Submitted to: Biological Control
Publication Type: Review article
Publication Acceptance Date: 8/8/2007
Publication Date: 7/1/2008
Citation: Kuldau, G., Bacon, C.W. 2008. Clavicipitaceous endophytes: Their ability to enhance grass resistance to multiple stresses. Biological Control. 46:57-71. Interpretive Summary: A scientist at the Toxicology and Mycotoxin Research Unit, Russell Research Center, and his collaborator were invited to review the role that a group of fungi play in the success of tall fescue and other grasses in defending itself from the many stresses encountered in the environment. Tall fescue is parasitized with a friendly fungus, referred to as an endophyte since it lives within the tissues of this and other grasses. This group of fungi, Neotyphodium coenophialum and related species, plays an important ecological role of this grass. These include establishment, persistence under grazing, drought tolerance, nitrogen efficiency, disease and resistances to fungal diseases, insects, and nematode pests. This grass has value as the only grass in the southeastern and northwestern United States that is used for livestock winter grazing forage, as well as turf and conservation uses. This group of fungi lives within a highly restricted area of grasses, between cells, referred to as the intercellular space or apoplasm. This review discusses the role of nutrients within the apoplasm, the need to develop plants with greater number or larger apoplasm to accomodate more fungal tissue for a higher quantitative response to environmental stresses. This review is the first to discuss the role of this niche as being important in the success of this grass, and also the first to suggest the need for plant breeding practices to be concerned with the apoplasm as a need for grass-endophyte improvements.
Technical Abstract: Grasses are noted for an absence of toxic secondary metabolites that can be used for their long-term survival and competitive strategies. However, fungi are notorious for their production of numerous secondary compounds, which do serve important functions in competition, while others relate to biotic and abiotic stress resistance. Co-habitation of specific fungi with grasses develop into specific mutualistic associations. The fungi of concern belong to a relatively small grouping of species within the Clavicipitaceae, and include species of the tribe Balansieae. This group of fungi shares a common feature in being endophytically associated with grasses, sedges and rushes as obligate biotrophic parasites. Specific attention is given to two related genera within this tribe, Epichloe and their anamorphic Neotyphodium relatives. The association of this group of fungi with grasses results in the accumulation of several classes of fungal metabolites that serve as relief mechanisms to biotic and abiotic stresses. These include drought tolerance, resistance to vertebrate and to invertebrate pests, fungal disease resistances, and tolerance to poor soil conditions. Resistances to multiple stresses can and often do occur simultaneoulsy within the same symbiotum. The resulting resistances to multiple stresses, along with inter- and intraspecific competitive advantages suggest the potentials for utilizing novel fungal endophytes and new host combinations for stress resistance in grasses in natural infected plants or for resistances to specific agronomic and ecological conditions, and soil remediations.