How mycorrhiza finds the root and what root exudates have to do with it
Before a mycorrhizal fungus can help a crop, root and fungus have to find each other in the dark. How their chemical conversation works, what encapsulated (bead) inoculants can and cannot do, and what it means in the field.
In brief
- Roots release sugars, acids and many other compounds, called root exudates. Among them, tiny amounts of strigolactones tell mycorrhizal fungi that a host root is near.
- Strigolactones make the fungal threads branch and switch on their energy supply. The fungus answers with its own signals, and the root builds a tunnel to let it in.
- The call works only over about a millimetre, and a crop short of phosphorus calls louder; heavy phosphate turns it down.
- Encapsulated beads keep the fungus alive in storage (still infective after 6 years at 4 °C in one study), but they open with moisture and soil microbes, not on a signal from the crop.
- Put VAM where roots will grow, into moist soil, with phosphorus by soil test, and keep the pack cool.
What root exudates are
Living roots constantly release substances into the soil around them: sugars, organic acids such as citrate and malate, amino acids and a long list of other compounds. Together they are called root exudates. Most of this material feeds the crowd of soil microbes around the root, and much of it does not last long: organic acids, for example, are quickly held by soil particles or eaten by microbes.2
A tiny part of the mix works as a message. For mycorrhizal fungi (AM fungi, sold as VAM) the most important message is a group of compounds called strigolactones.
The conversation between root and fungus
A mycorrhizal fungus cannot live for long without a root, and the root cannot see it. They find each other through a short exchange of chemical signals:3
- The spore wakes up. Moisture and warmth are enough; no root is needed. The spore grows a short thread, living on its own stored oil. If it finds no root, growth stops after a couple of weeks: the thread pulls its contents back and stays alive, ready to grow again later.4
- The root calls. Strigolactones released by the root, in extremely small amounts, make the fungal threads branch heavily5 and switch on their energy supply within an hour.6
- The fungus grows towards the root. Threads of one mycorrhizal fungus bent towards the roots of host plants and sensed them from about a millimetre away, but ignored plants that do not form mycorrhiza.7
- The fungus answers. It releases its own signals, called Myc factors,8 and the root's strigolactones make it release more of them.9 The root picks these up through a signalling pathway that legumes also use to form root nodules with Rhizobium.10
- The root opens the door. The thread forms a small attachment on the root surface, and the root cell itself builds a tunnel through which the fungus grows in.11 Inside, the fungus spreads through the outer root tissue and forms the tree-like arbuscules where it trades nutrients for the plant's sugars and fats (see types of mycorrhiza).
Strigolactones are not a mycorrhiza-only signal. They were first known as the compounds that make the seeds of parasitic weeds such as Striga germinate,5 so a crop that calls loudly for the fungus also calls these weeds.
Why a hungry crop calls louder
The plant decides how loudly to call. Tomato plants short of phosphorus made more strigolactones,12 and sorghum short of nitrogen or phosphorus released more of them from its roots.13 The reverse also holds: when phosphate is plentiful, the plant turns down the genes for the partnership, including those for making strigolactones, and colonisation falls.14
So heavy doses of soluble phosphorus do not just feed the crop; they also quieten its call to the fungus. That is one reason a VAM product used alongside a heavy phosphate dose may give little benefit.
Some crops cannot take part at all. The mustard and cabbage family has lost many of the genes the partnership needs,15 so mustard, rapeseed, cabbage and cauliflower do not form mycorrhiza, whatever is applied.
| Substance | Comes from | What it does |
|---|---|---|
| Strigolactones | Root | Make fungal threads branch and wake up their energy supply; released in extremely small amounts, more when the crop is short of phosphorus |
| Organic acids (citrate, malate) | Root | Free some nutrients from soil; quickly held by soil and eaten by microbes |
| Sugars and amino acids | Root | Mainly feed other soil microbes; the young fungal thread lives on its own stored oil |
| Myc factors | Fungus | Tell the root to prepare for the fungus |
| Bead (for example alginate) | Product | Protects the fungus in storage; opens with moisture and soil microbes |
Encapsulated mycorrhiza: what beads can and cannot do
Scientists have packed mycorrhizal fungi into small gel beads, mostly made of alginate from seaweed, since the early 1990s.16 The idea is to protect the fungus between the factory and the field. NanoKheti's Board Advisor describes one such concept in our VAM field note.
What beads have been shown to do. They keep the fungus alive in storage. Beads holding the fungus's storage bodies (vesicles) taken from colonised roots, stored at 4 °C, still colonised roots well after five years and were infective after six.17 Spores inside beads germinate, grow out and colonise roots.18
What has not been shown. It is sometimes said that root exudates "trigger" the bead to open, so that the fungus waits until a crop is near. The evidence does not support this. Spores in beads germinate without any plant present.18 Alginate beads swell with water, lose the calcium that holds them together to the soil water, and are broken down by soil microbes,19 whether or not a root is near. Strong citrate dissolves alginate in the laboratory, but the organic acids from roots are far more dilute and are held or eaten by soil within a short time.2 And reviews of encapsulated inoculants note that many formulations that work well in controlled conditions still fall short in the field.20
In short, a bead is a good way to keep the fungus alive until it is used. It does not replace putting it where roots will grow.
What this means on your farm
- Place the fungus where roots will grow. The root's call works over about a millimetre,7 so apply VAM on the seed, in the nursery, as a root dip or in the furrow, not scattered on the surface.
- Do not drown the call. Apply phosphorus according to a soil test; a heavy dose of soluble phosphate turns the partnership down.14
- Apply into moist soil. Spores wake up with moisture, and a thread that finds no root soon stops growing.4
- Keep products cool, beads or not. The fungus is alive; the best storage results for beads were in the cold.17 How to judge what is alive in a pack: see spores or infective propagules.
- Skip crops of the mustard and cabbage family. They cannot form the partnership.15
Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the summary, table, questions and links.
