Practice question

Caulogenesis is specific morphogenic pathway leading to shoot formation in vitro, term derived from caulis meaning stem. As branch of organogenesis it requires high cytokinin to low auxin ratio that suppresses root program and promotes shoot identity. At cellular level cytokinins like BAP, kinetin, zeatin bind histidine kinase receptors AHK2/3/4 initiating phosphorelay through AHP proteins to activate type-B Arabidopsis Response Regulators ARR1, ARR10, ARR12. These upregulate WUSCHEL organizing center, SHOOT MERISTEMLESS STM and CLAVATA3 feedback maintaining stem cell pool while inducing CYCD3;1 driving division in peripheral zone where leaf primordia initiate. Low auxin prevents activation of root specifiers LBD16 and ARF7. Histologically caulogenic focus organizes dome-shaped shoot apical meristem with L1, L2, L3 layers, leaf primordia and procambium differentiating from surrounding parenchyma. Practically caulogenesis underlies shoot multiplication phase of micropropagation, enabling exponential increase in shoot number per subculture before rooting, widely used for timber, ornamental and horticultural crops achieving commercial scale clonal fidelity.