Adapted to growing rampant in the field: A study uses the example of the North American rough-cropped water hemp to illustrate how agriculture itself contributes to weeds developing from wild plants. By comparing historical herbaria specimens with modern forms, the researchers were able to document that selection pressure led to mutations in several hundred genes in the plant. They give her better drought tolerance, accelerated growth and herbicide resistance. The study could make a fundamental contribution to understanding how harmless wild plants become problems for agriculture, say the scientists.
In order to provide the world's population with food, enormous areas of land on earth must be converted into productive areas shaped by people: the intensively cultivated monocultures are heavily cultivated, fertilized and additionally treated during planting and harvesting times. In order to ensure the highest possible yields, in addition to pests and pathogens, no competing plants must be allowed to spread among the useful plants. It stands to reason that these interventions exerted strong selective pressure on some of the unwanted plants. In order to track down the mechanisms by which these so-called weeds adapt, an international team of researchers with the participation of the Max Planck Institute for Biology in Tübingen has now looked at a plant that causes problems in North America, especially in corn and soybean cultivation.
Comparison with herbaria specimens
Rough-fruited water hemp (Amaranthus tuberculatus) was not always a problem there. Originally it grew near lakes and rivers, but then it began to spread more and more in agricultural crops. Mainly due to its resistance to weed killers, it can now hardly be contained there. In order to elucidate how the problematic plant might have adapted to agriculture, the researchers used the potential of traditional botany: they obtained and analyzed the genome of more than 100 water hemp specimens from herbaria dating back to 1820. They were then able to compare the data obtained with sequencing results from 187 water hemp samples from modern farms and neighboring areas.
In this way, the researchers were able to examine the history of the development of water hemp depending on the habitat over two centuries. Basically, they report, the particularly "weedy" version of rough-fruited water hemp has been shown to have spread from western to eastern North America. The plants exchanged parts of their genome with different local populations, which became apparent in the genetic test results. As the scientists explain, the water hemp was apparently able to adapt better to the different basic conditions of agriculture in the different regions.
"Weedy" adaptations are emerging
The team finally discovered mutations in several hundred genes in the genome of today's water hemp plants, which apparently spread as a result of selection pressure from agriculture. Mutations in hereditary traits that are known to be associated with drought tolerance, rapid growth and herbicide resistance were particularly common. "While water hemp originally grew near lakes and rivers, genetic modifications allowed the plant to survive in drier soil and grow rapidly to displace crops," explains co-author Sarah Otto from the University of British Columbia in Vancouver.
It turned out that some genetic variations were already found in some historical specimens. The majority of herbicide resistances, which meanwhile characterize the "success" of the plant, are apparently mostly newly created mutations: five out of seven herbicide resistance genes from more recent samples could not be detected in the historical specimens. "When sequencing plant genes, herbicides turned out to be one of the strongest agricultural filters, determining which plants survive and which die," says lead author Julia Kreiner from the University of British Columbia in Vancouver. "The genetic variants that help the plant thrive well in modern agricultural environments have increased remarkably rapidly to a high frequency since the intensification of agriculture in the 1960s," says Kreiner.
Co-author Detlef Weigel from the Max Planck Institute for Biology in Tübingen emphasizes the fundamental importance of the study: "Although it was carried out on North American plants, we have similar problems with herbicide resistance in Europe. Our study can therefore serve as a blueprint to better understand the transformation of harmless wild plants into problematic weeds in Europe as well,” says the scientist. Wright adds: "Extending this research to other scales and species could now advance our understanding of how agriculture and climate change are accelerating plant evolution." Finally, he emphasizes the value of the work of botanists of the past: "Our results underscore the enormous potential of studying historical genomes to understand the adaptation of plants," says Wright.
Source: University of British Columbia, Max Planck Institute for Biology Tübingen, specialist article: Science, doi: 10.1126/science.abo7293