L12. Stress tolerance in Crops Flashcards
What is meant by the term stress?
STRESS:
- An influence that is outside the normal range of homeostatic control
- An external condition that adversely affects growth, development and/or productivity
- Stresses trigger a wide range of plant responses:
- > Altered gene expression
- > Cellular metabolism
- > Changes in growth rates and crop yields
- Abiotic- physical or chemical environment e.g. water logging, drought, high or low temperatures, soil salinity…
- Biotic- imposed by other organism e.g. Herbivores and pathogens (fungi, bacteria)
How do plants respond to environmental stresses?
- Die
- Escape
- A strategy to avoid stress by rapidly completing the life cycle of the plant
- As the environment changes, the plant responds by initiating flowering and seed production
- By the time the stress is severe, the plant has reproduced and then dies or senesces
How do plants cope with drought and salinity stress?
- Drought and salinity cause similar physiological problems for plants
- In both cases, these stresses make it more difficult to keep water inside plant cells
- Water will move out of the cytoplasm of the cell when the salt concentration is higher outside the cell than inside
Protective mechanisms:
- Synthesis of proteins that have protective properties
- Activation of metabolic pathways to synthesise protective compounds
e. g. synthesise solutes which allow water to be retained inside the cell
What is Osmotic adjustment?
Osmotic adjustment
- A biochemical mechanism that helps plants acclimate to dry and saline conditions
- Many drought tolerant plants can regulate their solute concentration on periods of water stress by making osmotic adjustments
Osmotic adjustments= net increase in the number of solutes particles present in the plant cell
What are compatible solutes?
Osmoprotectants or compatible solutes
- are small molecules that act as osmolytes and help organisms survive extreme osmotic stress. In plants, their accumulation can increase survival under stress e.g. drought
Osmolytes
- are compounds affecting osmosis. They are soluble in the solution within a cell, or in the surrounding fluid, e.g. as plasma osmolytes. They play a role in maintaining cell volume and fluid balance
- Not just any compound will function as a solute
Compatible solutes are:
- uncharged at neutral pH
- non-toxic
- highly soluble in water
How do some of these compatible solutes confer stress tolerance?
Mannitol (a sugar alcohol)
- manmitol concentrations increase in response to osmotic stress
E.g. in celery, salt stress is down-regulated by NAD+ dependent mannitol dehydrogenase (enzyme that oxidises mannitol)
Proline (amino acid)
- protects membranes and proteins against the adverse effects of high concentrations of inorganic ions and temperature extremes
Glycine betaine/ GB (amino acid)
- synthesised and accumulated of GB promotes salt tolerance
- Plants differ in the solutes they produce in response to water deficit.
- Some accumulate mannitol, others accumulate glycine betaine
What practical approaches are available for engineering salinity/ drought stress tolerance in crop plants?
Transgenic drought tolerant plants
- Larger root system (edt1 mutant)
- Improving water use efficiency (HARDY)
- Transgenic expression of plant Nuclear Factor Y (NF-Y)
Transgenic drought tolerant plants:
Larger root system
- edt1 mutant improved root architecture so they are able to grow longer
Transgenic drought tolerant plants:
Improving water use efficiency
- The expression of the Arabidopsis HARDY gene in rice improves water use, so that they have roots with enhanced strength, branching, are drought resistant and salt tolerant
- HRD overexpression produces thicker leaves with more chloroplast- bearing mesophyll cells, which enhance photosynthesis.
Transgenic drought tolerant plants:
Transgenic expression of plant Nuclear Factor Y
- NF-Y increase in chlorophyll content increases photosynthesis, which decreases water loss
How biotechnology can make salinity-tolerant plants?
- introduce genes encoding enzymes leading to production of osmoprotective substances
eg. synthesis of Glycine-betaine
How are Glycine-Betaine levels enhanced?
- GB is a compatible osmotic solute produced by many organisms
- GB in plants occurs in two steps
- Choline oxidase enzyme was able to convert choline all the way to GB in one step
- A chimeric gene which expressed choline oxidase was constructed and put into plants
- Plant was more tolerant to salt and low temperatures
