Samenvatting Plant physiology H5: Cytokinins
Introduction:
• Balance between auxin & kinetin
o CK/IAA ratio high → Shoot differentiation
o CK/IAA ratio intermediate → callus
o CK/IAA ratio low → Root differentiation
• Kinetin = synthetic cytokinin / Zeatin = natural cytokinin
• 2 types of naturally occurring cytokinins
o Isoprenoid cytokinins → most abundant
o Aromatic cytokinins → in some plants
• Adenine is the backbone of CKs
Cytokinin Biosynthesis: → Not 1 major pathway
• Terpenoid biosynthesis brings the buildingblocks → Dimethylallyl diphosphate
(DMAPP) as precursor of cytokinin biosynthesis
• Adenosine phosphate-isopentyltransferase (IPT) catalyzes first step to form iP
precursors
o CYP735A converts the iP precursors into trans-zeatin (tZ) precursors
o Then there are 2 ways to get active iP or tZ
▪ 2-step pathway
• Dephosphorylation (nucleotide → nucleoside)
• Deribosylation (deglycosylation)
▪ 1-step pathway → via LONELY GUY (LOG) which directly catalyzes CK
activation
, • Interconversion between nucleotide & nucleoside precursor & cytokinins
o LOG is specific, others not
• Direct conversion of trans-zeatin (tZ) to dihydrozeatin (DZ) catalyzed by zeatin
reductase
• CK can also be produced from degradation of prenylated tRNA by tRNA-IPT through
CK precursor DMAPP → forms cis-Zeatin (cZ)
• There are different modified CK (conjugates = inactive CKs that have increased
stability and is seen as storage forms of active CKs)
• Cytokinin oxidase/dehydrogenase (CKX) can degrade CKs → irreversible
Introduction:
• Balance between auxin & kinetin
o CK/IAA ratio high → Shoot differentiation
o CK/IAA ratio intermediate → callus
o CK/IAA ratio low → Root differentiation
• Kinetin = synthetic cytokinin / Zeatin = natural cytokinin
• 2 types of naturally occurring cytokinins
o Isoprenoid cytokinins → most abundant
o Aromatic cytokinins → in some plants
• Adenine is the backbone of CKs
Cytokinin Biosynthesis: → Not 1 major pathway
• Terpenoid biosynthesis brings the buildingblocks → Dimethylallyl diphosphate
(DMAPP) as precursor of cytokinin biosynthesis
• Adenosine phosphate-isopentyltransferase (IPT) catalyzes first step to form iP
precursors
o CYP735A converts the iP precursors into trans-zeatin (tZ) precursors
o Then there are 2 ways to get active iP or tZ
▪ 2-step pathway
• Dephosphorylation (nucleotide → nucleoside)
• Deribosylation (deglycosylation)
▪ 1-step pathway → via LONELY GUY (LOG) which directly catalyzes CK
activation
, • Interconversion between nucleotide & nucleoside precursor & cytokinins
o LOG is specific, others not
• Direct conversion of trans-zeatin (tZ) to dihydrozeatin (DZ) catalyzed by zeatin
reductase
• CK can also be produced from degradation of prenylated tRNA by tRNA-IPT through
CK precursor DMAPP → forms cis-Zeatin (cZ)
• There are different modified CK (conjugates = inactive CKs that have increased
stability and is seen as storage forms of active CKs)
• Cytokinin oxidase/dehydrogenase (CKX) can degrade CKs → irreversible
