WITH OR WITHOUT FUNGI? FRUIT FRACTURE BIOMECHANICS OF LEPIDIUM DIDYMUM GERMINATION
Sperber, K.1a, Steinbrecher, T.2a, Graeber, K.2, Scherer, G.1, Clausing, S.1, Wiegand, N.1, Hourston, J.E.2, Kurre, R.3, Leubner-Metzger, G.2b* and Mummenhoff, K.1b*
1Department of Biology, Botany, University of Osnabrück, D-49076 Osnabrück, Germany
2School of Biological Sciences, Royal Holloway University of London, Egham, Surrey, United Kingdom
3Center for Advanced Light Microscopy, University of Osnabrück, D-49076 Osnabrück, Germany
Contact: James Hourston, www.seedbiology.eu, email@example.com
abThese authors contributed equally to this work
Plant/microbe interactions can play a huge role in the success or failure of plants at a crucial and vulnerable life stage. Examples of plants and microbes forming mutualisms which can improve germination are difficult to study and consequently rarely reported. We present evidence of an interaction between species of common fungi and the hard fruits of Lepidium didymum (Brassicaceae) which is fundamental to survival in distinct environments. Dispersal of fruits with hard pericarps (fruit coats) encasing seeds has evolved many times independently within taxa which have seed dispersal as their default strategy. The mechanisms by which the constraint of a hard pericarp determines germination timing in response to the environment are currently unknown. Here we show that the hard pericarp of Lepidium didymum controls germination solely by a biomechanical mechanism which is targeted by the fungi. We demonstrate fungal-mediated erosion of the pericarp tissues in a process guided by plant tissue architecture. Lepidium didymum’s hard pericarp acts as a mechanical barrier to delay germination, but it does not restrict water uptake or gas exchange. Mechanical dormancy is conferred by preventing full imbibition of the encased non-dormant seed. The lignified endocarp has biomechanically and morphologically distinct regions which serve as predetermined breaking zones. This pericarp-imposed mechanical dormancy is released by the activity of common fungi which weaken these zones by degrading non-lignified pericarp cells. We propose that the hard pericarp with this biomechanical mechanism contributed to the global distribution of this species in distinct environments and in association with the global distribution of the common fungi. Therefore, the fungal colonisation of fruits leads to a much faster onset and higher maximum germination as it effectively breaks this pericarp-imposed dormancy.