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.
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.
- 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.