How do plants produce their own nitrogen fertilizers?

A recent study sheds light on how some plants developed their ability to produce nitrogen through symbiotic relationships with bacteria. The discovery is important for efforts to develop nitrogen-fixing crops, reducing reliance on fertilizers.

Multiple genetic pathways

According to lead author Heather Rose Gates, she conducted the research Florida Museum of Natural HistoryThis understanding may improve crop improvement by considering multiple genetic pathways rather than focusing on a single model species.

„Breeding and crop improvement efforts often focus on single model species, which overlooks the evolutionary context of traits,” he said. Rather than focusing on just one method, this study indicates that multiple genetic approaches may be effective.

„Only seeing what you might think of as one version of the trait can limit the effectiveness of engineering that trait in other plants.”

Nitrogen-fixing microorganisms

Nitrogen, essential for all life on Earth, is abundant in the atmosphere but difficult to obtain due to intense competition in natural environments.

The air we breathe contains up to 78% nitrogen, but in a molecular form most organisms cannot directly use. Diazotrophs are the only microorganisms capable of fixing atmospheric nitrogen.

About 17,000 plant species form mutualistic relationships with diazotrophs. These microbes attack plant roots and induce the formation of nodule-like structures. Within these nodules, bacteria obtain sugar from the plant and, in return, provide usable nitrogen.

This mutualism is primarily found in a group of closely related plants known as the nitrogen-fixing clade, although even within this group, the trait is inconsistent.

Formation of root nodules

Most nitrogen-fixing plants are legumes such as soybeans, peanuts, and clover. Non-legume nitrogen-fixers include species of the birch family, the rose family, and some squash relatives.

Leading researchers theorize that forming nodules is genetically complex, a pattern that evolved once in this plant group. This suggests that a single gene mutation may enable nodulation in organisms that currently lack this trait, including many agricultural crops.

„When a trait involves a lot of genes and has a high cost to the plant in terms of energy, we know that forming root nodules, we expect there to be strong selective pressure against developing that trait. So, in that context, the single-origin hypothesis makes sense,” he said. Gates explained.

Nodular plants and their relatives

The scientists tested this hypothesis by using genetic data from nearly 15,000 species to reconstruct the evolutionary history of nodules in plants and their relatives. For this group they created a huge tree of life. They needed as much data as they needed to develop new organizational methods.

„It took us two years to collect 15,000 tissue samples from the nitrogen-fixing clade, sequence them and create a tree,” said co-author Robert Kuralnick, curator of biodiversity information at the Florida Museum.

Many of the samples were old and contained degraded DNA, but the team’s extraction and sequencing methods solved these problems. „We were very surprised by the generally high quality and quantity of recovery and usable genetic data from our samples,” noted Kuralnik.

Evolution of nitrogen fixation

The results suggest that the nodule formed in two steps. First, the group’s ancestors developed the basic genetic toolkit for making knots and passed it on to descendants.

However, additional genetic mechanisms were required to activate the knots, which evolved at least 16 times independently. Some species also lost their ability to knot on 10 separate occasions.

The findings show that the nodule is not controlled by a single genetic switch, but by a complex circuit breaker that requires multiple switches. The researchers identified and sequenced several genes involved in nodulation, and future research will focus on their functions and properties.

Genetics of nitrogen fixation

„The overall goal is to use what we’ve learned from these evolutionary studies to help us understand the basic genetics and processes involved in nitrogen-fixing symbiosis, and then apply that information to engineering,” said co-author Pam Soltis, the Florida superintendent. Museum.

Most commercial crops, such as wheat and rice, cannot form nodules and cannot rely on nitrogen fertilizers. Many bioengineering studies have focused on legumes, but co-author Doug Soltis says this may not be the best approach.

„Our phylogenetic tree suggests that you might want to look at other models. Nitrogen fixation may have evolved differently in legumes than it did in the rose family or the birch family, so there may be different pathways,” he concluded.

The study was published in the journal Natural communication.

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