What is the experience of growing up in a "honeypot"?

Microorganisms are the protagonists of the biological world.
Although they are not seen by us most of the time, they occupy almost every corner of the earth
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All creatures, including humans, have become their dwellings without exception, and as long as there is a gap, there is a place for them to live
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Fortunately, the vast majority of microbes have learned to live with us during their co-evolution with animals, including humans, and have even learned to adapt to the different microenvironments created by animals
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Gut microbes, for example, need to endure an oxygen-deficient environment in animals and obtain food in order to survive
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Gut microbes (Image credit: Sigma-Aldrich) These microbes that coexist with animals are collectively referred to as the animal's microbiome
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Like animals, plants have their own microbiomes, and microbes inhabit them wherever they are exposed
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In relatively broad areas such as the foliage, rhizosphere, etc.
, microorganisms in the environment will swarm from the surrounding area and then settle on it
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Plant phyllosphere and rhizosphere are the main gathering places for microorganisms (Image source: Wikipedia) Most of the microorganisms living in the phyllosphere do not receive gifts from plants, and they can only consume the substances captured by the leaves from the environment
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But plants also have a nutrient-rich place for microorganisms to inhabit, and that is the nectar of the flower heart
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It's the open reward that plants give to pollinators, and it's full of sugar, which can be earned by visiting flowers
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Such an open platform can naturally obtain microorganisms from the environment or from pollinators, and these microorganisms in nectar are equivalent to growing up in a "honey pot", so if you want to know how to grow up in a "honey pot" What kind of experience is it, just ask them! Surviving in a "honeypot" is no easy task, however, nor is it easy to survive in this "nectar pot", where the microbes that live in it face severe survival challenges
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The highly permeable environment in honey is one such difficulty
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We all know that the honey we buy is not easy to spoil, because the high sugar content of honey creates a highly permeable environment that is not conducive to microorganisms.
It is probably no easier to survive in it than to survive in high acid gastric juice
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Although not as concentrated as honey, nectar in plants is also a hyperosmotic environment that microorganisms must tolerate in order to survive
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In addition to this, the nectar of many plants contains toxic substances
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For example, the nectar of Jacaranda and Aconitum is bitter and contains toxic alkaloids, which are intolerant to small pollinators, and only larger pollinators can obtain nectar
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Likewise, this could be toxic to microbes that get nutrients directly from the environment
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The defensive substances contained in nectar (Image source: Reference 1) For plants, the microorganisms that live in nectar are also little thieves, because they break down the sugars in it, consume the rewards of pollinators, and reduce flowers Attraction of pollinators
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So many nectars contain antimicrobial substances
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For example, the nectar of tobacco contains unique nectar proteins that inhibit the survival of microorganisms in it
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However, despite the many barriers, nectar is still occupied by many microorganisms
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Scientists have isolated numerous microbes from nectar, and when grown in the lab, they all appear to be resistant to hyperosmolarity, peroxides, and antibacterial substances
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There are quite a few microbes that have fully adapted to the environment in nectar and even, like our gut flora, form a mutually beneficial relationship with plants
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Microorganisms isolated and cultured from nectar (Credit: UC ANR) For example, in an early spring bloom in Europe, the yeasts in the nectar live by decomposing the nectar, but their decomposing activity also produces heat, which can give The flowers of iron chopsticks that bloom in early spring are warmed up, which in turn increases the fruiting rate of iron chopsticks
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An iron chopstick that blooms in early spring relies on the metabolism of microorganisms in the nectar to heat the flower (Image source: Reference 2) Sometimes the two work in harmony to achieve each other, for example, a yeast in Clematis oleracea metabolizes nectar.
A unique volatile odor is produced, which acts as a signal for bumblebees to visit flowers and mediates the interaction between plants and pollinators
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A yeast-mediated pollination in Clematis glabrata (Image credit: Reference 3), although we know that nectar microbes can coexist harmoniously with plants
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But we still don't fully understand the mechanism by which they tolerate the nectar environment
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An obvious question is that although nectar is rich in carbohydrates (sugars), it lacks nitrogen and protein.
How do microorganisms solve the nutritional problem? It’s not enough to eat honey alone.
You have to eat some pollen.
The problem of nutritional balance has always plagued scientists: although there are trace amounts of amino acids in nectar, the growth density of microorganisms in it can reach 109 cells per milliliter, which is obviously not enough for survival.

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So scientists thought of pollen in nectar
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Both the natural dispersal of pollen and the activity of pollinating insects bring pollen into the nectar
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Pollen is rich in protein and enzymes, and bees even use pollen to raise their children during the breeding season.
Previous studies have confirmed that pollen in nectar can promote the growth of microorganisms
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But the pollen is covered in protective sporopollenin, so how do the microbes in the nectar get its nutrients? Electron microscope photos of pollen, which are composed of sporopollenin polymers (Image source: Wikipedia) The latest research found that the microorganisms in the nectar did not directly attack the city, but let the pollen germinate by itself, and then from the out of the shell
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The researchers chose a common plant, rhododendron, and let the pollen directly contact the common microorganisms in the nectar.
Seven kinds of microorganisms were selected and inoculated in the pollen solution, and they were treated at 15, 45 and 90 minutes after inoculation.
Perform imaging to capture the dynamics of pollen
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The test results showed that different microbial species and strains had different effects on the pollen germination and bursting of
C.

The germination (A) and blasting (B) of pollen after inoculation with different microorganisms.
Acinetobacter promotes blasting and germination.
In order to simulate the real natural environment, the researchers also used a 30% sucrose solution close to the nectar concentration to inoculate the nectar microorganisms.
, Acinetobacter also significantly promoted pollen germination and burst in such an environment
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C is the pollen germinated in Acinetobacter culture, D is the sterile control group (Image source: Reference 4) Did the nectar microorganisms get nutrients from the burst pollen? The researchers did another experiment.
They set the initial concentration of the cultured Acinetobacter (A.
pollinis SCC477) to the normal density in the nectar, and added the germinated and ungerminated pollen to it.
the growth of microorganisms under these conditions
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It was found that within 24 hours, the addition of germinated pollen significantly promoted the growth of Acinetobacter, their density was twice that of non-germinated pollen, and the protein content was also twice that of non-germinated pollen medium.

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At this point, a complete chain of evidence has come out.
Acinetobacter hidden in nectar is indeed eating pollen in an elegant way - they did not choose to be hard with the hard shell sporopollenin evolved from pollen, but broke them from the inside.
, using the natural mechanism of pollen germination to release the protein inside, to replenish nitrogen for the nitrogen-deficient environment of nectar, thereby promoting its own growth, and stably occupying the tiny ecological niche of nectar
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Schematic diagram of Acinetobacter eating pollen (Image source: Reference 4) Conclusion This also confirms that during the evolution of hundreds of millions of years, nectar, like our intestinal flora, has screened out adaptable organisms by its own environment.
Microbes (click here for the gut microbiome story)
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However, we are still at the beginning of understanding this tiny world.
Now we only know that microorganisms such as Acinetobacter in nectar supplement their nitrogen by eating pollen.
We still don’t know what the specific mechanism is.
Maybe pollen Germination, a delicately regulated process, we can also find clues in microbes
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What is more worth looking forward to is that there are still many microorganisms in nectar waiting for us to explore, such as yeast, which may have completely different survival strategies from Acinetobacter.
They do not explode pollen, and even inhibit pollen germination
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So how do these microbes interact with nectar and pollinators? We look forward to more exploration results
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References: [1] Heil, Martin.
"Nectar: ​​generation, regulation and ecological functions.
" Trends in plant science 16.
4 (2011): 191-200.
[2] Herrera CM , Medrano M .
Pollination consequences of mimicking intrafloral microbial warming in an early-blooming herb[J].
Flora, 2016:S0367253016301487[3] Yang, M.
, et al.
"Nectar yeasts enhance the interaction between Clematis akebioides and its bumblebee pollinator.
" Plant Biology 21.
4 (2019): 732- 737.
[4] Crowley, Bailey, and Avery Russell.
"Plant biology: Nectar bacteria grow by germinating and bursting pollen.
" Current Biology 31.
19 (2021): R1120-R1122.
[5] Christensen, Shawn M.
, Ivan Munkres, and Rachel L.
Vannette.
"Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness.
" Current Biology 31.
19 (2021): 4373-4380.
[6] Alvarez-Perez, Sergio,Carlos M.
Herrera, and Clara de Vega.
"Zooming-in on floral nectar: ​​a first exploration of nectar-associated bacteria in wild plant communities.
" FEMS Microbiology Ecology 80.
3 (2012): 591-602.