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Global Warming Effects on Mycorrhyzal Interactions

Effects of global warming on plants

• The effects of global change that directly affect plants are increased atmospheric CO₂, changes in precipitation, and increased temperatures.
  C3 plants)^[2] decreased precipitation and higher temperatures often have the opposite effect.^[3] The current altered precipitation and temperature regimes have increased the mortality rates of plants due to hydraulic failure and carbon starvation.^[4] Increases in temperature increase evapotranspiration rates, which increase the xylem
  ’s negative pressure, which can collapse the plant’s vascular tissue leading to hydraulic failure. To prevent hydraulic failure, plants close their stomata, but this can also bring negative effects. If closed for too long, it can lead to carbon starvation because the plant cannot get enough CO₂ to photosynthesize (mostly C3 plants).
• Elevated CO₂ induces stomatal closure, which decreases evapotranspiration and increases plant water use efficiency and soil water content in dry rangelands.[5] Unlike the stomatal closure due to increased temperatures, stomatal closure due to higher CO₂ levels does not lead to carbon starvation (C4 plants).

Effects of global warming on mycorrhizal fungi     

• Mycorrhizal fungi completely rely on plants as their sole carbon source.^[6] This means that they will be affected directly by global warming and indirectly by the response its plant host has in global warming.^[6] Plant mass die-off events due to forest fires, negatively impact the density, diversity, richness and evenness of, not only the plants, but also all its mycorrhizal fungi.^[7]       
• Global warming affects the structure and growth rate of mycorrhizal fungi. Different studies have observed various results; positive and negative. When studying AMF, Hawkes et al. (2008) ^[8] found that higher temperature decreased the number of vesicles and increased the number of hyphae. In this case, higher temperatures promoted hyphal growth. Kasai et al. (2000),^[9] found that higher temperatures decrease colonization in ECMF. While, Swaty et al. (1998), found that soil temperature increased ECMF colonization.

Effects of global warming on soil

• Due to increased temperatures and higher evapotranspiration rates, soil water content is expected to decrease in some areas,^[10] leading to drought. But in later studies, taking both elevated CO₂ and warmer temperatures into account,^[5] found that the two factors have opposing effects on soil water balance. They also found that elevated CO₂ can cancel out the desiccating effects of moderately warmer temperatures on dry rangeland soil.
• Fire events change the dynamic root/soil interface. Much of the biomass of mycorrhizal fungi exists in the top 4 inches of soil, a region likely to be affected by forest fire. Therefore, the damage sustained by the mycorrhizal fungi will affect the recovery time and productivity of the plants after the fire.

Effects on interaction

• In dry habitats, increased temperatures induce drought stress on plants. It has been found that plants with mycorrhizal symbioses are less vulnerable to drought.^{[1][11][12][13]} Mycorrhizal fungi improve plant resistance to drought by increasing root surface area for water absorption.^[1] Plant roots colonized by mycorrhizae more effectively scavenge for water in the soil and more effectively absorb water and nutrients.^[11] Also, they alter the physiology and expression of genes in plants to improve its drought resistance.^[12] AMF aid in the regulation of gene expression for the phytohormone abscisic acid (ABA), which promotes stomatal closure to minimize transpirational water loss.^[13][14] They also help to regulate plasma membrane aquaporins, which control membrane permeability to water.^[15][16]
• As CO2 is one of the substrates necessary for photosynthesis, increased CO2 is expected to increase photosynthetic rates. Depending on the plant type, C3 or C4, it allocates more carbon to the shoot or root system. C4 plants allocate more carbohydrates to mycorrhizal fungi, while C3 plants the increased available carbon to enhance biomass production.^[1]

Conclusion

• As global temperatures increase, the interaction of plants with mycorrhizal fungi become more crucial. Mycorrhizal fungi help plants cope with drought stress and plants are essential for these fungi to survive. Though it is important to mention that different regions and different organism will be affected differently by global change.

Barrett, Gracie, et al. "The Direct Response of the External Mycelium of Arbuscular Mycorrhizal Fungi to Temperature and the Implications for Nutrient Transfer." Soil Biology and Biochemistry, vol. 78, 01 Nov. 2014, pp. 109–117. EBSCOhost,

doi:10.1016/j.soilbio.2014.07.025.

Bray EA. 2002. Abscisic acid regulation of gene expression during water-deficit stress in the era of the Arabidopsis genome.  Plant Cell Environ. 25(2):153-161.