Cytokinins, Ethylene and Abscisic Acid | Biology for ACT PDF Download

Introduction of Cytokinins (CK)

• Cytokinin was discovered by Miller when he was working (in lab. of prof. Skoog) on tobacco pith culture. He added the contents of an old DNA-bottle (Herring fish sperm DNA) to the culture medium & observed that the tobacco pith callus could grow for longer period.

• Miller isolated an active substance from autoclaved DNA from Herring sperm, which stimulated cell division.

• He named this substance as kinetin.

• Term cytokinin By Letham, Phytokinin by Osborne and Kinin by Skoog. 

• The first natural cytokinin was identified & crystalized from immature corn grains by Letham & named as Zeatin.

• The most common cytokinin in plants are isopentenyl adenine zeatin.
  Structure of Cytokinin

• BAP (Benzylaminopurine), diphenylurea and thidiazuron are synthetic cytokinins.

• Cytokinin is a derivative of adenine base. 

• Root tips are major site of synthesis of CK (by mevalonic acid pathway). 

• Movement of cytokinin is polar &  basipetal.

• Coconut milk factor also performed activity like cytokinin, thus used in tissue culture.

• Zachau obtained cytokinins from serine-tRNA of yeast.       

Physiological Effects and Applications of Cytokinins                           

(2) Formation of interfascicular cambium and induce secondary growth.

(3) Morphogenesis: Morphogenetic changes induced by CK in presence of IAA.

High auxin + low CK - Root formation

(4) Counteraction of apical dominance: promotes growth of lateral buds.

(5) Breaking the dormancy of seeds: Like GA the dormancy of certain seeds can be broken by CK.

(6) Seed germination: Seeds of parasite plant (Striga) can germinate in the absence of host by CK treatment.

(7) Delay in senescence: (Richmond Lang Effect) The ageing process of leaves usually accompanies with loss of chlorophyll & rapid catabolism. This is called as senescence. senescence postponed by CK. (increase short life of plant parts)

(8) Lignin biosynthesis

(9) Parthenocarpy in some fruits

(10) Proplastids modification

(11) Phloem conduction (nutrients mobilisation) 

(12) Femaleness

(13) Flowering in SDP (also in long days)

(14) Induced stomatal opening

Bio–assay

(1) Tobacco pith cell division test 

(2) Chlorophyll preservation (retention) test (delay in senescence test) 

(3) Soyabean and Radish cotyledon cell division test.

Introduction of Ethylene

•  H.H Cousin first suggested, that ripened oranges are responsible for ripening of unripe bananas.

•  Ethylene is a gaseous pollutant hydrocarbon, but Burg reported it as a fruit ripening hormone.

• Pratt Goeschl – Recognized ethylene as a natural plant growth regulator.

•  Biosynthesis of ethylene takes place by methionine amino acid. Ethylene is synthesized in large quantity by ripening fruits and senescent organs. *

• Ethylene also formed in roots in water logged condition.

Physiological Effects and Applications of Ethylene

(1) Post harvest ripening of fruits: Citrus, oranges, banana, apple, tomato. today ethephon/CEPA (Chloroethyl Phosphonic acid) used at commercial level.

(2) Stimulation of senescence & abscission of leaves: Ethylene is synthesized in large quantity by ripening fruits and senescent organs.

(3) Flowering in pineapple

(4) Triple response on stem:

• Inhibition of stem elongation
• Stimulation of radial swelling of stem
• Horizontal growth of stem (ageotropism)

(5) Inhibits root growth: Ethylene is inhibitor of root growth but stimulates the formation of root hairs.

(6) Epinasty of leaves

(7) Femaleness

(8) Tightening of hooks of epicotyl and hypocotyl

(9) Inhibits the polar movement of auxin

Introduction of Abscisic Acid (ABA)

• First indication of growth inhibitors was given by Osborne.

• First growth inhibitor was identified by Bennet-Clark and Kefford (1953) from dormant potato tuber and called it b-inhibitor.

• Addicott & Okhuma (1963) obtained from mature cotton fruits and named as Abscisin II.(C15H20O4)

• Waring & Robinson – Isolated a growth inhibitor from old Betula leaves & called as dormin.
  Leaf Abscission

•  Later establised that b-inhibitor, Abscisin-II and dormin are same and called as Abscisic acid.

•  ABA is the most wide spread growth inhibitor in plants.

• ABA synthesized by Mevalonic acid pathway & oxidation of carotenes in chloroplasts.

• ABA also known as stress hormone, because it protects plants from adverse conditions like water stress. ABA increases tolerance of plants to various type of stresses.

Physiological Effects and Applications of

 Abscisic Acid

(1) Induce abscission: ABA causes ageing and abscission of leaves & fruits (antiauxin) (cellulase & pectinase genes induced by ABA)

(2) Induce bud & seed dormancy: ABA regulates (anti–GA) bud & seed dormancy.

ABA plays a major role in seed maturation enabling seeds to become dormant.

(3) Induce senescence:  ABA accelerates senescence of leaves.

(4) Inhibition of cell division & cell elongation: anti CK. 

(5) Stomatal closing: ABA causing the stomatal closing under the water stress conditions. Increases resistance to frost injury. (anti transpirant & stress hormone)

(6) Delaying of flowering in LDP

(7) Tuberisation in potato

(8) Inhibitor of a–amylase synthesis: Inhibition of seeds germination.

(9) Geotropism in roots

Old NCERT Syllabus

Other growth regulating substances

Wound hormone Traumatic Acid :- Induce callus formation on injured parts (healing of wounds) Chemically traumatic are auxin like substance.

Calines (Formative hormones)

(i) Rhizocalines :- Produced by leaves & induce formation of roots.

(ii) Caulocalines :- Produced by roots & induce formation of stem.

(iii) Phyllocalines :- (Self forming hormone) produced in cotyledons & leaves, induce division of leaf mesophylls.

Morphactins or (HFCA) These are, synthetic growth inhibitors, which are polyvalent (wide range) in action

(i) Inhibition of internode elongation

(ii) Reduction of apical dominance & promotion of lateral branching

(iii) Reduces the laminar area of leaf.

(iv) Abolition of phototropism

•  Chlormequat (CCC or Cycocel):- Growth inhibitor, which is used in bonsai.

•  Alar - 85 (B-Nine):- in floriculture

•  Agent orange :- Mixure of 2,4-D & 2,4,5-T used in bio–war. (Used by U.S.A. in Vieatnam war)

• Amo - 1618 :- in biowar.

•  Phosphon-D, Cycocel, Amo-1618, Alar-85, Ancymidol (A-REST) are antigibberellins and cause inhibition to stem growth.