A team of scientists from Oregon State University (OSU) has developed a new way to trace the global spread of disease-causing bacterium that harms major plant species in the world, reports Science Daily. Such a discovery could save commercial plant growers billions of dollars every year and is a promising development that can prevent ailments of over 100 plant species.
Halting the spread of Agrobacterium
Agrobacterium is a genus of Gram-negative bacteria that cause tumors in plants. It causes crown gall disease, which impacts over 100 plant species, including roses, fruit trees, nursery plants, shade trees, and grapevine. When combined, these species of plants are valued at more than $16 billion annually in the US alone, said the US Department of Agriculture (USDA).
Authors Alexandra J. Weisberg from the Department of Botany and Plant Pathology and colleagues focused their study on plasmids, extrachromosomal DNA molecules within cells that can replicate independently and are found in Agrobacterium. Once plasmids spread, it also amplifies the spread of the disease. For instance, plasmids spread the antibiotic resistance genes, which are now the pressing problem for human and animal health. The unique thing about the plasmids of Agrobacterium is that they can transfer a part of the plasmid into the plant cells and then genetically reprogram the host to cause hair root or crown-gall disease.
Crown gall in plants
Crown gall symptoms in plants include wart-like and round growths that appear at or above the soil line or on stems or the lower branches of the plants. Plants with several galls may be unable to move nutrients up the trunk and become unproductive, stunted, and weakened.
The plasmids have genes that enable Agrobacterium to transfer the whole plasmid horizontally from one bacterium to another instead of vertical inheritance, which is from a parent to a child. A train of Agrobacterium that is previously benign can end up as a pathogen lineage once it acquires a harmful plasmid, making it difficult to track the outbreak.
Developing the tracing system
So, the team combined the analyses of plasmids and chromosomal ancestry to determine their contributions as well as accurately model the global spread of disease. Prior to the team’s research, the generally accepted scientific view was that a large amount of genetic variation among Agrobacterium and the frequent transfer of genetic information among plasmids made it impossible to draw evolutionary relationships between the two. But without such information, it would be impossible to accurately track the disease outbreaks.
The team, therefore, focused on the two classes of plasmids, the root inducing and the tumor-inducing, both enabling the Agrobacterium to transfer a part of the plasmid into the plants and then cause disease.
Oregon State Plant Clinic’s director Melodie Putnam and others at the USDA-Agricultural Research Service and OSU provided hundreds of strains with plasmids from their collection and these helped the plant scientists analyze the large dataset.
Lead author Weisberg sequenced the 140 strains with plasmid and found that the plasmids all come from nine lineages only. The author added that as the team was armed with large genetic sequencing information on how to classify the Agrobacterium and plasmids, they could infer how the plasmids move among the bacteria as well as how the bacteria move among the nurseries.
The whole-genome sequences of Agrobacterium helped the researchers in linking the nurseries on the basis of possessing strains with the same plasmid sequences and genome, different genome sequences but same plasmid sequences, or different plasmid sequences but the same genome sequence. The team tracked at least seven cases in which the worldwide distribution of plants contributed to the transmission of the Agrobacterium strain-plasmid combination.
Importance of the study
They concluded that the strategy of inferring the transmission and evolution of virulence plasmids has potential applications in plant, animal, and human health and food safety. It can also be used in understanding the evolution and ecology of other plasmid-mediated processes, including mutualistic symbioses.
In a 2017 study by Abdullah M Al-Sadi from the Department of Crop Sciences in Sultan Qaboos University in Oman, it was pointed out that pant diseases have always been a challenge to crop production and plant growth in several parts of the world.
Although fungi, bacteria, and viruses that infect plants do not often cause infection in humans, the immune system of humans may be affected and it can induce a clinical symptom. This is why it is important to pay attention to bacterial species that are living in agricultural soils as human health can be affected in the process.
Economic loss from crown gall
Crown gall diseases caused by Agrobacterium remain a pathogen that is of considerable economic importance. It is responsible for field and nursery losses among a wide variety of plants. The highest losses from crown gall happen in young plants, such as those still in nursery. While the disease seldom kills plants, it can elicit reduced growth and lack of vigor. In Canada, losses in the nursery of ornamental and fruit plants amounted to $122 million in 1986 while the estimated damage in California totaled US$23 million in 1976. In 2015, biocontrol of crown gall contributed to approximately $100,000,000 to the Australian economy through the protection of almond trees, cherry trees, nectarine, peach, roses, and other plants, according to data from Bio-Care Technology, a global distributor of NOGALL biological treatment for crown gall.
On a global scale, we are losing a significant amount of food not just to plant diseases but also to pests at a time when the world needs to increase food production to feed the growing population. Over the years, there has been a general increase in both animal-based and plant-based protein. Scientific online publication Our World in Data shares that the daily per capita protein supply per person per day in the US in 2017 stood at 39.86g from plant-based food products and 73.87g animal-based protein compared to 32.22g plant protein and 62.99g animal protein per person per day in 1961.
Understanding the genetic basis of how pathogens diversify and emerge in the ecosystem is necessary to prepare the public against disease outbreak and also increase food security.