LIFE SCIENCE

Corn “gut microbiota”: an untapped source of biological nitrogen fixation


Corn wound fluid collection Courtesy of the Chinese Academy of Agricultural Sciences

Similar to the human microbiome, the plant microbiome is known as the second genome of plants and is essential for plant growth and development, nutrient absorption, pest and disease resistance, and so on.

Recently, scientists have discovered a highly conserved core bacterial microbiome that colonizes the xylem wound fluid of corn stems with nitrogen fixation ability, which provides nitrogen nutrition to maize and promotes root growth. The relevant research results were completed by the plant nutrition team of the Institute of Agricultural Resources and Regional Planning of the Chinese Academy of Agricultural Sciences (hereinafter referred to as the Institute of Resources and Regional Planning), and published in Nature-Communications.

The neglected corn “gut” microbiome

The microbiome in a plant is a collection of microorganisms that live in various parts of a plant, such as within leaves, roots, or stems, and their interactions with their hosts.

“In recent years, a large number of studies have begun to focus on the structure and function of plant microbial communities, but mainly focus on the two parts of the plant’s rhizosphere and leafline, because these are the two key interfaces of the plant’s interaction with the external environment (including microorganisms), while ignoring other parts of the plant.” Ai Chao, the corresponding author of the paper and an associate researcher at the Institute of Resource Planning, told China Science Daily that the xylem catheter of the plant stem is an effective long-distance transportation system driven by a hydrostatic pressure gradient between the root and the stem. This driving force ensures the smooth transport of water, nutrients and other substances needed by plants between different organs.

That is, relative to the rest of the plant, this conductive property of the xylem catheter of the plant stem makes it a highly dynamic internal environment, a key “channel” connecting the plant above and below ground. A small number of studies have reported that a small subset of bacteria selectively recruited by plant roots can spread to aboveground plant organs through the flow of fluid within the xylem of the stem.

The xylem injury fluid that Ai Chao and the team are concerned about mainly refers to the liquid flowing out of the sap of the plant transport organization (xylem) that is constantly moved in the catheter to the ground by the root pressure.

Based on previous findings, they hypothesized that “similar to the human gut microbiome, the xylem microbiome of plant stems may play an important role in plant growth and development.”

Localization of maize “gut” functional bacteria

Foreign studies have found that corn grown in the barren soil of the Hahuaca region of Mexico is rich in nitrogen-fixing bacteria in its aerogenic root mucus. The nitrogen energy captured by these nitrogen-fixing bacteria from the air provides 29 to 82% nitrogen nutrition for corn.

Will nitrogen-fixing microorganisms also exist in the xylem duct, the “gut” of corn?

Zhang Liyu, the first author of the paper and a doctoral candidate at the Institute of Resources and Planning, introduced that nitrogen-fixing bacteria are a general term for various prokaryotes that can fix biological nitrogen, and in general it can be divided into three categories – autogenerated nitrogen-fixing bacteria, symbiotic nitrogen-fixing bacteria and combined nitrogen-fixing bacteria.

Autogenerated nitrogen-fixing bacteria refer to a variety of free-living prokaryotes that can independently fix atmospheric nitrogen, and generally have weak nitrogen fixation ability.

Symbiotic nitrogen-fixing bacteria refer to prokaryotic microorganisms that must coexist with other organisms in order to fix nitrogen. For example, the rhizobia genus symbiosis with legumes, forming a nodule symbiote nitrogen fixation; Actinomycetes of the genus Frankensis symbiosis symbiosis with non-legumes, forming a nodule symbiote nitrogen fixation.

The combined nitrogen-fixing bacteria live in the root surface or root cortex of plants, which do not form nodules, but also have strong host specificity and are more efficient than autogenous nitrogen-fixing bacteria.

“The nitrogen fixation of rhizobia-legumes has indeed received widespread attention, and the combined nitrogen fixation effect of non-legumes, such as sugarcane, corn and other crops and nitrogen-fixing bacteria, has also been confirmed.” In the future, how to make full use of biological nitrogen fixation to improve nitrogen nutrition in non-legumes, thereby reducing nitrogen fertilizer application, has always been a frontier hotspot in scientific research. Ai Chao said.

Colonization characteristics of xylem endophytes in maize stem Courtesy of Chinese Academy of Agricultural Sciences

They found that, relative to other plant sites such as non-rhizosphere soils and plant roots, the xylem wound fluid of maize stem selectively recruited a highly conserved core microbial community dominated by γ‐Proteus, which contained some highly efficient nitrogen fixation and assisted nitrogen fixing bacteria. Its community structure hardly changes with changes in geographic and environmental distances.

Further using the abundance-possession model, they identified 25 core microbial communities within xylem that represent a group of microbial groups that can be stable at different sites, in different fertilization regimes, and in maize genotypes.

Untapped microbial resources

To verify the role of these core microbial communities in maize growth and development, they first constructed synthetic communities consisting of two core nitrogen-fixing bacteria and two auxiliary bacteria.

The colonization characteristics of synthetic communities in plants were confirmed by verification experiments, and the biological nitrogen fixation ability of synthetic communities was demonstrated by 15N isotope dilution method.

“After 65 days of maize growth, synthetic communities contributed 11.8% of the total nitrogen to maize stems.” Ai Chao said. Not only that, but synthetic communities also increased maize root biomass.

According to the whole genome sequencing results, the synthetic community has anabolic pathways such as indole-3-acetic acid (IAA) and ethylene, and these growth hormones may play a very critical role in plant root growth. The inoculated synthetic group lagged behind, and the maize root biomass increased by 13%.

“There is a multi-mechanism interaction relationship between the core endophytes and the maize host.” Ai Chao stressed that these neglected plant “gut” functional bacteria represent a class of untapped microbial resources that may play a key role in crop nitrogen fixation and pro-growth.

Moreover, studies have proved that the core functional bacteria of corn “intestine” exist universally under different environmental conditions and different corn genotypes, indicating that they may play a more important role in the growth process of corn.

Maize inoculation of nitrogen-fixing synthetic flora (SynCom) and uninsulated (Control) on day 35 growth. Courtesy of the Chinese Academy of Agricultural Sciences

“To our surprise, genomic data shows that there may be new species of bacteria that have not been discovered in the core strain, and we will further validate this result in the next research.” Ai Chao said.

Academician Zhou Wei’s team of the Institute of Resources Planning of the Chinese Academy of Agricultural Sciences has long been engaged in the research on the efficient utilization of nutrient resources, which confirms the nitrogen fixation and pro-growth function of the stem xylem microbiome by assessing the assembly mechanism and function of the maize microbiome of different soil types, climatic zones and genotypes, which has important reference significance and reference value for future plant probiotic bacteria research and microbial fertilizer research and development. (Source: China Science Daily Li Chen)

Related paper information:https://doi.org/10.1038/s41467-022-31113-w



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