Felipe Albornoz – Native soilborne pathogens equalize differences in competitive abilities

Hi everyone, this is Felipe and I’m here
today to try to convince you that pathogens are actually a good thing for plant diversity. The research I’m about to show you was conducted
in south Western Australia. One of the few plant diversity hotspots. We have been trying to figure out how is such
great diversity possible when these soils are so incredibly low on nutrients. Well there might be several mechanisms. Today I am gonna show you one, my favourite. First let’s talk a little bit about the
plants that live here in south Western Australia. Since the soils have so little phosphorus,
plants have developed very efficient strategies to acquire these nutrients. Here I will focus on only two. Cluster roots and symbioses with ectomycorrhizal
fungi. Cluster roots in this system are usually produced
by the Proteaceae family and they are the most efficient method for acquiring P. They
are fine delicate short lived roots that produced chemicals capable of mobilised otherwise inaccesible
P. On the other hand, a less efficient method is the formation of symbioses with ectomycorrhizal
fungi that in this study are formed by the Myrtaceae family. This symbiosis consists on a hyphal mantle
covering the root tip that increasing the P uptake. Now, since cluster roots are such delicate
roots they potentially are more susceptible to infection by pathogens, and in contrast,
the mantel produced by the ECM fungi could potentially work as a physical barrier against
pathogens. When we think about pathogens, the first thing
that comes to our mind is sickness, bad, detrimental. We are always trying to find ways to eradicate
pathogens. But, maybe, what if sometimes pathogens could be a good thing? What if we owe to pathogens for the beautiful
and rich biodiversity hotspot in south Western Australia? How is this even possible you ask? Well we propose the following model. In this model we have the two competitive plant species, on the right we have a cluster-root
plant, and on the left we have an ectomycorrhizal plant species from the Myrtaceae family. In the first scenario (the top panel). In absence of pathogen, Proteaceae plants
are more effective than Myrtaceae in acquiring nutrients and hence are the strong competitor
that negatively affects the Myrtaceae growth. But then, in the bottom panel. When pathogens are present. This could be more detrimental to cluster
roots than ECM roots given their potential susceptibility. Hence, reducing the competitive strength of
Proteaceae plants. This reduction in their competitive strength
of Proteaceae plants would then allow Myrtaceae plants to coexist. In this study we tried to provide evidence
for this model. We conducted two separate experiments in
order to answer the questions: do pathogens allow for plant coexistence between these
two families? On the first experiment we planted one seedling
in presence of absence of pathogen. This seedling could be one of the three plant
species for each family. For a total of six plant species. For the pathogen treatments we used absence
of pathogen, a mixture of weak strains of native Phytophthora, or inoculation with Phytophthora.
arenaria a known native and strong pathogen in this system. On the second experiment, we planted one Myrtaceae
seedlings competing with two different Proteaceae seedlings…in presence or absence of pathogen. Here we had some issues germinating two of
the six plant species so we only use 2 species for each family. Also since there were no differences between
the two pathogen inoculum from the first experiment, we only used presence or absence of pathogen. Now what did we find? Here you have some graphs showing seedling
biomass at the end of both experiments among the different pathogen inoculum treatments. All the top panels are Proteaceae species
and the bottom panels are the Myrtaceae ones. In the first experiment we found that for
all three Proteaceae species, the presence of either of the two pathogen treatments was
detrimental to their growth, while none of the Myrtaceae plants were negatively affected
by the presence of pathogen. This suggest that native species of Phytophthora
are only limiting the growth of the Proteaceae family and not the Myrtaceae ones. On the second experiment when the two plant
family were put to compete with each other, we found the same trend for Proteaceae plants:
being their growth hampered by the presence of pathogens. But when it came to Myrtaceae, we found something
very interesting. There was actually a positive effect of the
presence of Phytophthora for their growth. Pretty weird huh? This suggests a positive indirect effect of
the pathogens on Myrtaceae plant by restricting the growth of their competitors, the Proteaceae
plants. Now, the pathogens limiting the growth of
only Proteaceae and not Myrtaceae could be due to several reasons, could be host specifity,
pathogen defence provided by ECM fungi, higher susceptibility of cluster roots…all of the
above. Whichever the case is, our study cannot directly
elucidate this, but. let’s look at this graph for a minute. A standard way of measuring ECM fungal colonisation
is calculating the percentage of root tips that are colonised by the fungi. And typically, the more fungi the better. In this graph we correlated the percentage
of root tips that were colonised by ECM fungi against the seedling biomass. This was only done for the Myrtaceae seedlings
as Proteaceae are not colonised by ECM fungi. We found a positive correlation in both experiments
between these two variables, but only when pathogens were present. In absence of pathogen, there was no correlation
at all. This suggests ECM fungi’s main role here
in South West Australia could be pathogen defence, rather than nutrient uptake. Now, I want to stop here for a minute. These findings are very interesting. Until now, it was thought that the main role
of ECM fungi was just nutrient uptake. and with good reason. As I said before, soils in SWA are almost
entirely depleted from phosphorus and this element is crucial for plant survival and growth. But if this was the case, if ECM fungi’s
main role was indeed nutrient uptake we should have found also a positive correlation in
absence of pathogen. So regardless if the pathogens are there or
not, ECM fungi should be contributing to nutrient uptake but this was not the case. Now to finish, I would like to summarise the
main findings of this study: first there might be a trade-off between how good the plant
is at acquiring nutrients and how good it is at defending itself against pathogens. The other main finding is that the primary
role of ECM fungi might be pathogen defence rather than nutrient acquisition as previously
thought And finally, we found evidence that native
pathogens can relax competition between plant species. And this could potentially promote plant diversity
Thank you for listening and I hope I convinced you that being bad is a good thing. Pathogens in their native environments play
an important role in maintaining diversity. If you have any comments or questions you
would like me to answer just send me an email and I am happy to reply. That’s all for now, thank you, bye.bye