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Root Dynamics and Mycorrhizal Amelioration of Root Stress

Posted by: Scott Trimble
Oct. 20, 2021

The pollution caused by modern mining efforts is well documented, but the practice can also disturb soil structure and damage root systems, hampering restoration efforts in coal mines. The difficulty of studying roots and other below-ground parts of plants compounds the problems scientists face in remedying root stress. Using a miniaturized, scan-based rhizotron system, scientists followed root dynamics over time to show how arbuscular mycorrhizal fungi help damaged plants deal with root stress.

Soil Subsidence Causes Below-Ground Problems

Mining for coal takes place at the surface or underground, depending on the location of the resource. Underground mining involves excavating soil below the ground and if the roofs of underground mines are destroyed for any reason, the danger of topsoil subsidence is increased.

Soil subsidence is reported to create problems for above-ground vegetation in restoration projects of abandoned coal mines. During subsidence, soil becomes fissured. As a result, there is root damage or roots are exposed to the air, causing stress.

Any stress or damage to roots will affect plant growth. For successful restoration, plants need help to recover from the effects of subsidence.

The soil fissures and subsidence make in situ research to suggest remedies difficult. A team of Chinese scientists subverted this problem by conducting experiments in the laboratory by simulating soil fissures. These studies showed that the root damage due to fissures affects plant hormonal balance, which reduces biomass accumulation. The change in root morphology also affects the uptake of plant nutrients and the resistance plants have to stress.

Repairing Root Damage

Root morphology can be influenced not just by soil structure, but also by the availability of nutrients and arbuscular mycorrhizal fungi (AMF).

AMF stimulates the growth of lateral roots and root hairs. Hence, the use of AMF to help plants in restoration projects is a possible solution to repair root damage and help in biomass accumulation.

However, the interaction between roots and AMF is symbiotic and depends on give and take. When roots are damaged by fissuring, their functions are affected, and it is possible that they cannot provide the photosynthetic assimilates that AMF needs, like carbohydrates. In this scenario, the symbiotic interaction may not take place. Also, the relationship between AMF and the plant roots does not always have to be positive. The scientists were able to resolve this issue by showing that AMF does alleviate the negative effects of root damage due to fissures.

However, the way AMF achieved this remained a mystery. They also did not know the dynamics of the root response to damage and stress symbionts. 

Using a Root Damage Simulation Chamber

Five scientists from China’s Department of Coal Resources and Mining, and Department of Geology and Environment—Zhang, Bi, Song, Xiao, and Christie—designed a device that created the root damage stress and also allowed them to observe the subsequent root growth processes; see Figure 1. They also studied AMF interaction with plants and stress and how it affected hormonal balance, root and shoot growth, and also how it changed root morphology.

Fig. 1. “Ground fissure simulating device and the process of simulated ground fissure generation,” Zhang et al. (2021). (Image credits: https://doi.org/10.1016/j.ecolind.2021.107800)

At the start of the experiment, the scientists grew maize plants, cultivar Pinnuo 28. There were four treatments: damaged roots, with and without AMF inoculation and control, with and without AMF inoculation. The scientists used spores of Funneliformis mosseae as the AMF.  

The scientists installed minirhizotron tubes, 7 cm wide and 100 cm long, horizontally at depths of 20 and 40 cm. The tubes were 10 cm away from maize plants in the damaged and undamaged sections and the nozzles were outside the box so that a root imager could be used.

Before soil fissuring, root morphology, plant hormone levels, and leaf chlorophyll content and area were recorded.  

The scientists used the CI-600 In-Situ Root Imager to monitor root growth. They scanned the roots seven times per day, once before fissuring and then 1,2,3,4, 5, and 6 weeks/stages after fissuring. The CI-600 root imager was inserted into the minirhizotron tubes to get three images at a spacing of 20 cm in the tube. The images were sectioned and analyzed using the RootSnap! software that came with the Root Imager, manufactured by CID Bio- Science Inc. The scientists collected the total root length, average root diameter, root tip numbers, surface area, and volume from the Root Imager.  

To simulate fissuring, after the maize plants were 40 days old at maize jointing, the scientists removed the top part of the left side, which duplicated subsidence. The dimensions of the fissures and the damage caused to roots were recorded.

Leaf chlorophyll and total area, along with leaf and root hormone levels, were estimated 1, 2,  and 3 weeks after fissuring. After harvest, the root and shoot biomass was also estimated along with AMF colonization. See Figure 2 for a schematic representation of the steps in the experiment.

Figure 2: “ Flow chart of this study,” Zhang et al. (2021). (Image credits: https://doi.org/10.1016/j.ecolind.2021.107800)

AMF Improves Plant Response to Stress

By comparing AMF interaction in damaged and undamaged root systems, the scientists were able to find out how the symbiosis was useful for the maize plants after fissuring.

Mycorrhizal plants dampened the changes in root and leaf hormones due to fissuring. AMF also improved root growth, especially the percentage of fine root hairs. Similarly, there was also an increase in leaf area chlorophyll levels. Therefore, both shoot and root growth were enhanced by AMF.

Endogenous hormones are signaling substances that regulate plant physiological processes.

Before fissuring, there was no difference in hormone levels between the plants. After fissuring, damaged maize started showing a decline in levels of the auxin—Indole-3-Acetic Acid (IAA)—in both leaves and roots. This caused plants to slow their growth to reduce the effects of stress due to fissuring.

Cytokinin (CTK) is a hormone needed for leaf and shoot development and photosynthesis.  CTK levels were always positively correlated with leaf area and chlorophyll content in all plants. One week after fissuring, the damaged plants had less CTK in leaves and roots, and the leaf area and chlorophyll content were lower than in undamaged plants. Usually, CTK levels fall as plants mature, and this was seen in control plants. However, the CTK levels were much higher in damaged, mature maize because they still needed it to repair the damage.

Abscisic acid (ABA) is produced in response to stress. Its levels increased in both damaged plants, with and without AMF. As its levels increased in the initial stages, there was a reduction in root tips, length, area, volume, and leaf area and chlorophyll levels in damaged plants. In the second week, less ABA was found in the leaves of AMF plants than uninoculated plants; this indicates the plant is strengthening its roots systems at the expense of the shoots in response to stress. In later stages, after six weeks, the level of this hormone was similar in damaged and undamaged maize, showing that the plants had adapted to the fissuring stress.

The study showed that AMF, and not the normal plant response to fissuring, boosted plant growth. AMF reduces ABA production and increases IAA and CTK levels so that the overall hormone balance helps plants to resist stress and favor plant growth in comparison to control maize plants.

Figure 3: “Roots observed in the minirhizotron tubes of the treatments, Zhang et al. (2021). (Image credits: https://doi.org/10.1016/j.ecolind.2021.107800)

The root morphology studied through minirhizotrons showed that there were more roots of damaged plants near the tubes than of undamaged plants. This could be because the soil fissuring reduces the space for roots to grow in damaged areas so there is a higher concentration in unfissured areas; see Figure 3. This limits plants' access to nutrients.

A difference was recorded in fine root percentage in the treatments. At a depth of around 20 cm, the damaged plants, with and without AMF, had more fine roots than undamaged plants one week after fissuring. Moreover, the root length in AMF plants (M) was higher in damaged plants than in uninoculated plants (CK); see Figure 4. In undamaged plants, AMF did not influence root growth.

The scientists concluded the plants were producing more fine roots due to stress. The production of fine roots enabled the plants to search for more food in an effort to resist stress, and AMF was giving them a boost. Increases in IAA by AMF aids this process.

This study also indicated that AMF action is more helpful when the root damage is of low severity, but this aspect will need to be confirmed and quantified by further studies.
 

Figure 4: The number of root tips and fine root percentage at 20 cm depth is higher in AMF inoculated plants (M) than in control plants (CK) with damaged roots (RD). Zhang et al. (2021). (Image credits: https://doi.org/10.1016/j.ecolind.2021.107800)

Fissure-caused root damage reduces the root-to-shoot ratio. With a reduced root system, plants have fewer nutrients and water. The damaging effects of fissuring on roots start within a week, and the impact lasts until the end of the plant’s life. The biomass of damaged plants is far lower than undamaged plants. However, AMF improves root growth and fine root percentage, and along with the positive hormonal balance it creates, it can increase biomass.

Minirhizotrons Shed Light Below Ground

The study, once again, proves what scientists are discovering about the effects of root and shoot damage on plants. Plants recover more easily from shoot damage. Root damage and stress has a greater impact on plant growth and productivity. Therefore, technological advancements like minirhizotrons have far-reaching benefits, advancing our knowledge of below-ground plant dynamics.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Sources

Zhang, J., Bi, Y., Song, Z., Xiao, L., & Christie, P. (2021). Arbuscular mycorrhizal fungi alter root and foliar responses to fissure-induced root damage stress. Ecological Indicators, 127, 107800. https://doi.org/10.1016/j.ecolind.2021.107800

Prakash, A., & Gens, R. (2011). Chapter 14 - Remote Sensing of Coal Fres. In  G.B. Stracher,  A. Prakash, &  E. V. Sokol, Coal and Peat Fires: A Global Perspective (pp 231-253). Elsevier. https://doi.org/10.1016/B978-0-444-52858-2.00014-1


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