Chapter 9 and 10 Studyguide PB

🌱 Abiotic Stress

Non-living environmental stress that negatively affects plant growth and yield, such as drought, salinity, heat, and cold.

⚡ Why Stress-Tolerant Plants Are Needed

To stabilize growth under variable conditions, respond to climate change, and expand agriculture into harsh environments.

🔥 Reactive Oxygen Species (ROS)

Chemically reactive molecules produced under stress that can damage DNA, proteins, and lipids.

💧 Water Potential (WP)

Potential energy of water in a system; water moves from high to low WP (soil → plant → atmosphere).

💦 Turgor Pressure

Internal pressure in plant cells caused by water uptake, keeps cells firm and maintains structure.

🧪 Osmotic Potential (OP)

Ability of water to move through membranes based on solute concentration.

🌵 Water-Deficit (WD) Stress

Occurs when water demand exceeds supply, causing wilting, reduced photosynthesis, and protein damage.

💧 Osmotic Adjustment

Accumulation of solutes (osmolytes) by plants to lower osmotic potential and maintain turgor under drought.

🍬 Compatible Solutes (Osmoprotectants)

Non-toxic molecules that protect proteins and membranes during water deficit; include sugars (sorbitol, mannitol, pinitol) and zwitterions (glycine betaine).

🧬 Glycine Betaine (GB)

Highly soluble osmoprotectant that stabilizes proteins and membranes; produced via choline pathways in plants, algae, bacteria.

🌾 Transgenic Osmoprotectant Plants

Plants engineered to produce osmolytes like GB or trehalose to increase drought and salinity tolerance.

🧂 Salt Stress (Salinity)

High salt concentration reduces water uptake and is toxic to the cytoplasm; plants use vacuolar Na⁺ storage to tolerate it.

🌀 Na⁺/H⁺ Antiporter

Membrane protein that transports sodium into vacuoles using proton gradients, protecting sensitive plant tissues.

⚡ AVP1

Proton pump gene; overexpression in transgenic plants increases drought tolerance and aids salt tolerance.

❄️ Cold Stress and COR Regulon

Cold-responsive (COR) genes activated by CBF/DREB transcription factors stabilize cells under freezing stress.

🧬 CBF/DREB Transcription Factors

Activate multiple COR genes simultaneously, improving tolerance to cold, drought, and salinity.

🔥 Heat Stress and HSPs

Heat shock proteins act as chaperones to refold denatured proteins during heat stress; classes include HSP100, HSP90, HSP70, HSP60, and small HSPs.

🧩 Heat-Shock Factor (HSF)

Transcription factor that binds heat-shock elements (HSE) in promoters to activate HSP expression.

💥 ROS Damage

ROS cause lipid peroxidation, protein oxidation, and DNA damage under stress conditions.

🛡️ Plant ROS Defense

Enzymes (SOD, catalase, peroxidases) and antioxidants (vitamin C, vitamin E, carotenoids, glutathione) detoxify ROS.

🧬 SOD (Superoxide Dismutase)

Enzyme that converts superoxide radicals into less harmful molecules; types: Mn-SOD (mitochondria), Cu/Zn-SOD (cytosol/chloroplast), Fe-SOD (chloroplast).

🍊 Ascorbate (Vitamin C)

Antioxidant that scavenges ROS, reduces oxidative damage, and regenerates tocopherol.

🧴 Glutathione (GSH)

Antioxidant that detoxifies radicals, stabilizes membranes, and reacts with singlet oxygen.

🥥 Tocopherol (Vitamin E)

Membrane-bound antioxidant that stabilizes lipids and scavenges free radicals.

🌈 Carotenoids

Protect photosystems and detoxify ROS generated by light excitation.

🧬 Engineering Stress Tolerance Strategies

Genetic engineering can enhance WD tolerance via osmolytes and OS tolerance via ROS-scavenging enzymes; multi-gene approaches are most effective.

🌾 Crop Yield

Total harvestable biomass produced by a plant.

🥗 Crop Quality

Nutritional, sensory, and commercial value of harvested products, including flavor, vitamins, shelf life, and texture.

⚖️ Factors Affecting Yield and Quality

Yield depends on photosynthetic efficiency and harvest index; quality depends on nutritional content, flavor, shelf life, and suitability for processing.

☀️ Photosynthetic Efficiency

Ability of plants to convert sunlight into chemical energy (sugars) through photosynthesis.

📏 Harvest Index

Fraction of total plant biomass allocated to harvestable parts (e.g., grains, fruits).

🍅 Tomato as Model for Fruit Ripening

Chosen for study because it is climacteric, undergoing ethylene-triggered bursts of respiration and rapid ripening.

🍃 Ethylene Role in Ripening

Plant hormone that triggers color change, flavor development, and softening in climacteric fruits.

🧪 Key Ripening Enzymes

ACC oxidase (ethylene production), polygalacturonase (cell wall breakdown), phytoene synthase (lycopene synthesis/red color).

🧬 cDNA Cloning of Ripening Genes

Used to identify ripening-related genes expressed only in ripe fruit, like pTOM13, pTOM6, and pTOM5.

📝 Antisense Technology

Gene silencing approach where a reversed RNA fragment binds normal mRNA, preventing translation; used to delay fruit softening.

🍏 Polygalacturonase (PG) Role

Enzyme that breaks down pectin in cell walls during ripening, leading to fruit softening.

🔧 Other Cell-Wall Enzymes

PME, pectate lyase, cellulase, and xyloglucan hydrolase remodel cell walls, affecting texture; controlling multiple genes improves softening control.

⚙️ Ethylene Biosynthesis Pathway

SAM → ACC synthase → ACC → ACC oxidase → ethylene; suppression of ACC oxidase slows ripening.

🍅 FlavrSavr Tomato

First commercial GM tomato with antisense PG to delay softening while retaining flavor and color.

🍊 pTOM5 and Lycopene Synthesis

Phytoene synthase enzyme produces red pigment lycopene; manipulation affects color, dwarfing, and nutrition.

🌿 Dwarfing via GA Reduction

Overexpression of phytoene synthase lowers gibberellin, producing shorter plants that allocate more energy to grain/fruit, improving yield.

🌾 Green Revolution

Mid-20th century movement increasing food production with semi-dwarf wheat and rice, photo-insensitive varieties, and improved harvest index.

🌟 Golden Rice

Biofortified rice engineered to produce β-carotene in endosperm to combat vitamin A deficiency.

🧬 Golden Rice Genetic Engineering

Introduced psy (phytoene synthase), crtI (carotene desaturase), and lcy (lycopene β-cyclase) genes targeted to plastids; aphIV selectable marker used.

🌾 Golden Rice Outcome

Best lines accumulated 1.6 µg β-carotene/g rice, enough to supply ~100 µg retinol/day from 300g rice.

🥚 Protein Engineering for Nutrition

Increase essential amino acids (lysine, methionine, cysteine) by modifying storage proteins or transferring high-sulfur proteins from other species.

🌱 Glycinin Expression in Rice

Soybean lysine-rich protein expressed in rice endosperm; processed normally and improves nutritional value.

☀️ Photosynthesis and Yield

Yield depends on light capture, conversion efficiency, and assimilate distribution to sinks like seeds or fruits.

🌿 C3 vs C4 Photosynthesis

C3: Rubisco fixes CO₂, prone to photorespiration; C4: PEP carboxylase fixes CO₂ efficiently under high light/temperature.

🌞 Phytochromes

Light-sensitive proteins (PhyA–PhyE) regulating growth and shade response; Pr inactive absorbs red light, Pfr active absorbs far-red light.

🌿 Phytochrome Manipulation

Overexpressing PhyA or PhyB alters plant architecture, reduces stem elongation, and improves assimilate distribution and yield.

🍃 Delayed Leaf Senescence

Using SAG12 promoter linked to cytokinin synthesis gene (ipt) prolongs photosynthesis, increasing yield; may reduce nitrogen recycling.

🌱 Calvin Cycle Enhancement

Overexpressing rate-limiting enzymes FBPase and SBPase increases CO₂ fixation and biomass; example: cyanobacterial dual-function enzyme in tobacco.

✅ Integrated Yield Improvement Strategies

Genetic engineering improves yield, nutrition, and quality via delayed ripening, pigment/nutrient synthesis, amino acid balance, photosynthetic efficiency, and optimized plant architecture.