Could Beneficial Fungi Help Pine Trees Survive Extreme Heat?

As temperatures rise, trees cool themselves through transpiration—the release of water from their leaves and needles. They replenish that water by drawing moisture from the soil. But during prolonged heat, trees can lose water faster than they can replace it, creating dangerous heat stress.

One potential way to strengthen forests against heat and other environmental hazards is to add beneficial fungi to the soil. Scientists are investigating how mycorrhizal fungi can support endangered plants and ecosystems.

How ectomycorrhizal fungi help trees

Ectomycorrhizal fungi live on tree roots and form a mutually beneficial partnership known as symbiosis. The fungi expand the root system, helping trees absorb more water. In return, trees provide the fungi with sugars that supply energy.

Scientists at China’s Fujian Agriculture and Forestry University tested whether this partnership could help Masson pine seedlings withstand high temperatures.

Testing fungi on pine seedlings

For the experiment, researchers grew pine seedlings with and without ectomycorrhizal fungi in a greenhouse under different temperature conditions. They first grew a fungus called Cenococcum geophyllum on a gel-like substance called agar. At the same time, pine seedlings grew for 30 days before being transferred to pots filled with natural soil.

After 15 days in the pots, researchers introduced the fungi to the seedlings by spreading agar containing the fungi over a small, 1-by-1-centimeter area at the tip of each pine root. This process is called inoculation. As the seedlings grew, the researchers expected the fungal network to expand from the initial site and colonize new root tips.

The experiment included six types of C. geophyllum strains and one fungus-free control. To simulate heat stress, the researchers repeated the treatments at 25°C (77°F) and 35°C (95°F). Each treatment was repeated in three pots containing three seedlings, for a total of 126 seedlings.

Fungal partnerships improved growth during heat stress

After 60 days, the researchers harvested the pine seedlings and used a microscope to count the number of root tips colonized by fungi. They calculated the colonization rate by dividing the number of colonized root tips by the total number of root tips.

Seedlings with a colonization rate of at least 80% were considered to have successfully formed a symbiotic relationship with the fungus. The researchers then analyzed these seedlings to investigate the effects of the fungal partnership.

Heat-stressed pine seedlings grown at 35°C were 24% to 59% smaller than seedlings grown at 25°C. Their needles also turned yellow instead of remaining healthy green. However, seedlings that formed a symbiotic relationship with ectomycorrhizal fungi had greener needles and were 80% heavier than fungus-free control seedlings.

Fungi reduced signs of plant stress

The researchers also measured three compounds linked to plant stress: malondialdehyde (MDA), proline, and nitric oxide.

Plants produce MDA when their cells are damaged, so high MDA levels indicate greater stress. Plants produce proline and nitric oxide to help repair cells and retain water, meaning higher levels of these compounds can indicate that the plant is responding to stress and recovering.

To measure MDA and proline in the pine needles, the researchers mixed needle samples with acids that changed color depending on the amount of each compound present. To measure nitric oxide, they mixed the samples with a protein that converted the compound into a color-changing chemical.

Compared with control seedlings, those inoculated with fungi at 35°C had approximately 30% to 40% less MDA, 15% to 30% more proline, and 150% more nitric oxide in their roots.

Gene activity suggested stronger heat defenses

Finally, the researchers examined changes in gene activity in the inoculated pine seedlings. They identified active genes by analyzing RNA molecules from the roots, a process known as RNA sequencing or transcriptomics.

Seedlings inoculated with ectomycorrhizal fungi showed greater activity in genes associated with root strength, water retention, cell defense, and general plant protection than control seedlings.

Could mycorrhizal fungi strengthen forests?

By combining chemical analyses, growth measurements, and gene-activity data, the researchers concluded that partnerships with ectomycorrhizal fungi helped pine seedlings withstand heat stress.

During prolonged heating, inoculated seedlings maintained greener needles, grew more strongly, and showed lower levels of stress-related chemicals. The findings suggest that ectomycorrhizal fungi could become an ecological tool for helping trees retain water in a warming world.

The researchers recommended that future studies conduct field trials on inoculated trees to determine whether the results can be translated into practical forest-management strategies.

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