Perennial ryegrass (Lolium perenne L.) is used for forage in temperate regions throughout the world including Northern Europe, Pacific North West of USA, Japan, South-eastern Australia, and New Zealand [1,2,3,4,5]. It is the most commonly utilized pasture grass on dairy farms in Australia and has a high economic importance [6]. The asexual form of the endophytic fungus E. festucae var. lolii (previously known as Neotyphodium lolii and Acremonium lolii) is known to establish a symbiotic relationship with perennial ryegrass [7]. These interactions are beneficial to the pastoral agriculture industry as compounds produced by the endophyte confer resistance to biotic and abiotic stresses. For example, a select number of indole-diterpene class compounds, such the lolitrems A, B, and E that are present in endophyte-infected ryegrasses, are toxic to the larvae of the Argentine stem weevil (Listronotus bonariensis) [8,9]. The grass–endophyte association also produces secondary metabolites that are detrimental to grazing animals. The major toxins of concern are the alkaloids ergovaline and lolitrem B, which are present in old naturalized perennial ryegrass pastures containing the Standard Endophyte (SE) strain. Although both toxins are produced by endophyte-infected perennial ryegrass, ergovaline is normally most abundant in endophyte-infested tall fescue grass and causes the vasoconstrictive conditions fescue foot or summer slump disease [10,11]. The indole-diterpenes are predominant in endophyte-infected perennial ryegrass and lolitrem B is the end-point of the complex indole-diterpene biochemical pathway (Figure 1) [12]. Many of the indole-diterpene class of compounds, particularly the lolitrems, are reported as anti-mammalian alkaloids that significantly affect animal health. In particular, lolitrem B has been identified as a causative agent for perennial ryegrass staggers disease, a nervous disorder, notably of sheep and cattle that causes tremors [12]. However, despite the prevalence of perennial ryegrass in various geographic locations the toxicity reports are generally limited. Neurological signs associated with ryegrass staggers disease has been reported in animals, particularly sheep, grazing on perennial ryegrass in Australia, New Zealand, as well as Pacific northwest of USA and Europe [13,14]. This imposes a negative impact on industry, particularly dairy, meat, and wool production involving grazing animals [15]. Thus, forage improvement programs have been involved in selecting novel endophytes that do not produce the known toxins; however, complex mixtures of the intermediate compounds are still present in marketed forage grasses.

 

Figure 1. Indole-diterpene biosynthetic map sourced from KEGG pathways showing the enzymes responsible for producing indole-diterpene compounds at each step. Reproduced from [29,30,31].

 

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