G&D 4

CYTOKINESIS-

stimulate cell division

Cytokinins

Discovered* through efforts to find substances which stimulate cell division and promote buds/branching in callus culture

purine, adenine

isolated from degraded herring sperm DNA

Named – KINETIN (a by-product of degraded adenine)

naturally occurring cytokinin (1)

ZEATIN

Synthesized primarily in roots (especially in actively dividing cells i.e., in root apical meristem); but also in immature seeds and developing fruits

--Transported from roots to above ground parts primarily through xylem

Synthetic cytokinin (2)

Benzyladinine (BA)

Thidiazuron

Actions of Cytokinins:

Promote cell division-Cyto-kinesis

(Cytokinins affect/regulate activities of certain enzymes that catalyze various steps of the cell-cycle)

Growth and developmental effects of cytokinins in Horticulture

Promote lateral bud formation

Delay senescence

crown gall

Tumor/cancer (crown gall) caused by Agrobacterium tumefaciens

Transfers its T-DNA segment (and the genes*) into host plant’s DNA

*genes for cytokinin synthesis

genes for opine synthesis

Infected cells provide the tumor / gall for bacteria to thrive and enjoy good food

how do Scientists exploit crown gall

Bacteria trick the plant cells into playing a hospitable host: role of cytokinins

Scientists exploit this trick property of Agrobacterium by using it as a vehicle to transfer ‘genes of interest’- transgenic plants (GMOs) - 2013 World Food Prize

‘Gene editing’ - ‘CRISPR Cas9’

edits the sequence of a selected native gene (targeted deletion / insertion of DNA sequences / point mutation)

no transfer of a foreign gene(s) – less controversial

How CRISPR works

gene editing via CRISPR/Cas9 still uses transgenic approach, i.e. the inserting the gene editing apparatus

But the editing occurs at a precise location in the native gene (transformed)

The gene editing apparatus can then be removed by subsequent crossing while the desired mutation/editing of a target gene is maintained

Cytokinins Lateral bud formation and axillary branching

– Plant tissue culture

• Callus

• Individual shoots

•entire plant is produced

Auxin-Cytokinin Balancing Act ORGANOGENESIS

Multiple shoots produced when cytokinin is present

ORGANOGENESIS

Individual shoots transferred and grown on a medium with different hormonal composition (Auxins) to initiate ROOT FORMATION

Auxin-Cytokinin Balancing Act

Auxin-induced apical dominance can be overcome by cytokinin

think of witchs broom

• Extreme axillary branching

• Believed to stimulate overproduction of cytokinins which antagonizes auxins

• Apical dominance is not only countered but the lateral buds go crazy!

Delay senescence -cytokinins