Plants are key components of the terrestrial ecosystem carbon cycle. Atmospheric CO₂ is assimilated through photosynthesis and stored in plant biomass and in the soil. The use of turfgrass is expanding due to the increasing human population and urbanization. In this review, we summarize recent carbon sequestration research in turfgrass and compare turfgrass systems to other plant systems. The soil organic carbon (SOC) stored in turfgrass systems is comparable to that in other natural and agricultural systems. Turfgrass systems are generally carbon-neutral or carbon sinks, with the exception of intensively managed areas, such as golf course greens and athletic fields. Turfgrass used in other areas, such as golf course fairways and roughs, parks, and home lawns, has the potential to contribute to carbon sequestration if proper management practices are implemented. High management inputs can increase the biomass productivity of turfgrass but do not guarantee higher SOC compared to low management inputs. Additionally, choosing the appropriate turfgrass species that are well adapted to the local climate and tolerant to stresses can maximize CO₂ assimilation and biomass productivity, although other factors, such as soil respiration, can considerably affect SOC. Future research is needed to document the complete carbon footprint, as well as to identify best management practices and appropriate turfgrass species to enhance carbon sequestration in turfgrass systems.

Epichloë spp. are naturally occurring fungal endophytic symbionts of many cool-season grasses. Infection by the fungal endophytes often confers biotic and abiotic stress tolerance to their hosts. Endophyte-mediated disease resistance is well-established in the fine fescue grass Festuca rubra subsp. rubra (strong creeping red fescue) infected with E. festucae. Resistance to fungal pathogens is not an established effect of endophyte infection of other grass species, and may therefore be unique to the fine fescues. The underlying mechanism of the disease resistance is unknown. E. festucae produces a secreted antifungal protein that is highly expressed in the infected plant tissues and may therefore be involved in the disease resistance. Most Epichloë spp. do not have a gene for a similar antifungal protein. Here we report the characterization of the E. festucae antifungal protein, designated Efe-AfpA. The antifungal protein partially purified from the apoplastic proteins of endophyte-infected plant tissue and the recombinant protein expressed in the yeast Pichia pastoris was found to have activity against the important plant pathogen Sclerotinia homoeocarpa. Efe-AfpA may therefore be a component of the disease resistance seen in endophyte-infected strong creeping red fescue.

Managed turfgrass is a common component of urban landscapes that is expanding under current land use trends. Previous studies have reported high rates of soil carbon sequestration in turfgrass, but no systematic review has summarized these rates nor evaluated how they change as turfgrass ages. Here we conducted a meta-analysis of soil carbon sequestration rates from 63 studies globally, comprised mostly of C3 grass species in the U.S., including 24 chronosequence studies that evaluated carbon changes over 75 years or longer. We showed that turfgrass established within the last ten years had a positive mean soil C sequestration rate of 5.3 Mg CO₂ ha^-1 yr^-1 (95% CI = 3.7-6.2), which is higher than rates reported for several soil conservation practices. Areas converted to turfgrass from forests were an exception, sometimes lost soil carbon, and had a cross-study mean sequestration rate that did not differ from 0. In some locations, soil C accumulated linearly with turfgrass age over several decades, but the major trend was for soil C accumulation rates to decline through time, reaching a cross-study mean sequestration rate that was not different from 0 at 50 years. We show that fitting soil C timeseries with a mechanistically derived function rather than purely empirical functions did not alter these conclusions, nor did employing equivalent soil mass versus fixed-depth carbon stock accounting. We conducted a partial greenhouse gas budget that estimated emissions from mowing, N-fertilizer production, and soil N₂O emissions. When N fertilizer was applied, average maintenance emissions offset 32% of C sequestration in recently established turfgrass. Potential emission removals by turfgrass can be maximized with reduced-input management. Management decisions that avoid losing accrued soil C-both when turfgrass is first established and when it is eventually replaced with other land-uses-will also help maximize turfgrass C sequestration potential.

Many cool-season grasses have symbiotic relationships with Epichloë (Ascomycota, Clavicipitaceae) fungal endophytes that inhabit the intercellular spaces of the above-ground parts of the host plants. The presence of the Epichloë endophytes is generally beneficial to the hosts due to enhanced tolerance to biotic and abiotic stresses conferred by the endophytes. Many Epichloë spp. are asexual, and those infections always remain asymptomatic. However, some Epichloë spp. have a sexual stage and produce a macroscopic fruiting body, a stroma, that envelops the developing inflorescence causing a syndrome termed “choke disease”. Here, we report a fungal and plant gene expression analysis of choke stroma tissue and asymptomatic inflorescence tissue of Epichloë festucae-infected strong creeping red fescue (Festuca rubra subsp. rubra). Hundreds of fungal genes and over 10% of the plant genes were differentially expressed when comparing the two tissue types. The differentially expressed fungal genes in the choke stroma tissue indicated a change in carbohydrate and lipid metabolism, as well as a change in expression of numerous genes for candidate effector proteins. Plant stress-related genes were up-regulated in the stroma tissue, suggesting the plant host was responding to the epiphytic stage of E. festucae as a pathogen.

Strong creeping red fescue (Festuca rubra subsp. rubra) is a commercially important low-maintenance turfgrass and is often naturally infected with the fungal endophyte Epichloëfestucae. Epichloë spp. are endophytes of several cool-season grass species, often conferring insect resistance to the grass hosts due to the production of toxic alkaloids. In addition to insect resistance, a unique feature of the strong creeping red fescue/E. festucae symbiosis is the endophyte-mediated disease resistance to the fungal pathogen Clarireedia jacksonii, the causal agent of dollar spot disease. Such disease resistance is not a general feature of other grass/ Epichloë interactions. E. festucae isolates infecting red fescue have an antifungal protein gene Efe-afpA, whereas most other Epichloë spp. do not have a similar gene. The uniqueness of this gene suggests it may, therefore, be a component of the unique disease resistance seen in endophyte-infected red fescue. Here, we report the generation of CRISPR-Cas9 Efe-afpA gene knockouts with the goal of determining if absence of the protein in endophyte-infected Festuca rubra leads to disease susceptibility. However, it was not possible to infect plants with the knockout isolates, although infection was possible with the wild type E. festucae and with complemented isolates. This raises the interesting possibility that, in addition to having antifungal activity, the protein is required for the symbiotic interaction. The antifungal protein is a small secreted protein with high expression in planta relative to its expression in culture, all characteristics consistent with effector proteins. If Efe-AfpA is an effector protein it must be specific to certain interactions, since most Epichloë spp. do not have such a gene in their genomes.