Ageing and senescence

ageing and senescence in crop plants

1. Physiological and Molecular mechanism of
Ageing and Senescence

2. Senescence
•Senescenceorbiologicalageingisthechangeinthe
biologyofanorganismasitagesafteritsmaturity
•Suchchangesrangefromthoseaffectingitscellsand
theirfunctiontothatofthewholeorganism
•ThewordsenescenceisderivedfromtheLatinword
senex,meaningoldman,oldage,oradvancedin
age
•Senescence:Aprograminwhichthefunctionof
organorwholeplantnaturallydeclinestodeath.This
isanessentialphaseofthegrowthanddevelopment
inplant

3. Ageing
Ageing is the accumulation of changes in an organism
or object over time
In biology, senescence is the state or process of ageing
Plant senescence is the study of aging in plants

4. Cellular senescence
•Cellularsenescenceisthephenomenonbywhich
normaldiploidcellslosetheabilitytodivide,normally
afterabout50celldivisionsinvitro.
•Somecellsbecomesenescentafterfewer
replicationscyclesasaresultofDNAdoublestrand
breaks,toxins,etc.
•Thisphenomenonisalsoknownas"replicative
senescence",the"Hayflickphenomenon",orthe
HayflicklimitinhonourofDr.LeonardHayflickwho
wasthefirsttopublishthisinformationin1965.

5. •InresponsetoDNAdamage(includingshortened
telomeres),cellseitherageorself-destruct
(apoptosis-programmedcelldeath)ifthedamage
cannotbeeasilyrepaired.
•Inthis'cellularsuicide',thedeathofonecell,ormore,
maybenefittheorganismasawhole.
•Forexample,inplantsthedeathofthewater-
conductingxylemcells(tracheidsandvessel
elements)allowsthecellstofunctionmoreefficiently
andsodeliverwatertotheupperpartsofaplant.
•Theonesthatdonotself-destructremainuntil
destroyedbyoutsideforces

6. Plants exhibit various types of
senescence
Senescenceoccursinavarietyoforgansandin
responsetomanydifferentcues.
Manyannualplants,e.g.wheat,maize,soybean,
abruptlyyellowanddiefollowinggrainproduction.
Senescenceoftheentireplantafterasingle
reproductivecycle=>monocarpicsenescence

7. Monocarpic senescence in soybean
Left:
Entire plant underwent senescence
after flowering and production of
fruits.
Right:
Plant remained green and vegetative
because its flowers were continually
removed.

8. Other types of senescence
1.Senescence of aerial shoots in herbaceous perennials
2.Seasonal leaf senescence (e.g. deciduous trees)
3.Sequential leaf senescence (e.g. leaves die when they reach a
certain age)
4.Senescence (ripening) of fleshy fruit; senescence of dry fruit
5.Senescence of specialized cell types (e.g. trichomes, tracheids)
6.Senescence of storage cotyledons and floral organs

9. Stages of flower senescence in morning glory

10. Types of plant senescence
Overall Senescence
Senescence occurs in
whole plant body, such
as annuals which
senesces to death after
flower and setting.

11. Top Senescence
The part aboveground dies with the end of
growth season,but the part underground is
alive for several years.
Perennial weeds , corm and bulb lily
In summer In winter

12. Deciduous senescence
The leaf falls in summer or
Winter-Deciduous trees

13. Progressive senescence
Senescence only occurs in older organ or
tissue.Neworgan or tissue develops while
old those are senescing.

14. Simple model for leaf growth and senescence

15. •Theinvolvementofdifferentsetsofgenesduringleafsenescence
affectedbyvarioussenescencefactors.
•Leafsenescenceisaffectedbyseveralfactorsandinvolvesthe
inductionofdifferentsetsofgenes.
•Apparentsymptomsofsenescencemaylookthesame,thedetailed
molecularstatesofsenescentleavesaredifferentdependingonthe
senescencefactors.

16. Triggers of senescence
Internal → monocarpic senescence
External → day length and temperature in autumnal leaf
senescence of deciduous plants
→ abiotic and biotic stress
Regardless of the initial stimulus, different senescence
patterns share common internal programs in which,
regulatory senescence genes initiate a cascade of secondary
gene expression that brings about senescence and death.

17. Three stages of Leaf Senescence

18. Physiological and biochemical events
Senescence is genetically encoded, allowing a predictable course of
cellular events.
Some organelles are destroyed while others remain active.
Chloroplast –first organelle to deteriorate during onset of leaf
senescence (destruction of thylakoid protein components and
stromatal enzymes)
Nuclei remain structurally and functionally intact until the late
stages of senescence
Senescent tissues carry out catabolic processes that require de novo
synthesis of
•proteases
•nucleases
•lipases
•chlorophyll-degrading enzymes

19. Senescence is an ordered series of Physiological
and biochemical events
Senescence down-regulated genes (SDGs)–their expression decreases
during senescence
e.g. photosynthetic genes
Senescence-associated genes (SAGs)–their expression is induced during
senescence
Group A:proteases, ribonucleases, lipases, ACC synthase, ACC oxidase
Group B: glutamine syntethase (converts NH
4
+
to glutamine, nitrogen
recycling from leaves)

20. Differential gene expression during leaf senescence
Senescencedown-regulatedgenes(SDCs)includechlorophylla/b-bindingprotein
gene(CAB),Rubiscosmallsubunitgene(SSU).
SAGs-expressionup-regulatedduringleafsenescence.
ClassISAGs-expressedonlyduringsenescence(senescence-specific).
ClassIISAGs-havebasallevelofexpressionduringearlyleafdevelopment,but
expressionincreasesduringsenescence.
Gan & Amasino (1997)

21. Physiological mechanism of Senescence on
Bio macromolecules

22. Physiology and biochemistry
Senescence-associatedgenes(SAGs)
Senescenceiscontrolledbyspecialgenes.
Twokindsofgenescanbefoundduringsenescence.
Senescence-downwardgenesmostofgenescode
enzymesrelevanttophotosynthesis,energymetabolism
andothersynthesis.
Senescence-upwardgenesmostofgenescodeenzymes
forhydrolase,suchasDNase,RNase,Protease,
phospholipase

23. Senescence-associatedgenesSAGsreferstotheir
mRNA levelsincreasewithsenescence
proceeding.Theyfunctioninmetabolismof
biomacromoleculedegradationandmobilization.
Morethan40geneshavebeencloned:
ProteasesinMaize,A.thaliana,rape.
SAG2,LSC7,SAG12,LSC790,LSC760,RNS1,RNS2,RNS3
inA.thaliana,
PEPC,MDH,MS,ICL,GAPDH,F-1,6-P,aldolaseand
β—galactosidaseinrape,cornandcucumber.

24. Degradationofbiomacromolecules
1.DNAdegradesRNAchangesinqualityand
quantity.
RNAbreakdownfasterthanDNAdoesduring
senescence,especiallyrRNA,whichismore
sensitivetosenescence.
RNaseactivity rises and DNA—RNA
polymerase activity declines.

25. 2.Protein synthesis decreases and its
degradation increases
Soluble protein-----Rubiscodecreases by 85%,
thylakoidmembrane protein decreases by 50%,
and cytochromef,balso decreases fast
3. Biomembranebreakdowns and loses its
function.

26. Senescence is a recycling process
Someofthereleasednutrients(N)
suchasnitrogenaretransportedto
developingseedsandyoungorgans
attheshootapex.
Senescenceproceedsfromleafmargins
towardthecenter.Cellssurroundingthe
vasculartissuessenescerelativelylateto
facilitatenutrientmobilizationfrom
adjacentsenescingcells.
Gan & Amasino (1997)
Plant Physiol. 113: 313

27. (a)Senescingleavescanberecognizedbytheircharacteristiclossofchlorophyll.
Often,thelastareasofaleafthatsenesceareclosetoveins,presumablybecause
theseareneededfornutrientexport.
Thetop-leftleafisjuststartingtosenesce;thebottom-rightleafisinthemost-
advancedstageofsenescence.
(b)Asaleafsenesce,nutrientssuchasnitrogen,phosphorusandmetalsare
reallocatedtootherpartsoftheplantsuchasdevelopingseedsandleaves.

28. A model for regulatory pathways in
flower senescence
PCDsignalisgeneratedbyboth
externalandinternalstimuliand
transducedbysomesignals
resultinghormonalimbalanceinthe
cell
Alteredlevelofhormonesactivates
severalcascadeandtranscriptional
regulation
Initiationofsenescencestartswith
expressionofseveralSAGslike
proteases, nucleases, wall
degradingandoxidativeenzymes
Laterstageofsenescence
symptomsbecomevisibleand
ultimatelyleadstocelldeathof
flowers

29. Program for plant senescence
Senescence can occur at different levels:
-cell, tissue, organs and whole plant.
Cell senescence
Membrane and organelle senescence
Senescence in cell membrane
Lipid phase change
Biomembranechanges -liquid-crystalline state to
solid-gel state.
Hard and inflexible, fluidity decreases and cohesion
increases.

30. Degradation and peroxidation of lipid lead to
decrease in lipid content
Synthesis ↓, lipase ↑,
Phospholipase lipoxygenase and active O
2↑
MDA (malonyldialdehyde) ↑

31. Phospholipid
↓phospholipaseAorB
Polydouble-boundfatacid
↓Lox(lipoxygenase)
Organicfreeradicals---?

32. Biomembranedegradation and leakage.
Loss equilibrium of ions and disorder of
metabolism
Organelle senescence
Ribosome and rough ER↓
chloroplasts breakdown
mitochondria cristaeswollen ↑
vacuole broken .
Autophagyoccurs and cell senesces and
degrades.

33. OrganSenescence
Leafsenescence
Photosynthesisdeclines-----slowerphaseand
fasterphase
Decreaseinactivityandcontentofphotosynthetic
keyenzyme(Rubisco)
Decreaseinactivityofphotoelectrontransportand
photophosphorylation.
Decreaseinstomatalconductance.
Decreaseinchlorophyll.Leafyellow.
Organelledegradation

34. Impactofsenescenceonplastidultrastructureinleavesofwild-type
andastay-greenmutantoftheC
3grassX.Festulolium.
(A)Priortosenescence,thechloroplastsofawild-typeandmutantplantscontainnumerous
grana,stacksofappressedthylakoidmembranes.(B)Theseinternalmembranestructuresare
lostduringsenescenceofawild-typemesophyllcell,andelectron-denselipiddroplets
knownasplastoglobuliaccumulate.(C)Retentionofintrinsicthylakoidmembraneproteins,
pigments,andotherhydrophobiccomponentsgivesthegerontoplastsofmutanttissuesa
distinctiveappearance,withpersistentgranastacksandfewplastoglobuli.

35. Chlorophyll a and its
breakdown products.
Subcellular compart-
mentation of the pheo-
phorbide, a pathway of
chlorophyll catabolism
in leaf mesophyll cells.

36. (A)Activityofkeychlorophyll-catabolizingenzymePaO(pheophorbideaoxygenase)is
stronglyinducedinsenescingtissuesofwild-typeX.Festuloliumbutundetectablein
presenescentleavesandinastay-greenmutant.(B)Inductionofchlorophylldegradationin
wild-typetissueisaccompaniedbylossofthepigment-bindingmembraneproteinLHCP,as
shownbyWesternblottinganalysis.InthemutantasecondformofLHCPIIprogressively
accumulatesassenescenceproceeds.Asillustratedinthecartoon,degradationofLHCPII
whichprotrudesfromthethylakoidmembraneintothestroma.(C)StabilityofRubisco,the
majorstromalprotein,isenhancedveryslightlyinthemutantcomparedtowildtype.

38. Seedaging
Theviabilityofseedlosesinverselyfrommatureto
death
Degradationandleakageofbiomembrane:
MitochondriaandERbecomeswollen,plasmic
membranecontactsanddepartfromcellwall.
DNAinjurybroken
Enzymeactivitydecreases:dehydrogenase
Storagematterexhausting,freefatacidrising.

39. Programmed cell death is a specialized type
of senescence
Senescence can occur at the level of:
whole plant (monocarpic senescence)
•organ (leaf senescence)
•cell (tracheary element differentiation)
Process whereby individual cells activate an intrinsic senescence program
= Programmed Cell Death (PCD)
In animals, PCD may be initiated by specific signals (errors in DNA
replication during division)
-involves expression of a characteristic set of genes, resulting in cell
death
-accompanied by morophological and biochemical changes
(apoptosis, Greek: “falling off”)
-during apoptosis, cell nucleus condenses and DNA fragments in a
specific pattern

40. Programmed cell death is a specialized type
of senescence
PCDinplants,lesswellcharacterized
-PCDoccursduringdifferentiationofxylemtrachearyelements,during
whichnucleiandchromatindegradeandcytoplasmdisappears→
activationofgenesencodingnucleasesandproteases
-protectionagainstpathogenicorganisms
-infectionbypathogencausesplantcellstoquicklyaccumulatehigh
concentrationsoftoxicphenoliccompoundsanddie(it’snotquiteas
simple)→deadcellsformsmallcircularislandofcelldeath(necrotic
lesion)
-necroticlesionsisolateandpreventinfectionfromspreadingto
surroundinghealthytissuesbysurroundingthepathogenwithatoxicand
nutritionallydepletedenvironment(hypersensitiveresponse)

41. Programmed cell death (PCD)
The organism controls the initiation and execution of
the cell death process, these types of cell death are
referred to as programmed cell death (PCD)
PCD can appear in all organelles of cell

42. Celldeathoccursin
almostallplantcellsand
tissues.
PCDisinvolvedin
numerousprocesses,including
thefollowingillustratedinthis
figuregameteformation,
including
Megasporeformation(1);
Embryo development(2);
Degenerationoftissuesinthe
seedandfruit(3);
Tissueandorgandevelopment
(4)through(6);
Senescence(7);and
Responsestoenviron-mental
signalsandpath-ogens(8and9).

43. The inflorescences of maize contain flowers that are initially bisexual, but PCD
results in the death of male or female tissues to give rise to female inflore-scence
(ear) or male inflorescence (tassel), respectively. In the tasselseed2 mutant
(A),female tissues in the tassel do not undergo PCD, and the resulting tasselflowers
are mostly pistilate. A wild-type tassel in included for comparison (B).

44. OnevisibleexampleofPCD
inplantsisseeninthe
ornamentalplantMon-stera
deliciosa.
Theleavesofthisplantexhibit
deepindentationsandholeinthe
lamina,whichresultfromthepro-
grammeddeathofspecificregions
oftissueinthedevelopingpromor-
dia.Astheleafexpands,these
areasarenotreplaced,andthe
resultingleaflaminahasthe
characteristicpatternthatinspires
thecommonname,‘‘Swisscheese
plant’’

45. Utilizationofgermplasm resistantto
senescence
selectionofvarietiesandcultivarsresistto
senescence
Transgenic plant for resistant to senescence
ACC synthase gene, nr,(ipt1, kn1)
Control of senescence

46. Tobacco plants over-expression of the kn1gene
•Overexpressionofkn1in
Tobacco
•35S:kn1 plantsare
characterizedbyareduction
inleafandplantsize,altered
leafshape,lossofapical
dominance,delayin
senescence,andformationof
ectopicmeristems.Atleftis
aleaffromanontransformed
plant;atrightisa35S:kn1
branch.

47. Salicylic acid has a role in regulating gene
expression during leaf senescence
Morris et al. 2000
Plant J 23: 677
pad4
ArabidopsismutantsdefectiveinSA-signaling:
npr1=NONEXPRESSER OFPRGENES1(ankyrinrepeatprotein)
pad4=PHYTOALEXIN-DEFICIENT4(lipase-likeprotein)
NahG=expressessalicylatehydroxylase,unabletoaccumulateSA
Asleavesstartedtosenesce,chlorosisbecame
visibleattheedges,graduallyspreadingupthe
leaf.Thischlorosiswasrapidlyfollowedby
necrosis,thesenescingtissuenormallydyingwell
beforetheentireleafbecamechlorotic.
Plants grown at 12h L/8h D.
pad4showeduniformchlorosisoverthe
entireleaflamina,butnegligiblesignsof
necrosis.
Effectofpad4mutationappearstobeto
delayoreveninhibitnecrosisinthe
senescingtissue.