Brief note about plant vector

What is plant vector ?

Plant Vectors

A vector is a circular DNA molecule capable of independent existence and replication within a host cell. In case of plants, Ti and Ri are the two most commonly used plasmids which are used as vectors. Plant cells as such do not possess any endogenous plasmids. But two plasmids called pTi and pRi, are present naturally in the bacteria, Agrobacterium tumefaciens and Agrobacterium rhizogenes, respectively. These plasmids provide a naturally occurring transformation system. A part of the plasmid DNA, called as T-DNA, is transferred into the genomes of most dicot and some monocot plants. pTi stands for tumor inducing plasmid and pRi stands for root inducing plasmid. The infection of Agrobacterium tumefaciens is mediated by transfer of a segment of pTi called as T-DNA into the plant cell. Various bacterial chromosomal genes, such as chvB, exo genes, cell genes, are concerned with the biosynthesis of cell attachment polysaccharides due to which the bacterial cells adhere firmly to the plant cells. While two chromosomal genes are expressed constitutively in bacterial cells that is expressed at all the times inside a cell, which are responsible for virulence associated aspects, Agrobacterium tumefaciens Ti plasmid produces tumor like growth from which roots / shots may sometimes be produced. The infected cells are able to grow in culture on a medium devoid of any growth regulator while uninfected normal plant cells need exogenous auxin or cytokinin. These plasmids also carry genes for IAA (Indole Acetic Acid - auxin) and cytokinin production which is the reason for indefinite growth on a growth regulator free culture medium. When pTi is introduced into Rhizobium trifolii, it gains the ability to produce galls and to utilize opines. The crown gall root cells also synthesize unique nitrogenous compounds called opines, which are not produced by normal plant cells, which are not infected, nor are they utilized. The infected cells use opines as their carbon and nitrogen source. The type of opine produced depends on the bacterial strain. Agrobacterium tunefaciens strain produces either octopine or nopaline which the Agrobacterium rhizogenes produce either agropine or mannopine.