LECTURE 21- Hormone: regulators of development III

what are the different kinds of cytokinins

• Adenine: part of DNA

• Kinetin

• Zeatin

• 6-benzylamino purine (BAP)

• Isopentenyl adenine (i^6 Ad)

whats the most common cytokinin in plants

zeanine

where is cytokinin synthesised

primarily in root tips

how is cytokinin transported

transported in xylem from roots to shoots

what are the effects of cytokinin

• Cell division

• Shoot formation

• Leaf senescence

• Apical dominance, root development

how do cytokinins and auxins regulate plant tissue

cytokinins are responsible for shoot induction

auxin is responsible for root induction

• people use cytokinins and auxin for plant cloning and propagation

what happens when both cytokinin and auxin is absent from plant tissue

plant tissue will die

what happens when there is too much auxin in plant tissue

more auxin = roots may develop

too much auxin = becomes toxic. plant stops developing

what happens when there is too much cytokinin in plant tissue

more cytokinin = shoot formation, but no callus growth

too much cytokinin (no auxin) = no root tissue formation

what happens when there is too much cytokinin and auxin in plant tissue

cytokinin + auxin = callus growth and cell division

too much = shoots won’t grow from too much auxin, and cytokinin suppresses root induction

relationship between cytokinin and senescence

cytokinin production only increases in old leaves, which keeps old leaves alive

• cytokinin can prevent senescence (aging)

what is ethylene made of

a gas made of two C atoms and hydrogen double bound together

what is the site of biosynthesis for ethylene

responsive to stress (tissues ripening or senescence)

how is ethylene transported

moves through diffusion

• because ethylene is a gas, it can move out of the plant

what are the effects of ethylene

• Fruit ripening (especially apple, bananas and avocados)

• leaf/flower senescence

• leaf/fruit abscission

what is the precursor of ethylene biosynthesis

methionine

what is the intermediate of ethylene biosynthesis

1-aminocyclopropan-1-carboxylic acid (ACC)

• non-volatile

• converted into ethylene once transported to necessary site in plant

what are etiolated seedlings

when plants are grown in the dark, the leaves stay closed and the hypocotyl gets really long as it tries to find sunlight

what happens when there’s an increase in ethylene in etiolated seedlings

triple response occurs:

• Hypocotyl shortens

• Hypocotyl thickens

• Hypocotyl bends over

what happens to mutants in the presence of ethylene

when plants are grown with ethylene, they can become ethylene resistant (etr) and receptors will stop recognising and responding to ethylene. plant hypocotyl will bend and shorten

how does ethylene affect fruit ripening

fruits like bananas, avocados, or apples will produce lots of ethylene and cause the fruit to ripen quickly.

• ripening triggers more ethylene production

• surrounding fruits will also ripen

what does abscisic acid look like

Molecule has acid group and is also hydrophobic (can cross membrane)

chemical nature of abscisic acid

synthesised from carotenoids

what is the site of biosynthesis for abscisic acid

• Important in water stress

• Found in mature leaves and roots

• Ensures stomata is closed

how is abscisic acid transported

• Leaves: exported through phloem

• Roots: exported through xylem

what are the effects of abscisic acid

• Stomata closure

• Induction of photosynthate transport from leaves to developing seeds

• Storage-protein synthesis in seeds

• Embryogenesis

• Seed dormancy

what is the chemical nature of gibberellins

fungal product

what is the site of biosynthesis for gibberellins

• Young tissues of shoots

• Developing seeds

how are gibberellins transported

• Unknown

• Most likely transported through xylem and phloem

what are the effects of gibberellins

• Cell elongation

• Germination

• Elongation of maize and cereals

gibberellins can cause death in plants like rice. without gibberellins, plants are shorter and stronger and better protected against wind

what hormone induces leaf abscission

ethylene

• ethylene mixed with auxin can cause death in abscission layer