The  growth  and  development  of  a  plant  are  influenced  by:    

• Gene:c  factors  • External  environmental  factors  • Chemical  hormones  inside  the  plant  

 Receptor   Ac:va:on  or  repression  of  sets  of  genes  in  the  nucleus  

Secondary  messengers  mediate  the  phytohormone  responses  

1.  Involve  in  the  transfer  informa:on  from  sources  to  targets  

2.  Amplify  the  signal  produced  by  the  phytohormone  

 Hormone,  from  the  Greek  word  horman,  meaning  "to  s:mulate”.    

Hormones  control    • Growth    • Development  • Movement.  

Phytohormones  

Plant  hormones  are  chemical  messengers  

Hormones  are  organic  compounds  that  are  effec:ve  at  very  low  concentra:on    (1g  20,000  tons-­‐1)  

Usually  synthesized  in  one  part  of  the  plant  and  are  transported  to  another  loca:on.      

They  interact  with  specific  target  :ssues  to  cause  physiological  responses  • Growth  • Fruit  ripening  

Each  response  is  oWen  the  result  of  two  or  more  hormones  ac:ng  together.  

Phytohormones  

• Hormones  s:mulate  or  inhibit  plant  growth  

• Growth  regulators  rather  than  s:mulators.    

Major  groups  of  hormones:    1.  Auxins    2.  Gibberellins  3.  Ethylene  4.  Cytokinins  5.  Abscisic  acid  6.  Brassinostereoids  7.  Salicylic  acid  8.  Polyaminas  9.  Jasmonates  10. Systemin  11. Nitric  oxide  

Phytohormones  

Results on growth of coleoptiles of canary grass and oats suggested that the reception of light in the tip of the shoot stimulated a bending toward light source.  

EARLY EXPERIMENTS ON PHOTROPISM SHOWED THAT A STIMULUS (LIGHT) RELEASED CHEMICALS THAT INFLUENCED GROWTH

Phytohormones  

Auxin  causes  several  responses  in  plants:  

       *  Bending  toward  a  light  source  (phototropism)  

       *  Downward  root  growth  in  response  to  gravity  (geotropism)  

       *  Promo:on  of  apical  dominance  

       *  Flower  forma:on  

       *  Fruit  set  and  growth  

       *  Forma:on  of  adven::ous  roots  

       *    Differen:a:on  of  vascular  :ssues  (de  novo  or  repairing  existent  vascular  :ssue)    

Precursors  of  hydathodes  produced  auxins  

Auxins  

Addi:on  of  auxins  produce  parthenocarpic  fruit.  

Natural  or  ar:ficially  induced  produc:on  of  fruit  without  fer:liza:on  of  ovules.  

Stenospermocarpy    Biological  mechanism  that  produces  seedlessness  in  some  fruits  (many  table  grapes,  watermelon)  

diploid  +  tetraploid  parent  =  triploid    seeds  

vegeta6ve  parthenocarpy  Plants  that  do  not  require  pollina:on  or  other  s:mula:on  to  produce  parthenocarpic  fruit  (cucumber)    

Auxins  

polar  auxin  (IAA)  transport  

Acropetal  transport  

Basipetal  transport  

• Passive  diffusion  • Ac:ve  transport  

Auxins  

Synthe:c  auxins    

Widely  used  in  agriculture  and  hor:culture  •  prevent  leaf  abscission  •  prevent  fruit  drop  •  promote  flowering  and  frui:ng  •  control  weeds  

Agent  Orange  -­‐  1:1  ra:o  of  2,4-­‐D  and  2,4,5-­‐T  used  to  defoliate  trees  in  Vietnam  War.    

Dioxin  usually  contaminates  2,4,5-­‐T,  which  is  linked  to  miscarriages,  birth  defects,leukemia,  and  other  types  of  cancer.      

Auxins  

In  1964,  zea:n  was  the  first  naturally  occurring  cytokinin  isolated  from  corn.  

Zea:n  and  zea:n  riboside  are  found  in  coconut  milk.    

All  cytokinins  (ar:ficial  or  natural)  are  chemically  similar  to  adenine.  

Cytokinin  from  Cytokinesis  

Natural  

Synthe:c  

Cytokinins  

Cytokinins  

Callus  development  <[auxin]  =  roots  <[kine6n]  =  buds  

• shoot  ini6a6on  • roots  • seeds    • fruits  • young  leaves    • chloroplast  development  

Lateral  bud  development  

Move  non  polarly  in  xylem,  phloem,  and  parenchyma  cells.    

Ethylene    • Is  found  only  in  the  gaseous  form  • Induces  ripening  (e.g.,  green  bananas).  • Epinasty  (causes  leaves  to  droop  in  the  fall  )    • Non  polarly  in  xylem,  phloem,  and  parenchyma  cells.    • Causes  abscission  of  fruits  and  flowers  

Ethylene  was  first  detected  as  flower  buds  opened  and  exhibited  diurnal  oscilla:ons  with  peak  produc:on  prior  to  petal  abscission  

Plants  oWen  increase  ethylene  produc:on  in  response  to  stress  

Is  found  in  high  concentra:ons  within  cells  at  the  end  of  a  plant's  life.  

Methionine  is  the  precursor  of  ethylene  

Most  produced  organic  compound    107  million  metric  tons  in  2005  

Ethylene    

Once  fruit  ini:ated  red  colora:on,  ethylene  release  becomes  elevated  in  a  linear  fashion  (without  diurnal  fluctua:ons).  

Abscisin  is  made  from  carotenoids  and  moves  non-­‐polarly  through  plant  :ssue.    

Abscisic  acid  (ABA)  is  a  general  plant-­‐growth  inhibitor.    

Induces  dormancy  Prevents  seeds  from  germina:ng    

High  concentra:ons  of  ABA  in  guard  cells  during  periods  of  drought  stress  play  a  role  in  stomatal  closure  

Abscisic  acid  (ABA)    

Viviparous  mutants  Reduced  sensi:vity  for  ABA  

In  1930's,  Ewi:  Kurosawa  and  colleagues  were  studying  plants  suffering  from  bakanae,  or  "foolish  seedling"  disease  in  rice.    

• Synthesized  in  apical  por:ons  of  stems  and  roots  • Transport  is  non-­‐polar  

Can  be  readily  obtained  in  large  quan::es  from    fermenta:ons  of  the  fungus  Gibberella  fujikuroi  

• cell  division  and  elonga:on  • break  seed  dormancy  • speed  germina:on  

Gibberellins  

Thompson  seedless  grapes  (Vi/s  vinifera)  

bol6ng  

Gibberellins  

External  factors  and  plant  growth.  Tropic  responses  

Tropic  responses  Direc:onal  movements  by  growth  in  response  to  a  direc:onal  s:mulus  

Phototropism  is  a  response  to  blue  light  Phytphormone:  Auxin    Second  mesenger:  Calcium  

Geotropism  or  gravitropism  Phytphormones:    cytokinin  and  auxin  

Starch  -­‐  statolith  hypothesis:  Gravity  has  caused  amyloplast  to  fall  inside  this  statocyte,  lodging  them  against  other  cell  components.    Protoplast  pressure  hypothesis:  

The  weight  of  the  en:re  mass  of  the  protoplast  is  involved  in  the  percep:on.  

Tesengrity  model  

Tension  between  structural  components  

External  factors  and  plant  growth.  Geotropism  

Thigmotropism  is  direc:onal  growth  response  to  contact  with  an  object.  

tendrils  

Greek  thigma  =  touch  Rapid  response  Diferen:a:on  in  the  elonga:on  of  the  cells  

External  factors  and  plant  growth.  Thigmotropism    

External  factors  and  plant  growth.  Nas6c  movements  and  heliotropism    

Nas:c  movements  are  independent  of  the  s:mulus'  posi:on  

Touch,  mechanical,  electrical,  thermal  or  chemical    

In  Mimosa  pudica.    change  in  the  osmo:c  poten:al  (turgor)  of  cells  of  the  pulvinus  due  to  the  movement  of  K+  ions  and  contrac:on  of  the  cytoskeleton  1cm/sec.  

Venus  Flytrap,  Dionaea  muscipula.  Acid  growth  (cells  that  to  elongate  or  expand  quickly  at  low  pH)  and  turgor  (>  1  second)  

External  factors  and  plant  growth.  Heliotropism  

Heliotropism  is  the  diurnal  mo:on  of  plant  parts  (flowers  or  leaves)  in  response  to  the  direc:on  of  the  sun.  

During  the  night,  the  flowers  may  assume  a  random  orienta:on    

Heliotropism  is  a  response  to  blue  light.  

External  factors  and  plant  growth.  Circadian  rhythms  

Roughly  24-­‐hour  cycle  in  the  biochemical,  physiological,  or  behavioral  organisms’  processes    

Hours  of  light  

Endogenous  and  synchronized  by  the  environment  Biological  clock  

Photosynthesis  Movement  

Night  Day  

According  to  their  photoperiods,  plants  can  be  classified  into  three  groups:  

a)  Short  day  plants.  Examples:  Nico7ana  tabacum,  soybean,  strawberry,  and    chrysanthemum.  

b)    Long  day  plants.  Examples:  Onion,  carrot  and  spinach.  c)  Day  neutral  plants.  Examples:  Tomatoes,  cucumbers,  sunflower,  dandelions  and  

coqon.    

Phytocromes  are  the  photoreceptors  for  photoperiodism  

External  factors  and  plant  growth.  Photoperiodism