Aspen Leaves as a “Chemical Landscape” for Fungal Endophyte Diversity—Effects of Nitrogen Addition

Abstract

Abiotic and biotic factors may shape the mycobiome communities in plants directly but also indirectly by modifying the quality of host plants as a substrate. We hypothesized that nitrogen fertilization (N) would determine the quality of aspen (Populus tremula) leaves as a substrate for the endophytic fungi, and that by subjecting the plants to N, we could manipulate the concentrations of positive (nutritious) and negative (antifungal) chemicals in leaves, thus changing the internal “chemical landscape” for the fungi. We expected that this would lead to changes in the fungal community composition, in line with the predictions of heterogeneity–diversity relationship and resource availability hypotheses. To test this, we conducted a greenhouse study where aspen plants were subjected to N treatment. The chemical status of the leaves was confirmed using GC/MS (114 metabolites, including amino acids and sugars), LC/MS (11 phenolics), and UV-spectrometry (antifungal condensed tannins, CTs), and the endophytic communities were characterized using culture-dependent sequencing. We found that N treatment reduced foliar concentrations of CT precursor catechin but not that of CTs. Nitrogen treatment also increased the concentrations of the amino acids and reduced the concentration of some sugars. We introduced beetle herbivores (H) as a second treatment but found no rapid changes in chemical traits nor strong effect on the diversity of endophytes induced by herbivores. A few rare fungi were associated with and potentially vectored by the beetle herbivores. Our findings indicate that in a controlled environment, the externally induced changes did not strongly alter endophyte diversity in aspen leaves.

Can nitrogen supersede host identity in shaping the community composition of foliar endophytic fungi in an alpine meadow ecosystem?

“…Tables 1 , 2 ; Figures 2 , 4 ). The findings agree with culture-based experimental evidence from Sweden recently (Witzell et al, 2022 ) that N input did not alter the total richness and Shannon diversity of FEFs in 12 aspen genotypes. We suggest that in the alpine meadow ecosystems, total fungal communities harbored in diverse groups of plants tend to persist under increasing N deposition.…”

Diversity and abundance of culturable endophytes in leaves of susceptible and resistant alternate hosts of Cronartium pini and C. ribicola

“…Endophytes infect the plants from the surrounding environment (Gomes et al, 2018) and the dominance of certain fungi in the culturable fraction may thus re ect the high frequency of these fungi in the environmental inoculum in the region of the study area. The fact that the same morphotypes dominated in the samples from the greenhouse-grown I. balsamina supports this view -in greenhouses, the environmental inoculum is limited to spores that are most abundant in the incoming air (Witzell et al, 2022).…”

“…The high colonization rate of Melampyrum and Veronica species indicates that they were suitable substratum for endophytic fungi. The quality of leaves as a habitat for endophytic fungi could not be analysed in this study, but it has been suggested that especially the secondary metabolites, such as potentially antifungal phenolics could be a factor directing the microbiome composition (Witzell et al, 2022). In previous studies, some of these compounds have been found to be high in rust-resistant alternate host species (Kaitera & Witzell, 2016;Piispanen et al, 2023).…”

Molecular studies of rust on European aspen suggest an autochthonous relationship shaped by genotype

“…Fungal communities in tree foliage are not uniformly distributed or stationary ( Toju et al., 2019 ); their composition is influenced by several factors including host genotype ( Albrectsen et al., 2010b ), growth site ( Siddique et al., 2021 ), seasonal factors ( Albrectsen et al., 2010b ), ontogeny ( Bourassa et al., 2005 ; Boutigny et al., 2013a ), tissue type ( Siddique et al., 2021 ), and host-associated consumers ( Arnold et al., 2003 ; Albrectsen et al., 2010b ; Siddique et al., 2021 ). The mechanisms underpinning these relationships are not fully understood but it has been suggested that both the host’s chemical defense responses ( Ullah et al., 2017 ; Witzell et al., 2022 ) and competition between members of the mycobiome may help shape the microbial community ( Wemheuer et al., 2019 ; Siddique et al., 2021 ). In the present study, we also obtained results suggesting the occurrence of a transition in the composition of the fungal community.…”