Judith Lundberg-Felten holding a tray with small aspen trees in jarsPhoto: Johan Gunséus We are studying the development of ectomycorrhizal symbioses. This type of symbiosis forms naturally between the majority of temperate and boreal forest trees and soil fungi. Ectomycorrhizal fungi exploit the soil very efficiently to absorb nutrients (N, P) through their extensive hyphal networks. A part of these nutrients can be exchanged with tree partners for photosynthetic sugars. The nutrient exchange can benefit ree and fungus. Despite the importance of ectomycorrhizal symbiosis for the health of the forest (soil) ecosystem, the molecular mechanisms that trigger ectomycorrhiza establishment remain largely unknown.

Ectomycorrhizal roots (ECM) (Figure 1) are characterized by three tissues: a fungal mantle surrounding the root from which extramatrical hyphae reach out into the soil to gather nutrients (N, P). These nutrients are exchanged with the plant for photosynthetic derived sugars in the Hartig Net, where a number of specific plant and fungal transport proteins are expressed (Martin and Nehls, Current Opinion in Plant Biology 2009). The structure of the Hartig Net is characterized by fungal hyphae that invade the apoplastic space between root epidermis/cortex cells (Figure 1C).

Collage of three photos showing ectomycorrhizal roots of Populus and Laccaria in three different resolutionFigure 1: (A) Ectomycorrhizal roots of Populus and Laccaria bicolor. Note the swollen and short nature of the ectomycorrhizal roots. (B) Light microscopy image of a transverse section through an ectomycorrhizal root. (C) Magnified light microscopy image of the Hartig Net. Cortex (Co), Epidermis (E), Mantle (M), Hartig Net (HN). Pictures: Judith Lundberg-Felten

Hartig Net development requires loosening of the radial wall between adjacent epidermis cells. This process involves degradation of the middle lamella between these cells. It has been proposed that fungus- and plant-derived enzymes from the Carbohydrate Active Enzyme (CAzyme) family, which have the potential to modify cell wall polymers, could mediate the cell wall release. Fungal genes coding for CAzymes have been identified in Laccaria bicolor and Tuber melanosporum ECM (Balestrini et al., Current Genetics 2012; Veneault-Fourrey et al., Fungal Genetics and Biology 2014; Sillo et al., Planta 2016, Chowdhury et al. 2022) and some of these are induced within the Hartig Net (Hacquard et al., Environmental Microbiology 2013). 

Ectomycorrhizal fungi secrete effectors such as Mycorrhiza induced Small Secreted Protein 7 (MiSSP7), which can be taken up into the plant and trigger plant responses that may contribute to cell wall release (Plett et al., Current Biology 2011). MiSSP7 is required for Hartig Net formation and fungal strains of L. bicolor that lack this peptide form only a very shallow Hartig Net, suggesting that even plant-triggered processes are required for Hartig Net formation. Fungal auxin is yet another fungal factor that is likely to contribute to cell wall remodelling and Hartig Net formation (Gay et al., New Phytologist 1994), but again functional studies are needed to prove this assumption and how auxin may interact with effectors and CAzymes remains to be investigated. The different categories of actors potentially contributing to cell wall remodelling and Hartig Net formation are depicted in Figure 2.

Illustration about the processes involved in Hartig Net formation. Figure 2: Processes involved in Hartig Net formation. Fungal hyphae release polysaccharide material and adhere to the root surface. Apoplastic fungal effectors, fungal CAzymes and fungal auxin are released into the apoplastic space and may contribute to the degradation of the middle lamella, permitting fungal invasion of the apoplastic space. The fungus also releases cytoplasmic effectors that are taken up through endocytosis into the plant cell. These effectors trigger plant responses. Such responses could lead to production/release of CAzymes and plant-derived auxin that may participate in the degradation of the middle lamella. Picture: Judith Lundberg-Felten

The aim of my research is to uncover the nature of cell wall remodelling during Hartig Net establishment and to reveal the crucial molecular factors behind this process and their interplay. In my group we are using an elegant combination of state of the art cell wall analysis techniques together with microscopy, hormone metabolomics, transcriptomics and cell biology on material from ectomycorrhiza from gymnosperm and angiosperm trees with fungi having different mycorrhization capacity, to reveal the nature of cell wall remodelling required for Hartig Net establishment.

In 2022 we demonstrated that a pectin methylesterase from the fungus L. bicolor is involved in Hartig Net formation with hybrid aspen trees (Chowdhury et al. 2022). L. bicolor harbours two types of pectin modifying enzyme families. Our work together with the one by Zhang et al. (2022) is showing that at least one member of each of these families is required for normal Hartig Net formation and that therefore L. bicolor actively contributes to the separation of adjacent cells for Hartig Net formation with Populus trees roots.

Key Publications

  • Chowdhury J, Kemppainen M, Delhomme N, Shutava I, Zhou J, Takahashi J, Pardo AG and J Lundberg-Felten Laccaria bicolor pectin methylesterases are involved in ectomycorrhiza development with Populus tremula × Populus tremuloides. New Phytologist 2022, 236(2):639-655
  • Kemppainen M, Chowdhury J, Lundberg-Felten J and A Pardo Fluorescent protein expression in the ectomycorrhizal fungus Laccaria bicolor: a plasmid toolkit for easy use of fluorescent markers in basidiomycetes. Current Genetics 2020 66(4):791-811.
  • Felten J, Hall H, Jaumot J, Tauler R, de Juan A and A Gorzsás. Vibrational spectroscopic image analysis of biological material using multivariate curve resolution-alternating least squares (MCR-ALS). Nature protocols 2015, 10 (2) 217-240
  • Vayssieres A, Pencik A, Felten J, Kohler A, Ljung K, Martin F and V Legué. Development of the poplar-Laccaria ectomycorrhiza modifies root auxin metabolism, signalling and response. Plant Physiology 2015, 169 (1), 890
  • Ditengou FA, Müller A, Rosenkranz M, Felten J, Lasok H, Miloradovid van Doorn M, Legue V, Palme K and JP Schnitzler. Volatile signalling by sesquiterpenes from ectomycorrhizal fungi reprograms root architecture. Nature Communications 2015, 6: 6279-6279
  • Felten J, Martin F and V Legué. Signaling in ectomycorrhizal symbiosis. in S. Perotto and F. Baluska (eds.), Signaling and Communication in Plant Symbiosis, Signaling and Communication in Plants 11, Volume 10, 2012, pp 123-142, Springer-Verlag Berlin Heidelberg
  • Felten J, Legué V and F Ditengou. Lateral root stimulation in the early interaction between Arabidopsis thaliana and the ectomycorrhizal fungus Laccaria bicolor: Is fungal auxin the trigger? Plant Signaling & Behavior 2010, 5(7) pp 864-867
  • Felten J, Kohler A, Morin E, Bhalerao RP, Palme K, Martin Ditengou, FA and V Legue. The ectomycorrhizal fungus Laccaria bicolor stimulates lateral root formation in poplar and Arabidopsis through auxin transport and signaling. Plant Physiology 2009, 151 (4) pp 1991-2005