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Agrobacterium rhizhogenes and Ri Plasmid

Agrobacterium rhizhogenes is a Gram negative soil bacterium that incites hairy root disease in dicot plants. A plasmid called Ri plasmid (root hair inducing plasmid) is responsible for this disease.
Hairy root formation by Agrobacterium rhizhogenes
Ri plasmids are analogous to Agrobacterium tumefaciens Ti plasmid.
Ri plasmid organization:
  • Large plasmid of 250kb
  • Has 2 T-DNA fragments Tr-DNA and Tl-DNA (left and right) separated by 15 kB segment
  • Tr-DNA is similar to T-DNA. It has genes for auxin and agropine synthesis
    (a type of opine)
  • Tl-DNA has 4 genes: rolA, rolB, rolC and rolD (rol=root loci)
  • Rol A: responsible for hairy root formation
  • Rol B: induces root initiation and callus formation
  • Rol C: promotes root growth
  • Rol D: suppresses callus growth
  • Then Ri plasmid has a virulence region that mediated T DNA transfer and an ori same as Ti plasmid.
Uses
  • Ri plasmids can be used as substitutes for Ti plasmids to transfer foreign genes to plants
  • Ri plasmids  exchange DNA segments between plants and bacteria and hence they could be study gene exchange between the organisms.
  • They could be used to induce rooting in clones where rooting from callus or explant is difficult.
  • They could also be used to increase nodulation frequency of plantlets during micropropagation.

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