skip to main content

Key Takeaways

  • Drought reshapes the soil and root microbial community. 
  • In this study, root-associated Actinobacteria, particularly Streptomyces, became much more abundant under drought stress. These bacteria have characteristics that may help turfgrass tolerate drought.
  • Several microbial functions associated with stress tolerance were predicted to increase under reduced irrigation. These include functions linked to ethylene regulation, antioxidant activity and nutrient cycling.
  • A similar study conducted on turf-type tall fescue also showed that cultivars with better drought tolerance had an abundance of certain soil and root microbes – namely Actinobacteria, Basidiomycota or Glomeromycota.
  • Management practices that support soil biological activity may help maintain a root microbiome that helps turfgrass tolerate drought and other stresses.
     

Water conservation and drought tolerance are increasingly important considerations for golf courses. ‘TifTuf’ bermudagrass, drought-tolerant varieties of turf-type tall fescue, and other grasses that offer better drought tolerance are widely used to maintain high-quality playing surfaces while using less irrigation. The ability of certain grasses to tolerate drought better than others has traditionally been attributed to characteristics such as deep rooting, efficient water use, and other genetic traits that give plants the ability to maintain good turf quality under water-limited conditions. These characteristics are the result of decades of hard work by turfgrass breeders across the country. However, another potential contributor to the drought tolerance of ‘TifTuf’ and other improved grass varieties is often overlooked – the turfgrass microbiome.

Drought Tolerance: A Look Below the Surface

Every turfgrass root is associated with a diverse microbial community, including bacteria and fungi living in the rhizosphere surrounding the roots and the endosphere within roots. Collectively, these microorganisms are referred to as the plant microbiome. Rather than simply existing in the soil, many of these microbes interact closely with plants and can influence plant growth, health and responses to environmental stress.

Among these microbes are groups commonly referred to as plant growth promoting microorganisms (PGPM). These beneficial microorganisms can enhance plant growth and help plants cope with environmental stress through a variety of mechanisms. Some PGPM improve nutrient acquisition by increasing nitrogen availability or phosphorus solubilization. Others produce plant growth hormones such as auxins and cytokinins that stimulate root development. Certain microbes can also regulate stress-related plant hormones such as ethylene, helping plants maintain growth during drought conditions. In addition, many beneficial microorganisms produce antioxidants or stimulate antioxidant activity within plants, helping protect plant cells from oxidative damage caused by environmental stress (Farooq et al., 2009; Inbaraj, 2021; Naylor & Coleman-Derr, 2017).

An increasing number of studies have shown that plants do not passively tolerate drought stress; instead, they actively reshape the microbial communities associated with their roots. During drought, plants often release different compounds (exudates) from their roots that favor microbes that are better adapted to dry environments and are capable of providing stress-protective functions. Microbiologists use the terms “Gram-positive” and “Gram-negative” to classify bacteria into two broad groups distinguished by differences in cell wall structure, which can influence how they respond to environmental stresses such as drought. Studies across multiple plant systems have shown that drought often enriches Gram-positive bacterial groups, especially Actinobacteria (Naylor et al., 2017; Santos-Medellin et al., 2017; Veach et al., 2020). Actinobacteria are especially interesting because many possess characteristics that make them naturally well suited for drought conditions. Compared with many Gram-negative bacteria, they generally have thicker cell walls, greater resistance to desiccation, and the ability to produce spores or filamentous structures that improve survival under environmental stress (Ebrahimi-Zarandi et al., 2023). Many members of this group, including Streptomyces, are also recognized as important PGPM because they can produce plant growth promoting compounds, antimicrobial metabolites, osmoprotectants (molecules that help balance osmotic potential), and enzymes involved in nutrient cycling.

“An increasing number of studies have shown that plants do not passively tolerate drought stress; instead, they actively reshape the microbial communities associated with their roots.”

Studies have also shown that drought-induced microbial shifts are often more pronounced within root tissues than in surrounding soil. Plants can influence which microorganisms colonize root tissues through changes in root exudates, root chemistry and root physiology during drought stress (Compant et al., 2020; Pascale et al., 2019). These selective plant-microbe interactions may play an important role in helping plants adapt to stressful environmental conditions.

Although these plant-microbe interactions have been extensively studied in agricultural crops such as rice, sorghum, wheat and poplar, relatively little information has been available for turfgrass systems. Understanding how the turfgrass microbiome contributes to drought resilience may have important implications for future water conservation and turf management strategies.

Creating Water Deficits in the Field

To investigate how the soil and root microbiome changed under various levels of drought, we conducted a study using ‘TifTuf’ bermudagrass where plots were put under various levels of drought and then soil cores with roots were removed for microbial analysis. The study was conducted at the Lake Wheeler Turfgrass Field Laboratory at North Carolina State University using established ‘TifTuf’ bermudagrass plots. Rather than imposing drought with rainout shelters, irrigation was adjusted according to evapotranspiration (ET) replacement rates. Plots received irrigation twice per week at rates equivalent to 0%, 40%, 60%, 80%, 100% or 120% of ET losses (Figure 1a). This irrigation strategy allowed us to create a realistic range of water-deficit conditions similar to the reduced irrigation conditions commonly encountered in golf course management. Microbial communities in the following three microhabitats were analyzed using high-throughput sequencing: bulk soil, rhizosphere soil (i.e., the soil tightly associated with roots), and root endosphere (Figure 1b). A related study on turf-type tall fescue that found similar drought-induced changes in the root microbiome is discussed later in the article.

The Root Microbiome Was the Most Responsive

One of the most interesting findings was that irrigation rate had a limited effect on the total abundance of bacteria within each microhabitat. Instead, drought primarily altered the composition of the bacterial community, particularly within the root endosphere.

Microbial communities in the root endosphere changed substantially as irrigation decreased, even at the phylum level, whereas microbial communities in surrounding soil showed comparatively smaller changes (Figure 2). This suggests that the plant was not simply experiencing drought; it was actively reshaping the microbial community associated with its roots. Therefore, drought tolerance may be influenced not only by turfgrass genetics and soil physical properties, but also by beneficial microorganisms living in and around roots that help plants adapt to water-limited conditions.

“Therefore, drought tolerance may be influenced not only by turfgrass genetics and soil physical properties, but also by beneficial microorganisms living in and around roots that help plants adapt to water-limited conditions.”

A Dramatic Increase in Actinobacteria

Among all microbial groups examined, Actinobacteria showed the most striking response to drought. Actinobacteria already represented a substantial portion (approximately 34%) of the root microbiome under well-watered conditions. However, as irrigation decreased, their abundance increased dramatically. Under the driest treatment, Actinobacteria accounted for approximately half of the bacterial community living inside roots, compared with roughly one-third under full irrigation (Figure 2). At the same time, Proteobacteria declined.

To further identify the Actinobacteria most responsive to drought, we examined the top 20 bacterial operational taxonomic units (OTUs) in the root endosphere. An OTU represents a group of closely related DNA sequences and is commonly used as a proxy for a bacterial species in microbiome studies. These top 20 OTUs accounted for approximately 65% of the root endosphere bacterial community (Figure 3). Among them, eight belonged to Actinobacteria, representing about 27% of the community. Four Actinobacteria-associated OTUs were enriched under no irrigation or lower irrigation treatments and were negatively correlated with soil moisture. These OTUs belonged to Micromonosporaceae, Streptomyces, Actinosynnemataceae, and Amycolatopsis, suggesting that multiple Actinobacterial groups may contribute to the drought response of ‘TifTuf’ roots.

This result was especially intriguing because similar drought-induced enrichment of Actinobacteria has been reported in crops such as rice, sorghum, wheat and poplar. In other words, ‘TifTuf’ appears to share a microbial drought-response strategy observed across many plant systems.

Microbial Functions That May Help Turf During Drought

In our study, the increase in Actinobacteria was accompanied by increases in predicted microbial functions that could plausibly improve plant performance during drought. These included genes associated with reducing plant stress (ACC deaminase), protecting cells from oxidative damage (superoxide dismutase), breaking down organic matter (carbon degradation enzymes) and transforming nutrients into forms that plants can use (extracellular nitrogen enzymes). To better understand these potential functions, we used a software tool that predicts microbial functions based on the types of bacteria present in a sample (called PICRUSt2) to predict bacterial functional genes related to phytohormone regulation, antioxidant activity, and nutrient transformations in the root endosphere. Although these results are predictive rather than direct measurements of gene activity, they provide useful insight into how drought-enriched microbes may help ‘TifTuf’ tolerate drought stress.

One of the most notable responses was the increase in predicted ACC deaminase genes under lower irrigation rates. ACC, or 1-aminocyclopropane-1-carboxylic acid, is the precursor of ethylene, a plant hormone that often accumulates during environmental stress. Soil microorganisms capable of producing ACC deaminase can promote plant growth by sequestering and cleaving plant-produced ACC, thereby reducing ethylene levels in the plant. Because excessive ethylene production during drought can inhibit root growth and accelerate stress responses, microorganisms with ACC deaminase activity may help turfgrass better tolerate water deficits. In our study, the predicted abundance of ACC deaminase genes increased under lower irrigation rates and was associated with drought-enriched Actinobacterial taxa.

Our models also predicted increases in genes related to antioxidant activity and nutrient cycling. The SOD1 gene, which encodes superoxide dismutase type I, was promoted under 0% and 40% ET irrigation and was negatively correlated with soil moisture. This enzyme helps mitigate oxidative damage that occurs during environmental stress and can damage plant cells. In addition, genes encoding extracellular enzymes involved in nitrogen mineralization, such as chitinase and lysozyme, as well as enzymes involved in the degradation of starch, cellulose and hemicellulose, were also promoted by water stress. Together, these predicted functions suggest that drought-associated microbial communities may help ‘TifTuf’ regulate stress hormones, reduce oxidative damage, and maintain nutrient acquisition under water-limited conditions.

A Phenomenon Not Unique to Bermudagrass

A similar study conducted on turf-type tall fescue at N.C. State also found that drought substantially altered the root and soil microbiome (Hu et al., 2023). Beginning in 2018, research plots containing six different varieties of tall fescue were maintained with precipitation as the only source of water. After three years without irrigation, drought increased the abundance of Actinobacteria throughout the soil-root system and enriched Basidiomycota and Glomeromycota in the soil and root microbiome. Importantly, tall fescue cultivars with good drought tolerance consistently harbored greater abundances of these microbial groups than cultivars with a lower tolerance for drought.

Functional predictions using the same modeling as in the ‘TifTuf’ study indicated that drought-enriched microbial communities also contained more genes associated with phytohormone production, antioxidant activity and nutrient acquisition. Once again, the genus Streptomyces was strongly associated with these beneficial functions, suggesting that drought-tolerant tall fescue varieties may change microbial communities to enhance stress tolerance. Together, these findings demonstrate that drought-associated shifts in beneficial root microbiomes are not unique to bermudagrass and may represent a broader mechanism across turfgrass species.

Implications for Golf Course Management

Plant genetics remain a major driver of drought performance. ‘TifTuf’ bermudagrass, improved varieties of turf-type tall fescue, and other drought-tolerant grasses have important physiological traits that contribute to drought resilience. However, our study suggests that the microbial community associated with roots, particularly beneficial bacteria living inside root tissues, may also be an important part of the overall drought response.

Practices that maintain healthy root systems and support soil biological activity will help preserve potentially beneficial plant-microbe relationships that develop under water-limited conditions. These findings also suggest that turf response to drought should not be evaluated only by aboveground appearance. Important biological changes may already be occurring within the root system before visible symptoms appear.

“These findings also suggest that turf response to drought should not be evaluated only by aboveground appearance.”

Overall, our study showed that drought substantially reshaped the microbial community living in and around ‘TifTuf’ roots. As water became limiting, Actinobacteria became increasingly dominant, and microbial functions associated with stress tolerance, antioxidant activity and nutrient acquisition were predicted to become more abundant. ‘TifTuf’ has been widely adopted by golf courses due to its enhanced drought tolerance, which is the result of decades of hard work by turfgrass breeders. These findings suggest that part of what gives ‘TifTuf’ and other improved grass varieties their enhanced drought resilience may come from microorganisms living inside the roots. Although specific microbiome management recommendations are not yet available, practices that support soil biological activity may help maintain a beneficial root microbiome.

References

Compant, S., Cambon, M.C., Vacher, C., Mitter, B., Samad, A., & Sessitsch, A. (2020). The plant endosphere world – bacterial life within plants. Environmental Microbiology, 23(4), 1812-1829.

Ebrahimi-Zarandi, M., Etesami, H., & Glick, B.R. (2023). Fostering plant resilience to drought with Actinobacteria: Unveiling perennial allies in drought stress tolerance. Plant Stress, 10, 100242.

Farooq, M., Wahid, A., Kobayashi, N., Fujita, D., & Basra, S.M.A. (2009). Plant drought stress: effects, mechanisms and management. Agronomy for Sustainable Development, 29(1), 185-212. https://doi.org/10.1051/agro:2008021

Hu, J., Miller, G., & Shi, W. (2023). Abundance, diversity, and composition of root-associated microbial communities varied with tall fescue cultivars under water deficit. Frontiers in Microbiology, 13, 1078836.

Inbaraj, M.P. (2021). Plant-microbe interactions in alleviating abiotic stress – a mini review. Frontiers in Agronomy, 3. https://doi.org/10.3389/fagro.2021.667903

Naylor, D., & Coleman-Derr, D. (2017). Drought stress and root-associated bacterial communities. Frontiers in Plant Science, 8, 2223. https://doi.org/10.3389/fpls.2017.02223

Naylor, D., DeGraaf, S., Purdom, E., & Coleman-Derr, D. (2017). Drought and host selection influence bacterial community dynamics in the grass root microbiome. The ISME Journal, 11(12), 2691-2704. https://doi.org/10.1038/ismej.2017.118

Pascale, A., Proietti, S., Pantelides, I.S., & Stringlis, I.A. (2019). Modulation of the root microbiome by plant molecules: The basis for targeted disease suppression and plant growth promotion. Frontiers in Plant Science, 10, 1741. https://doi.org/10.3389/fpls.2019.01741

Santos-Medellin, C., Edwards, J., Liechty, Z., Nguyen, B., & Sundaresan, V. (2017). Drought stress results in a compartment-specific restructuring of the rice root-associated microbiomes. mBio, 8(4). https://doi.org/10.1128/mBio.00764-17

Veach, A.M., Chen, H., Yang, Z.K., Labbe, A.D., Engle, N.L., Tschaplinski, T.J., Schadt, C.W., & Cregger, M.A. (2020). Plant hosts modify belowground microbial community response to extreme drought. mSystems, 5(3). https://doi.org/10.1128/mSystems.00092-20