Introduction
Plant diseases are the main factors affecting the health of natural grassland worldwide, which reduce the yield and quality of forages, and seriously restrict the sustainable utilization of grassland and the development of animal husbandry (Liu et al., 2016b; Zhang et al., 2020). In recent years, soil-borne microorganisms (such as saprophytic fungi Trichoderma ) have been reported to stimulate plant growth (Ji et al., 2021), produce antibiotics (Liu et al., 2016a) and activate plant defense response to phytopathogens (Shoresh et al., 2005), and have been applied in the control of agricultural diseases as an environmentally friendly alternative to the massive use of chemical pesticides (Zin and Badaluddin, 2020). Besides soil-borne microbiota, endophytes widely live and reproduce in the plant kingdom without causing prominent diseases, protect host plants from environmental pathogenic microorganisms, and are essential potential resources for mining new biocontrol agents (Silva et al., 2019).
In natural grasslands, most cold-season grasses can form symbiotic relationships with endophytic fungi of the genus Epichloë (Clay, 1990). The fungal hyphae colonize the intercellular spaces of sheaths and leaves of the host (Clay and Schardl, 2002). Plants serve as hosts and provide nutrients to their endophytes, and the endophytic fungi protect them from biotic and abiotic stresses, such as drought (Ren et al., 2011), herbivores (Bastías et al., 2017), and pathogenic infections (Xia et al., 2018).
The positive effect ofEpichloë infection on the disease resistance of the host grass has been reported not only in cultivated grasses such as Lolium perenne (Pańka et al., 2013a) and Festuca arundinacea (Chen et al., 2017); but also in natural grasses, such as Festuca pratensis (Sabzalian et al., 2012), Leymus chinensis (Wang et al., 2016), Festuca rubra (Niones and Takemoto, 2014) andAchnatherum inebrians (Xia et al., 2016). However, little is known about the involved mechanisms of resistance. It is well known that endophytic infections can enhance herbivore resistance of the host grasses due to the production of alkaloids (Bastías et al., 2017). However, alkaloids are not likely directly associated with fungal pathogenic resistance (Holzmann-Wirth et al., 2000). Several in vitro studies have identified that endophytes could produce inhibitory metabolites against plant pathogenic fungi, such as Epichlicin (Seto et al., 2007) and cyclosporin T (Song et al., 2015). Whether these inhibitory metabolites are also effective in endophyte-infected grasses has not been reported up to now.
In addition to metabolites produced by endophytes themselves, endophytic infections could reprogram the metabolism of the host grasses. Phenolic compounds are essential secondary metabolites involved in the regulation of the plant immune system against disease (Daayf et al., 2012). Researchers found that endophytes upregulated gene expression levels related to phenylpropanoid synthesis in the host L. perenne cv Samson (Dupont et al., 2015) and significantly induced total phenol accumulation in L. perenne (Rasmussen et al., 2008). Pańka et al. (2013a, 2013b) studied the disease resistance of tall fescue and perennial ryegrass and confirmed that endophytic infections might increase the resistance of the host by the accumulation of total phenols. However, there are numerous phenolic compounds in plants (Balasundram et al., 2006), which compounds that play a vital role in the endophytes effecting host disease resistance, are yet to be elucidated.
Plant immune responses to diseases include systemic acquired resistance (SAR) induced by pathogenic microorganisms and induced systemic resistance (ISR) enhanced by beneficial microorganisms (Vallad and Goodman, 2004). In general, SAR mainly relies on the activation of signal molecules such as salicylic acid (SA), pipecolic acid (Pip), and azelaic acid (Klessig et al., 2018); in comparison, ISR needs to rely on jasmonic acid (JA) and ethylene (ET), independent of SAR signal molecules to be triggered (Pieterse et al., 2014). A wide variety of mutualists, including Bacillus , Trichoderma , andmycorrhiza species, sensitize the plant immune system for activated ISR by JA/ET (Cameron et al., 2013; Gutjahr, 2014; Pieterse et al., 2014). As for Epichloë endophytes, Schmid et al. (2017) studied the transcriptome of L. perenne cv. Nui established that endophytes upregulated genes involved in SA, JA, and ET biosynthesis. However, whether endophytes activate ISR in grass hosts or not remains unknown.
Achnatherum sibiricum is a perennial, sparse bunchgrass widely distributed in northern China’s natural grasslands, which harbors two different Epichloë species, Epichloë gansuensis andEpichloë sibiricum (Zhang et al., 2009). In the previous study, Niu et al. (2016) found that both species of endophytes could increase the resistance of A. sibiricum to common pathogens in grassland. In order to study the disease-resistance mechanism of endophytes to protect the host, the metabolome was used in this study. Herein, endophyte-infected (EI) and endophyte-free (EF) A. sibiricumplants were used as plant materials; Curvularia lunata was chosen as the pathogen, which is a common and severe disease that affects crops and forages worldwide (Chang et al., 2020; Nookongbut et al., 2020; Wonglom et al., 2019). Based on the evidence thatEpichloë endophyte could significantly improve the resistance ofA. sibiricum to C. lunata (Shi et al., 2020), the following two questions were investigated: 1) What are the significantly differential metabolites (DMs) in the host caused by endophytic infections before and after pathogen inoculation?; 2) What are the possible mechanisms by endophyte for enhancing host’s disease resistance?