Practice question

Cytokinins constitute class of N6 substituted adenine derivatives including natural trans zeatin, isopentenyl adenine and synthetic benzylaminopurine BAP, kinetin, that primarily drive shoot proliferation, cell division and delay senescence. Perception occurs through membrane localized histidine kinase receptors AHK2, AHK3, AHK4 autophosphorylating upon cytokinin binding, transferring phosphate to histidine phosphotransfer proteins AHPs that shuttle to nucleus phosphorylating type-B Arabidopsis Response Regulators ARR1, ARR10, ARR12 transcription factors. Type-B ARRs directly induce expression of WUSCHEL maintaining stem cell niche, SHOOT MERISTEMLESS and KNAT1 KNOX genes preventing differentiation, and D-type cyclin CYCD3;1 accelerating G1 to S transition. Cytokinins also antagonize apical dominance by promoting axillary bud outgrowth and counteract ethylene and ABA induced senescence by stabilizing chloroplasts and photosynthetic enzymes. In tissue culture BAP 1-5 mg per L induces multiple shoots from single explant enabling exponential multiplication; high cytokinin low auxin ratio ensures caulogenesis while balanced ratio maintains callus. Thus cytokinins are essential tool for commercial clonal propagation of orchids, banana and forestry species.