Practice question

Low auxin combined with high cytokinin directs differentiation toward shoot regeneration, fundamental to shoot multiplication media. Cytokinin dominance activates signaling cascade: cytokinin binding to sensory histidine kinases AHK2, AHK3, AHK4 in endoplasmic reticulum triggers histidine-to-aspartate phosphorelay via AHPs to nucleus where type-B ARR transcription factors ARR1, ARR10, ARR12 switch on genes for shoot meristem identity. Key targets include WUSCHEL maintaining stem cell pool, SHOOT MERISTEMLESS KNOX gene preventing differentiation and CYCD3;1 promoting cell cycle in peripheral zone. Simultaneously cytokinin represses auxin transport by downregulating PIN1 expression in shoot apex and upregulates type-A ARR which negatively modulates cytokinin oversignaling creating balanced meristem size. Reduced auxin prevents lateral root program mediated by ARF7/19 and LBD16 from dominating. Morphologically resulting tissue organizes shoot apical meristem dome with leaf primordia and provascular strands. Routinely formulation containing BAP 2-5 mg per L and NAA 0.1-0.2 mg per L achieves caulogenesis across diverse species including banana, apple, rose, coffee enabling rapid multiplication.