Next Generation Biosensor Reveals Gibberellin's Critical Role in Legume Nitrogen-Fixation

Jul 25, 2024

Researchers at the University of Cambridge have demonstrated that the plant hormone gibberellin (GA) is essential for the formation and maturation of nitrogen-fixing root nodules in legumes and can also increase nodule size.

Cereal crops like wheat, maize and rice are nitrogen-hungry crops and depend heavily on synthetic fertilizers to meet their nitrogen needs. However, synthetic nitrogen fertilizers require an enormous amount of energy to manufacture, are expensive for farmers and cause negative environmental impacts like water pollution.

Unlike cereals, legumes, like peas, beans and pulses, can obtain their own nitrogen through a natural symbiotic relationship with nitrogen-fixing bacteria, forming lateral root-derived organs called nodules. This nitrogen-fixing ability also leads to higher protein content in legume crops, making them more nutritious for human consumption.

However, legume crops stop producing root nodules when the soil has relatively high concentrations of nitrogen and, as a result, potentially miss out on producing higher yields.

Scientists around the world are working on how to both boost legume yields and transfer nitrogen-fixing abilities from legumes to cereals, but this involves unraveling and understanding the complex genetic and biochemical pathways involved in nodule formation and nitrogen fixation.

In research published in The Plant Cell, Dr. Alexander Jones' research group at the Sainsbury Laboratory Cambridge University (SLCU) and Professor Giles Oldroyd's group at the Crop Science Centre have made a major step towards this goal by revealing the GA dynamics that govern the development, morphology and function of nitrogen-fixing root nodules.

Dr. Jones said, "There were some confusing and conflicting reports about the function of GA in nodule symbiosis. Experiments showed that adding GA reduces nodulation and removing GA increases nodulation in legumes like Medicago truncatula, which suggests GA is antagonistic towards nodulation. But there is also a legume mutant in peas that produces less GA and has fewer nodules, which suggests that GA is somehow required for nodulation."

"These conflicting results suggest there is probably something going on with spatial-temporal GA patterning. For example, there may be specific places where GA needs to be and some places where it needs to be absent. Or that the precise concentration of GA is important."

Using the highly sensitive next-generation biosensor nlsGIBBERELLIN PERCEPTION SENSOR 2 (GPS2) developed by the Jones Group, Dr. Colleen Drapek was able to visualize exactly where and when GA was present and in what relative concentrations it occurred. She found GA accumulated in the nodule primordium (the zone in the root cortex where cells start dividing in the early stages of nodule formation) in Medicago infected with rhizobium bacteria.

Dr. Drapek said, "Right at the beginning of nodule formation you start to see an accumulation of GA in the nodule primordia, but very little GA anywhere else in the root. As the root nodule further develops, you see GA accumulating at quite high concentrations and remaining at high levels in the mature nodule."

Dr. Drapek used GA and symbiotic Medicago mutants to further test what GA was doing by targeting overexpression of enzymes that break GA down or synthesize GA. The result for the former was that no nodules formed and the latter had larger nodules. She adds, "This shows GA is very important for nodules, but that its function is specific to zones where the nodule is being initiated and not surrounding areas. We know that low GA is good for the initial rhizobium infection of the roots, but then later you need GA to be present for the nodulation process to proceed and for nodules to mature."

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Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.