1. Plants with Neotyphodium endophytes almost always show no external signs of presence of the endophyte.
2. The relationship between Neotyphodium endophytes and host grasses generally is considered mutualistic, with both partners gaining benefits.
3. The endophyte grows as if it were a plant tissue, a tissue that provides a protective chemical capability that the plant lacks.
4. Neotyphodium endophytes are transmitted vertically through seed, although novel associations may be established by inoculating seedlings. Vertical transmission is highly efficient (nearly 100%) in most natural grass-Neotyphodium associations.
5. Hyphae are present in the embryo at the time of seed maturation.
6. Neotyphodium endophytes are biotrophs, with all nutrient requirements coming from the host grass.
7. In nature, Neotyphodium endophytes are found only in symbiosis with host grasses, although nearly all can be grown in culture.
8. Hyphae are confined to spaces between the grass host cells.
9. Hyphae within the shoot apical meristem grow into each new leaf and tiller.
10. All aboveground parts of the plant can be colonized. Leaf blades of at least some U.S. tall fescue cultivars may be endophyte free or have low concentrations of hyphae.
11. Vascular bundles seldom contain hyphae.
12. Hyphae within the shoot apical meristem colonize the inflorescences as soon as they commence development. Hyphae do not have to grow up through the flowering stem to colonize the inflorescence.
13. The growth of the hyphae is strongly regulated, being synchronized with that of the host plant. Hyphae stop growing when the leaf ceases growth.
14. The known features of grass colonization by Neotyphodium endophytes are explained by hyphae growing at their tips and branching when among dividing plant cells, then elongating by intercalary growth when among enlarging plant cells, and finally ceasing growth altogether when among mature plant cells.
15. The concentration of hyphae present in leaves is influenced by temperature, with fewer hyphae being produced in winter than summer.
16. Hyphae remain alive and metabolically active for the life of the leaf.
17. The intercellular spaces within host grasses provide Neotyphodium endophytes with a stable environment and an ongoing supply of nutrients with little competition from other microorganisms.
18. The presence of endophyte hyphae apparently does not induce a host defense response, although a range of host defense reactions may be elicited in some artificially established symbioses.
19. There is constant selection pressure for grass-endophyte compatibility, especially during seedling establishment. Any association that does not have fitness will be outcompeted by more vigorous associations or will be unable to tolerate biotic and abiotic stresses. Likewise, any mutation that adversely affects the association or its ability to survive will be eliminated.
20. Neotyphodium fungi are at the extreme end of the continuum of interactions between fungi and plants and provide a unique opportunity to investigate interactions between grasses and fungi.
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