Science China Life Sciences | 2019
Epichloë festucae var. lolii endophyte affects host response to fungal disease progression in perennial ryegrass (Lolium perenne)
Abstract
Perennial ryegrass (Lolium perenne) is a globally important forage and turf grass species that commonly forms symbiotic associations with the asexual fungal endophyte—Epichloë festucae var. lolii. Epichloë endophytes mutualistically interact with host plants by providing major fitness enhancements and protection from both biotic and abiotic stresses (Young et al., 2013). Previous studies have shown that Epichloë festucae var. lolii inhibits the growth of plant pathogenic fungi and fungal spore germination in vitro. Additional studies conducted in the lab or field confirmed that endophytes inhibited pathogen invasion and lesion development in perennial ryegrass. However, endophytes are not always beneficial, and may even weaken the host plant and reduce the disease resistance of the host. Such inconsistent or contrasting results in Festuca spp., Achnatherum inebrians and Leymus chinensis have also been reported (Wang et al., 2016). Our previous studies showed that perennial ryegrass cultivar ‘Pinnacle’ had high resistance to leaf spot disease (Tian et al., 2008). In order to characterize this cultivar more comprehensively and understand the host physiological response mechanism during disease progression, this continuous study inoculated plants germinated from seeds collected from E+ and E– perennial ryegrass cultivar ‘Pinnacle’ plants used in a previous study (Tian et al., 2008). Two pathogens (Alternaria alternata and Bipolaris sorokiniana) were used in this study based on their ability to produce lesions on perennial ryegrass plants. The plants’ pathogen spore inoculation experiment showed that no disease symptom was observed in the control plants (CK). Both A. alternata and B. sorokiniana caused disease symptoms in perennial ryegrass plants regardless of their endophyte status. The disease symptom occurred in plants with pathogens’ inoculation from 3 days after inoculation. E+ plants developed fewer diseased leaves than E– plants, indicated by the lower percentage of diseased leaves and lower lesion length, which suggest that endophyte can increase host disease resistance in perennial ryegrass (Table S1 in Supporting Information). This is consistent with previous studies (Tian et al., 2008; Ma et al., 2015). However, the significant difference between E+ and E– plants was detected only at the later stages-inoculated after 11 and 13 days of disease progression, which suggests that mutualistic interactions between the endophyte and host are complex and influenced by environmental and genetic factors. A possible explanation might be that pathogens proliferate rapidly, while the endophyte grows slowly and takes time to induce host physiological symptoms. The inhibition effects of this physiological response will delay the generation of symptoms. The variation in phenylalanine ammonia-lyase (PAL) and polyphenoloxidase (PPO) enzyme activity were also measured in the host during disease progression. During disease progression, PAL and PPL enzyme activity (Table 1) in all E + and E– plants inoculated with pathogens significantly increased, and then significantly decreased after peaking at 9