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Origin and Early Evolution of Plant Symbionts

Final Report Summary - SYMBIONTS (Origin and Early Evolution of Plant Symbionts)

Fungi and fungi-like microorganisms are thought to be of key importance to the development of life on land. They entered into relations with plants to varying degrees (SYMBIONTS), helping plants colonize the land with major consequences for the evolution of life on Earth. Their fossilised remains are known to be preserved within the stems, roots and leaves of plant fossils, yet they have not been systematically studied. My goal was to determine the extent and nature of microorganism/plant associations during the early development of life on land. This was addressed by making new use of historic collections of fossil plants in The Natural History Museum, London.

Objectives
1. Undertake a novel systematics search of historic collections of fossil plants to locate plant-associated saprophytes, symbionts and pathogens focussing on the earliest plants to live on land.
2. Describe the characteristics of the fungi and other microorganisms, including their life cycles, biology, affinities, and their interactions with the plants using state of the art imaging methods.
3. Establish how they relate to living forms.
4. Understand the origin and evolution of key microorganism/plant associations and their significance to the evolution of life on land.

SYMBIONTS website: http://www.nhm.ac.uk/research-curation/earth-sciences/fossil-plants/fossil-plant-research/symbionts/index.html

Work carried out to achieve objectives
When plants become fossilised, their internal tissues are frequently preserved in minerals, a process which also preserves associated microorganisms. Fossil plants have been studied for over 100 years by making thin sections, but the associated microbial remains have been mostly overlooked. The first step of my research was to survey these historic fossil plant slide collections for the microrganisms they contain. Because the collections are so large (over 15,000 slides) my initial survey involved a representative sample of 200 slides.

These were examined using microscopy (Leica M205C stereomicroscope ; Nikon Eclipse LV100ND compound microscope). I found that each fossil plant slide contained a large number of microorganisms, so a digital record was made of each significant find, which can be linked back to the original microscope slide. Many of these represent newly discovered organisms or newly discovered plant/microorganism associations.

In addition to using standard microscopy, I used state-of-the art Confocal Laser Scanning Microscopy (CLSM). This technique was originally developed for use in biology to image in three dimensions the structural components of living tissues and cells. This is the first application of the technique to fossil fungi, enabling me to better characterize their features and thereby their relations to living species. I was able to obtain detailed three-dimensional images of fossil microorganisms within 407 million year
old plants, which are among the earliest known plants to live on land.

Results
1. New microorganisms related to endomycorrhizal fungi: These are fungi that live symbiotically within most plants today helping them to obtain nutrients from the soil. I discovered an important group new to the fossil (Mucoromycotina) forming associations with the earliest plants. I was also able to show that it lived together within the same plants as another group of endomycorrhizal fungi (Glomeromycota). My results were presented at scientific meetings in 2014 : (1) 33th New Phytologist Symposium, Zurich (Switzerland), (2) European Paleobotanical and Palynological Conference (Italy) and (3) International Palaeontological Congress, Mendoza (Argentina). They were published recently in New Phytologist. Results also formed part of a review paper on the evolution of roots published in Plant Physiology. These are both leading international scientific journals.

2. New microorganisms related to chytrids: New associations were found between chytrid fungi and the earliest plants. Results were presented at two international meetings in 2014 : (1) European Paleobotanical and Palynological Conference (Italy) and (2) International Palaeontological Congress, Mendoza (Argentina). A manuscript will be submitted for publication in November.

3. New microorganisms related to the oomycetes (water molds): Today this group includes some important plant pathogens (e.g. Phytophthora infestans Potato Blight). My discoveries add to a growing body of data indicating that oomycetes were an important component of microorganism diversity in early land ecosystems. My longer term goal is to use these new data to show when key events in oomycete evolution took place. Results were presented at three international meetings in 2013 and 2014: (1) The Botanical Society of America (USA), (2) Agora Palaeobotanica Arino (Spain) and (3) Oomycete Molecular genetics network meeting (UK). They are being prepared for publication.

Conclusions
1. The reliance of plants on fungi to obtain nutrients from the soil (i.e. endomycorrhizae) dates back to their earliest known land-dwelling ancestors. It involved ancestors of major groups of living fungi, including Glomeromycota and Mucoromycotina.
2. The earliest plants had no roots. Their endomycorrhizae functioned in a way that was more similar to primitive living plants called liverworts than to more complex living forms such as ferns and conifers.
3. Oomycetes, which include serious pathogens of modern plants, were diverse in the earliest plant communities. However, it is likely that the earliest oomycetes were not pathogens.
4. Historic slide collections of fossil plants are an important new source of information on the evolution of microorganisms associated with plants.

Potential impact and use of results
1. Scientific Community : Results are principally of interest to the scientific community. They will help establish when important groups of fungi and oomycetes evolved and how with plants they influenced the evolution of key Earth Systems (e.g. the carbon cycle). Together with colleagues in the UK and France, I obtained funding (£10,000 New Phytologist Trust) to host an international workshop on the co-evolution of plants and fungi, which was held at NHM (London) in September 2014. One outcome is that we are working with French and American colleagues to develop an interdisciplinary reserach project bridging palaeontology and genomics. A second outcome is an invitation to write a major review paper for New Phytologist on this topic.
I gave five seminars directed to students and colleagues in UK, France and Estonia

2. Civic Society : I wrote an article for Jardins de France, the journal of the Société Nationale d’Horticulture de France which was published in May 2013 and I co-authored an article for the popular French Scientific magazine La Recherche on this research, which was publised in November 2013 and. My work has been presented to the public at several outreach events in The Natural History Museum (London) and at four other venues in the UK.