Growth of Neotyphodium and Epichloë is tightly regulated within host grasses. Colonization of leaves by these endophytes takes place while the leaf is growing, and the production of hyphae ceases when leaf growth ceases (Christensen et al., 2002; Schmid et al., 2000; Tan et al., 2001). Thus the number of hyphae in any one region of a leaf remains constant throughout the life of the leaf (Tan et al., 2001) even though there is an ongoing supply of nutrients. This synchronization of growth can be observed in plants infected with strains of E. festucae and N. lolii that have been genetically modified to express green fluorescent protein (GFP). In the leaf sheaths present in the pseudostem of a tiller, the number of hyphae was similar in each even though their ages varied (Fig. 14-24). Without this synchronization the number of hyphae would continue to increase as the leaf aged, so the endophyte would likely become a cost to the host grass. The regulation of hyphal growth within the Neotyphodium/Epichloë-grass complex is discussed further in Christensen et al. (2002, 2008), Christensen and Voisey (2007), Schmid et al. (2000), Takemoto et al. (2006), Tan et al. (2001), and Tanaka et al. (2006).
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Fig. 14-24. Cross section through a perennial ryegrass tiller infected with Epichloë festucae expressing GFP. The number of hyphae in each sheath (LS) is similar, and hyphae are not found in the vascular bundles. The red color (see enclosed CD) is autofluorescence of chloroplasts.
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