- The T-DNA region of the Ti plasmid of Agrobacterium tumefaciens harbors two genes: X and Y. Mutation of gene ‘X’ produces a rooty tumour while mutation of gene ‘Y’ produces shoots in the tumor. Based on the above information, which one of the following statements is correct?
(1) Gene ‘X’ encodes auxins and gene ‘Y’ encodes cytokinins
(2) Gene ‘X’ encodes cytokinins and gene ‘Y’ encodes auxins
(3) Gene ‘X’ and gene ‘Y’ both encode auxins
(4) Gene ‘X’ encodes opines while gene ‘Y’ encodes cytokininsThe correct statement is (2) Gene ‘X’ encodes cytokinins and gene ‘Y’ encodes auxins.
Logic from tumour phenotype
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A rooty tumour implies a high auxin : low cytokinin ratio in the transformed plant tissue, which promotes root formation.
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If mutation of gene X makes the tumour rooty, that mutation must reduce cytokinin while auxin production from gene Y remains, shifting the balance to auxin dominance. Thus, gene X normally encodes cytokinin biosynthesis.
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A shooty tumour implies high cytokinin : low auxin. Mutation of gene Y produces shoots, so Y must normally provide auxin; when Y is inactivated, auxin level drops, cytokinin from gene X dominates, and shoots form. Therefore, gene Y encodes auxin biosynthesis.
So: X → cytokinin, Y → auxin, matching option (2).
Option‑by‑option explanation
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Gene ‘X’ encodes auxins and gene ‘Y’ encodes cytokinins – Incorrect
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If X made auxin, mutating X would reduce auxin, leaving relatively more cytokinin and giving a shooty, not rooty, tumour. This contradicts the question.
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Gene ‘X’ encodes cytokinins and gene ‘Y’ encodes auxins – Correct
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Loss of cytokinin (X mutant) leaves auxin high → rooty tumour.
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Loss of auxin (Y mutant) leaves cytokinin high → shooty tumour.
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Gene ‘X’ and gene ‘Y’ both encode auxins – Incorrect
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Mutating either gene would mainly lower auxin; both mutations should shift towards cytokinin‑type (shooty) growth, not give opposite rooty vs shooty outcomes.
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Gene ‘X’ encodes opines while gene ‘Y’ encodes cytokinins – Incorrect
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Opine synthesis genes affect nutrient supply for Agrobacterium, not directly the auxin–cytokinin ratio controlling root vs shoot formation. This cannot explain the contrasting rooty versus shooty phenotypes from X and Y mutations.
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SEO‑oriented introduction (for article use)
The T‑DNA region of the Agrobacterium tumefaciens Ti plasmid carries genes for auxin and cytokinin biosynthesis, and the relative activity of these genes determines whether infected plant cells form rooty or shooty tumours. When a mutation in gene X yields rooty tumours and a mutation in gene Y yields shooty tumours, the only consistent interpretation is that gene X encodes a cytokinin‑biosynthesis function and gene Y encodes an auxin‑biosynthesis function, making option (2) the correct choice.
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