Disruption of root carbon transport into forest humus stimulates fungal opportunists at the expense of mycorrhizal fungi

Ectomycorrhizal fungi dominate the humus layers of boreal forests. They depend on carbohydrates that are translocated through roots, via fungal mycelium to microsites in the soil, wherein they forage for nutrients. Mycorrhizal fungi are therefore sensitive to disruptive disturbances that may restrict their carbon supply. By disrupting root connections, we induced a sudden decline in mycorrhizal mycelial abundance and studied the consequent effects on growth and activity of free living, saprotrophic fungi and bacteria in pine forest humus, using molecular community analyses in combination with enzyme activity measurements. Ectomycorrhizal fungi had decreased in abundance 14 days after root severing, but the abundance of certain free-living ascomycetes was three times higher within 5 days of the disturbance compared with undisturbed controls. Root disruption also increased laccase production by an order of magnitude and cellulase production by a factor of 5. In contrast, bacterial populations seemed little affected. The results indicate that access to an external carbon source enables mycorrhizal fungi to monopolise the humus, but disturbances may induce rapid growth of opportunistic saprotrophic fungi that presumably use the dying mycorrhizal mycelium. Studies of such functional shifts in fungal communities, induced by disturbance, may shed light on mechanisms behind nutrient retention and release in boreal forests. The results also highlight the fundamental problems associated with methods that study microbial processes in soil samples that have been isolated from living roots.

Ectomycorrhizal Cortinarius species participate in enzymatic oxidation of humus in northern forest ecosystems

In northern forests, belowground sequestration of nitrogen (N) in complex organic pools restricts nutrient availability to plants. Oxidative extracellular enzymes produced by ectomycorrhizal fungi may aid plant N acquisition by providing access to N in macromolecular complexes. We test the hypotheses that ectomycorrhizal Cortinarius species produce Mn-dependent peroxidases, and that the activity of these enzymes declines at elevated concentrations of inorganic N. In a boreal pine forest and a sub-arctic birch forest, Cortinarius DNA was assessed by 454-sequencing of ITS amplicons and related to Mn-peroxidase activity in humus samples with- and without previous N amendment. Transcription of Cortinarius Mn-peroxidase genes was investigated in field samples. Phylogenetic analyses of Cortinarius peroxidase amplicons and genome sequences were performed. We found a significant co-localization of high peroxidase activity and DNA from Cortinarius species. Peroxidase activity was reduced by high ammonium concentrations. Amplification of mRNA sequences indicated transcription of Cortinarius Mn-peroxidase genes under field conditions. The Cortinarius glaucopus genome encodes 11 peroxidases - a number comparable to many white-rot wood decomposers. These results support the hypothesis that some ectomycorrhizal fungi - Cortinarius species in particular - may play an important role in decomposition of complex organic matter, linked to their mobilization of organically bound N. [KEYWORDS: carbon sequestration Class II peroxidases decomposition ectomycorrhizal fungi high throughput sequencing nitrogen limitation priming effect transcription SOIL ORGANIC-MATTER DEGRADING HEME PEROXIDASES MYCORRHIZAL FUNGI BOREAL FORES

Vertical distribution of ectomycorrhizal fungal taxa in a podzol profile

Studies of ectomycorrhizal fungal communities in forest soils are usually restricted to the uppermost organic horizons. Boreal forest podzols are highly stratified and little is known about the vertical distribution of ectomycorrhizal communities in the underlying mineral horizons. Ectomycorrhizal root tips were sampled from seven horizons in three continuous columns of a 52-cm deep podzol profile. Root tips were sorted into morphological groups and the colonising fungi identified by sequencing of the rDNA ITS region. The vertical distribution of mycorrhizal taxa was examined. A relationship between ectomycorrhizal species composition and soil horizon was found. Tomentellopsis submollis , three Piloderma species and Dermocybe spp. were found predominantly in the upper horizons while Suillus luteus , Lactarius utilis and three undescribed Piloderma species were associated with the mineral horizons. Two thirds of the root tips were found in the mineral soil and half of the taxa were restricted to the mineral horizons. The results highlight the need to include the mineral soil in order to gain a more accurate representation of the ectomycorrhizal community

A tribute to Sally E. Smith

International audienceProf. Sarah (Sally) E. Smith, a long‐standing New Phytologist Advisory Board member, and friend to the journal, died in September 2019. Sally will be remembered not only for her outstanding body of work, but for her friendship, mentorship and leadership of the mycorrhizal research community. By way of tribute we invited colleagues of Sally to share their recollections, and we publish this Virtual Issue in her memory.Sally was born in 1941, and received Bachelor and PhD degrees from Cambridge University, UK, before relocating to Adelaide, Australia in the late 1960s, alongside her husband Andrew (FA) Smith. Upon arrival in Adelaide, Sally undertook a number of positions at the University of Adelaide’s Botany Department, followed by research work at the Waite Research Institute. In 1991 she was appointed Senior Lecturer in the Department of Soil Science, and she received a Doctorate of Science from the University of Adelaide that same year. Sally was appointed Professor in 1995.Sally was elected a Fellow of the Australian Academy of Science in 2001, and, among numerous awards, she received an Australian Centenary Medal for contribution to Australian society and services to biology (2003), the Taylor (2006) and Prescott (2012) medals of the Australian Society of Soil Science, and the International Mycorrhiza Society’s Eminent Mycorrhiza Researcher Award (2019). Sally was also Honorary Professor at the Research Centre for Eco‐Environmental Sciences (Chinese Academy of Sciences), and an Honorary Research Professor at the Chinese Agricultural University, Beijing.Sally officially retired in 2006, but remained very active, holding an Adjunct and later Emeritus Chair at the University of Adelaide, contributing to many international meetings, including the 2014, 33rd New Phytologist Symposium ‘Networks of Power and Influence: Ecology and Evolution of Symbioses between Plants and Mycorrhizal Fungi’ (Bender et al., 2014), and continuing to act as an Advisory Board Member at New Phytologist, providing much‐valued critical insight and advice to our Editors. Her reviews, even when she disagreed, were always supportive and positive; she often provided much detailed advice and her large view of the literature to the authors.In January 2019, New Phytologist published a Profile of Sally (Smith, 2019), which outlines her achievements in more detail, but importantly, it also highlights her many personal qualities. In the profile, Sally outlined the successes and challenges she faced throughout her career, but she also dwells on the many friendships and relationships she developed; her warmth, and generosity, is evident throughout the piece, and also in the personal recollections outlined below (Fig. 1). In a way, Sally is still here with us. We are still prepared to hear her unforgettable voice at conferences; we still use the precious and irreplaceable textbook ‘Mycorrhizal symbiosis’ she wrote with David Read, which will forever remind us of the first steps of our research community and the broad view she had of the symbiosis (Smith & Read, 2008). Sally will be greatly missed, as a scientist, friend, colleague and mentor