Outbreak of Drepanopeziza fungus in aspen forests and variation in stand susceptibility: leaf functional traits, compensatory growth and phenology

Changes in the foliar fungal community between oak leaf flushes along a latitudinal gradient in Europe

AIM

Leaves support a large diversity of fungi, which are known to cause plant diseases, induce plant defences or influence leaf senescence and decomposition. To advance our understanding of how foliar fungal communities are structured and assembled, we assessed to what extent leaf flush and latitude can explain the within- and among-tree variation in foliar fungal communities.

LOCATION

A latitudinal gradient spanning c. 20 degrees in latitude in Europe.

TAXA

The foliar fungal community associated with a foundation tree species, the pedunculate oak Quercus robur.

METHODS

We examined the main and interactive effects of leaf flush and latitude on the foliar fungal community by sampling 20 populations of the pedunculate oak Quercus robur across the tree's range. We used the ITS region as a target for characterization of fungal communities using DNA metabarcoding.

RESULTS

Species composition, but not species richness, differed between leaf flushes. Across the latitudinal gradient, species richness was highest in the central part of the oak's distributional range, and foliar fungal community composition shifted along the latitudinal gradient. Among fungal guilds, the relative abundance of plant pathogens and mycoparasites was lower on the first leaf flush, and the relative abundance of plant pathogens and saprotrophs decreased with latitude.

CONCLUSIONS

Changes in community composition between leaf flushes and along the latitudinal gradient were mostly a result of species turnover. Overall, our findings demonstrate that leaf flush and latitude explain 5%-22% of the small- and large-scale spatial variation in the foliar fungal community on a foundation tree within the temperate region. Using space-for-time substitution, we expect that foliar fungal community structure will change with climate warming, with an increase in the abundance of plant pathogens and mycoparasites at higher latitudes, with major consequences for plant health, species interactions and ecosystem dynamics.

Systematic identification and functional characterization of the CFEM proteins in poplar fungus Marssonina brunnea

Proteins containing Common in Fungal Extracellular Membrane (CFEM) domains uniquely exist in fungi and play significant roles in their whole life history. In this study, a total of 11 MbCFEM proteins were identified from Marssonina brunnea f. sp. multigermtubi (MULT), a hemibiotrophic pathogenic fungus on poplars that causes severe leaf diseases. Phylogenic analysis showed that the 11 proteins (MbCFEM1-11) were divided into three clades based on the trans-membrane domain and the CFEM domain. Sequence alignment and WebLogo analysis of CFEM domains verified the amino acids conservatism therein. All of them possess eight cysteines except MbCFEM4 and MbCFEM11, which lack two cysteines each. Six MbCFEM proteins with a signal peptide and without trans-membrane domain were considered as candidate effectors for further functional analysis. Three-dimensional (3D) models of their CFEM domains presented a helical-basket structure homologous to the crucial virulence factor Csa2 of Candida albicans. Afterward, four (MbCFEM1, 6, 8, and 9) out of six candidate effectors were successfully cloned and a yeast signal sequence trap (YSST) assay confirmed their secretion activity. Pathogen challenge assays demonstrated that the transient expression of four candidate MbCFEM effectors in Nicotiana benthamiana promoted Fusarium proliferatum infection, respectively. In an N. benthamiana heterogeneous expression system, MbCFEM1, MbCFEM6, and MbCFEM9 appeared to suppress both BAX/INF1-triggered PCD, whereas MbCFEM8 could only defeat BAX-triggered PCD. Additionally, subcellular localization analysis indicated that the four candidate MbCFEM effectors accumulate in the cell membrane, nucleus, chloroplast, and cytosolic bodies. These results demonstrate that MbCFEM1, MbCFEM6, MbCFEM8, and MbCFEM9 are effectors of M. brunnea and provide valuable targets for further dissection of the molecular mechanisms underlying the poplar-M. brunnea interaction.

The co-existence of multiple oak leaf flushes contributes to the large within-tree variation in chemistry, insect attack and pathogen infection

Many plant species produce multiple leaf flushes during the growing season, which might have major consequences for within-plant variation in chemistry and species interactions. Yet, we lack a theoretical or empirical framework for how differences among leaf flushes might shape variation in damage by insects and diseases. We assessed the impact of leaf flush identity on leaf chemistry, insect attack and pathogen infection on the pedunculate oak Quercus robur by sampling leaves from each leaf flush in 20 populations across seven European countries during an entire growing season. The first leaf flush had higher levels of primary compounds, and lower levels of secondary compounds, than the second flush, whereas plant chemistry was highly variable in the third flush. Insect attack decreased from the first to the third flush, whereas infection by oak powdery mildew was lowest on leaves from the first flush. The relationship between plant chemistry, insect attack and pathogen infection varied strongly among leaf flushes and seasons. Our findings demonstrate the importance of considering differences among leaf flushes for our understanding of within-tree variation in chemistry, insect attack and disease levels, something particularly relevant given the expected increase in the number of leaf flushes with climate change.

Rapid and Specific Detection of the Poplar Black Spot Disease Caused by Marssonina brunnea Using Loop-Mediated Isothermal Amplification Assay

Marssonina brunnea is the main pathogen that causes poplar black spot disease, which leads to the decrease of the photosynthetic efficiency and significantly affects the production and quality of timber. Currently, no in-field diagnostic exists for M. brunnea. Here, we described a loop-mediated isothermal amplification (LAMP) assay for the rapid and sensitive detection of M. brunnea. A set of six oligonucleotide primers was designed to recognize eight distinct sequences of the internal transcribed spacer (ITS) region of M. brunnea. The LAMP assay was optimized by the combination of high specificity, sensitivity, and rapidity for the detection of less than 10 pg/μL of target genomic DNA in 60 min per reaction at 65 °C, whereas with PCR, there was no amplification of DNA with concentration less than 1 ng/μL. Among the genomic DNA of 20 fungalisolates, only the samples containing the genomic DNA of M. brunnea changed from violet to sky blue (visible to the naked eye) by using hydroxynaphthol blue (HNB) dye. No DNA was amplified from the eight other fungus species, including two other Marssonina pathogens, three other foliar fungi pathogens of poplar, and three common foliar fungal endophytes of poplar. Moreover, the detection rates of M. brunnea from artificially and naturally infected poplar leaves were 10/16 (62.5%) and 6/16 (37.5%) using PCR, respectively, while the positive-sample ratios were both 16/16 (100%) using the LAMP assay. Overall, the ITS LAMP assay established here can be a better alternative to PCR-based techniques for the specific and sensitive detection of M. brunnea in poplar endemic areas with resource-limited settings.

Distinctive Gene Expression Profiles and Effectors Consistent With Host Specificity in Two Formae Speciales of Marssonina brunnea

The knowledge on the host specificity of a pathogen underlying an interaction is becoming an urgent necessity for global warming. In this study, the gene expression profiles and the roles of effectors in host specificity were integrally characterized with two formae speciales, multigermtubi and monogermtubi, of a hemibiotrophic pathogen Marssonina brunnea when they were infecting respective susceptible poplar hosts. With a functional genome comparison referring to a de novo transcriptome of M. brunnea and Pathogen-Host Interaction database functional annotations, the multigermtubi strain showed abundant and significant differentially expressed unigenes (DEGs) (more than 40%) in colonizing the initial invasion stage and in the necrotrophic stage. The monogermtubi strain induced less than 10% of DEGs in the initial invasion stage but which abruptly increased to more than 80% DEGs in the necrotrophic stage. Both strains induced the least DEGs in the biotrophic stage compared to the initial invasion and necrotrophic stages. The orthologs of the effector genes Ecp6, PemG1, XEG1, ACE1, and Mg3LysM were exclusively induced by one of the two formae speciales depending on the infection stages. Some unigenes homologous to carbohydrate lytic enzyme genes, especially pectate lyases, were notably induced with multigermtubi forma specialis infection but not expressed in the monogermtubi forma specialis at an earlier infection stage. The extraordinary differences in the functional genome level between the two formae speciales of M. brunnea could be fundamental to exploring their host specificity determinant and evolution. This study also firstly provided the fungal transcriptome of the monogermtubi forma specialis for M. brunnea.

Secondary leaves of an outbreak-adapted tree species are both more resource acquisitive and more herbivore resistant than primary leaves

The magnitude and frequency of insect outbreaks are predicted to increase in forests, but how trees cope with severe outbreak defoliation is not yet fully understood. Winter deciduous trees often produce a secondary leaf flush in response to defoliation (i.e., compensatory leaf regrowth or refoliation), which promotes fast replenishment of carbon (C) storage and eventually tree survival. However, secondary leaf flushes may imply a high susceptibility to insect herbivory, especially in the event of an ongoing outbreak. We hypothesized that in winter deciduous species adapted to outbreak-driven defoliations, secondary leaves are both more C acquisitive and more herbivore resistant than primary leaves. During an outbreak by Ormiscodes amphimone F. affecting Nothofagus pumilio (Poepp. & Endl.) Krasser forests, we (i) quantified the defoliation and subsequent refoliation by analyzing the seasonal dynamics of the normalized difference vegetation index (NDVI) and (ii) compared the physiological traits and herbivore resistance of primary and secondary leaves. Comparisons of the NDVI of the primary and second leaf flushes relative to the NDVI of the defoliated forest indicated 31% refoliation, which is close to the leaf regrowth reported by a previous study in juvenile N. pumilio trees subjected to experimental defoliation. Primary leaves had higher leaf mass per area, size, carbon:nitrogen ratio and soluble sugar concentration than secondary leaves, along with lower nitrogen and starch concentrations, and similar total polyphenol and phosphorus concentrations. In both a choice and a non-choice bioassay, the leaf consumption rates by O. amphimone larvae were significantly higher (>50%) for primary than for secondary leaves, indicating higher herbivore resistance in the latter. Our study shows that secondary leaf flushes in outbreak-adapted tree species can be both C acquisitive and herbivore resistant, and suggests that these two features mediate the positive effects of the compensatory leaf regrowth on the tree C balance and forest resilience.

Timing and mode of simulated ungulate herbivory alter aspen defense strategies

Timing of herbivory or selection of specific plant tissues (mode of herbivory) by different ungulate herbivore species are likely to have important influences on plant defense strategies. In this study, we devised two different modes of simulated herbivory, representing a selective ungulate feeding strategy (defoliation: leaf tissue removal only) and a bulk feeding strategy (clipping: leaves, twigs and meristems taken together). We applied these contrasting herbivory treatments to juvenile aspen suckers (Populus tremuloides Michx.) regenerating underneath aspen stands in early summer (June), late summer (August) or at both times to determine the effects of herbivory mode, timing and frequency on regenerating aspen. In response to the simulated herbivory treatments, we measured traits related to three plant defense strategies: tolerance (aboveground biomass and stem diameter), resistance (foliar phenolic glycosides) and vertical escape (sucker height and average leader length). There was no evidence that mode, timing or frequency of simulated herbivory induced or repressed phenolic glycoside production. Early summer herbivory was more detrimental than late summer herbivory on aspen tolerance and escape. Repeat herbivory in late summer did not amplify the negative effects of early summer herbivory. Clipping and defoliation tended to have similar effects on tolerance but clipping was more detrimental than defoliation on vertical escape. These results suggest that different ungulate herbivore species may have disparate impacts on the plant communities by selecting different tissues of the same plant, or browsing the plant at different times in the growing season.