For thousands of years, humans have influenced the genetics of plants. Selection of certain traits is intentional, such as choosing to save seed from the most flavourful or largest fruit. In the process of seed saving, however, there is an unconscious selection for other traits. For example, there is selection for seeds that do not “shatter” easily or early. Shattering is the process in which pods burst open or seed fall off stalks. If peas burst out of their pods as soon as they mature or grain falls off the stem easily, it’s difficult to save the seed. Millennia of saving seed has led to crops that are generally larger, more uniform, tastier to the human palate (sweeter, less bitter, etc.), and easy to grow, harvest and propagate.
Modern plant breeding has accelerated the change in plants. Selection is often done in field plots where weeds are controlled by herbicides and nutrients supplied by synthetic fertilizers. More recently, plant breeding is conducted in laboratories where strains are grown in pots containing sterile media. Field trials remain a component of plant breeding but this is almost always done on non-organic fields.
One valuable trait that has been largely lost during plant breeding is the function for symbiosis, the development of mutually beneficial relationships with other species. Porter and Sachs (2020) investigated the relationships between crops and beneficial microbes, particularly arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (rhizobia). They found that modern plant breeding has often selected crops with less potential to form symbiotic relationships. Sometimes this happens because there is a slight cost to symbiosis—the plant provides carbohydrates to the microorganisms, which then help the plant access more nutrients. In the wild, stressed systems and on organic farms, this slight cost (which may reduce growth or yield) is more than compensated for the nutrient advantage. But in systems where high levels of soluble nutrients are applied, the plants are well irrigated and pests are controlled by pesticides, the symbiotic relationship has little or no advantage. When plant strains lose the ability to develop these relationships, applications of mycorrhizal fungi and nitrogen-fixing bacteria have no effect on the crop.
In future plant breeding, the researchers suggest that breeders consider the genes that lead to symbiosis. Older varieties of crops, as well as wild ancestors of crops and “feral crop populations,” often contain the genes that allow them to form mutually beneficial relationships with soil microorganisms.
“Resolving the impact of domestication processes on symbiosis function in crops, and maximizing the benefits of symbiotic function in elite cultivars, has the potential to have a pivotal role in meeting the challenges to food security we face in the coming century,” conclude Porter and Sachs (2020).
Source: Porter, Stephanie S. and Joel L. Sachs. 2020. Agriculture and the Disruption of Plant–Microbial Symbiosis. Trends in Ecology & Evolution. March issue. 14 pages.
— Janet Wallace