Practice question

The concept of auxin cytokinin interaction governing morphogenesis was established by Skoog and Miller using tobacco pith cultures, showing distinct concentration ratios produce different developmental outcomes. Intermediate ratio where both hormones are present in approximately equal proportions sustains cell division without organ determination, leading to formation of unorganized callus mass. Mechanistically balanced signaling maintains expression of D-type cyclins particularly CYCD3;1 and cyclin dependent kinases driving G1 to S transition while preventing dominant activation of shoot promoting type-B ARRs or root promoting ARF-LBD module. Auxin supports cell expansion through acid growth and induction of expansins and cell wall biosynthesis enzymes, while cytokinin enhances cytokinesis by regulating phragmoplast formation and suppressing KRP inhibitors. Neither pathway overwhelms the other, so cells stay in dedifferentiated meristematic condition comprising thin-walled parenchyma with dense cytoplasm and prominent nuclei capable of later redifferentiation. This callus state provides reservoir of totipotent cells exploited for subsequent directed differentiation into shoots, roots or embryos upon changing hormonal balance in subculture sequence.