How Minirhizotrons for Nematode Detection Improve Root Health Monitoring

• Early studies used minirhizotron to understand how nematode-roots interaction impacts yield.
• Minirhizotrons have also been used to quantify nematode damage and cultivar selection.
• Minirhizotrons can show symptoms like root swelling and root trait changes at infection sites.

Root studies have increased in number recently due to novel data collection techniques, such as imaging through minirhizotron systems. Most studies focus on gathering information on root morphological traits and temporal growth dynamics. Minirhizotron applications to study pathogen infestations are less common but can significantly help in controlling root diseases, as proven by the research discussed in this article. Find out how scientists are using minirhizotron systems for nematode infection detection.

1. Potato Cyst Nematodes Effect on Cultivars

An early study on root diseases using minirhizotrons was published in 1995. The study by de Ruijter & van Oijen (1995) was on cyst nematodes or Globodera spp, which can cause significant yield losses, depending on cultivar, nematode density, and soil type. How the factors reduced the yield was unknown. So, the scientists studied the effect of the nematodes on plant growth parameters like leaf area, photosynthesis, and senescence in soils with three different compaction (loose, lightly compacted, and severely compacted).

The scientists conducted the experiments over two years—in 1991, they used two G. pallida-resistant cultivars, and in 1992, they used two susceptible cultivars. Half the area was fumigated before transplanting plantlets, and nematode counts were taken 2-4 weeks after fumigation.

Minirhizotrons were placed in all plots except those with intermediated soil compaction to a depth of 0.85 meters between plants to be equidistant to the four nearest plants. Imaging used videos. The root parameters estimated with imaging were root length and root senescence rate, which were compared by comparing the presence and absence of roots between readings. The root length data was used to find the roots’ vertical and horizontal spatial distribution.

Figure 1: “Time courses of total root length (km m-2), for various treatments. -o- Non-compacted & fumigated; -●- compacted & fumigated; -Δ- non-compacted & not fumigated; -▲- compacted & not fumigated,” de Ruijter & van Oijen (1994). (Image credits: Ann. appl. Bioi. (1995), 127:499-520)

The results showed that nematode density and soil compaction effects were additive and reduced yield in all cultivars except one resistant variety-Elles.

Root-shoot ratios, soil compaction, and nematode density increased biomass allocation to roots. Nematodes increased specific root length in all cultivars, but also root senescence.

Nematodes reduced crop growth, specific leaf area, and photosynthesis and increased leaf senescence. Though soil compaction did not affect stem and tubers, it reduced carbon partitioning to leaves in the early stages but increased it later.

Cv. Elles had the least root senescence and the highest total root length and grew thicker roots. The nutrient absorption was also best in Elles. Cultivar Elles had better nematode resistance because it maintained thick roots and high root length density, but these aspects didn’t help against soil compaction.

These kinds of early studies were able to show the root-shoot connections that change due to nematode attacks.

2. Nematode Affects Grass Rooting Dynamics

Figure 2: “Microplot study of rooting dynamics of bermudagrass and St. Augustinegrass genotypes using minirhizotron method,” Aryal et al. 2015.   (Image credits: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4755707/)

St. Augustinegrass (Stenotaphrum secundatum) and Bermudagrass (Cynodon spp.) are popular perennial turf grasses used in the tropics on sports fields. Nematodes, which directly affect roots, are the most important pests of turfgrasses in Florida. Belonolaimus longicaudatus is the major nematode that causes root damage and makes grasses susceptible to drought and heat, affecting growth and aesthetics.

Since only a few chemicals can control nematodes in golf courses, scientists are developing cultivars that tolerate B. longicaudatus.

Aryal et al. (2015) experimented on five bermudagrass and two St. Augustinegrass cultivars with varying tolerance to B. longicaudatus to see how the pathogen affects their root systems and evaluate their tolerance.

The study used minirhizotrons, which allowed non-destructive tracking of root dynamics over two years in a microplot (see Figure 2). The transparent root tubes went 45 cm deep, and root images were captured using a camera between soil depths of 1.0 cm to 15 cm once a month between April and September. The root parameters checked were root length, diameter, surface area, and volume.

Three bermudagrass and one St. Augustinegrass cultivar that were nematode-tolerant showed significant differences in root characteristics at different soil depths and two tolerance patterns compared to susceptible cultivars.

Bermudagrass ‘Celebration’ and ‘PI 291590’ have a more extensive root system and perform better during nematode infection than the standard Tifway cultivar. Cultivar ‘TifSport’ had minimal root loss due to B. longicaudatus infection. ‘Celebration,’ ‘PI 291590’, and ‘TifSport’ had more root diameter, increasing root surface area and volume. They also had more fine roots, adding to the total root length and average root diameter. Similarly, St. Augustinegrass cv Floratam had higher values for the root parameters than susceptible cv ‘FX 313.’

Most nematodes in turf ecosystems occur in the upper 5 cm of soil depth, while most roots are 0-10 cm deep. This allows water and nutrient absorption from deeper roots, where nematode damage is less.

Usually, grass height is mowed low in golf courses, reducing the photosynthetic rate and allocation of resources to develop deeper roots. The tolerant cultivars’ genetically driven deeper root systems thus improve nutrient and water uptake efficiency, prevent drought from nematode attacks, and reduce irrigation and nutrient inputs.

3. Root-knot Nematode Infection Observation

Root-knot nematodes (Meloidogyne spp.) are a major global threat to fresh produce, including eggplants, legumes, melons, and cruciferous vegetables. Nematodes cause severe infection and damage and billions of dollars are spent controlling them.

The parasitic nematodes establish permanent feed sites in host roots, causing galls on roots that reduce nutrient and water absorption and translocation. The galls can be 1 to 20 mm and coalesce to distort roots. Root damage also leads to secondary infection by other pathogens. Above-ground symptoms appear much later, including poor vegetative growth, necrosis, and wilting due to a lack of water.

Lu et al. (2020) decided to use adaptive minirhizotrons (or microrhizotrons) to observe the changes in tomato roots caused by Meloidogyne incognita. Tomato seedlings and several adaptive minirhizotrons preset in situ were planted in the field. The root images captured were transferred and processed to get data on root diameter and root hair changes due to the nematode infection. Root architecture was quantified and developed using a 3D architecture digitizer.

The results showed that roots began swelling 25 days after the infection. Meanwhile, the lengthening of the root hair in the swelling root sections began after 20 days—also, lateral roots formed evenly in control plants at depths over 100 cm. However, lateral root growth was more clustered in formation and concentrated in shallower depths of up to 10 cm in infected plants. Shoot height was also affected. However, the adaptive minirhizotron provided early detection and estimation of the nematode infection before symptoms appeared in shoots.

Minirhizotrons Systems

Minirhizotron systems consist of transparent root tubes pre-installed in the soil, allowing roots to grow naturally around them. A camera inserted into the root tube can take images at the required soil depths to scan roots. As the studies show, the method is non-destructive, and pictures can be taken over months and years without disturbing the roots to understand root dynamics. The CID Bio-Science Inc. offers imagers of two sizes- the CI-600 In-Situ Root Imager for a 6.35 cm root tube and the CI-602 Narrow Gauge Root Imager for a 5 cm root tube diameter.

These tools can help scientists phenotype roots for cultivar selection, understand pathogen-root dynamics, and quantify nematode damage to suggest preventive and control measures.

Sources

Aryal, S. K., Crow, W. T., McSorley, R., Giblin-Davis, R. M., Rowland, D. L., Poudel, B., & Kenworthy, K. E. (2015). Effects of infection by Belonolaimus longicaudatus on rooting dynamics among St. Augustinegrass and Bermudagrass Genotypes. Journal of Nematology, 47(4), 322.

Lu, W., Wang, X., Wang, F., & Liu, J. (2020). Fine root capture and phenotypic analysis for tomato infected with Meloidogyne incognita. Computers and Electronics in Agriculture, 173, 105455.

de Ruijter, F. J., & van Oijen, M. (1995). The effect of potato cyst nematodes and soil compaction on growth of some potato cultivars (No. 6). AB-DLO.