Q.Describe the role of Agrobacterium tumefaciens in transforming a plant cell.

Agrobacterium tumefaciens is a soil bacterium that naturally infects plants and causes crown gall disease by transferring a segment of its DNA into the plant genome. Scientists exploit this natural genetic engineering ability to create transgenic plants.

The bacterium carries a large plasmid called the Ti plasmid (tumor-inducing plasmid). Within this plasmid lies a specific region known as T-DNA (transfer DNA), which gets integrated into the chromosomal DNA of infected plant cells. In nature, the T-DNA carries genes that cause uncontrolled cell division, forming tumors. …

The story of Agrobacterium tumefaciens is one of nature's most elegant examples of horizontal gene transfer, and it has become the workhorse of plant genetic engineering. This soil bacterium is the causative agent of crown gall disease, a condition that produces tumorous growths on infected plants. But what makes this pathogen remarkable is not the disease itself — it is the molecular machinery the bacterium uses to hijack the plant's genetic system.

Agrobacterium carries a large plasmid called the Ti plasmid (tumor-inducing plasmid). Within this plasmid lies a specific region known as T-DNA, or transfer DNA. When the bacterium infects a plant cell, usually at a wound site, it does something extraordinary: it transfers a copy of this T-DNA segment directly into the plant cell's nucleus. The plant cell then integrates this foreign DNA into its own chromosomes. Once integrated, the T-DNA genes are expressed by the plant's cellular machinery, leading to the production of plant growth hormones that cause uncontrolled cell division — the tumor. The T-DNA also encodes enzymes that produce unusual amino acid derivatives called opines, which the bacterium uses as a nutrient source, essentially turning the plant cell into a food factory for Agrobacterium.

Biotechnologists recognized that this natural gene transfer system could be repurposed. The strategy is straightforward: remove the tumor-causing genes from the T-DNA region but retain the border sequences that signal where transfer begins and ends. Into this disarmed T-DNA, scientists insert the gene of interest — perhaps a gene for insect resistance, herbicide tolerance, or improved nutritional content. The modified Ti plasmid is reintroduced into Agrobacterium, which is then used to infect plant cells or tissue explants in culture.

Method 2 -- as a numbered sequence of molecular events, not a description

Break the natural infection-and-gene-transfer process into an ordered timeline of discrete molecular events, the same way a mechanism is taught in a stepwise diagram:

1. A wound on a dicot plant releases phenolic compounds that are sensed by Agrobacterium.
2. This signal activates the plasmid's vir (virulence) genes.
3. The vir gene products nick the T-DNA at its border sequences and excise a single-stranded copy of the segment between them.
4. This T-DNA strand, coated by vir-encoded proteins, is transferred into the plant cell, guided toward and through the nuclear membrane.
5. The T-DNA integrates into the plant's chromosomal DNA (largely at random sites) via the plant's own DNA-repair machinery. …