Agrobacterium tumefaciens, a crown gall bacterium, is called as the natural genetic engineer of plants.

Explanation:
Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants. This bacterium has the unique ability to transfer a portion of its DNA, known as the Ti (tumor-inducing) plasmid, into the host plant's genome. This transfer of genetic material from the bacterium to the plant is known as horizontal gene transfer.

Key Points:
- Agrobacterium tumefaciens is a gram-negative soil bacterium that infects a wide range of plants, including dicots and some monocots.
- It is called the natural genetic engineer of plants because it can transfer its DNA into the genome of the host plant, leading to the formation of crown galls.
- Crown galls are tumor-like growths that occur on the stems, roots, or other parts of the plant. They are caused by the integration of the Ti plasmid DNA into the plant's genome.
- The Ti plasmid contains several genes that are involved in the transformation process. These genes include the vir genes, which are responsible for the transfer of DNA from the bacterium to the plant, and the oncogenes, which promote uncontrolled cell division and the formation of tumors.
- The transfer of DNA from Agrobacterium to the plant occurs through a process called conjugation. The bacterium attaches to the plant's cells and injects a segment of its DNA into the plant's nucleus. This DNA integrates into the plant's genome and is replicated along with the plant's own DNA during cell division.
- Once the DNA is integrated into the plant's genome, it can be expressed and lead to the production of specific proteins, such as enzymes involved in the synthesis of plant growth hormones.
- Agrobacterium-mediated transformation has been widely used in genetic engineering to introduce desirable traits into plants. Scientists can modify the Ti plasmid to carry specific genes of interest, such as genes for herbicide resistance or insect resistance, and then infect plants with the modified bacterium. This allows for the production of genetically modified plants with desired traits.
- Agrobacterium-mediated transformation is a powerful tool in plant biotechnology and has revolutionized agriculture by enabling the development of genetically modified crops with improved traits, such as increased yield, disease resistance, and nutritional quality.

In conclusion, Agrobacterium tumefaciens is called the natural genetic engineer of plants because of its unique ability to transfer its DNA into the genome of host plants, leading to the formation of crown galls and allowing for the introduction of desirable traits through genetic engineering.