Browsing by master's degree program "Magisterprogrammet i växtbiologi"

As biodiversity is being lost worldwide at an accelerating rate due to anthropogenic activities, the frequency and severity of many infectious diseases has been observed to increase. Together these patterns have brought forth an urgent need to understand the possible linkages between biodiversity and disease risk. Two contradicting hypotheses have been proposed to explain the diversity-disease relationship. The dilution effect hypothesis suggests that increasing host community species diversity ‘dilutes’ disease risk, whereas the amplification effect hypothesis predicts disease risk to increase with increasing diversity. Even though most of the studies support the dilution effect, there remains an intensive debate regarding the generality of this effect. As most of the research efforts to understand the relationship between diversity and disease have focused on animals and crop plants or have been carried out experimentally, one of the research gaps is how relevant the dilution effect is in wild plant communities. In nature, plants and their diseases are affected simultaneously by multiple abiotic and biotic environmental factors that might confound or supersede the effects of diversity. It is also poorly understood, whether we might expect dilution effects to occur not only on diversity gradients driven by anthropogenic diversity loss, but also on natural diversity gradients. To study the possible association between host community species diversity and disease risk in the wild and to test whether this association could be detected after accounting for the effects of abiotic factors, I surveyed grassland vascular plant communities for their species diversity and foliar disease symptoms along a natural diversity gradient driven by elevation. I also recorded data on the mean soil surface temperature in the surveyed plant communities and used structural equation modelling to differentiate and compare the effects of biotic and abiotic variables on disease risk. The data were collected on Mount Calanda in the Swiss Alps during summer 2019. In this thesis I show that host community species diversity and disease risk are negatively associated with each other along a natural diversity gradient driven by elevation. Furthermore, this negative effect can be detected even after accounting for the effects of elevation and mean soil surface temperature on disease. Together the results support the occurrence and the ecological relevance of the dilution effect in wild plant communities along natural diversity gradients and suggest that diversity might protect wild plant communities from increased disease risk. Future studies should aim to identify the exact mechanisms of the association to help us better understand when and where we might expect dilution effects to occur in the wild. This knowledge can be used to predict how epidemics, that affect the well-being of ecosystems, humans and wildlife, are born in the changing world.

The aim of this study was to examine the leaf endophytic bacteria in Plantago lanceolata. The first aim was to get a comprehensive picture of the bacterial diversity, by screening for the different bacterial genera inside the leaves. Furthermore, I aimed to examine the effect of soil and maternal genotype on the endophytic community within P. lanceolata leaves and search for clues of vertical inheritance of endophytes from parent to offspring via seeds. I studied the endophytic bacteria by extracting DNA from the plant leaves and by trying to amplify any bacterial DNA present to get a view of the bacterial diversity in the leaves. My aim was to compare the bacterial community of the mother plants to that of their offspring and also to compare the bacterial communities of plants grown in different soil conditions. Furthermore, I tried to study how the soil conditions affect the growth of P. lanceolata seedlings. I collected seeds and leaf samples of P. lanceolata from Åland, Southwestern Finland, from a population that is part of the ongoing long-term metapopulation research started in Åland in the early 90’s. I marked 21 plant individuals (hereafter referred to as the “mother plants”) in the field in June, when collecting the first leaf samples. In August I collected all seeds from the same plant individuals and a second set of leaf samples. I also collected soil samples from the same location. With the seeds collected from the wild population I executed a growth experiment in Viikki, Helsinki. I grew one set of seeds in twice autoclaved sand (hereafter referred to as the “sterile soil”) and another set in twice autoclaved sand mixed with soil collected from the Åland population (hereafter referred to as the “environmental soil”). I surface sterilized all seeds and then sowed each in their own growth pot and placed them in a growth chamber. During the experiment I took measurements of the leaves. At end of the growth experiment, I took samples of the leaves and surface sterilized them to exclude any epiphytic microorganisms from the analysis. I also surface sterilized the leaf samples taken from the mother plants. I then extracted DNA from the leaf samples and run PCR to amplify certain regions of the bacterial 16S rDNA gene, that is widely used in bacterial taxonomy. The obtained DNA reads where then clustered into Operational Taxonomic Units (OTUs) and assigned taxonomy using SILVA reference database. Mitochondria and chloroplasts of eukaryotic organisms also harbour 16S rDNA regions, so the challenge of studies looking at endophytic bacteria is to exclude the 16S regions of mitochondria and chloroplasts. This proved to be a problem in my study also. More than 86% of all DNA reads obtained turned out to be from P. lanceolata mitochondria and more than 12% from P. lanceolata chloroplasts. Only a bit more than 1% of the reads were eubacterial. This effectively hindered reliable analysis of the endophyte community. I nevertheless analysed the observed eubacterial diversity although the results must be taken as only preliminary and with utmost caution. The eubacterial reads clustered into 218 OTUs, representing 71 different bacterial genera. Six most common genera constituted over 83% of eubacterial reads. Most of these bacteria, most notably Shewanella, Ralstonia and Halomonas, could be identified as being clearly contaminants and not real endophytes. For the 65 less common bacterial genera I performed community analysis using Bray-Curtis Dissimilarity index and Analysis of Similarities (ANOSIM). The results showed that there was a significant difference between the different soil treatments (P = 0.014, R = 0.3787) and also between the two growth chambers (P = 0.011, R = 0.5493). I found no effect of maternal genotype on the bacterial community. Therefore, I observed no sign of vertical inheritance of endophytes. The growth experiment results showed that germination percentage was significantly lower in the environmental soil than in the sterile soil for all genotypes (F = 10.78, P = 0.0012). However, seedling in the environmental soil grew bigger than the seedlings in the sterile soil (F = 10.91, P < 0.0001). For future studies on similar topics, validating molecular methods before large scale sequencing could yield more reliable results. Size fractionating the DNA products of the first PCR round could exclude most mitochondrial sequences and hence allow better analysis of endophytes. This would enable studying interesting questions on coevolution and ecology of host-endophyte interactions. Although I did find some differences in the bacterial communities of different treatments, these results must be considered with caution and as only preliminary.

Ecophysiology and ecology in plants are strongly affected by the conditions surrounding them. Adaptation aids plants to survive and to succeed in the prevailing conditions. Winter is a challenge to plants, particularly in northern latitudes and higher altitudes, because it exposes plants to cold and drought, for example. Plants survive from winter on species level with the help of genetic adaptations and as individuals also with the help of acclimation. Woodland strawberry (Fragaria vesca) has been observed to grow separate winter leaves. This allows it to continue photosynthesis in mild conditions during winter, thus improving its energy balance, and to start growing earlier than other species in the spring, which is beneficial in interspecific competition. Fragaria vesca is a species that has wide distribution in the northern hemisphere, and its genotypes are found from very different locations and conditions. However, adaptive traits such as producing a new set of leaves for winter can turn out to be a disadvantage if environmental conditions change rapidly. Climate change brings about changes that are difficult to predict, and these changes are advancing at a fast pace when compared to the developmental history of plants. The aim of this thesis was to study the effect of temperature on summer and winter leaf development, stolon formation and summer and winter leaf chlorophyll, flavonol and anthocyanin content in different Fragaria vesca genotypes. Leaf chlorophyll and secondary compound content give information about leaf development and stress reactions in plants. Plants are known to produce anthocyanins in order to protect the photosynthetic apparatus during chlorophyll recovery in leaf senescence. Anthocyanins are also produced as a response to low temperatures. Research increases knowledge of the ecophysiological and winter ecology-related processes in Fragaria vesca and in the commercially valuable Rosacea-family as well as provides information about the possible responses of these organisms to climate change. Material for the study consisted of twelve European Fragaria vesca genotypes, which had originally been collected from five countries: Norway, Finland, Germany, Italy and Spain. The genotypes had been collected from different latitudes, and they also expressed altitudinal differences. In this study, these genotypes were kept in two temperature treatments, warm (+16°C) and cold (+11°C/six weeks, after which +6°C/four weeks) at a greenhouse. Leaf development was studied by measuring summer and winter leaf middle leaflet width and length, and petiole length. Stolons from each plant individual were counted on a weekly basis and observations about stolon production in relation to the timing of summer leaf senescence and winter leaf development were made at the same time. Leaf chlorophyll and secondary compound content was measured with a Dualex-meter, which provided values for chlorophyll, flavonol and anthocyanin content. The underlying assumption was that cold temperature would induce winter leaf development and summer leaf senescence. The results show that there were differences in summer leaf size between genotypes. Winter leaves had differences between genotypes, but also within genotypes at different temperature treatments. Stolon count was lower and stolon production ceased slightly earlier in the cold treatment. Moreover, summer leaf chlorophyll content decreased in both treatments, but the summer leaves senesced earlier in the warm room. Summer leaf flavonol and anthocyanin values were generally higher in the cooler temperature treatment. Anthocyanins were also produced by winter leaves in the cooler temperature treatment. Based on the results, Fragaria vesca genotypes had differences related to their origin, but temperature also had an effect on winter leaf development, stolon production and the production of secondary compounds. The effect of cold temperature on the size of developing winter leaves was clear. In the cooler temperature treatment, the winter leaves were smaller than in the warmer treatment. The anthocyanin content of summer leaves was higher than in the winter leaves, and the summer leaf anthocyanin content was higher in the colder temperature treatment, where the stress related to the photosynthetic apparatus and low temperatures was combined. Nevertheless, lower temperature did not explain all the responses observed in the genotypes of the study, and thus it is likely that acclimation and winter leaf development in Fragaria vesca are affected by some other factor in addition to temperature, e.g. light regime. A possible continuation for this work would be to study the effect of light conditions or milder winters on winter leaf development in Fragaria vesca genotypes and on the physiology of the species.