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QUORUM SENSING AND AUTO INDUCERS IN THE SOIL MICROBIOME

One of the true rock stars in the soil ecology world is our good colony—compared to when there are multiple colonies of
friend Dr. Christine Jones. She is an internationally renowned many kinds of bacteria.
and highly respected groundcover and soils ecologist and has Quorum sensing in the soil microbiome enables multi-spe-
taught here at our farm several times in recent years. She has cies crops and pastures to function more effectively than
a wealth of experience working with innovative landholders monocultures. Once the diversity of plants and hence the
to implement regenerative land management practices that diversity of functional groups of soil microbes reaches a
enhance biodiversity, increase biological activity, sequester certain threshold—or quorum—everything changes. The
carbon, activate soil nutrient cycles, restore water balance,
improve productivity, and create new topsoil. microbial community begins to function as a coordinated
“super-organism” and can perform tasks that individual
In human society, a quorum is the number of members of an microbes cannot achieve alone. The lights come on, not
organization that must be present in order for decisions to be unlike the bioluminescent marine bacteria that suddenly
made and business to be transacted. In the microbial world, shine brightly in a dark ocean.
the term quorum sensing (QS) refers to density dependent
coordinated behavior that regulates gene expression in the Quorum sensing also helps explain how biostimulants im-
microbial population and/or in the host plant or animal. prove plant health, even at very low concentrations. The
Quorum sensing was first described in the 1960s in relation biochemical signals mimic plant and microbial diversity,
to the expression of bioluminescence in the marine bacteri- resulting in the production of growth stimulating and plant
um Vibrio fischeri. When free-living in the ocean, V. fischeri protection hormones.
is non-luminescent, but when populations reach a critical Disease-causing organisms use quorum sensing to express
population density they “shine”... but only in the dark. The virulence and pathogenicity. The good news is that once the
bacteria know “how many” of them there are—and they configuration of the signals has been determined, they can
also know that it’s dark. be scrambled and rendered ineffective by a process termed
Microbes can’t see, think, or hear. But by means of chemi- “quorum quenching” (QQ). Quorum quenching is proving
cal signals, called auto-inducers, they have the capacity to to be more effective than antibiotics and fungicides, which
detect how many others are in their vicinity—both of their kill everything, good or bad.
own species and of other species. In the last decade, research
into quorum sensing has grown exponentially. It is now rec- In soils, both QS and QQ are important for the function and
ognized that quorum sensing is utilized by bacteria, archaea, resilience of plant communities, not only in the face of biotic
fungi, and viruses in all habitats—in water, on land, in plants, stresses (e.g. pests and diseases) but also in regards to pro-
on plants, in the soil, and in animals and humans. moting health, abundance and resilience in the face of abiotic

Social insects like ants and bees also use signals to com- stress (such as drought, frost, and nutrient deficiencies).
municate. A single bee behaves very differently to a colony There is much to be gained by applying our understanding of
of bees. Similarly, a single bacterium behaves very differ- quorum sensing in the agricultural space. QS is the only pro-
ently to a colony of bacteria. And even a colony of one cess that adequately explains the extraordinary results (such
kind of bacteria behaves very differently when it is the only as abundant nutrient availability and enhanced drought tol-
erance) observed once plant diversity—and hence microbial
diversity—each a critical threshold, or tipping point.
The flip side to quorum sensing is that when there are not
enough microbes to form a quorum, nothing happens.
No matter whether it is in the human or animal gut—or
in the soil—when microbial populations do not attain a
Photos by Phill Lee quorum some very important genes (that plants, animals,

and people require for immunity, for example) get switched

today in human, animal, plant and soil health.
Left: Crops seeded into chemically-fallowed soil in the presence of high rates of N have bare off. The lights go out... which is precisely what’s happening
roots. In the absence of a microbial quorum there is no protection from pests and diseases
and no soil building. We need to figure out how to turn the lights back on...
Right: The roots of crops direct drilled into diverse cover without the use of high-analysis and fast.
fertilizers support a protective, soil-building microbial quorum.

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