The extraordinary underground intelligence of plants, and what tropical gardeners are only beginning to discover.
We spend our lives looking at leaves. We inspect them for damage, disease, discolouration and insects. We admire the flowers, argue over fertilisers and wonder why one plant flourishes while another, growing only metres away, struggles.
Yet some of the most remarkable things a plant ever does are happening on the few millimetres of root we never see.
Scientists are discovering that the area immediately surrounding a living root is far more sophisticated than we once imagined.
Some plants can even release their own DNA into the soil to trap invading pathogens.
Roots can cast DNA nets
At the growing tips of many plants, specialised cells known as border cells separate from the root and enter the surrounding soil.
These cells release a complex mixture of mucilage, proteins, metabolites and extracellular DNA.
That DNA can form microscopic nets capable of entangling certain pathogens, helping prevent them from reaching vulnerable root tissues.
Scientists call these structures root extracellular traps.
There is a remarkable parallel here with human immunity. Our neutrophils, a type of white blood cell, can also release extracellular DNA to form nets that capture pathogens.
A plant root, it turns out, has some extraordinary defensive abilities of its own.
And that raises an interesting question. What happens to these defences when the root is growing in compacted, chemically stressed or biologically impoverished soil?
A sick plant can change who comes to its rescue
Plants are not passive occupants of the ground.
When attacked by pathogens or exposed to drought, salinity or nutrient stress, they can alter the chemistry of the substances released through their roots.
These substances, known as root exudates, include sugars, organic acids, phenolics and other compounds.
Changes in these exudates can influence which microorganisms gather around the root.
Researchers have given this phenomenon the memorable name the cry for help hypothesis.
It is more complicated than a plant simply calling for rescuers. The chemicals released, the microorganisms present and the surrounding soil conditions all influence what happens next.
Nevertheless, the discovery changes how we should think about a garden.
A plant can actively influence the biological community immediately surrounding its roots.
Plants communicate using pulses of calcium
Here is another discovery that deserves attention.
Plants generate electrical signals and calcium oscillations. The frequency and pattern of these signals can carry biological information.
When certain beneficial microorganisms approach a root and begin establishing a symbiotic relationship, repeated pulses of calcium can occur inside the nuclei of root cells.
The plant's cellular machinery interprets these patterns.
Research published in Nature Plants in 2025 demonstrated that altering the frequency of these calcium oscillations could influence root symbiosis and nutrient acquisition.
The researchers even described components of the signalling machinery as a heart and a pacemaker.
Think about the implications.
The relationship between a plant and a microorganism involves recognition, signalling and a coordinated biological response.
When we talk about soil microorganisms, we are not talking about organisms wandering around underground performing random good deeds.
There is communication occurring at a cellular level.
What else is travelling with that mite?
Anyone who grows orchids or collects tropical plants knows that mites can be troublesome.
But some mites carry something considerably more serious than the damage caused by their feeding. Certain tiny flat mites belonging to the genus Brevipalpus transmit plant viruses.
These include viruses associated with orchid fleck disease and, overseas, citrus leprosis.
Australian biosecurity information records that one strain of orchid fleck virus, OFV-Orc1, is already widespread in Australia and has been detected in orchids and other plant families.
Other related viral strains remain biosecurity concerns.
So when you discover mites on a treasured orchid, there is another question worth asking. What else is travelling with that mite?
The microscopic predators in your soil
Ginger enthusiasts have another reason to take an interest in underground biology.
The guava root-knot nematode, Meloidogyne enterolobii, is a serious plant-parasitic nematode of concern to tropical and subtropical horticulture.
It has been reported in Queensland, the Northern Territory and NSW, and can affect ginger and other susceptible plants.
But not all nematodes are plant parasites.
Soils can contain predatory nematodes, bacteria and fungi that attack plant-parasitic nematodes or their eggs.
Australian agricultural guidance recognises that organically amended soils can support some of these natural antagonists.
It is a fascinating reminder that the organisms we cannot see are not necessarily our enemies.
Some are predators. Some are decomposers. Some form intimate relationships with roots. Others influence nutrient availability and soil structure.
Understanding this underground ecology is a different proposition from treating every unwanted organism as an isolated enemy.
The top of your tropical tree may be cooking
Have you noticed how the upper leaves of a tall tropical plant or tree can appear scorched while the foliage below remains relatively unaffected?
There may be more happening than simple exposure to sunshine.
Air temperature is not leaf temperature.
In a major tropical forest study published in Nature, researchers found that the average critical temperature at which the photosynthetic machinery of tropical tree leaves begins to fail was approximately 46.7°C.
Individual upper-canopy leaves were recorded exceeding 40°C.
A gardener might quite reasonably say that the day was only 36 degrees, without realising that exposed leaves high above their head could be considerably hotter.
There is another process involved.
As atmospheric dryness increases, the air draws more moisture from plants. Scientists describe this demand using vapour pressure deficit, or VPD.
The plant must maintain a continuous movement of water from the soil, through the roots and conducting tissues, into the leaves.
Under severe stress, air blockages called embolisms can develop inside the water-conducting system, interfering with that movement.
Australian research has linked severe hydraulic impairment with canopy browning and dieback.
So when the top of a tropical tree looks burnt, perhaps the interesting question isn't simply what landed on the leaf.
Did the plant's hydraulic system fail to keep that leaf cool?
And where did that journey of water begin? In the soil.
The world around a root deserves another look
As Australia's summer approaches, some gardeners will contend with prolonged heat and dry conditions, while others may face heavy rainfall and saturated ground.
Both extremes bring us back to the physical and biological environment surrounding roots.
Water must enter the soil, oxygen must remain available, nutrients must cycle, and microorganisms must be able to carry out their many different functions.
At Earthfood, our work with living soil microorganisms has made this underground world a particular interest of mine.
The more we learn about root biology, the more remarkable it becomes.
A plant can alter the chemistry of its surroundings, interact with microorganisms through cellular signals and deploy DNA-based defences against pathogens. Its survival above ground is closely connected to processes occurring below it.
And we are still discovering how much of this works.
Perhaps the next time you admire a magnificent tropical plant, spare a thought for the few millimetres of living soil surrounding its roots.
There is an extraordinary amount happening there.
Earthfood Functional Soil microbes are the only ones of their kind to make sure the Mother Natures repair tool kit keeps her plants functioning the way they should. Every herbicide, every insecticide and high nitrogen fertiliser kills your soil microbe - now you know what to do to get the garden you love and dreamed of.
Bronwyn Holm, Founder, Earthfood®
Farmers' Friend • Gardeners' Guide • Soil Advocate • Growers' Voice