Molecular physiology of Plant. stress that is catagories into Abiotic and Biotic factors.

Molecular physiology of plant.

1. Drought Productivity and stability of plants - affected -by water stress. Cell expansion and growth - first processes -to decline under water stress. With progressive water deficit, photosynthesis is adversely affected. Membranes and proteins damaged by a reduction in -hydration and an increase ROS

2. Tolerance Reduction in water loss: → closure of stomata (reduces transpiration) → maintained by ABA→ABA synthesis - from carotinoid by ABAsynthesizing enzyme in root tip. → enters into xylem vessel from root tip to leaves. →ABA regulates - metabolisın and stomatal behaviour under water stress

3. Protection of photosynthetic machinery:→ wilting of leaves – protects photosynthetic machinery - direct sun rays. → stomatal closureiii ) Osmotic adjustment :→ synthesis and accumulate compatible solutes and ions- under water stress. → proline and glycine betaine highly compatible solutes - increase cellular osmotic pressure. → protect against water loss in leaves

4. Heat temperature Serious threat -above - adapted. occurs plant experience temperature Plant exposed to high temperature - changes occur at molecular level - altering the gene expression. Leads to synthesis -Heat shock proteins. Important adaptive strategy.

5. Tolerance Anatomical changes:→ reduced cell size→closure of stomata→curtailed water lossil ) Physiological changes :→ plant water status - important - water stress. → minimize water loss and synthesize compatible solutes (glycine betaine). In most species, rate of photosynthesis declines above35 C. → As temperature increases-rate of photosynthesis decrease and Dark & photorespiration increase

6. Integrity and functions of biological membranes sensitive. As temperature Increase alters quaternary structures of membrane proteins. tertiary andlil ) Molecular changes:→temperature increases- increase HSPs production. →HSPs - associate with particular structure- cell wall, chloroplast, ribosome and mitochondria. → eg. In tomato plant- HSPs aggregate into granular structure in cytoplasm- protects the protein biosynthesis machinery. → other proteins or mRNAs -increase (not considered as HSPs) - includes - glycolytic enzymes, protein kinases and ubiquitin.

7. Chilling and freezing Above 0°C or below some threshold temperature (unique for each species) – Chilling stress.Below O*C - Freezing stress. Chill sensitive plants- cotton, soybean, maize and rice.Plants face 3 major problems:→perturbation of membrane. →slow down - chemical and biochemical reaction. → changes in water status and availability. Plants-tolerate -extracellular freezing. Intracellular ice crystals - lethal.They pierce- plasma membrane.

8. Tolerance Stabilization of membrane:→ changes in lipid composition - increase membrane stability against freezing stress. ii) Cryopreservation:→ soluble sugars cryoprotective function. other and osmolytes have→ they protect cell membranes and organells during freezing. → sugar replace water and decrease degree of freeze induced dehydration.

9. UV radiation Reduction in stratospheric 03 radiation (280-320nm) Increase solar UV-B Cause - decrease in growth and other physiological response in many crops. ToleranceIDNA repair:→ UV-B radiation targets DNA. →UV radiation induces lesions in DNA. Eg.Pyrimidine dimmers→ dimmers repaired (photolyase) excision repair.

10. Tolerance Accumulation of secondary metabolites:→ flavonoids/anthocyanins exposure. induced by UV-B→ accumulate in epidermis - keep UV radiation away from reaching photosynthetic tissues. ← polyamines contribute -UV tolerance. waxes, specific alkaloidsiii ) Morphological changes:→Leaf curling is a photomorphogenic response observable at low fluences of UV-B. →A protective function - hypothesized - for leaf or epidermal thickening

11. Salinity Presence of excess ions affect many plant processes. Plants are subjected to salinity are subjected to two groups: Halophytes: withstand 20% of salt in soil. Non-halophytes: limited growth sodium salt (usually 0.01%).

12. Mechanism of tolerance Minimizing entry and conc. of salt in cytoplasm. Two strategies. Stress avoidance: barriers that negative effect of stress. Stress tolerance: successful survival despite effect of stress.

13. Salt exclusion:-plants can limit salt accumulation in its tissues -by inhibiting- root uptake.-Strategies evolved sensitive organs. restrict salt transport into cytoplasm ii) Salt excretion :halophytes have anatomical eliminate excess salt ions. structure tosalt glands and salt bladders salt glands: embedded in surface of leaves- salt bladders: specialized trichomes.

14. Heavy metal Gaining importance chain. impact in human health → foodSeveral vegetables, fruits and cereal crops – accumulate heavy metals, Plants growing on soil contains high level of metal metallophytes.

15. Tolerance Compartmentation: isolation of metal ions in tissues ( cell wall of roots and leaves) or cellular compartments (vacuoles) - which are less sensitive to metals. they are away from metabolically active compartments (cytosol, mitochondria or chloroplast) ii) Metal excretion:metals -excreted in salt crystals, released through salt glands of some halophytes.

16. Biotic stress - occurs as a result of damage done to an organism by other living organisms. (such as bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants). Herbivore Low levels damage - can be completely compensated by plants in terms of fitness. Further increments the intensity of damage result in a decreasing ability to maintain complete tolerance.

17. Plant defense against Herbivory Trait that increases plant fitness when faced with herbivory. Two categories: Tolerance and Resistance. Resistance: Constitutive or induced defenses.Physical defenses (e.g. Thorns on roses, spines on a cactus). Chemical defenses are secondary metabolites - Carbon-based defenses and Nitrogen-based defenses. Some plants can attract natural enemies to herbivores - indirect defense (e.g. ants reduce herbivory)

18. Tolerance Photosynthetic enhancement Growth enhancement Advancing the timing of bud break Delaying the senescence of infected tissue:(due to increased levels of cytokinins ) Increasing nutrient uptake

19. Allelopathy Allelopathy interference mechanism by which plants release molecules (allelochemicals)- that affect seed germination, plant phisiology , growth and survival of other plants. Tolerance of allelopathic compounds in plants Exclusion (root or leaf) Compartmentation (deposit these compounds in nonmetabolic compartments) (vacuole) Excretion Detoxification

20. Crosstalk Reactive Oxygen Species (ROS): Act as secondary messengers in both stress types, regulating defense responses and stress acclimation. Salicylic Acid (SA): Mainly involved in pathogen defense, but also plays a role in drought and salt stress responses. Jasmonic Acid (JA) & Ethylene (ET): Crucial for insect and pathogen resistance; also involved in responses to drought and wounding. Abscisic Acid (ABA): Primarily an abiotic stress hormone (drought, salt, cold), but can modulate biotic stress pathways by interacting with SA and JA signaling. Mitogen-activated protein kinases (MAPKs) integrate both stress signals to fine-tune cellular responses. Calcium ions (Ca²⁺) serve as secondary messengers, activating downstream responses to both stress types. Calcium-dependent protein kinases (CDPKs) mediate cross-adaptation to biotic and abiotic stresses.