Summary of Plant Development and Hormonal Regulation
Plant Development and Hormonal Regulation: A Student Guide
Introduction
Stress physiology studies how plants perceive, respond and adapt to adverse environmental conditions that impair growth, survival or reproduction. Abiotic stresses (water deficit, flooding, temperature extremes, salinity, metal toxicity, light imbalance) and biotic stresses (herbivores, pests, pathogens) trigger signaling cascades that change metabolism, gene expression and structure. Understanding these responses helps improve crop resilience and management.
Definition: Abiotic stress — environmental conditions (e.g., drought, salinity, temperature extremes, flooding) that negatively affect plant growth and physiology.
Overview of stress responses: framework and outcomes
Plants respond to stress at several levels:
- Perception: sensors in membranes and cell walls detect physical or chemical changes.
- Signaling: rapid secondary messengers (Ca$^{2+}$, reactive oxygen species (ROS), hormones such as abscisic acid (ABA) and ethylene) transmit information.
- Response: short-term reversible adjustments (stomatal closure, osmotic adjustment), acclimation (inducible protective mechanisms), and long-term adaptation (selection for genetic changes across generations).
Definition: Acclimation — reversible physiological or structural changes that increase tolerance to a stress when exposure is repeated or prolonged.
Definition: Adaptation — heritable genetic changes in a population that increase survival or reproduction under persistent stress.
Short-term vs long-term responses
- Short-term/reversible: osmotic adjustments, stomatal closure, transient induction of HSPs and antioxidants.
- Repetitive/long-term: enhanced protective metabolite pools, structural changes (e.g., thicker cuticle), epigenetic imprinting of seeds.
Key abiotic stresses and physiological effects
Water deficit (drought and low humidity)
- Primary cause: low soil water availability, low soil water potential due to texture or high salinity.
- Immediate effects:
- Reduced cell turgor and cell expansion (less growth)
- Stomatal closure → reduced CO$_2$ uptake and lower carbon fixation
- Imbalance between light harvesting and carbon reactions → excess excitation and ROS production
- Membrane mechanical stress and damage
- Typical responses:
- ABA accumulation → stomatal closure, reduced transpiration
- Ethylene increases → leaf epinasty and abscission when damage is severe
- Osmotic adjustment: accumulation of compatible solutes (proline, certain amino acids, polyols)
- Increased antioxidant defenses and chaperones (HSPs)
Definition: Osmotic adjustment — accumulation of ions in vacuoles and compatible solutes in the cytosol to lower cellular osmotic potential without disrupting metabolism.
Water flooding (waterlogging)
- Roots become oxygen-limited, shifting metabolism to fermentation (lactic and alcoholic).
- Consequences: intracellular acidification, ATP shortage, membrane and oxidative damage.
- Adaptive responses:
- Formation of aerenchyma (gas-filled tissues) to transport O$_2$ to roots (ethylene-mediated)
- Induction of antioxidant systems and metabolic reconfiguration
Temperature extremes
- Membrane fluidity is highly temperature-sensitive and controls many processes (electron transport, ion transport, membrane protein function).
- High temperatures increase membrane fluidity (liquid state) and can denature proteins, impair photosynthesis, and increase ROS.
- Low temperatures reduce fluidity (gel/solid state) impairing transport and enzyme kinetics.
- Low temperature specifics:
- Chilling (non-freezing low T) reduces metabolism, causes membrane damage, necrosis, and ROS accumulation in sensitive species (e.g., tomato, pepper).
- Freezing causes ice formation in apoplast and/or symplast:
- Gradual cooling: extracellular ice formation lowers apoplast
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Stress Physiology
Klíčové pojmy: Plants perceive stress and signal via Ca$^{2+}$, ROS and hormones like ABA and ethylene, Drought reduces turgor, causes stomatal closure and ROS; key response is osmotic adjustment, Flooding causes root hypoxia and fermentation; adaptive response includes aerenchyma formation, High and low temperatures disrupt membrane fluidity; acclimation alters membrane lipids and protective solutes, Salinity combines osmotic stress and ion toxicity; plants exclude, compartmentalize or chelate ions, Heat shock proteins (HSPs) act as chaperones across many stress types (cross protection), ROS function both as damaging agents and second messengers (e.g., H$_2$O$_2$ induces Ca$^{2+}$ influx), Biotic defenses include mechanical, chemical and inducible responses mediated by jasmonic acid, ethylene and salicylic acid, Specific pathogen resistance uses R gene recognition and hypersensitive response (localized cell death), Acclimation is phenotypic and often reversible; adaptation involves heritable genomic changes, Osmotic adjustment requires vacuolar ion sequestration plus compatible solute accumulation, Management applications: breeding for traits (osmotic adjustment, aerenchyma), agronomy (irrigation, drainage)