Discussing the evolution of plant symbioses

For the second seminar of the term, Dr Pierre-Marc Delaux discussed his research on the evolution of two plant symbioses. The symbiosis with arbuscular mycorrhizal (AM) fungi increases plant nutrient uptake and is found in around 80% of land plants1. In contrast, the symbiosis with nitrogen-fixing bacteria, which provide nitrogen for the host plant, is restricted to just four orders of Angiosperms2. These two symbioses share many genes which control signalling between plant and microbe. By studying symbiosis-related genes and their distribution amongst land plant lineages, Dr Delaux hopes to understand when and how the symbioses evolved, and how they were subsequently conserved or lost in different groups of plants.

Dr Delaux’s study of plant genomes shows that two key regulatory genes of the AM symbiosis are found in algae, the closest relatives to land plants, suggesting a preadaptation to the symbiosis, with further key genes evolving in land plants3. Loss of the AM symbiosis correlates with the loss of a symbiotic gene set4. In contrast, there is no definitive evidence as to whether the nitrogen-fixing symbiosis arose once, or many times following a precursor adaptation. However, Dr Delaux’s research indicates that there were multiple independent losses of this symbiosis, which correlate with the loss or fragmentation of one of two regulatory genes5. Further work aims to characterise the mechanism in which such symbiosis-related genes are lost.

One interesting question arising from Dr Delaux’s work is why such potentially beneficial symbioses have been lost multiple times. Were some past soils rich enough for plants to cope without additional nutrient input, or did poorly-performing microbes arise that were no longer worth investment from the plant? Going forward, Dr Delaux hopes to determine if key symbiosis genes can re-engineer symbioses in green algae. Ultimately this work could enable the introduction of beneficial symbioses into novel crop species, which, given the need to reduce global fertiliser use, could be an important step in making agriculture more sustainable.  

 

Further readings:

  1. Parniske, M. Arbuscular mycorrhiza: The mother of plant root endosymbioses. Nat. Rev. Microbiol. 6, 763–775 (2008).
  2. Soltis, D. E. et al. Chloroplast gene sequence data suggest a single origin of the predisposition for symbiotic nitrogen fixation in angiosperms. Proc. Natl. Acad. Sci. U. S. A. 92, 2647–2651 (1995).
  3. Delaux, P.-M. et al. Algal ancestor of land plants was preadapted for symbiosis. Proc. Natl. Acad. Sci. 112, 13390–13395 (2015).
  4. Delaux, P. M. et al. Comparative Phylogenomics Uncovers the Impact of Symbiotic Associations on Host Genome Evolution. PLoS Genet. 10, (2014).
  5. Griesmann, M. et al. Phylogenomics reveals multiple losses of nitrogen-fixing root nodule symbiosis. Science (80-. ). 361, eaat1743 (2018).

 

Laura is a DPhil student researching the the legume-rhizobium symbiosis and how plants regulate this interaction.

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