Conference Paper

Canopy height and litter-derived spore sources affect composition and function of tropical endophyte communities

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Abstract

Background/Question/Methods Foliar endophytic fungi are environmentally-acquired symbionts, whose diverse communities reside cryptically within the healthy photosynthetic tissue of all plant species sampled to date. Endophyte colonization can benefit host plants by enhancing defense against pathogens and herbivores, but with demonstrated costs including latent pathogenicity and lowered host photosynthetic rate. A critical question in both endophyte ecology and broader ecological theory is to what extent do abiotic and biotic factors affect community composition? Moreover, does community assembly of endophytes carry functional consequences for plant hosts? We hypothesized that canopy height as well as leaf litter presence and identity would influence endophyte community assembly, and that hosts with different endophyte communities would vary in their resistance to pathogen infection. We manipulated litter exposure and canopy height of endophyte-free Theobroma cacao (cacao tree) seedlings on Barro Colorado Island, Panama. After two weeks, we used fungal culturing and sequencing techniques to compare the composition of early endophyte communities among treatments. Subsequently, we inoculated seedlings with a common pathogen of T. cacao (Phytophthora sp.) and compared pathogen damage among treatments. Results/Conclusions Both litter and canopy height significantly affected the community structure of endophytes inhabiting plant tissue. Plants in the upper canopy were the least colonized by endophytes. Plants without litter were colonized by both a greater number and diversity of endophytes than seedlings treated with conspecific (T. cacao) or heterospecific (mixed tree species) litter. Further, in our analysis of pathogen infection following endophyte colonization, we found that seedlings treated with conspecific litter were more resistant to infection by Phytophthora than seedlings treated with heterospecific or no litter. Control, endophyte-free seedlings were the most susceptible to pathogen infection. These results are consistent with recent studies showing enhancement of host defense by dominant endophytes of healthy conspecific tissues. We suggest that that leaf litter, particularly from healthy conspecifics, results in a priority effect in the early stages of endophyte community assembly. This would promote colonization by the endophyte species that are the most effective at inhibiting further endophyte colonization as well as reduce pathogen infection. This phenomenon has both ecological and applied implications, especially in a global cash crop such as cacao.

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a ARVALIS -Institut du Végétal, 6 chemin de la côte vieille, F-31 450 Baziège; b INRA, UR1264 MycSA, 71 avenue Edouard Bourleaux, F-33 883 Villenave d'Ornon; c INRA, UMR1290 BIOGER, Avenue Lucien Brétignières, F-78 850 Thiverval Grignon; d ARVALIS -Institut du Végétal, Station expé rimentale de Boigneville, F-91 720 Boigneville; and e CNRS, INRA UR1264 MycSA, 71 avenue Edouard Bourleaux, F-33 883 Villenave d'Ornon, France Fungal interactions of Fusarium verticillioides and F. graminearum in maize ears and the impact on fungal development and toxin accumulation were investigated in a 2-year field study at two locations in France. Maize ears were inoculated either with a spore mixture of F. graminearum and F. verticillioides or using a sequential inoculation procedure consisting of a first inoculation with F. graminearum followed by a second with F. verticillioides 1 week later. Toxin and fungal biomass were assessed on mature kernels, using HPLC and quantitative PCR. Correlation between the levels of DNA and toxin was high concerning F. graminearum DNA and deoxynivalenol (R 2 = 0AE73) and moderate for F. verticillioides DNA and fumonisin (R 2 = 0AE44). Fusarium graminearum DNA either decreased in mixed inoculations or was not influenced by subsequent inoc-ulations with F. verticillioides, compared to single inoculations. In contrast, F. verticillioides DNA either significantly increased or was not affected in mixed and sequential inoculations. In two of the replicates, it can be assumed that natural contamination by F. verticillioides was favoured by previous contamination with F. graminearum. Overall, the results sug-gest that F. verticillioides has competitive advantages over the F. graminearum strains. Additionally, the data provide, for the first time, key evidence that previous contamination by F. graminearum in maize ears can facilitate subsequent infections by F. verticillioides.
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A species ofMycosphaerella with aPseudocercospora anamorph was collected on overwintered fallen leaves of Japanese beech,Fagus crenata. Based on comparison of morphology withMycosphaerella species on Fagaceae, the fungus was newly described asMycosphaerella buna. ThePseudocercospora anamorph derived from a single ascospore of the fungus was morphologically identical to an endophytic anamorph isolated from asymptomatic living leaves of Japanese beech.
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Endophytes are fungi that form inapparent infections within leaves and stems of healthy plants. Closely related to virulent pathogens but with limited, if any, pathogenic effects themselves, many endophytes protect host plants from natural enemies. Animal herbivores and, in some cases, pathogenic microbes are poisoned by the mycotoxins produced by endophytes, "Constitutive mutualism' is the relatively faithful association, usually with grasses, of endophytes that infect host ovules and are propagated in host seed; substantial fungal biomass with probable high metabolic cost develops throughout the aerial parts of the host plant. "Inducible mutualist' endophytes are not involved with host seed and disseminate independently through air or in water. Infecting only vegetative parts of the host and remaining metabolically inactive for long periods with relatively little fungal biomass, inducible mutualists grow rapidly and produce toxins against herbivores when damaged host tissues provide new sites for infection. -from Author