Inferring field performance from drought experiments can be misleading: The case of symbiosis between grasses and Epichloë fungal endophytes

GUNDEL, P.E.; IRISARRI J G N; FAZIO, L.; CASAS, C.; PEREZ, L.I.

JOURNAL OF ARID ENVIRONMENTS

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2016

0140-1963

Epichlo?e fungal endophytes (Clavicipitaceae, Hypocreales) formpersistent symbioses with grasses of the subfamily Po?oideae (Poaceae),including many important cultivated and wild forage species(Leuchtmann et al., 2014). The interaction has been labelled as adefensive mutualism, since fungal alkaloids protect host plantsagainst herbivores (Clay, 1988; Schardl et al., 2007; Saikkonenet al., 2013). Along with herbivore resistance, tolerance to droughtis commonly mentioned as an additional benefit for host plants.However, while the bioactivity of fungal alkaloids on herbivoresis well-established (see e.g., Wilkinson et al., 2000), the physiologicalmechanisms underlying the endophyte effect on host planttolerance to drought are not so clear (reviewed in Malinowskiand Belesky, 2000). The idea of positive effects of endophytes onplant tolerance to drought has been used to explain ecological patternsin which high symbiosis incidence is associated with lowlevels of precipitation (Lewis et al., 1997; Malinowski and Belesky,2006; Afkhami et al., 2014). However, a recent global survey ofendophyte incidence including a larger number of grass and fungalendophyte species in a wider environmental gradient revealed adifferent pattern (Semmartin et al., 2015). By reviewing the literatureand adding information to the drier extreme of the gradient,they found a positive relationship between endophyte incidenceand mean aboveground net primary productivity (ANPP), a variableclosely associated with mean precipitation (Sala et al., 1988). Moreover,increasing ANPP leads to an increase in herbivore pressureand biological complexity (McNaughton et al., 1989; Thrall et al.,2007, Fig. 1). In clear opposition with the notion of fungal endophytesconferring tolerance to drought, the low endophyte averageincidence in the arid extreme of the gradient (which was associatedwith a large variance) seems to be more than an exceptional rarityamong patterns of symbiosis prevalence and environmental variables(see Novas et al., 2007; Gundel et al., 2011a).Here,we propose a reinterpretation that can help solving some ofthese apparent contradictions from the literature and takes into accountthree issues: (i) past biases in experimental systems and specieschoice; (ii) an influence of those experimental results in theinterpretation of regional patterns; and (iii) the dynamics of the interactionbetween grasses and their endophytes in arid ecosystems.