post harvest physiology related to senescence

ppt on senescence

1. Senescence Associated Genes and Gene Products Zuby gohar Ansari Dept. of Crop Physiology

2. INTRODUCTION : Senescence is a highly ordered process regulated by genes known as senescence associated genes (SAGs). Till now, more than 30 SAGs have been isolated, cloned and characterized. Using genetic molecular and OMICS analysis Onset, progression and completion of senescence in highly coordinated

3. Two transcription families NAC, WRKY NAC: contain N-terminal NAC DNA binding domains have 117 members specific to dicots and eukaryotes role: formation of 2ry wall formation biotic , abiotic stress responses WRKY: 60 aminoacid DNA-binding WRKY domain have zinc-finger motif contain 74 members role: biotic abiotic stress response growth & devolopment seed germination

4. Genes for macromolecular degradation export of nutrients mobilization of N2 breakdown of lipids, transport mobilization of other nutrients maintenance of cell integrity of leaves PR proteins (against infection) against free radicle induced damage

5. Genes for macromolecular degradation during leaf senescence : These genes participate in degradation of proteins and then mobilization of the breakdown products from senescing leaves to other parts of the plants.

6. Tomato Leaf Senescence- cysteine proteinases Cysteine protease having a high degree of homology with oryzain y and aleurain Expression of the genes is also observed in young and mature leaves, however with their enhanced expression during senescence. Enhanced expression of genes for synthesis of nucleases and lipases for degradation of nucleic acids and lipids. The genes express only after initiation of the process.

7. Genes for export of nutrients : Maturity of the leaf is likely to send signals for a decline in photosynthesis, induction of senescence and subsequent mobilization of resources to other growing regions. At late stage of senescence with respiratory burden the plants, rejects it by the process of leaf abscission. The mobilization process like degradation of macromolecules, interconversion of metabolites to their transportable forms, and their energy-dependent movement are tightly regulated.

8. Mobilization of nitrogen: The function of GS1 is to convert ammonia to glutamine, a known transportable form of nitrogen that is remobilized from senescing leaves to growing parts of plants.

9. Breakdown of lipids, its conversion to sugars and their transport: sugar starvation leading to the activation of conversion of lipids to sugars. The expression of genes for the enzymes participating in the process of gluconeogenesis for production of sucrose plays an important role during senescence. Lipid conversion to sugar is a major event favouring the proposition of energy maintenance of senescing leaves.

10. Mobilization of other nutrients: The enzyme ATP sulphurylase is known to participate in the biosynthetic pathway of methionine and cysteine . Cysteine conversion to glutathione plays a key role in minimizing the level of toxic oxygen free radicals produced during senescence storage and transport of sulphur from senescing leaves to growing parts of the plants.

11. Maintenance of cell integrity of senescing leaves: Senescence increase the level of expression of a cDNA clone having sequence similarity with catalase gene, Expression of the genes against pathogen attack and up-regulation of the genes against metal toxicity

12. Against infection: During leaf senescence of  Brassica  and  Lycopersicon , a significant increase in the expression of homologues of pathogen-related (PR) genes has been observed. PR proteins are known to protect the plants against pathogen attack and genes for the proteins. Since senescing leaves are relatively prone to pathogen attack, the PR protein may protect the leaves against infection.

13. Against free radical-induced damage: The metabolic significance of senescence-induced increase in the expression of homologues of ferritin genes. Degradation of some of the cellular macromolecules may result in release of free metals and can increased pool of free iron. The free iron is known to bring about a metal catalysed reaction for production of oxygen-free radicals, leading to the damage of senescing cells.

14. Metallothionin may bind with free metal ions released from protein breakdown and thus make them available for storage and transport. Senescence-induced enhanced expression of the catalase gene. Cu/Zn seperoxide dismutase -oxygen radicle scavenging enzyme Calcium- calmodulin binding protein expressed at senescence stages.

15. Abundant SAGs Protienases Lipases Nucleases Chlorophyllase Glutamate synthase Cystein proteases To identify non abundant SAGs use SSH ( supression subtractive hybridization)

16. Signals for expression of senescence-associated genes: The down-regulation of photosynthesis as a possible signal for induction of senescence-related genes A reduction in the level of cytokinins , a possible hormonal signal may lead to induction/enhancement of foliar senescence. metabolism of ethylene during fruit ripening signalling molecule regulating foliar senescence development of reproductive structures in plants Jasmonic acid PCD pathway is induced by low cell density Involvement of Ca 2+  and protein phosphorylation in this signal transduction

17. FUNCTIONAL AND ULTRA STRUCTURAL CHANGES DURING SENESCENCE

18. CHLOROPLAST : Earliest sites of catabolism Loss of plastid integrity is 1 st sign of leaf senescence Gerontoplasts devolop only from mature chloroplasts cannot devide donot retain any biosynthetic activity lost own DNA Degradation on RUBISCO

19. Increase in plastoglobuli ( osmophilic globuli ) Increase in carotenoid content of the plastoglobuli Grana Disorientation Dialation Of Thylakoids Reduced Density And Size Of Ribosomes Breakdown Of Proteins (Extrinsic And Intrinsic) Loss Of Electron Transport Capability Decrease in spongy mesophyll cells number and activity

20. Figure 1. Leaf senescence in grapevine plants. Ruberti C, Barizza E, Bodner M, La Rocca N, et al. (2014) Mitochondria Change Dynamics and Morphology during Grapevine Leaf Senescence. PLoS ONE 9(7): e102012. doi:10.1371/journal.pone.0102012 http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102012

21. MITOCHONDRIA : Can remain until late stages of senescence Mitochondria were less dynamic Respiration rate increased at senescence Glutamate dehydrogenase Gtl -D rate increased to peak Morphology and dynamics of mitochondria represent a reliable senescence marker for plant cells. mitochondria regulate PCD by releasing pro-apoptotic factors such as cytochrome c Swelling of cristae

22. Mitochondrial morphology and distribution in mature and senescent leaves of grapevine long branched structures formed by mitochondria in a mature leaf

23. OTHER FUNCTIONAL & ULTRASTRUCTURAL CHANGES: Loss of starch Distortion of endocarp and mesocarp cell in pea ovary Degradation of ER (RER) Changes in lipid phase of senescing membranes Lipid bilayer : liquid crystalline to gel phase (leaky)

24. Nucleus : condensation of chromatin ( propidium iodide stained nuclei) Endonucleolytic fragmentation (late stages) Vacoule : cells become more vacoulate change in permeability of tonoplast triggering Autophagic processes

25. Cell Injury and Cell Death

26. Cell Injury If the cells fail to adapt under stress, they undergo certain changes called cell injury . The affected cells may recover from the injury ( reversible ) or may die ( irreversible ).

27. Causes of Cell Injury Oxygen deprivation (hypoxia) Physical agents Chemical agents Infections agents Immunologic reactions Genetic defects Nutritional imbalances

28. Morphology of Cell Injury Reversible: Cellular swelling and vacuoles formation (Hydropic changes) Changes at this stage EM - blebbing of the plasma membrane, swelling of mitochondria and dilatation of ER Nuclear alterations Fatty change

29. Sequence of events in reversible injury reduced oxidative phosphorylation and ATP production in the mitochondria increased anaerobic metabolism (glycolysis)  reduced glycogen stores and increased production of Lactic acid decreased intracellular pH - clumping of nuclear DNA decreased activity of Na+ pump (ATP-dependent) generalized edema (increased intracellular Na+ and H 2 0) detachment of ribosomes from ER - reduced protein synthesis surface blebs, mitochondrial swelling

30. Cloudy swelling, Kidney Hydropic change or vacuolar degeneration. Appears whenever cells are incapable of maintaining ionic and fluid homeostasis. The first manifestation of almost all forms of cell injury. Reversible injury.

31. IRREVERSIBLE INJURY disruption of plasma membrane disintegration of DNA, RNA, and phospholipids rupture of lysosomes and leakage of lysosomal enzymes into the cytoplasm  enzymatic digestion of cellular components influx of Ca+2 into the intracellular space

32. MECHANISMS OF FREE RADICAL-INDUCED CELL INJURY Free radicals are chemical species with a single unpaired electron in an outer orbit. Free radicals are highly reactive with cell membranes and nucleic acids. They may by initiated by: absorption of radiant energy enzymatic metabolism of exogenous chemicals or drugs redox reactions of normal metabolism: superoxide anion radical from mitochondria and cytochrome p450 hydrogen peroxide from peroxysome catylases hydroxyl ions from ionization of water transition metals nitric oxide

33. MECHANISMS OF FREE RADICAL-INDUCED CELL INJURY Free radicals cause: lipid peroxidation of membranes oxidation of proteins DNA damage

34. THANK-Q