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?