Aerial root mucus hides “heart scheming”

The roots that occur on the stems or leaves of plants are called aerial roots, which are a very special root type and metamorphosis organ, and some plants have a large amount of mucus attached to them, thus forming a magical microenvironment.

Recently, Xu Peng’s team, a researcher at the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences (hereinafter referred to as the Banna Botanical Garden), cooperated with researchers from Zhejiang University and the Shenzhen Institute of Agricultural Genomics of the Chinese Academy of Agricultural Sciences to find that microorganisms living in the aerial root mucus of vine plants are closely related to nitrogen fixation and plant growth, and they have even discovered potential pathogenic bacteria control methods in it. The research results were published online in the international journal Microbiome.

Vine wild peony aerial root mucus. Photo courtesy of Banna Botanical Garden

“Natural Medium” for Nitrogen Fixation

In 2018, Xu Peng accidentally found a vine plant vine wild peony in the Banna Botanical Garden, which grew a lot of aerial roots at the stem nodes and was wrapped in a lump of mucus, which was very strange.

Vine wild peony aerial root mucus. Photo courtesy of Banna Botanical Garden

Xu Peng said that the aerial roots of vine plants have nothing more than two functions, one is to climb, and the other is to grow to the right time and plunge into the soil. “Aerial roots in crops like corn and sweet potatoes also attach mucus, but in the past we thought it was to provide lubrication and protection to the root tip cells, so that aerial roots can better penetrate the soil.”

But this is not the case with the vine wild peony. “Its aerial roots are mostly exposed, and it secretes so much mucus so laboriously, what is it for?” Xu Peng secretly speculated that it may provide microbes with a “natural medium” and therefore perform some special biological function.

Coincidentally, a recent study at the time showed that the presence of large carbohydrates in the aerial root secretions of Mexican corn, as well as the nitrogen-fixing bacteria that inhabit them, can even contribute 29%-82% of the biological nitrogen fixation of the host plant, which is an essential element of life for plants. This study is the first to suggest that the aerial rooting mucus-microbial system of monocots plays an important role in meeting plant nitrogen needs.

The study directly inspired Xu Peng and the research team. They examined the chemical composition of aerial root mucus of the vine wild peony and found that the carbohydrate content in it was nearly a hundred times higher than that of underground root exudates. Subsequently, they further discovered through microbiome sequencing and isotope nitrogen labeling culture system that the mucus microhabitat is also rich in a large number of nitrogen-fixing bacterial taxa, which contributes 37-54% of the nitrogen fixation of plant organisms, which better supports the growth of plants.

Combining molecular biological evidence such as plant genome and transcriptome, the research team found for the first time that aerial roots of dicots can perform the biological function of nitrogen fixation through functional microorganisms in mucus and realize the two-way carbon and nitrogen exchange process with the host plant, and the results also expand the scope of “rhizosphere” defined in traditional plant ecology research.

“Guard” the fungus to maintain microbial homeostasis

The difference is that the aerial root research of Mexican corn stops there, while Xu Peng does not.

“We don’t know the biological mechanisms of aerial rooting mucus secretion and nitrogen fixation.” Xu Peng is curious, in the mucus environment, why is it not a “big outbreak” of various microorganisms, but dominated by nitrogen-fixing bacterial groups?

At this point, the research team proposed a hypothesis: the presence of certain components or special microorganisms in the mucus microbial community can selectively allow the growth of nitrogen-fixing bacteria while suppressing “undesirable” environmental and pathogenic microorganisms.

The researchers screened 56 fungi and bacteria isolated from mucus samples for their broad-spectrum antimicrobial activity. As a result, they were pleasantly surprised to find that there is indeed a fungus, F-XTBG 8 (Chaetomella raphigera), which has broad-spectrum antibacterial activity that can inhibit the growth of more than 100 common plant pathogens and fungal growth in the environment, but only allow the growth of nitrogen-fixing microbial groups in mucus to maintain the homeostasis of microorganisms in mucus and the balance of microhabitats. F-XTBG 8 is precisely the “defending” fungus that is “recruited” and associated by the host.

This made Xu Peng excited. “We have found important applications in this study of biological mechanisms.”

“We can artificially synthesize the microbial flora that fixes nitrogen in mucus, convert it into biological agents, and make seed coating agents to improve nitrogen fixation capacity and promote plant growth.” He said.

As a scholar who has been engaged in rice breeding research for a long time, Xu Peng is very concerned about crop pest control, and most diseases in plants and animals are caused by microorganisms. “If the resistance mechanism and secondary metabolite components of F-XTBG 8 can be thoroughly studied, and a microbial resistance agent can be produced through artificial synthesis or proliferation, it will play an important role in promoting disease resistance in multiple hosts of crops such as rice, corn, wheat and tomato.”

Xu Peng told China Science News that at present, this broad-spectrum resistant strain F-XTBG8 has applied for a biocontrol strain patent, and it has also made preliminary progress in the study of the disease promotion and disease resistance effect of crops. (Source: Hu Minqi, China Science News)

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