PhD defense: Feng Long

Feng Long defends her thesis:

The Fraxinus excelsior mycobiome

Zoom link

Supervisors:
Professor Lene Rostgaard Nielsen, IGN
Professor Erik Dahl Kjær, IGN
Associate Professor Chatchai Kosawang, IGN
Associate Professor James Doonan, IGN

Assessment Committee:
Professor Isabella Børja, NIMBIO and NMBU – Norway
Associate Professor Rimvydas Vasaitis, SLU - Sweden
Lektor Hans Jørgen Lyngs Jørgensen (chair), PLEN

Summary (shortened):
In recent years, a growing body of evidence has demonstrated the role of fungal symbionts in modulating plant host resistance against abiotic and biotic stress, such as drought tolerance and defense against fungal pathogens. These studies have generated interest in tree-associated fungal communities, also known as the tree mycobiome. The most well-studied tree-fungi relationship is that of mycorrhizal fungi, which enhance tree vigor through nutrient uptake facilitation and pathogen antagonism. Despite the widespread focus on mycorrhizal fungi, some endophytic fungi also have protective roles against a variety of plant pathogens, through, for example, the production of anti-fungal secondary metabolites. However, mycobiome communities must survive in the face of changing environmental conditions. For instance, climatic factors, including drought and high temperature, can alter the fungal species within each community. Climate change can also affect soil properties, such as pH and nutrient availability, which can again modify the mycobiome community structure. Trees can shape the structure of their associated mycobiome, and microbes can be transferred from the mother to the next generation, and trees with different genetic backgrounds may select distinct fungi, even within the same species.

In Manuscript I, I hypothesized that mother ash trees disperse different sets of mycobiomes via their seed wings. The rationale behind testing the hypothesis was to better understand the early dispersal of the seed mycobiome of ash trees with different levels of ash dieback susceptibility. In common ash, the function of the samara is to enhance seed dispersal and it is spread together with the seed itself, while most of the seed stalk remains on the mother tree. The seeds were collected from two genotypes of common ash. The endophytic fungal community of seed wing and stalk were profiled and analyzed with ITS1-based amplicon sequencing in this manuscript. Seed wing had a higher richness than seed stalk within genotypes, while neither the diversity between the mycobiomes from the two tissue types nor the two genotypes were significantly different. Furthermore, the mycobiome composition differed between seed wings of the two genotypes, while seed stalks were not significantly different. Therefore, the results suggested that F. excelsior mother trees dispersed different sets of endophytic fungi through their samaras. 

Finally, in Manuscript IV, I conducted a greenhouse study to explore the function of forest soils in ash seedling growth and  drought tolerance in comparison to other soil types. I hypothesized that the forest soil mycobiome could benefit ash seedling growth in comparison to potting and agriculture soil. I further hypothesized that soil type and drought stress can affect the rootassociated fungal community and furthermore that these communities can recover after rewatering. The rationale behind testing the hypothesis was to explore the implications of expected extreme weather conditions, including droughts, on ash seedling vigor. The results showed that soil type and drought stress influenced the photosynthesis of ash seedlings, and the seedlings grown in forest soil had higher stomatal conductance (gs), CO2 assimilation, and transpiration. In contrast, the total
CO2 assimilation was lower in forest soil than in the other two soil types under drought conditions. Moreover, soil type affected ash growth in response to drought stress and rewatering. The total plant biomass in forest soil was lower after rewatering, while the root/shoot ratio increased to be higher than in the other two soil types after rewatering. Furthermore, soil type affected leaf C, N, P, K, Mg content and C/N ratio (P < 0.001) in response to drought stress, but not leaf Ca. The mycobiome composition of the root endosphere and rhizosphere was significantly affected by soil types and drought stress, although only the root endosphere mycobiome was significantly influenced by rewatering. The correlation between the root endo- and rhizosphere fungal
community and soil properties indicated that soil pH, C, N, and C/N ratio could explain the variations. These findings suggest that ash seedlings modulate the root-associated mycobiome via plant-soil-microbe interaction during drought scenarios.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk before the defence. After the defence the thesis will become available from the Royal Danish Libary at kb@kb.dk.