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The Role of Agrobacterium in Plant Biotechnology

BY: Padma Kumar | Category: Agriculture | Submitted: 2010-08-22 23:46:40
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Article Summary: "This article describes what makes gene exchange possible between unrelated species using Agrobacterium. GM plants have genes of other species artificially introduced in them; the general idea being to put in a new trait for a particular benefit. .."


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GM plants have genes of other species artificially introduced in them--- the general idea being to put in a new trait for a particular benefit. As regards this process, we human beings cannot be credited with inventing plant biotechnology that revolves around genetic engineering. On the contrary, the microorganism Agrobacterium tumefaciens should be credited with inventing plant biotechnology that revolves around genetic engineering because this organism has been indulging in genetic manipulation as a natural process.

What is Agrobacterium?

The word Agrobacterium denotes a soil bacterium Agrobacterium tumefaciens (small motile bacterial rods) that causes the crown gall disease in plant stems. This is a tumor and Agrobacterium is responsible for it.

Uses of Agrobacterium

Agrobacterium can totally transform plants and it does so by altering the plant genome.
Using Agrobacterium the following can be done
a) Make transformation protocols for new plant species, for different cultivars and variants of it.
b) Transformation protocols for non-plant species(as for example in yeast, fungi and algae)

How does Agrobacterium genetically transform their hosts? It does so by delivering external DNA to plant cells.

What makes gene exchange possible between unrelated species using Agrobacterium?

Nature (using Agrobacterium) has developed its own variety of genetic engineering. For example, consider the crown gall disease in plants which usually causes tumors at the point where the stem meets the roots. Crown gall disease is actually plant cancer and the direct result of genetic manipulation by Agrobacterium tumefaciens that resides in the soil. This bacterium gets into the plant if the plant has any wound that's adjacent to the soil. The next step is, it causes growth of tumors.

Why does this happen?

That's because of a genetic factor to crown gall disease which makes the plant cells replicate uncontrollably. Researchers found that these diseased cells contained opines (low molecular weight compounds that are source of nitrogen and energy for the bacterium). So what happens is, in the diseased plant, the plant metabolism was altered to make nutrients that the bacteria wanted by specific gene conscription and so even if antibiotics were used to kill the bacteria the cancer would continue to grow. The question is why did the normal plant cell turn into a tumor oriented cell? Researchers discovered that it was plasmids in the bacteria Agrobacterium tumefaciens that caused the tumors.

Plasmids are ring shaped DNA sections that can usually be found only in cell nucleus and certain cell processes are controlled by genetic information in plasmids. It was found that when agrobacterium infection occurs, a part of Agrobacterium tumefaciens DNA was fused with the host plant's genome. This is how they enter other cells. In other words, the plant tumor cells contain transferred bacterial DNA. This is how Agrobacterium tumefaciens could change plant cells into tumor cells. Significantly, this is also the reason as to why gene exchange could be made possible between unrelated plant species. This opened a window for using the natural genetic engineering capabilities of the Agrobacterium bacteria as a tool to make transgenic plants. Modern plant biotechnology revolves around this aspect.

Another significant point to be noted is, if Agrobacterium tumefaciens did not contain the plasmids then it cannot cause the tumors a fact that has been experimentally proved by using Agrobacterium tumefaciens without plasmids. That's how the terminology tumor-inducing plasmid or Ti plasmid originated

So the genes belonging to cancer causing bacteria in plant tumor cells could be first localized. Then it could be substituted with other genes using Agrobacterium. Then what you get is newer plants with different characteristics such as more yield, resistance to disease, and environmental stresses. In all these situations Agrobacterium functions as a carrier of genes.

An example of Agrobacterium mediated transformation

In semi- arid land the ideal serial crop that could be grown is sorghum. Apart from the grain it has multi-usage like for example the stover could be used as fodder.
But unfortunately as regards sorghum, tissue culture and transformation are difficult to achieve. Amongst the earlier research efforts, DNA was introduced into sorghum protoplasts by electroporation and other methods. But this didn't bring about plant regeneration. However Agrobacterium mediated transformation of sorghum could be achieved.

Agroinfiltration

For overexpression of recombinant proteins in plants, the technology of using agrobacterium genetic transformation is the most preferred. The methodology used is agroinfiltration.

By infecting host tissues (usually plant leaves) and using that process to produce recombinant proteins in plant cells--this technology is called agroinfiltration. As this does not involve stable integration of T-DNA into the host genome, within a few days after infection protein expression could be achieved. But the only problem is very low levels of proteins expression in leaves as compared to transgenic plants. That is mainly because only a small part of the plant is transfected. Agroinfiltration is vested with speed and convenience unlike other plant transformation methods.

Non-Agrobacterium

Rhizobium sp. NGR234, Sinorhizobium meliloti and Mesorhizobium loti are bacteria (different from Agrobacterium genus) that can achieve gene transfer to different plants when carrying a disarmed Ti plasmid and binary vector. These non- Agrobacterium technologies could achieve stable transformation in tobacco, rice and Arabidopsis plant species and have been helpful in getting over the patent issues surrounding Agrobacterium.

For transient protein expression, an alternative to agrobacterium is viruses based vectors (like recombinant plus sense RNA viral vectors) since they can move from cell to cell and replicate host cells autonomously.

Conclusion

From the research point of view, Agrobacterium is an important plant pathogen that is required for biological genetic transformation in plants and Agrobacterium transformation technologies were developed by leading international crop science players such as Bayer and others. So there are several patent impediments that restrict their usage.

Perhaps a paradigm shift is required from this perspective so that these bacterial species (and other non- Agrobacterium species) can widely be used in future at least for research purposes so that more and more transformaton protocols for newer plant species can be developed. Besides, these technologies are important to farmers for improving their crop plants. Any restriction of intellectual property rights in Agrobacterium based technology is bound to be counter productive although IP protection is an important issue as well.

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