- The plant hormones, auxins and cytokinins, and their interactions play an important role in regulating apical dominance_ The following figure represents a” experiment related to the study of gene interactions that influence axillary bud outgrowth or dormancy. Q,Z and M represent genes involved Phytohorrnone pathway.
Based on the above figure, the following statements were made:
- ‘X’ is an auxin that maintains expression of ‘Q’ and ‘Z’ and represses ‘M’.
B. ‘Y’ is a cytokinin that promotes axillary bud growth and is induced by ‘M’.
C Decapitation (removal of apex) activates}
D. ‘X’ is a cytokinin that represses ‘M’. ‘X’
Which one of the following options represents correct staternent(s)?
(1) A and C only (2) B and D only
(3) A and B only (4) C only
Quick reconstruction of the figure logic
From the figure (DEC 2018 CSIR pattern):
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With intact apex and hormone X present, genes Q and Z are “on” and M is “off”; axillary bud remains dormant.
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After decapitation, X is removed, Q and Z go “off” and M turns “on”; bud grows.
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Supplying Y (a second hormone) restores bud growth even in conditions where X is low/absent, indicating that Y is a cytokinin induced by M and promoting axillary bud outgrowth.
Thus:
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X behaves like auxin coming from the shoot apex: it maintains expression of Q and Z (repressors of bud growth) and keeps M off.
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Y behaves like cytokinin, whose synthesis or signaling is linked to M and which directly stimulates bud growth.
Detailed evaluation of each statement
Statement A
“‘X’ is an auxin that maintains expression of ‘Q’ and ‘Z’ and represses ‘M’.”
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In intact plants, high auxin from the shoot apex enforces apical dominance by sustaining transcription of genes that keep axillary buds dormant and by repressing genes that would promote bud activation.
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The figure shows that when X is present, Q and Z are “on” and M is “off”; when X is removed (decapitation), Q and Z go “off” and M turns “on”.
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This matches the classic role of auxin from the apex: maintaining bud dormancy via downstream repressors and preventing expression of bud‑activating modules.
→ Statement A is correct.
Statement B
“‘Y’ is a cytokinin that promotes axillary bud growth and is induced by ‘M’.”
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After decapitation, M becomes “on” and the axillary bud grows; this is consistent with M activating cytokinin synthesis/signaling.
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Cytokinins are well known to promote axillary bud outgrowth and to antagonize auxin‑mediated apical dominance.
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The figure’s rescue experiment (exogenous Y restoring bud growth) fits Y being a cytokinin whose production is downstream of M.
→ Statement B is correct.
Statement C
(paraphrased from figure) “Decapitation (removal of apex) activates ‘X’.”
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Removing the apex removes source of auxin X; thus X levels drop, not increase.
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In the figure, decapitation clearly corresponds to loss of X and consequent switching off of Q and Z and activation of M; so saying decapitation “activates X” is opposite to what happens.
→ Statement C is incorrect.
Statement D
“‘X’ is a cytokinin that represses ‘M’.”
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Repression of M by X is correct, but identifying X as cytokinin contradicts the physiological pattern.
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A hormone produced at the apex that maintains apical dominance and suppresses axillary buds is auxin, not cytokinin. Cytokinins generally relieve dominance and promote bud growth.
→ Statement D is incorrect.
Final selection
Only A and B accurately describe the roles of the hormones and genes in the experiment, so the correct option is:
(3) A and B only.
