The internet has been chattering about gut bacteria recently. We’re realizing that our bodies are full of other creatures necessary for our own survival. Almost 5lbs of us is gut flora. Hopefully it’s leading people to the realization that life is full of interconnections - our own body is a community.
For plants these interconnections are even more profound. Fixed in place, plants depend on the surrounding environment to provide all resources. From their distant history plants have been experts at plugging in and muddying the waters.
I Am Many
Plants themselves cannot conduct photosynthesis. They depend on an organism they trapped inside their cells around 1-2 billion years ago. Scientists are not sure if a parasitic event turned mutual, or food escaped being dinner, but an independent single-celled organism got locked inside plants from that day forward. We call them chloroplasts.
These organisms were a species of cyanobacteria, and over billions of years they lost more and more of their bodily integrity. Eventually they were reduced to being an organelle in plant cells. Although chloroplasts can swim around inside the cell, send signals to the plant to adjust its metabolic behavior, and contain their own DNA, they can no longer survive outside of a plant cell (Note: This is closely mirrored by the evolution of mitochondria in our own lineage). This is because necessary components of their genetics responsible for key compounds (and the ribosome which manufactures them) are held within the plant nucleus. When removed from their host cell, the chloroplast cannot live or even die properly, and the plant starves. The two are interwoven too deeply to survive apart.
Chloroplasts are just one example of unrelated cells that plants depend on. These formerly independent organisms turned organelles are called plastids. They perform a wide range of functions including producing pigment, aging and autumn leaf colors, and the storage of nutrients.
If these features were removed from plants, what would they be? Uncolored, non-photosynthetic organisms which did not produce or store nutrients. They wouldn’t be plants anymore. If a plant is only a plant if other organisms live inside of it, what does it mean to be a plant at all?
Associations Become Family
As has been covered in exhausting detail on the internet, many plants also interconnect with a vast network of fungi . These are soil fungi which tap inside of plant roots like a mushroom IV (this is known as Mycorrhiza) to trade nutrients back and forth. The fungal communities are composed of individual fibrous cells called Hyphae that grow into spiderweb-like networks of Mycelia. Hyphae continuously send signals back and forth as they mine the soil for minerals and trade nutrients with surrounding plants. These signals have been compared by some scientists as being akin to language.
Plants can also shuttle nutrients and chemical signals through the mycorrhizal network. A single tree can use the soil fungi to warn of drought, disease, and insect attacks. These multi-species webs can become deeply symbiotic, as in the case of Douglas Firs and Paper Birch trees which appear to be interdependent. This interconnects whole landscapes as mutualistic context-aware memory-forming mega-organisms called Holobionts (Note that none of these traits necessarily suggest sentience, although it does make one wonder what it means to be a thinking being to begin with).
Plants are excellent at producing nutrients from photosynthesis but they need minerals from the soil to complete their metabolism. Although they can and do absorb these from their roots, fungal communities are even more proficient and gladly trade for the plant’s carbohydrates.
Plants and fungi are not always altruistic. At times the fungi can be purely parasitic, taking nutrients without returning minerals. Plants too can act selfishly. Dying trees begin sending their stored nutrients into the mycorrhizal network, but nepotistically will send nutrients preferentially to their kin.
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The plant community does not cease this interconnection above the soil. Many species began to form co-evolutionary relationships with animals. Although this is widely known in the context of pollinator - flower relationships, the interdependence goes deeper.
As they are mostly immobile, plants cannot take steps to defend themselves against outside invaders (although they have significant chemical defenses available). Some species evaded this problem by recruiting insects as body guards patrolling along the plant, acting not unlike our white blood cells.
Inside of flowers are structures called nectaries, which produce the sweet liquid insects collect during pollination (this is the main ingredient in honey). This structure has opened a wide range of plant-insect interactions.
When the common aphid feeds on the low-nutrient venous liquid of plants, their soft squishy bodies need to constantly poop water in order to fit more food in their stomachs. These excretions are called honeydew and still contain a small amount of sugar.
Ants at some point recognized that honeydew is a great food source. Once an ant colony finds aphids, they begin to march around eating the honeydew and defending against any potential predators. We call this behavior aphid farming. Plants understandably are not benefited by a parasitic infection defended by personal security. So they began to fight back using a diplomatic soft-power approach.
Nectar provides a very similar nutrition to honeydew, and plants began to grow nectaries outside of the flower (called extrafloral nectaries or EFNs) to provide an aphid-alternative. These nectar sources also attract numerous predator insects like wasps and lady beetles, which then attack herbivores which threaten the plant. Plant communities with high enough rates of EFN-bearing plants create islands of arthropod abundance as a result of the available nectar.
Scientists believe that predatory insects are not the defenders plant seek. Instead, they appear to be redirecting ants away from aphid farming and towards patrolling their aerial parts. From an ants perspective an EFN is not significantly different than an aphid farm. They march around, attack anything that threatens their meal, and eat to their hearts content. (Plants must play this line carefully, in a poorly orchestrated system the ants will both accept the free lunches from the EFN and from the aphids. This means double the nutrient loss from the plant host). In some plants this mutualist bribery evolved into something closer to blackmail.
Life forms have each evolved unique specialties to survive. Animals became athletes with powerful movement, bacteria learned to cheat exams by swapping genetic notes, and plants specialized in chemistry. In EFNs, plants utilize their chemical skills to produce a broad range of compounds designed to entice and control the behavior of insects.
Like any other animal, ants require a balanced diet of macronutrients including carbohydrates, fats, and proteins. A lack of protein triggers a hunting drive to close the nutritional gap. Plants which sense herbivorous insect damage begin to rapidly drop their protein content in EFNs. This sends a hoard of aggressive hungry ants swarming the area looking for a quick meal.
Plants also have a more manipulative trick up their sleeve. Scientists have found that the chemicals used by ants to signal nest defense can also be released by plant EFNs. The theory suggests that when confronted by a large mammalian herbivore, this chemical triggers an all out response by the ant body guards - including rushing and biting the feet and spraying formic acid.
EFNs clearly benefit the plants which produce them. However, in agricultural contexts these relationships can break down (despite evidence suggesting there are pest management benefits which can reaped!) . In cotton fields sprayed to remove ants, the EFNs only attracted damaging herbivorous insects. Moreover many EFNs have been bred out of commercial crops - sometimes accidentally and sometimes intentionally. This means wild-type plants utilize tools for self defense which are lost in their domestic and more needy agricultural counterparts.
Humans Myopia
Plants aren’t individuals. They never have been. The web of life that links ecologies together include species as a single bead in a larger chain. When we isolate our focus on plants to individual cultivars in arbitrary locations, we lose the context that actually makes plants thrive and function.
What is a tree without its fungal community? It cannot talk to its siblings to warn them of danger. The carbon inheritance from dying trees is denied by the silence in the soil.
What is a plant without its insects? They are its external immune system, integral to their gonads and virility. They are inert, impotent, and diseased when isolated.
What is a non-native plant in a foreign land? They speak a different ecological language, are denied the community that enables them to thrive. A plant will learn to communicate with its new neighbors in time. But a flash of a human’s short life will never see the millennia of coevolution necessary to choreograph the relationship.
When we fail to view species as strands in a web of life, we fail to see the fullness and richness of what that individual actually is. If we want to champion life we must work with tapestries instead of threads.