Q.31 In nature, Agrobacterium tumefaciens mediated infection of plant cells leads to
P. crown gall disease in plants
Q. hairy root disease in plants
R. transfer of T-DNA into the plant chromosome
S. transfer of Ri-plasmid into the plant cell
(A) S only (B) P and R only (C) Q and S only (D) Q only
Correct Answer: (B) P and R only. Agrobacterium tumefaciens causes crown gall disease through T-DNA transfer and integration into the plant chromosome.
Natural Infection Process
Agrobacterium tumefaciens, a soil bacterium, infects wounded dicot plants at the crown (soil line), inducing neoplastic growths via its Ti plasmid. The T-DNA segment transfers into plant nuclei, integrating randomly and expressing oncogenes for auxin/cytokinin overproduction, causing uncontrolled proliferation.
Correct Statements Explained
P. Crown gall disease in plants: Characteristic tumors form at infection sites due to phytohormone imbalance from T-DNA genes iaaH/iaaM (auxin) and ipt (cytokinin).
R. Transfer of T-DNA into the plant chromosome: Vir genes (virA/B/D/E) orchestrate single-stranded T-DNA export via type IV secretion, enabling stable genomic insertion and heritability.
Q false: Hairy root disease results from Agrobacterium rhizogenes and its Ri plasmid, distinct from Ti-mediated galling.
S false: Ri-plasmid belongs to rhizogenes; tumefaciens transfers Ti-plasmid T-DNA only.
Option Analysis
Option (A) S only: Incorrect—Ri-plasmid association belongs to hairy root pathogen, not tumefaciens.
Option (C) Q and S only: Incorrect—both hairy root/Ri references mismatch tumefaciens pathology.
Option (D) Q only: Incorrect—hairy root disease links to rhizogenes, not tumefaciens crown gall.
Biotechnology Applications
Disarmed Ti plasmids enable precise T-DNA mediated plant transformation for GM crops. Understanding natural infection informs protocols optimizing vir induction by acetosyringone and wound-responsive attachment.
