Grasses and Epichloë Endophytes

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Kendall Lee Carroll Lee	+ 1940 word(s)	1940	2021-10-26 08:46:48	\|
2	h	Camila Xu	Meta information modification	1940	2021-11-02 05:09:48	\|

This entry is adapted from the peer-reviewed paper 10.3390/microorganisms9112186

Epichloë species of fungi that confer abiotic and biotic stress tolerances to the grass.

endophyte Epichloë forage mechanisms stress

1. Introduction

Poaceae is a large taxonomic family of grasses. Most grasses used for forage originated in Europe, Africa, and western Asia, with over 600 species from this family being used worldwide for livestock forage ^[1]. Typically, these grasses are cross-pollinated polyploids with complicated genomes. In America, grasses occupy over 238 million ha of land used for forage production ^[2]. Forage grasses are essential to livestock operations and can be native or introduced, perennial or annual, and cool-season or warm-season depending on the region and farmer preference. Pasture-fed beef and dairy are gaining popularity in the United States and are already the primary mode of livestock production in places like New Zealand, South America, and Europe. The hardiness and endurance of the grass is relied on by livestock farmers. Persistence under grazing pressure, high nutritive value, drought resistance, insect resistance and high yield are important criteria for the sustainability of forage production and climate resilience of these species. Temperate grasses, also known as cool-season, are an important class of forages that sustain the supply of most of the world’s beef and dairy products ^[3]. Several cool-season grasses are known to establish a fungal association with Epichloë species of fungi that confer abiotic and biotic stress tolerances to the grass. Biotic stressors refer to living elements and include weeds, insects, and diseases that affect plant health and compromise yield. Abiotic stressors refer to non-living elements that could harm the crop, such as drought and flood. A major difference between forage grasses and other crops is that grazing livestock is the main biotic stressor. Withstanding grazing pressures is one of the most important stress tolerances forage grasses need to possess. Producers often choose not to spray chemical pesticides for insects in grazing situations due to the cost versus return of the treatment, but spraying weeds is more common ^[4]. Abiotic stress tolerance is necessary for forage grasses as a less rigorously managed crop. Most pastures are subjected to drought, flood, nutrient deficiency, high temperature, frost, etc., without much intervention from producers. This leads producers to search for forage grasses that are naturally tolerant to many of these stresses.

2. Evolution of Epichloë Mutualistic Association with Grasses

Epichloë fungi often form symbiotic mutualistic relationships with cool-season grasses. Nearly 30% of cool-season grasses across the globe are known to form a relationship with Epichloë species ^[5]. The fungus provides the plant with protection through secondary metabolite production and the plant provides nutrients and accommodation to the fungus. The alkaloids produced by Epichloë species have four major classes called lolines, indole-diterpenes, peramines, and ergot alkaloids. These different alkaloids confer protection against a variety of abiotic and biotic stressors. Peramines and lolines are known for their insect deterrence and insecticidal properties, whereas ergot alkaloids and indole diterpenes have mammalian toxicities ^[6].

Epichloë fungi can have sexual and/or asexual transmission. While sexual Epichloë mutualism with grasses does occur, such as E. typhina with Dactylis glomerata (orchard grass), it is more common that asexual forms of the fungus inhabit perennial cool season species ^[7]. Some common asexual Epichloë relationships with cool-season grasses include E. coenophiala with Festuca arundinacea (tall fescue), E. festucae var. lolii with Lolium perenne (perennial ryegrass), and E. occultans with Lolium multiflorum (annual ryegrass).

Phylogenetic, genetic, and physiological evidence suggest that Epichloë endophytes and cool-season grass hosts co-adapted together over a long period of time ^[8]^[9]. Most of the mutualistic Epichloë species are asexually reproduced by vertical transmission. Evolutionary theory states that vertical transmission is an indicator of a strong mutualist as asexuality leads to very little response to selection due to decreased recombination and genetic diversity ^[8]^[10]. This contrasts with the host that reproduces sexually and therefore can quickly respond to selective pressures. This system of the host having increased gene flow compared to the symbiont is consistent with Law’s Hypothesis, which states that loss of sexual reproduction is often a precursor to mutualism, as it reduces the chance of the symbiont becoming pathogenic again. Many asexual Epichloë species are interspecific hybrids that arose from sexual progenitors. The exact mechanism by which hybridization occurred is unclear, but it likely arose from the fusion of hyphae and nuclei or abnormal segregation in mating between sympatric Epichloë species ^[11]. The change from sexual reproduction to asexuality eliminates a primary mechanism of pathogenicity ^[12]. The asexual interspecific hybrid species often show increased fitness compared to their sexual progenitors by alkaloid gene loci pyramiding ^[13], which likely led to more than half of Epichloë species that are now hybrids ^[12]. Post-hybridization, co-adaptation of the fungus and grass occurs in a host-specific manner. These facts present a strong argument for a long evolutionary history of mutualism between Epichloë species and their grass hosts.

It is likely that asexual hybrid Epichloë symbionts co-diverged at the same time ^[8]^[9]. Previous research using these genes has found that many Epichloë species display co-phylogeny with their grass host ^[14]. Schardl et al. ^[9] found that early cladogenesis events corresponded between Pooidae (a subfamily of Poaceae) and Epichloë. They also found that within the tribe Poeae, a group that contained Lolium species showed mirrored topology to their Lolium-associated-clade endophytes. These results provide evidence for co-divergence between the species.

3. Mechanisms of Interaction between Grass and Endophyte

Epichloë species are known for producing bioprotective compounds known as alkaloids ^[15]. The most commonly produced alkaloids are lolines, indole-diterpenes, ergot alkaloids, and peramines. Genes or gene clusters have been identified for these alkaloids. These gene clusters consist of 10 to 12 genes for EAS (ergot alkaloid), IDT (indole-diterpene) and LOL (loline), depending on the species ^[16]. A single gene, PER, was found to be responsible for peramine synthesis ^[17]. The pathways for each alkaloid are complex, with many intermediate metabolites produced that affect alkaloid composition ^[18]. Endophyte-infected (E + ) grasses have been documented to gain protection from drought, insects, nematodes, cold, flooding, heavy metals, pathogens, and mammalian herbivory ^[19]. Loline and peramine alkaloids are known insect deterrents and insecticides ^[20]^[21]. Ergot alkaloids and indole-diterpenes alkaloids have mammalian toxicity properties as well as some insect protection ^[13]. E+ plants can withstand drought and edaphic stress at a higher level than uninfected plants (E-) ^[21]. Research shows that endophytes can interact with the grass host to modify the physiology and biochemistry of the plant, such as rolling and shedding leaves under drought stress ^[22]. These types of modifications likely translate to other stressors as overall yield and persistence in a variety of environments are higher for E+ grasses than E- grasses ^[21]^[23]^[24]. The exact protections that the endophyte provides depend on alkaloid genes present as well as the genetic interplay between the endophyte and the host.

The exact mechanisms that lead to the successful mutualism between Epichloë species and cool-season grasses are not well defined. Most plants, when faced with a fungal invasion, will activate defense responses such as a hypersensitive response, which triggered cell death. Epichloë species do elicit defense responses from non-host plants ^[25]. However, endophyte-infected host grasses do not produce these responses against the endophyte. The endophyte lives and grows in the intercellular space within the plant, in synchrony with the plant maturation ^[20]. The fact that the plant does not react to the endophyte as a pathogen, and that the endophyte only grows at the same pace as the grass, suggests a significant crosstalk between the two species. There are a few mechanisms by which this may occur. A recent study proposed that the fungus secretes an inhibitor to papain-like cysteine proteases (PLCPs). PLCPs are a part of the innate plant immune response that acts to trigger signaling cascades when met with pathogens. Some pathogens utilize plant cysteine protease inhibitors to decrease the PLCP response. Passarge et al. ^[26] found that E. festucae acted similarly by reducing four active PLCPs in perennial ryegrass (L. perenne). They hypothesized that a yet-to-be-identified fungal secreted effector inhibits PLCPs. Other research has shown that endophyte-infected grasses have higher levels of phenolic compounds and other antioxidants compared to endophyte-free grasses ^[27]. Other studies have postulated that the grass recognizes the endophyte through increased production of resveratrol and chitinase, as well as other phenolics ^[28]. Reactive oxygen species (ROS) have been shown to regulate the fungal growth within perennial ryegrass. Tanaka et al. ^[28] identified a fungal mutant with an insertion in the NADPH oxidase gene noxA, which leads to dramatically increased fungal growth within the plant. NoxA is a hydrolase enzyme that regulates superoxide production. They detected ROS in the non-mutant mutualistic fungus, but not in the noxA mutant, leading them to conclude that ROS restricts fungal growth. Further ROS production is regulated by SakA (stress-activated mitogen-activated protein kinase A). It was found that when SakA is deleted, ROS production increases ^[25]. It is well known that grasses and endophytes can both change the metabolite pathways within themselves to produce different compounds ^[29], but research has shown that in some cases, the plant and endophyte can work together to produce a metabolite that could not be synthesized by either species alone ^[30]. There is also evidence that the crosstalk will vary between plant parts. Transcriptome analysis found that fungal gene expression differs between floral and vegetative parts of Epichloë-infected grasses ^[31]. This finding provides further evidence for the complicated nature of this mutualism.

Other essential factors in the mechanisms of mutualism have been found. Two Rho GTPases, Cdc42 and Rac4 have been implicated in intercalary growth regulation and hyphal network formation ^[32]. ProA, a transcriptional regulator, appears to work in tandem with symB/symC, membrane-associated proteins, to act on NOX to regulate fungi growth ^[33]. Additionally, Green et al. ^[34] found a component of STRIPAK (striatin-interacting phosphatase and kinase) called MobC to be important for mutualism and hyphal cell to cell fusion (Figure 1).

Figure 1. A visual representation of putative enzymes, genes and proteins necessary for Epichloë endophyte and grass mutualism. SakA and NoxA regulate ROS production. Cdc42 and Rac4 regulate intercalary and hyphal network growth. ProA works with SymB/SymC to regulate endophyte growth. MobC is necessary for the symbiosis of the endophyte and grass.

Specific genes have been found to be essential in the change from pathogenic infection to a mutualistic relationship in the endophyte and the host grasses. Eaton et al. ^[25] disrupted components of the Nox complex and SakA, previously revealed to be necessary for mutualism, in L. perenne infected with E. festucae and determined the transcriptomic changes that occurred (Figure 1). They found that certain classes of genes were up or down-regulated in the grass, most notably genes involved in pathogen defense, transposon activation, hormone biosynthesis, and response ^[25]. Further work carried out by Eaton et al. ^[35] identified 182 specific fungal genes associated with mutualism in the L. perenne and E. festucae system. They identified the upregulation of genes that encode degradative enzymes, transporters, and primary metabolism and the downregulation of genes that encode for secondary metabolism and production of small-secreted proteins. These are genes involved with nutrient-starvation response and indicate that disruption of the mutualism results in the endophyte, taking in more nutrients from the host, including cell wall components, consistent with pathogen feeding patterns. Further evidence of transcriptome changes that aid in mutualism has been found. Schmid et al. ^[36] discovered gene expression changes in fungi in growing grass compared to those in mature grasses with genes shifting from hyphal growth to alkaloid production ^[36]. New genes important for symbiosis continue to be found. Hettiarachchige et al. ^[17] recently identified novel genes at different stages of seedling development with Epichloë infection. They found that the first 0–4 h of growth showed the highest amount of differentially expressed genes, suggesting that genetic cascades for symbiosis are triggered quickly. Additionally, they found different novel candidate genes for secondary metabolites between the different strains of endophyte studied. This provides evidence of the highly specific nature of the endophyte-grass association.

References

Hanson, J.; Cantrell, J.; Proud, J.; Debouck, D.; Cuervo, M.; Schultze-Kraft, R.; Jorge, A. Forage: Grass. 2021. Available online: https://www.genebanks.org/resources/crops/forages-grass/ (accessed on 1 July 2021).
Sanderson, M.; Jolley, L.W.; Dobrowolski, J. Pasture and hayland in the USA: Land resources, conservation practices, and ecosystem services. In Conservation Outcomes from Pastureland and Hayland Practices: Assessment, Recommendations, and Knowledge Gaps; Allen Press: Lawrence, KS, USA, 2012; pp. 25–40.
Wilkins, P.W.; Humphreys, M.O. Progress in breeding perennial forage grasses for temperate agriculture. J. Agric. Sci. 2003, 140, 129–150.
Moser, L.E.; Hoveland, C.S. Cool-Season Grass Overview; Moser, L.E., Buxton, D.R., Casler, M.D., Eds.; ASA, CSSA, SSSA: Madison, WI, USA, 1996; Volume 34.
Leuchtmann, A. Systematics, distribution, and host specificity of grass endophytes. Nat. Toxins 1993, 1, 150–162.
Young, C.A.; Charlton, N.D.; Takach, J.E.; Swoboda, G.A.; Trammell, M.A.; Huhman, D.V.; Hopkins, A.A. Characterization of Epichloe coenophiala within the US: Are all tall fescue endophytes created equal? Front. Chem. 2014, 2, 95.
Faeth, S.H.; Hamilton, C.E. Does An asexual endophyte symbiont alter life stage and long-term survival in a perennial host grass? Microb. Ecol. 2006, 52, 748–755.
Easton, H.S. Grasses and neotyphodium endophytes: Co-adaptation and adaptive breeding. Euphytica 2007, 154, 295–306.
Schardl, C.L.; Craven, K.D.; Speakman, S.; Stromberg, A.; Lindstrom, A.; Yoshida, R. A novel test for host-symbiont codivergence indicates ancient origin of fungal endophytes in grasses. Syst. Biol. 2008, 57, 483–498.
Frank, S.A. Genetics of mutualism: The evolution of altruism between species. J. Theor. Biol. 1994, 170, 393–400.
Tsai, H.F.; Liu, J.S.; Staben, C.; Christensen, M.J.; Latch, G.C.; Siegel, M.R.; Schardl, C.L. Evolutionary diversification of fungal endophytes of tall fescue grass by hybridization with Epichloë species. Proc. Natl. Acad. Sci. USA 1994, 91, 2542–2546.
Moon, C.D.; Craven, K.D.; Leuchtmann, A.; Clement, S.L.; Schardl, C.L. Prevalence of interspecific hybrids amongst asexual fungal endophytes of grasses. Mol. Ecol. 2004, 13, 1455–1467.
Saikkonen, K.; Young, C.A.; Helander, M.; Schardl, C.L. Endophytic Epichloë species and their grass hosts: From evolution to applications. Plant Mol. Biol. 2016, 90, 665–675.
Schardl, C.L.; Leuchtmann, A.; Chung, K.R.; Penny, D.; Siegel, M.R. Coevolution by common descent of fungal symbionts (Epichloe spp.) and grass hosts. Mol. Biol. Evol. 1997, 14, 133.
Bacon, C.W.; Siegel, M.R. Endophyte parasitism of tall fescue. J. Prod. Agric. 1988, 1, 45–55.
Young, C.A.; Felitti, S.; Shields, K.; Spangenberg, G.; Johnson, R.D.; Bryan, G.T.; Saikia, S.; Scott, B. A complex gene cluster for indole-diterpene biosynthesis in the grass endophyte Neotyphodium lolii. Fungal Genet. Biol. 2006, 43, 679–693.
Hettiarachchige, I.K.; Elkins, A.C.; Reddy, P.; Mann, R.C.; Guthridge, K.M.; Sawbridge, T.I.; Forster, J.W.; Spangenberg, G.C. Genetic modification of asexual Epichloë endophytes with the perA gene for peramine biosynthesis. Mol. Genet. Genom. 2019, 294, 315–328.
Young, C.A.; Schardl, C.L.; Panaccione, D.G.; Florea, S.; Takach, J.E.; Charlton, N.D.; Moore, N.; Webb, J.S.; Jaromczyk, J. Genetics, genomics and evolution of ergot alkaloid diversity. Toxins 2015, 7, 1273–1302.
Xia, C.; Li, N.; Zhang, Y.; Li, C.; Zhang, X.; Nan, Z. Role of Epichloë endophytes in defense responses of cool-season grasses to pathogens: A Review. Plant Dis. 2018, 102, 2061–2073.
Freitas, P.P.; Hampton, J.G.; Rolston, M.P.; Glare, T.R.; Miller, P.P.; Card, S.D. A tale of two grass species: Temperature affects the symbiosis of a mutualistic Epichloë endophyte in both tall fescue and perennial ryegrass. Front. Plant Sci. 2020, 11.
Bouton, J.H.; Gates, R.N.; Belesky, D.P.; Owsley, M. Yield and persistence of tall fescue in the southeastern coastal plain after removal of its endophyte. Agron. J. 1993, 85, 52–55.
Malinowski, D.P.; Belesky, D.P. Adaptations of endophyte-infected cool-season grasses to environmental stresses: Mechanisms of drought and mineral stress tolerance. Crop. Sci. 2000, 40, 923–940.
West, C.P.; Izekor, E.; Turner, K.E.; Elmi, A.A. Endophyte effects on growth and persistence of tall fescue along a water-supply gradient. Agron. J. 1993, 85, 264–270.
Hume, D.E.; Cooper, B.; Panckhurst, K. The role of endophyte in determining the persistence and productivity of ryegrass, tall fescue and meadow fescue in Northland. In Proceedings of the New Zealand Grassland Association, Waitangi, New Zealand, 1 January 2009.
Eaton, C.J.; Cox, M.P.; Scott, B. What triggers grass endophytes to switch from mutualism to pathogenism? Plant Sci. 2011, 180, 190–195.
Passarge, A.; Demir, F.; Green, K.; Depotter, J.R.L.; Scott, B.; Huesgen, P.F.; Doehlemann, G.; Misas Villamil, J.C. Host apoplastic cysteine protease activity is suppressed during the mutualistic association of Lolium perenne and Epichloë festucae. J. Exp. Bot. 2021, 72, 3410–3426.
Malinowski, D.; Belesky, D. Epichloë (formerly neotyphodium) fungal endophytes increase adaptation of cool-season perennial grasses to environmental stresses. Acta Agrobot. 2019, 2, 72.
Tanaka, A.; Christensen, M.J.; Takemoto, D.; Park, P.; Scott, B. Reactive oxygen species play a role in regulating a fungus–perennial ryegrass mutualistic interaction. Plant Cell 2006, 18, 1052–1066.
Pimentel, M.R.; Molina, G.; Dionísio, A.P.; Maróstica Junior, M.R.; Pastore, G.M. The use of endophytes to obtain bioactive compounds and their application in biotransformation process. Biotechnol. Res. Int. 2011, 2011, 576286.
Pan, F.; Hou, K.; Gao, F.; Hu, B.; Chen, Q.; Wu, W. Peimisine and peiminine production by endophytic fungus Fusarium sp. isolated from Fritillaria unibracteata var. wabensis. Phytomedicine 2014, 21, 1104–1109.
Nagabhyru, P.; Dinkins, R.D.; Schardl, C.L. Transcriptomics of Epichloë-Grass symbioses in host vegetative and reproductive stages. Mol. Plant-Microbe Interact. 2018, 32, 194–207.
Kayano, Y.; Tanaka, A.; Takemoto, D. Two closely related Rho GTPases, Cdc42 and RacA, of the en-dophytic fungus Epichloë festucae have contrasting roles for ROS production and symbiotic infection synchronized with the host plant. PLoS Pathog. 2018, 14, e1006840.
Green, K.A.; Becker, Y.; Tanaka, A.; Takemoto, D.; Fitzsimons, H.L.; Seiler, S.; Lalucque, H.; Silar, P.; Scott, B. SymB and SymC, two membrane associated proteins, are required for Epichloë festucae hyphal cell-cell fusion and maintenance of a mutualistic interaction with Lolium perenne. Mol Microbiol 2017, 103, 657–677.
Green, K.A.; Becker, Y.; Fitzsimons, H.L.; Scott, B. An Epichloë festucae homologue of MOB3, a component of the STRIPAK complex, is required for the establishment of a mutualistic symbiotic interaction with Lolium perenne. Mol. Plant Pathol. 2016, 17, 1480–1492.
Eaton, C.J.; Dupont, P.-Y.; Solomon, P.; Clayton, W.; Scott, B.; Cox, M.P. A core gene set describes the molecular basis of mutualism and antagonism in Epichloë spp. Mol. Plant-Microbe Interact. 2014, 28, 218–231.
Schmid, J.; Day, R.; Zhang, N.; Dupont, P.-Y.; Cox, M.P.; Schardl, C.L.; Minards, N.; Truglio, M.; Moore, N.; Harris, D.R.; et al. Host tissue environment directs activities of an Epichloë endophyte, while it induces systemic hormone and defense responses in its native perennial ryegrass host. Mol. Plant-Microbe Interact. 2016, 30, 138–149.

© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license; additional terms may apply. By using this site, you agree to the Terms and Conditions and Privacy Policy.

Upload a video for this entry

Information

Subjects: Plant Sciences

Contributor MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Kendall Lee Lee

View Times: 479

Update Date: 02 Nov 2021

Table of Contents

Video Upload Options

Confirm

1. Introduction

2. Evolution of Epichloë Mutualistic Association with Grasses

3. Mechanisms of Interaction between Grass and Endophyte

References