[9-10] - Abiotic Stress + Drought Tolerance Flashcards
Explain how two other conditions besides drought can act as water deficiency stresses
SALINITY and FREEZING:
Salinity
- Results in water deficit because the high concentration of salt reduces water potential of the soil
- This makes it harder for roots to extract water from soil
- 6% of total land surface area is salt affected (i.e., >1500 ppm Na+)
- 13% of cultivated land is salt affected, and 50% of irrigated land (common if irrigation is poorly managed, e.g.. saltwater is used)
- This reduces yields on cultivated land and reduces expansion to new areas
Freezing
- As ice formation begins in the soil, the water potential outside of plant cells decreases
- The resulting high solute concentration outside the cell draws water out via osmosis, causing dehydration and cell death
- Many crop plants are low-temperature sensitive, limiting distribution and reducing yield due to the shortened growing season
[Background] - Briefly explain the current significance of water security and drought
Frequency of major droughts and heat waves is increasing, and will continue to do so with climate change
-> this has affected food production and increased food prices
-> also increased dependency on irrigation (40% of world’s food now relies on it)
Explain the three important “potentials” in the equation and how they relate to each other
Water Potential = Solute Potential + Pressure Potential
Solute potential (osmotic pressure) is the concentration of solutes dissolved in water -> the higher the solute potential, the lower the water potential
Pressure potential (turgor pressure) is the physical force exerted by water on the cell membrane and cell wall -> positive pressure (turgor) increases water potential [e.g., during high water status, water enters the cell and makes it turgid, while under water loss, turgor is lost and the cell becomes flaccid]
Water potential gradients determine the direction of flow, and result in a continuous transpirational pull of water up through the plant into the atmosphere, where WP is lower than in the soil (only 2-5% of the water taken from the soil stays in the plant)
What are the common physiological impacts of water deficit on plants?
SHORT-TERM (REVERSIBLE):
- Leaf wilting
- Lower rate of photosynthesis (reduced CO2 uptake)
- Reduced cellular/metabolic activities
LONG-TERM EFFECTS OF CONTINUED DEFICIT (IRREVERSIBLE):
- Leaf abscission (so fewer organs to support)
- Concentration of cellular contents e.g., ions (cytotoxicity)
- Severely inhibited photosynthesis and respiration
- Localised cell death, vacuole splitting, chloroplast/mitochondria/nuclear envelope breakdown
At this stages, some viable seeds may still be recovered if rehydrated
Describe the effect of high salt concentration in the soil on plant cells
When Na+/K+ ratios are altered, high [Na+] can outcompete K+ for transport into the cell, leading to ion-specific stress, cytotoxicity and K+ deficiency
There are various non-selective channels and K+ transporters, many of which will take up Na+ if it is in high extracellular concentration
Na+ can be cytotoxic as many plant enzymes are sensitive to it, while reduced K+ is also detrimental as it is an essential nutrient
[Possible segue sentence in an essay] -> the goal of developing crops for water deficit stress tolerance poses a significant challenge to breeders and genetic engineers, as water use efficiency is an extremely complex trait, with hundreds of genes playing a role in it. (Free 5/5)
Cool beans
Describe the four broad strategies for engineering Water Deficit Stress Tolerance in plants
- OSMOPROTECTANTS (metabolic or molecular)
- STRESS-RESPONSIVE TRANSCRIPTIONAL REGULATION
- OSMOREGULATION VIA ION OR WATER TRANSPORTERS
- WATER USE-EFFICIENT CARBON FIXATION
Describe some of the ways in which plants have evolved to withstand droughts or water deficit
Many drought tolerant plants (i.e., xerophytes) are perennials with specialised morphology:
- Thickened cuticles (reduced water loss, reduced leaf breakage from wilting)
- Shiny cuticles (reflect light and heat)
- Trichomes (leaf hairs which reflect light and trap damp air above sunken stomata to reduce water loss)
- Reduced surface area (very small leaves, or no leaves at all)
- Altered metabolism to prevent photorespiration, e.g., Crassulacean acid metabolism (CAM)
Succulent traits:
- Water storage organs containing enlarged cells with high-volume vacuoles
- Low transpiration traits
- Thick cuticles
ALSO OSMOPROTECTANTS (see other FCs)
Give one example of “engineering succulence”
A grape-derived transcription factor known to be involved in cell expansion (CEB1) was overexpressed in Arabidopsis
Mutants showed increased cell size, succulence and reduced intercellular air space
Overexpressing lines also showed increased salinity tolerance due to reduced uptake of salts and increased dilution
Explain what is meant by “resurrection plants” and how they achieve this
Resurrection plants such as C. plantagineum, L. brevidens and R. hydrometrica can tolerate extreme dessication, and can recover from the dry state in 12-15 hours upon rehydration
In these plants, dehydration induces significant quantities of OSMOPROTECTANTS for membrane and protein stabilisation, and turgor maintenance
Osmoprotectants (also known as compatible solutes or osmolytes) are small products of metabolism, which act like “antifreeze” to protect membrane and protein structure and prevent denaturation
Give some key examples of osmoprotectants
Amino acids (e.g., proline, glutamate)
Carbohydrates (e.g., trehalose)
Sugar alcohols (e.g., inositol, sorbitol)
Quarternary ammonium compounds (e.g., glycine betaine)
and Tertriary Sulphonium Compounds (e.g., dimethylsulphoniopropionate)
Why are osmoprotectants sometimes called “Compatible Solutes”?
They are ‘compatible’ as they do not interfere with cellular structure or function (as opposed to perturbing ions such as Na+ and Cl-)
More specifically:
They are small, non-toxic compounds which do not disrupt hydration shells around proteins, but stabilise proteins and membranes, and allow turgor maintenance of cells and tissues to prevent water loss
Explain how Osmoprotectants can be a target of GE in plants
This is the first of the four broad GE strategies to engineer water deficit stress tolerance
Can be metabolic (e.g., Trehalose) or molecular (i.e., proteins e.g., HSPs and CSPs which act as chaperones to protect protein conformation)
METABOLIC - e.g., TREHALOSE:
- Trehalose is a non-reducing disaccharide sugar present at high concentrations in desiccation-resistant resurrection plants
- It is synthesised from UDP-glucose and Glucose-6-phosphate via two enzymes: Trehalose-6-phosphate synthase (TPS) and Trehalose-6-phosphate phosphatase (TPP)
- Previously, expression of osmoprotectant genes has successfully increased tolerance of drought, salinity and freezing tolerance in many plant species, BUT often with stunted growth under non-stress conditions
-> Strategy for high expression of trehalose without reducing yield:
- Expression of both TPS and TPP as a fusion proteing (TPSP) on a single plasmid, under either a chloroplast-specific (rice rbcS) or stress-dependent (ABA-inducible) promoter
- Using the stress-induced promoter, transgenic rice showed improved relative water content (RWC), chlorophyll content, K+/Na+ ratio, stomatal conductance and photosynthetic efficiency compared to the WT under stress conditions such as high pH, high EC and severe drought
MOLECULAR:
- Some osmoprotectants are proteins (e.g., HSPs and CSPs) which function as molecular chaperones to stabilise and protect conformation of proteins, RNAs and cell membranes
- One example is CspB from bacteria - an RNA chaperone which regulates translation and cell growth under stress conditions
- CsbB expression using an actin promoter in various plants, including maize, has been shown to increase tolerance of cold, heat and drough
- This is the basis for DroughtGard, the first commercial drought-tolerant GM crop, first approved in 2010
Explain how Stress-Responsive Transcriptional Regulation can be used in GE to improve drought tolerance
This is the second of the four broad strategies to improve stress tolerance via GE
CONTEXT:
Plants use the hormone abscisic acid (ABA) - the stress hormone - to perceive and respond to water deficit stresses
- ABA biosynthesis increases in response to these stresses, ABA then upregulates TFs such as MYB, NAC, HD-Zip, as well as ABF which binds the ABA response element
- Altogether, this induces many stress-responsive genes:
-> Osmoprotectants (e.g., sugars, proline, glycine betaine)
-> Molecular chaperones (e.g., HSPs + CSPs)
-> Oxidative stress responses (peroxidase, superoxide dismutase)
-> Movement of water and ions (aquaporins, ion channels)
[NOTE: there is also ABA-independent signalling via CBF4 which binds the Drought Response Element]
GENETIC ENGINEERING:
- HaHB4
- Overexpression of the ABA-inducible transcription factor HaHB-4 increased drought tolerance in Arabidopsis
- Field study later showed that transgenic wheat expressing HaHBF from sunflower outyielded WT wheat by 6% across a range of environments and had 9% greater water use efficiency
- Verdeca developed soybeans engineered to express this gene, and both soybean and wheat versions have now been approved in some countries - CBF3
- Expression of CBF3 (a transcription factor) in Arabidopsis under a RD29A stress-induced promoter showed both drought and freezing tolerance without stunted growth
Explain how Osmoregulation can be a target for GE in plants
This is the third of the four broad strategies to improve drought tolerance -> aim to reduce Na+ import via manipulating ion transporters
There is a fundamental challenge when pursuing this strategy:
-> due to redundancy of transporters, targeting one transporter may simply result in import via a different transporter
-> AND there is a risk of reducing necessary import of K+
A more effective strategy may be to increase EXPORT of Na+ from the cell, or to store it in vacuoles (using a proton pump to energise these processes by creating a H+ gradient)
AtNHX1 is a vacuolar antiport protein which transports Na+ into vacuoles, while transporting H+ into the cytosol
-> Overexpression of AtNHX1 in tomato using 35S promoter resulted in Na+ accumulation in leaves, but NOT in the root or fruit
-> This greatly improved salinity tolerance, and allowed the transgenic tomatoes to grow in up to 200mM NaCl soil
Another strategy - AVP1
- AVP1 is a H+ pump which can create a H+ gradient via transport into the vacuole, which energises H+-coupled ion transporters such as NHX1
- Overexpression of AVP1 in tomatoes improved drought tolerance and increased root mass
