Most humans feel pretty adept and evolved in their ability to communicate, however, our skills pale in comparison to microbiology. Imagine, down in the soil, away from light, there is no need for eyes. Instead, microbes have incredible sensory systems to determine; are you food, friend, or foe?
If you zoom into a cell, you’ll see that every single cell wall, in microbes, plants and animals, has hundreds of thousands of receptors ready to bind to signaling molecules. These receptors function like keys, turning gene expression on or off. This switching process can influence cell metabolism, growth, group behaviors, and responses to nutrient, energy, hormonal, pathogens, and environmental cues.
The number and nature of the understandings around plant and biological “infochemicals” is a rapidly growing field of study. There is an incredible diversity of cross-talk between plants and microbes, plants to insects, and microbes to each other, that we can harness to improve our food production outcomes. Appreciating how small these signals are, and how subtle these soil systems can be, provides access for new actions and understanding.
Plant root exudates are a conductor for this underground microbial chatter, sending out packages containing amino acids, sugars, hormones, secondary metabolites, and organic acids, altering microbial biomass and species diversity. Root exudates provide diverse benefits, as payments to stimulate the microbes providing beneficial services, reducing competition from other plants, or as signals to suppress and defend against pathogens and pests. These exudates also play a key role in aggregate stability and carbon and nitrogen dynamics.
One group of molecules produced by plants and microbes are called volatile organic compounds (VOCs). These VOC can easily move through gas and water-filled pores. VOC provides multiple services to plants, triggering plant immunity, promoting plant growth, antimicrobials, food source, attractants, or as infochemicals.
Microbes are also streaming chemical and electrical signals to communicate with each other, and with their vital support system, the plants. Some microbes produce a huge variety of communication VOC (over 800 have been identified thus far). Many of the stronger odors you will be familiar with — feces (indole), urine (ammonia), vomit (butyric), rotting cabbage/farty smells (Methyl mercaptan), rotten eggs (hydrogen sulfide gas) and rotten potatoes (dimethyl disulfide).
The distinctive “funky” aroma of the mycorrhizal truffle is in part due to dimethyl disulfide and from a molecule called androsterone, a hormone produced by male pigs. This in part explains why pigs can make such good truffle hunters. The fungus sends out this signaling scent to pigs, and us, to dig it up at full maturation to aid in completing its reproductive cycle, which is limited by sporulating underground. These spores pass through the digestive tract, remaining viable when anything, or anyone, poops in the woods.
Geosmin is what gives beetroot its taste and what creates the smell after a shower of rain meets the road on a hot summer’s day. The smell of geosmin released by Streptomyces has been found to attract collembola, (springtails) which then aid in the dispersal of its spores. We can smell geosmin at 5 parts per trillion, while a shark can smell a single drop of blood in an Olympic swimming pool (1ppm). This means human noses are 200,000X more sensitive to the scent of geosmin, than a shark is to blood!
Some bacteria and plants, produce large quantities of indole, a signaling molecule between cells, species, and even kingdoms. Indole provides benefits by inhibiting biological communication and virulence and is an important hormone for plant growth. Insects sense indole, which works two-fold, firstly by attracting natural enemies of insect pests such as predators and parasitoids. Secondly, indole can also regulate plant defense systems, repelling pests.
Testate amoebae, the ones who fashion themselves in hardened shells for protection, have been observed hunting together as a pack, to track down and attack bacteria- feeding nematodes. Their behaviors are determined by these signaling molecules.
The ability for plants to communicate with microbiology is influenced by:
• Plant nutrition
• Plant diversity
• Photosynthetic by-products
• Plant stress, climate and disturbance
• Plant breeding
• Growth phase
Different plants during different growth stages, release a diverse cocktail of root metabolites. Plant stress has a huge influence on a plant’s ability to share resources. Annuals, perennials, and early succession plants send different signals depending on if they are in an active growing phase, under attack, or ending their life cycles.
Insecticides, fungicides, and herbicides including Captan, Maneb, Chlorpyrifos, and Paraquat impact directly, or indirectly, on photosynthesis and on the root exudates necessary for quorum signaling. Enzymes and signals involved in nitrogen fixation and transforming free amino acids into complete proteins are compromised by these chemicals.
Plant breeding has altered specific communications from plant roots, so that some cultivars, such as wheat varieties, no longer signal to beneficial protists for nutrient cycling or bacterial control, or no longer initiate symbioses with mycorrhizae.
We can enhance these communication pathways through buying seed that has been grown in healthy soil systems with minimal chemical use, or by biopriming seeds. We find fungal dominant composts like the Johnson-Su compost, quality vermicast, plant/weed extracts and seaweed are all rich sources of secondary metabolites for plant and microbial health. These can be applied by soaking seeds in a solution, during seeding, or applied as a foliar spray. Results from these applications at parts per trillion can be extraordinary. You can trial these for yourself using brix (measured with a refractometer) and rooting responses as response indicators. Always err on the side of “less is more,” as at high rates these metabolites can suppress growth, inhibit plant and microbial communications.
How can you tell if these signals have been disrupted? Digging holes and using all your senses; smell, sight, touch, feel, is the first step. Ignoring the underground signals, the next messages to show up will be the indicator plants (weeds), pests and diseases, natures repair team. As we deepen our ability to tune in, check is your land whispering sweet songs of vibrant love, or yelling at you to get off?! How little can it take and how subtle can we be, to improve our soil health outcomes?
Nicole Masters will be speaking on Both Days at Groundswell, more info here. She will also be signing her book “For the Love of Soil”