Plants can protect their wood against the spread of air bubbles (emboli) and pathogens by forming specialised structures called 'thylls'. This is what an international team has just demonstrated in a fossil wood from the end of the Devonian period. This discovery - in which the EDDYlab of Liège and the Royal Belgian Institute of Natural Sciences participated - published in the journal Nature Plants, provides a better understanding of the evolutionary history of this defence mechanism and the biology of the first woody trees.
he fossil record is an invaluable source of information about past ecosystems. Surprisingly, sometimes a tiny detail can reveal important aspects of the ecology of an entire organism and its ecosystem. This is what happened to a team of Belgian, French, Irish and German researchers, led by Dr Anne-Laure Decombeix, a palaeobotanist at the AMAP Laboratory (University of Montpellier), who have just uncovered a very elaborate drought protection mechanism in plants dating back to the Devonian period (360 million years ago) by studying fossils collected in south-eastern Ireland. This phenomenon, known as tylosis, results from the formation of thylls - parenchyma cell outgrowths in the wood - which can completely block certain conductive cells and thus protect the rest of the wood from the propagation of pathogens or air bubbles.
Cyrille Prestianni, a palaeontologist at the Evolution & Diversity Dynamics Lab (EDDyLab / Faculty of Science) of ULiège and the Royal Belgian Institute of Natural Sciences, helped collect, date and study numerous specimens of Callixylon, the fossil plant that was studied. "The fossils were collected in the autumn of 2021 and then sent to Montpellier where they were prepared using a technique specially developed for this work. This technique makes it possible to observe fossil plants in great detail."
Callixylon is particularly important because it is part of the now extinct group (the archaeopteridales), which was one of the first groups to form large trees in the Devonian. These plants reproduced by spores - like ferns today - but produced wood resembling that of the gymnosperms (a sub-branch, nowadays almost exclusively formed by the conifers: pines, cypresses, junipers, etc.). By studying the organisation of this wood in detail, the researchers were able to highlight the presence of a very special structure called tylosis. These structures, which constitute a development of parenchymal cells in an adjacent water-conducting cell, enable the plant to overcome one of its weak points, the risk of embolism.
"This discovery is exceptional in several ways. Firstly, because of the remarkable level of preservation of the fossil and the degree of detail that is observed. Secondly, because of the fact that the authors are highlighting details of the functioning of the oldest forests known to the earth", explains Cyrille Prestianni. This study thus illustrates how fossils can provide detailed information about certain physiological processes that are even hundreds of millions of years old. This type of information allows us to understand fossil plants as once-living organisms and to trace the deep origin of key biological processes that still exist today.
Decombeix A-L, Harper CJ, Prestianni C, Durieux T, Ramel M, Krings M. 2023. Fossil evidence of tylosis formation in Late Devonian plants. Nature Plants., 20 april 2023.