closes stomata
- drought sensitive tomato mutant notabilis lacks ABA-biosynthesis gene NCED1, wilts much more easily
- over expression of NCED1 in transgenic petunia, can tolerate 14 days of drought
other functions of ABA
- promotes fruit ripening
- promotes cold hardiness
- seed treatment for hybrid seed production
- increases shelf life and improves plant tolerance for transplanting seedlings
salicylic acid
- stress-induced hormone
- increases in infected plants
- gene nahG from Pseudomonas encodes sacylic acid degradation
- Arabidopsis plants with nahG transgene cannot accumulate sacylic acid, so much more susceptible to pathogens
other functions of salicyclic acid
- thermotolerance
- stomatal aperture
- nodulation
- growth and development
- seed germination
- leaf senescence
- fruit yield
- respiration
- DNA damage repair
- transgenerational memory
- human use as a cleanser
auxin and shoot growth
- diffusible growth promoter
- experiments in oat seedlings in 1920s using coleoptile (pointed protective sheath covering the emerging shoot)
- auxin stimulates proton pump ATPases in plasma membrane
- leads to acidification of wall (H+ pumped in)
- lower pH in wall causes expansin proteins to loosen wall polysaccharides, leads to cell expansion
auxin herbicides
- earliest herbicide, 2,4D
- synthetic chemical with auxin effects
- disrupts cell growth functions in treated broad leaved weeds but grass less sensitive
- widely used in lawn weedkiller
auxin herbicides in vietnam war
- 2,4-D and 2,4,5-T (synthetic auxins) components of agent orange (also toxin dioxin)
- 19m gallons defoliated vietnam forests in late 1960s
auxin and 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 and minerals)
- auxin and cytokinin control differentiation
- mid auxin, mid cytokinin = callus (undifferentiated)
- low auxin, high cytokinin = shoots
- high auxin, low cytokinin = roots
hormones in callus and tumours
- callus can form when trees are wounded or suffer tumours
- undifferentiated mass of plant cells
- mid auxin and cytokinin
hormones in crown gall disease
- infected plant cells incorporate auxin and cytokinin biosynthesis genes of Agrobacterium Ti-plasmid
- plant cells produce opines, unique nutrient source
- only known natural prokaryote-to-eukaryote DNA transfer
hormones in horticulture
- promotion of roots by auxin, rooting garden products
