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Diversity and beneficial properties of bacterial endophytes of arctic plants

Final Report Summary - ARCTIC ENDOPHYTES (Diversity and beneficial properties of bacterial endophytes of arctic plants)

Bacterial endophyte communities in arctic plants

Plants in all climates and in all ecosystems are associated with a diverse array of microbes. Endophytic bacteria inhabit the internal plant tissues, and have been isolated from a large diversity of plants, where they form non-pathogenic, often mutualistic, relationships with their hosts. Although the presence of endophytic bacteria has been known for long, awareness of their abundance and potential benefits has grown rapidly in recent years. The reported plant beneficial traits include growth stimulation, nutrient mobilisation, nitrogen fixation, detoxification of pollutants and biocontrol of plant pathogens. There are only a handful of studies on diversity of endophytic bacteria in natural ecosystems, and currently there is no data on endophytic bacterial diversity in arctic flora.

Arctic and subarctic climates are highly demanding environments for plants, due to low temperature average and extreme cold, short growth season (1-4 mo) and low solar radiation, with high annual fluctuations. Water stress, either in the form of drought or flooding, is common. As recycling of nutrients, especially nitrogen and phosphorus, in cold climates is very slow, the soil is often very nutrient-poor. To survive in this highly demanding environment, arctic plants employ diverse anatomical and physiological adaptations like low-growth morphology, dominantly vegetative propagation, low optimal temperature for photosynthesis and physiological hardening to prevent frost damage.

The study characterised the taxonomic and functional diversity of endobacterial flora of three arcto-alpine plant species (Oxyria digyna, Diapensia lapponica and Juncus trifidus) in the low Arctic in Kilpisjärvi area, northwestern Scandinavia (69°03'N). The endobacterial flora was isolated from plant samples collected from three distinct sites, and endophyte community diversity was analysed by cultivation and DNA-based methods. Functional diversity of culturable endobacteria was analysed by activity assays and by PCR amplification of functional genes.

Analysis of bacterial endophyte library of over 350 isolates as well as 6 clone libraries each encompassing 90- sequences revealed a high diversity of bacteria living in association with arctic plant species, representing Actinobacteria, Bacteroides, Firmicutes, Acidobacteria and a- , ß- and ?- proteobacteria. The most common taxonomic groups were a- and ß- proteobacteria and Actinobacteria. Taxonomic distribution of the culturable isolates as well as the clone libraries were dependent mainly on host plant species, but also on (properties of) sampling sites.

Several bacterial groups associated tightly with specific plant species: for example, Burkholderia spp. dominated D. lapponica and J. trifidus samples, but were nearly absent from O. digyna. Sphingomonas sp. were common in D. lapponica as well as in O. digyna, and sequence alignment of Sphingobacterial isolates revealed their plant host specificity. Moreover, Sphigomonas sp. from different plant hosts showed divergent metabolic profiles.

The isolated bacteria are well adapted to low temperatures; this is reflected in both taxonomy, with the closest relatives often representing psychrophilic or -tolerant isolates - over 50 % of closest relatives were isolated from glaciers, snow, polar or alpine soils or other cold environments - as well as in their physiology: most isolates grow well at +4 degrees Celsius. Phosphate solubilisation is a very common trait in isolates analysed. In contrast, cellulase or amylase activity were detected in less than 10 % of the analysed isolates. Many of the isolates retained full enzymatic activity at +2 - +5 degrees Celsius.

This was the first comprehensive study on endobacterial diversity in the Arctic, and will be continued and expanded based on the results obtained in this study. Additionally, the endophyte isolate library will be used to study potential plant beneficial effects. In addition to scientific data and papers, the project produced one meeting organised by the participants of the MC project.