Principle: hormone balance and organogenesis

In plant tissue culture, the ratio of cytokinins to auxins largely determines morphogenesis from callus. A high cytokinin / low auxin ratio favors shoot bud formation (caulogenesis), while a high auxin / low cytokinin ratio promotes root formation (rhizogenesis); intermediate or balanced levels encourage callus proliferation without clear organ formation. This classic rule underlies the answer choices in the question and is widely used in micropropagation protocols.

Why option (1) is correct: low auxin and high cytokinin

When the callus (undifferentiated mass of cells) is transferred to a medium with high cytokinin and low auxin, cells are pushed toward shoot meristem differentiation. Cytokinins (e.g., BAP, kinetin, zeatin) stimulate cell division in shoot-forming regions and promote the development of shoot primordia, while a small amount of auxin maintains cell competence without driving root or callus dominance. Therefore, to initiate shoots from undifferentiated tissue, the medium must contain low auxin and high cytokinin.

Why the other options are incorrect

1. (2) High auxin and high cytokinin

   • When both hormones are present at high and roughly similar levels, the balance often supports vigorous callus growth rather than clear organ formation.

   • Such media are typically used for callus induction and maintenance, not for directed shoot regeneration. Shoots may be few or abnormal, so this is not considered the “must contain” combination for reliable shoot initiation.

2. (3) High auxin and low cytokinin

   • A high auxin / low cytokinin ratio shifts development toward root formation and/or compact callus, a condition frequently used for rhizogenesis after shoots have been formed.

   • This is essentially the opposite of what is needed for initiating shoots from callus, so option (3) is characteristically used for roots, not shoots.

3. (4) Low auxin and low cytokinin

   • With both hormones low, the culture often shows little morphogenetic response: callus may stop proliferating, or explants simply survive with minimal growth.

   • Without a sufficiently strong cytokinin signal, shoot buds rarely initiate from undifferentiated tissue, so this combination is unsuitable for shoot induction.

SEO‑oriented introduction (for article use)

In plant tissue culture, successful shoot initiation from callus depends critically on the auxin–cytokinin ratio in the regeneration medium. For most species, a medium containing low auxin and high cytokinin is ideal because it redirects undifferentiated callus cells toward shoot meristem formation, whereas higher auxin levels tend to favor roots or callus proliferation rather than organized shoot development.