Although the output of sweet potato in China ranks first in the world, sweet potato is not native to China. Sweet potato is a kind of sweet potato in convolvulaceae, and its origin is in the south-central United States on the other side of the distant ocean. For local aborigines, sweet potatoes have been planted for thousands of years. But it was not until15-16th century that Spanish colonists brought sweet potatoes out of America for the first time. It was not until the Ming Dynasty more than 400 years ago that it was introduced to China through Southeast Asia-hence the word "Fan" of sweet potato.
Since sweet potato has been widely planted all over the world, it has become one of the most important potato crops in China and even in the world, and its importance is second only to potato. Sweet potato contains a lot of starch, which can be directly roasted, boiled and steamed, and can also be processed into many types of food-the vermicelli and vermicelli we eat are mainly processed from sweet potato. In addition, a large number of sweet potatoes are used as feed for livestock.
Because sweet potato is common in vegetable market, people will not doubt its "naturalness" when choosing and eating it. Recently, however, a research result may give some people the impression that sweet potato is "pure natural"-researchers found DNA fragments transferred from bacteria in the genome of sweet potato.
Agrobacterium-a master of gene transfer. So, why did the DNA fragment of Agrobacterium enter the genome of sweet potato? This has to start with the characteristics of Agrobacterium.
Although Agrobacterium is widespread in soil, it prefers to parasitize in plant tissues-after all, it is much easier to obtain nutrients in plants than to decompose organic matter. When they are parasitic in plant tissues, they will also make plant tissues grow special structures such as tumors or hairy roots for them. These structures can produce a special amino acid derivative (called crown gall base) for Agrobacterium, which cannot be synthesized by plants themselves. According to these different structures, Agrobacterium can be divided into two categories, Agrobacterium tumefaciens and Agrobacterium rhizogenes.
Scientists have found that once plants begin to form tumors and hairy roots, the subsequent growth of these structures and the synthesis of crown gall can continue without the presence of Agrobacterium. This means that Agrobacterium relies on changing the regulatory mechanism of tissue cell growth to achieve the purpose of inducing special structural development and crown gall synthesis. Related experiments have proved that Agrobacterium contains a huge circular DNA, which does not depend on the genome of the bacteria itself. It is called tumor-inducing plasmid (Ti plasmid), and in Agrobacterium rhizogenes, it is called root hair-inducing plasmid (Ri plasmid), which contains a mobile DNA sequence. This sequence contains many genes that stimulate plant cell division and guanine synthesis. When Agrobacterium infects plants, this DNA sequence can be transferred into plant cells and integrated into plant genome, thus stimulating plants to form tumors or hairy roots and synthesize crown gall. So this DNA is called DNA (T-DNA), which is also the origin of the names of two fragments in sweet potato.
The "bacterial DNA fragment" in the sweet potato genome comes from Ghent University (IPC). China Agricultural University and USDA Plant Genetic Resources Unit found that there are two gene fragments in the sweet potato genome, which are highly similar to the sequence called T-DNA in soil bacteria Agrobacterium. These two fragments are called IbT-DNA 1 and IBT-DNA 2(IB respectively (IB is the abbreviation of the scientific name of sweet potato). The similarity between the two sweet potato genomes and Agrobacterium DNA sequences is high enough for researchers to believe that they were indeed transferred from Agrobacterium to sweet potato genome.
To determine when this transfer occurred, the researchers analyzed hundreds of sweet potato samples collected from different regions of South America, Central America, Africa, Asia and Oceania. These samples include cultivated sweet potato, wild sweet potato and related species of sweet potato. The results showed that all the 29 1 cultivated sweet potatoes collected by researchers contained IbT-DNA 1 fragment, which did not exist in other wild sweet potatoes or related species. In addition, IbT-DNA2 fragment exists in some cultivated sweet potatoes, wild sweet potatoes and related species [2].
This result shows that, at least for the fragment of IbT-DNA 1, the time of its transfer to sweet potato is highly consistent with the time when cultivated sweet potato began to propagate-only in this way can we explain why it exists in all cultivated sweet potatoes but not in its wild "ancestors". In addition, there are several complete and expressible genes in the Agrobacterium DNA fragment of cultivated sweet potato. Through testing, the researchers found that these genes can be expressed in different tissues of sweet potato. They speculate that this gene transfer event may provide some "characteristics" for sweet potato breeding, so that it can be preserved and spread through selection.