Tall fescue pastures in Texas dominated by E+ plants (94% infestation) had greater persistence than low-endophyte (12% infestation) pastures (Read and Camp, 1986). Greenhouse and small-plot field experiments showed that E+ plants had enhanced drought tolerance (Arachevaleta et al., 1989; West et al., 1993). The mechanisms enabling this tolerance are complex and probably involve numerous factors. Protecting growing points (apical meristems) from irreversible desiccation is one way to ameliorate the impact of drought. Osmotic adjustment, that is, promotion of active solute accumulation in cell sap, was greater in the base of vegetative tillers, where cell growth occurs, of E+ than of E- plants, with the magnitude of adjustment related to tiller survival (Elmi and West, 1995). Cell solute accumulation slowed the loss of cell turgor in or near growing points, protecting tissue capable of regenerating after drought. More simple sugars accumulated in leaf sheaths of drought-stressed E+ than E- plants; this could help minimize desiccation or freezing damage to cells (Richardson et al., 1992). In a field trial, water content of the outer leaf sheath of vegetative tillers was greater in E+ than E- plants, suggesting that water loss would be less and the plant would be protected from wilting (Elbersen and West, 1996). Shoot mass and tiller numbers often are greater in E+ than in E- plants (Arachevaleta et al., 1989; De Battista et al., 1990; Hill et al., 1990; West et al., 1993), but some tall fescue genotypes are unaffected or grow less when endophyte infected (Belesky et al., 1989). A greater rate of tillering by certain E+ plants delays tiller number decline (West et al., 1993).
An indirect means by which endophyte enhances host drought tolerance is by deterring herbivory. The production of certain chemical constituents, including a wide array of alkaloids, could cause grazing animals to avoid endophyte infected tall fescue (see Chapter 13). Plants could then retain leaves, tillers, and roots and be better able to survive stressful conditions. Grazing reduces the amount of leaf area available for photosynthesis, reducing the ability of the plant to regrow and sustain energy reserves in tiller bases and crown tissue. Overgrazing can lead to energy depletion, weaken the root systems, and diminish the ability to acquire water, especially when precipitation is limited. Drought stress appears to enhance production of ergot alkaloids in E+ tall fescue (Arechavaleta et al., 1992), thereby amplifying the endophyte protection effect. Likewise, deterrence of root-feeding organisms by E+ plants can enhance water acquisition by the host. When endophyte was removed from tall fescue, a large increase in root-knot nematode (Meloidogyne marylandi Jepson & Golden) occurred on roots, reducing root mass and lowering osmotic adjustment in the tiller growth zone (Elmi et al., 2000).
Fig. 4-2. Some physiological responses of endophyte infected tall fescue plants.
Proposed mechanisms of endophyte-enhanced host plant persistence include a range of cellular metabolism and whole-plant responses (Fig. 4-2). Stomata of E+ plants appeared to close earlier at the onset of drought compared with E- plants grown under highly controlled conditions (Belesky et al., 1987; Elmi and West, 1995). In contrast, field-grown E+ plants tended to have greater stomatal conductance and less leaf rolling, a symptom of water deficit stress, than E- plants of the same genotype (Elbersen and West, 1996). This might reflect the ability of E+ plants to develop large root systems that improve access to soil water, delaying onset of drought stress symptoms. Various abiotic stresses, including drought, heat, cold, nutrient deficiency, and high solar radiation, can incite excess production of reactive oxygen species, or free radicals, which can damage cell organelles and membranes and impair normal plant function. Endophytes appear to enhance production of the antioxidants superoxide dismutase (Fike et al., 2001; West et al., 2005) and total phenolics (Malinowski et al., 1998a). Endophytes occurring in the leaf sheaths and meristematic zone of vegetative tillers during summer water-deficit events may thus produce signals at the hypha-plant cell wall interface that translate into plant cytoplasmic biochemical responses responsible for cell turgor maintenance and free-radical quenching. The endophyte effects on host fitness are not always positive. Host and endophyte genotypes and environment, including management, interact in often unpredictable ways to modulate phenotypic expression, such that endophyte symbiosis can be neutral or even detrimental to the host (West, 2007).
The creation of novel host-endophyte associations that were devoid of ergot alkaloids was a significant technological advance to help mitigate the occurrence of fescue toxicosis in grazing livestock (Bouton and Easton, 2005; Nihsen et al., 2004). Bouton et al. (2002) demonstrated superior stand survival of ‘Jesup' tall fescue inoculated with endophyte AR542, a novel endophyte that does not produce ergot alkaloids compared with tall fescue infected by wild, toxic endophyte. Lamb weight gains when grazing the novel host-endophyte plants were similar to those achieved when grazing E- pastures. Novel, nontoxic endophytes now provide a means to understand and benefit from Neotyphodium endophytes as mechanisms of improving host persistence (see Chapter 20).
Another recent advance in tall fescue technology has been the development of cultivars from Mediterranean germplasm that exhibit partial summer dormancy as a means of improving summer drought survival (Norton et al., 2006). Traditional tall fescue cultivars used in temperate climates are considered "summer-active"-growth continues in summer as long as soil moisture is plentiful. Cultivars such as Kentucky 31 (KY-31) apparently need the Neotyphodium symbiosis to persist during drought. Endophyte infection did not influence growth and survival of summer-dormant fescue populations in northwest Arkansas (Underwood et al., 2008). In fact, summer-dormant types showed virtually complete stand survival after drought, irrespective of endophyte infection status. Endophyte infection did benefit summer-dormant tall fescue in the semiarid environment of north-central Texas (Malinowski et al., 2005c). These authors concluded that summer-dormant types show potential for survival in the subhumid to semiarid transition zone of the southern Great Plains of Oklahoma and Texas, where summer-active types do not persist.
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