Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 1: Spartina BiologyFUNGAL SYMBIOSIS:APOTENTIAL MECHANISM OF PLANT INVASIVENESSR.J. RODRIGUEZ 1 ,R.S.REDMAN 2,3 ,M.HOY 2 , AND N. ELDER 41 U.S. Geological Survey, 6505 NE 65th Street, Seattle, WA 98115; Rusty_Rodriguez@usgs.gov2 Department <strong>of</strong> Biology, University <strong>of</strong> Washington, Seattle, WA 981953 Department <strong>of</strong> Microbiology, Montana State University, Bozeman, MT 597174 U.S. Geological Survey, Marrowstone Station, Nordland, WA 98358We propose that fungal endophytes provide a mechanism for <strong>the</strong> habitat expansion <strong>of</strong> invasiveplants including Spartina species. We have determined that Spartina spp. are symbiotic wi<strong>the</strong>ndophytic fungi and have begun to assess <strong>the</strong> role <strong>of</strong> fungal endophytes in <strong>the</strong> invasiveness <strong>of</strong> <strong>the</strong>seplants. Preliminary data indicate that <strong>the</strong> fungal endophytes in Spartina spp. in Puget Sound arenative to that region. The role <strong>of</strong> symbiosis in <strong>the</strong> invasion <strong>of</strong> invasive species and <strong>the</strong> adaptation <strong>of</strong>plants to high stress habitats is discussed.Keywords: symbiosis, fungi, lifestyle, plant ecology, fungal ecology, Colletotrichum, Curvularia,SpartinaINTRODUCTIONIt is estimated that thousands <strong>of</strong> plants are introducedinto non-native habitats every year, however, only a smallpercentage <strong>of</strong> <strong>the</strong>se plants become invasive. Interestingly,invasions may result from intercontinental orintracontinental plant movements and may involve ei<strong>the</strong>rvast or very short distances. Several <strong>the</strong>ories have beenproposed to explain <strong>the</strong> invasiveness <strong>of</strong> plants (Daehler2003; Callaway and Ridenour 2004). However, noneadequately explain why plants can achieve high densities innew habitats (Daehler, 2003; Silvertown 2004). It doesappear that invasion is context dependent and may involve anumber <strong>of</strong> biotic and abiotic factors (Daehler 2003).Although <strong>the</strong>re have been numerous studies on invasiveplants, few include fungal symbiosis as a component <strong>of</strong> plantbiology. Yet, all plants in natural ecosystems are thought tobe symbiotic with mycorrhizal and/or endophytic fungi.These fungi differ in distribution, biology and physiology.Mycorrhizal fungi are limited to colonizing roots and growout into <strong>the</strong> rhizosphere effectively expanding root systemsby transporting nutrients and water that would o<strong>the</strong>rwise beunavaiable to root systems (Read 1999).Endophytic fungi reside entirely within plant tissues andmay occur in specific tissues (roots, crowns, stems, leaves,seed coats or seeds) or throughout <strong>the</strong> plant. Althoughmycorrhizal fungi do not colonize all plants, it is thoughtthat all plants are colonized by endophytic fungi. Fungalendophytes can be divided into two major groups(Rodriguez et al. 2009b): 1) a small number <strong>of</strong> fastidiousspecies that are restricted to a small number <strong>of</strong> monocothosts (Clay and Schardl 2002) and 2) a large number <strong>of</strong>tractable species with broad host ranges (Stone et al. 2004).Both groups <strong>of</strong> fungal endophytes are known to be importantto <strong>the</strong> structure, function, and health <strong>of</strong> plant communities.In fact, without fungal symbioses, plant communities do notappear to survive many environmental stresses (Arnold et al.2003; Dingle and McGee 2003; Ernst et al., 2003; Redmanet al. 2002b).FUNGAL SYMBIOTIC LIFESTYLESCollectively, fungal symbionts express a variety <strong>of</strong>symbiotic lifestyles including mutualism, commensalism,and parasitism. These lifestyles are based on positive,neutral or negative fitness benefits to <strong>the</strong> host and symbiont(Table 1, Lewis 1985). Mutualistic fungi have been shown toincrease plant growth and productivity (Marks and Clay1996; Varma et al. 1999; Read 1999; Redman et al. 2002a;Rodriguez et al. 2009a), and confer tolerance to abiotic andbiotic stresses including drought, metals, salt, temperature,pathogens and herbivores (Bacon and Hill 1996; Bacon1993; Read 1999; Carroll 1986; Redman et al. 2001;Redman et al. 2002b; Latch 1993; Rodriguez et al. 2008).We have demonstrated that endophytic fungi also have <strong>the</strong>ability to switch lifestyles in response to host genotypes andthat <strong>the</strong> host range <strong>of</strong> fungal endophytes is typically greaterthan previously thought (Redman et al. 2001). For example,fungi from <strong>the</strong> genus Colletotrichum are classified as plantpathogenic fungi. However, several species have <strong>the</strong> abilityto asymptomatically colonize plants not previously known tobe hosts and express non-pathogenic lifestyles includingmutualism. When Colletotrichum species expressmutualistic lifesytles <strong>the</strong>y confer disease resistance againstfungi that express pathogneic lifestyles in <strong>the</strong> respective hostplant. Therefore, one fungal isolate may be a virulentpathogen on one host and a disease protecting mutualist onano<strong>the</strong>r.Symbiotically conferred disease resistance is correlatedwith <strong>the</strong> activation <strong>of</strong> host defense systems (Redman et al.1999). When exposed to virulent pathogens, non-symbioticplants slowly activated defense systems over a four-dayperiod and by day five <strong>the</strong> plants were dead. Symbioticplants activated defense systems within 24 hours <strong>of</strong> exposureto a virulent pathogen and completely terminated pathogen-39-