Analysis of Epichloë festucae small secreted proteins in the interaction with Lolium perenne

Abstract

19Epichloë festucae is an endophyte of the agriculturally important perennial 20 ryegrass. This species systemically colonises the aerial tissues of this host 21 where its growth is tightly regulated thereby maintaining a mutualistic symbiotic 42 proteins are either not required for the interaction at the observed life stages or 43 that there is redundancy between effectors expressed by E. festucae. 44 45 53 Cladosporium fulvum [2-4], as well as Pit2 and Rsp3 from the corn smut 54 pathogen Ustilago maydis [5, 6]. More specifically, Avr4 binds to chitin 55 molecules present in the cell wall of invading hyphae to prevent hydrolysis by 56 host chitinases [2, 7], while Ecp6 sequesters chitin oligosaccharides released 57 from the cell wall of invading hyphae to prevent detection by host chitin immune 58 receptors [4, 8]. Rsp3 binds to the cell wall of invading hyphae to protect them 59 against two antimicrobial mannose-binding host proteins [6], while both Pit2 60 and Avr2 inhibit host cysteine proteases to prevent degradation of fungal 61 proteins [3, 9-11]. A well-characterized example of an intracellular fungal 62 effector protein is Cmu1 from U. maydis, which functions as a chorismate 63 mutase to redirect the metabolism of chorismate away from the production of 64 the defense signaling hormone salicylic acid [12]. 4 65 Like plant-pathogenic fungi, plant-beneficial fungi must also deploy 66 effector proteins to promote host colonization [13, 14]. However, to date only a 67 small number of effectors from this group of fungi have been identified and 68 functionally characterized. Furthermore, of those that have been functionally 69 characterized all are from mycorrhizal fungi. One such example is MiSSP7 from 70 Laccaria bicolor, an ectomycorrhizal fungus of poplar roots. In plants (including 71 poplar), JAZ proteins repress the expression of jasmonic acid (JA)-related 72 defense genes, with the degradation of these proteins leading to the activation 73 of JA-related defense genes [15]. During infection of poplar, MiSSP7 is 74 transported into the plant nucleus where it interacts with and stabilizes the JAZ 75 protein PtJAZ6 to prevent its degradation, and thus prevent the expression of 76 JA defence-related genes [16, 17]. A second example is SP7, which is 77 produced by the arbuscular mycorrhizal fungus, Glomus intraradices. SP7, 78 which like PtJAZ6 is also transported into the plant nucleus, influences defense 79 gene signaling by interacting with the pathogenicity-related transcription factor 80 ERF12 [18]. A third example is Fgb1 has been characterized from the 81 endophytic root-colonizing fungus, Piriformospora indica. Fgb1 is a β-glucan-82 binding lectin that modulates the fungal cell composition and prevents β-glucan-83 triggered immunity in plants, presumably through a similar mechanism to that 84 shown for as Ecp6 and other LysM domain-containing effectors from plant-85 pathogenic fungi [19]. 86 There is little amino acid sequence conservation between effector 87 proteins of plant-associated fungi, and mo...