Core concept: hormone ratio and organ formation

In classic tobacco pith callus experiments, the relative levels of auxin and cytokinin determine whether the callus forms roots, shoots, or remains callus.

• A high auxin to cytokinin ratio favours rhizogenesis, i.e., adventitious root formation.

• A low auxin to cytokinin ratio favours caulogenesis, i.e., shoot formation.

• Intermediate or balanced levels generally maintain callus without clear organs.

Therefore, if the auxin to cytokinin ratio is increased (more auxin relative to cytokinin than before), the system moves toward the “root side” of this balance, causing adventitious roots to develop from the callus.

Option-by-option explanation

1. Adventitious roots will form – CORRECT

• Increasing the auxin to cytokinin ratio means either raising auxin, lowering cytokinin, or both.

• Auxin promotes root primordia initiation; when it dominates over cytokinin, callus cells differentiate into root tissues, giving adventitious roots on the callus surface.

2. Adventitious shoot will form – INCORRECT

• Adventitious shoots require the opposite hormonal environment: relatively high cytokinin and low auxin.

• When auxin is higher relative to cytokinin, shoot bud formation is suppressed, so this option contradicts the known response.

3. There will be no root formation – INCORRECT

• If auxin is increased relative to cytokinin, root formation becomes more likely, not less.

• “No root formation” would be expected either at very low auxin (favouring shoots) or at a hormone balance that maintains undifferentiated callus.

4. There will be no shoot formation – PARTIALLY TRUE BUT INCOMPLETE

• It is correct that increasing auxin to cytokinin ratio tends to inhibit shoot formation, but the key observable effect tested in this question is positive organogenesis (what forms), not what fails to form.

• Since the hallmark response is appearance of adventitious roots, option (1) is the best and fully accurate choice, while option (4) only gives a negative consequence and ignores root induction.

SEO‑optimized introduction (for article use)

In plant tissue culture, organogenesis from tobacco pith callus is controlled by the auxin to cytokinin ratio in the culture medium. When this ratio is increased so that auxin dominates over cytokinin, the hormonal environment favours adventitious root formation, whereas lower auxin relative to cytokinin promotes shoot regeneration from the same callus.