- Agrobacterium tumefacienscauses crown gall diseases in dicot plants. Which phytohormone genes are present of T-DNA?
(1) Auxin and cytokinin
(2) Auxin only
(3) Cytokinin only
(4) Cytokinin and Gibberellic acid
Correct answer: (1) Auxin and cytokinin.
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Agrobacterium tumefaciens carries a Ti (tumor‑inducing) plasmid whose T‑DNA segment integrates into the plant genome and contains genes for overproduction of both auxin (IAA) and cytokinin. This simultaneous, unregulated synthesis of the two hormones causes uncontrolled cell division and enlargement, leading to crown gall tumors.
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(2) Auxin only – incorrect, because cytokinin genes are also present and essential for tumor formation.
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(3) Cytokinin only – incorrect for the same reason; auxin genes are also encoded on T‑DNA.
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(4) Cytokinin and gibberellic acid – incorrect, as gibberellin biosynthesis genes are not part of the canonical T‑DNA hormone region; the key pair is auxin plus cytokinin.
