Hyphae of Neotyphodium and Epichloë are transmitted maternally in host seed and colonize ovaries as they are developing (Fig. 14-7) and thereafter the embryos early in their development. When the seed is mature, the endophyte occupies the embryonic axis, including the shoot apical meristem (SAM) region (Fig. 14-8 to 14-10) and also the scutellum (Fig. 14-8).


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Fig. 14-7. Perennial ryegrass ovary containing many hyphae of a genetically modified Epichloë festucae strain expressing GFP.



Fig. 14-8. Perennial ryegrass seed infected with a GFP-expressing Epichloë festucae strain. Hyphae are present in the embryonic axis (EA) and the scutellum (S) as well as close to the seed coat.



Fig. 14-9. Cross section through the embryo within a tall fescue seed infected with Neotyphodium coenophialum. At seed maturity hyphae are present in the tissue that will become the seedling shoot apical meristem and the first leaf (L).



Fig. 14-10. Neotyphodium coenophialum hyphae (-->) within leaf tissue of the tall fescue embryo shown in Fig. 14-9.



Fig. 14-11. A layer of hyphae (-->) is present between the aleurone layer (A) and the seed coat. Hyphae are not present within the endosperm.


Hyphae are present also in the crushed remains of the nucellus between the seed coat and the aleurone layer (Fig. 14-11). Those near the aleurone layer are not thought to play a direct role in the symbiosis with the germinating seedling other than to produce alkaloids that may provide protection against feeding insects. In nature, the only known route of dissemination of the asexual Neotyphodium species is via vertical transmission, the infection of host embryos by fungal hyphae in inflorescences. Epichloë endophytes, however, are capable of horizontal transmission through the production of infective ascospores after sexual reproduction via fusion of compatible haploid conidia. For further reading on seed transmission of Neotyphodium endophytes refer to Freeman (1904), Majewska-Sawka and Nakashima (2004), Philipson and Christey (1986), Selosse and Schardl (2007), and White et al. (1991).


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