Diversity, taxonomic composition, and functional aspects of fungal communities in living, senesced, and fallen leaves at five sites across North America

Jana M U’Ren,A Elizabeth Arnold

doi:10.7717/peerj.2768

Abstract

BackgroundFungal endophytes inhabit symptomless, living tissues of all major plant lineages to form one of earth’s most prevalent groups of symbionts. Many reproduce from senesced and/or decomposing leaves and can produce extracellular leaf-degrading enzymes, blurring the line between symbiotrophy and saprotrophy. To better understand the endophyte–saprotroph continuum we compared fungal communities and functional traits of focal strains isolated from living leaves to those isolated from leaves after senescence and decomposition, with a focus on foliage of woody plants in five biogeographic provinces ranging from tundra to subtropical scrub forest.MethodsWe cultured fungi from the interior of surface-sterilized leaves that were living at the time of sampling (i.e., endophytes), leaves that were dead and were retained in plant canopies (dead leaf fungi, DLF), and fallen leaves (leaf litter fungi, LLF) from 3–4 species of woody plants in each of five sites in North America. Our sampling encompassed 18 plant species representing two families of Pinophyta and five families of Angiospermae. Diversity and composition of fungal communities within and among leaf life stages, hosts, and sites were compared using ITS-partial LSU rDNA data. We evaluated substrate use and enzyme activity by a subset of fungi isolated only from living tissues vs. fungi isolated only from non-living leaves.ResultsAcross the diverse biomes and plant taxa surveyed here, culturable fungi from living leaves were isolated less frequently and were less diverse than those isolated from non-living leaves. Fungal communities in living leaves also differed detectably in composition from communities in dead leaves and leaf litter within focal sites and host taxa, regardless of differential weighting of rare and abundant fungi. All focal isolates grew on cellulose, lignin, and pectin as sole carbon sources, but none displayed ligninolytic or pectinolytic activity in vitro. Cellulolytic activity differed among fungal classes. Within Dothideomycetes, activity differed significantly between fungi from living vs. non-living leaves, but such differences were not observed in Sordariomycetes.DiscussionAlthough some fungi with endophytic life stages clearly persist for periods of time in leaves after senescence and incorporation into leaf litter, our sampling across diverse biomes and host lineages detected consistent differences between fungal assemblages in living vs. non-living leaves, reflecting incursion by fungi from the leaf exterior after leaf death and as leaves begin to decompose. However, fungi found only in living leaves do not differ consistently in cellulolytic activity from those fungi detected thus far only in dead leaves. Future analyses should consider Basidiomycota in addition to the Ascomycota fungi evaluated here, and should explore more dimensions of functional traits and persistence to further define the endophytism-to-saprotrophy continuum.

Highlights

Fungal endophytes inhabit symptomless, living photosynthetic tissues of all major plant lineages to form one of earth’s most prevalent groups of symbionts (e.g., Arnold et al, 2010; U’Ren et al, 2012; Zimmerman & Vitousek, 2012; Davey et al, 2013; Balínt et al, 2015)
Many endophytes blur the line between symbiotrophy and saprotrophy, creating a challenge for estimating trophic modes and the scale of fungal diversity based on species richness in particular functional groups
ITS rDNA partial LSU rDNA data obtained from 2,064 isolates of Ascomycota comprised 555 unique genotypes (100% sequence similarity) and 306 putative species (i.e., Operational taxonomic units (OTU) at 95% sequence similarity) (Table 1)

Summary

Introduction

Fungal endophytes inhabit symptomless, living photosynthetic tissues of all major plant lineages to form one of earth’s most prevalent groups of symbionts (e.g., Arnold et al, 2010; U’Ren et al, 2012; Zimmerman & Vitousek, 2012; Davey et al, 2013; Balínt et al, 2015). Fungal communities in living leaves differed detectably in composition from communities in dead leaves and leaf litter within focal sites and host taxa, regardless of differential weighting of rare and abundant fungi. Some fungi with endophytic life stages clearly persist for periods of time in leaves after senescence and incorporation into leaf litter, our sampling across diverse biomes and host lineages detected consistent differences between fungal assemblages in living vs non-living leaves, reflecting incursion by fungi from the leaf exterior after leaf death and as leaves begin to decompose.

Objectives

Methods

Results

Discussion

Conclusion