What we are reading in March, 2018
By: Camille Truong and C. Guillermo Bueno
Seed dispersal is a key feature for the survival and reproduction success of both fungi and plants. Dispersal strategies have been studied extensively in plants, much less in fungi, and almost never in plant-fungi interactions! In general, it is believed that seedling survival increases with distance, as competition with siblings and the density of specific enemies (pathogens, parasites, herbivores) decreases. This interesting study postulated that, in mycorrhizal plants, such dispersal advantages constitute a trade-off because established mycorrhizal networks close to the mother tree facilitate the inoculation of seedlings with compatible mycorrhizas. Seedlings with a quick access to nutrients and protection against pathogens will be favored compared to seedlings establishing further away from this network.
Dispersal of mycorrhizal fungi is believed to be largely stochastic but the reality is that we don’t know much about it! Possible mechanisms for long-distance dispersal (LDD) include wind, the co-dispersal of mycorrhizal spores attached to seeds, and via animal feces after the ingestion of fruiting bodies – a well-known phenomenon in truffles or truffle-like ectomycorrhizal fungi.
The authors collected information about LDD strategies from almost 2000 plant species belonging to 685 genera and 124 families, based on the presence of specialized structures. They also classified them as obligate mycorrhizal (arbuscular mycorrhizas, ectomycorrhizas, orchid mycorrhizas and ericoid mycorrhizas), facultative mycorrhizal (arbuscular mycorrhizas) or non-mycorrhizal plants. Plant mycorrhizal status and type were tested against the presence and type of LDD strategies, while taking into account the phylogenetic relatedness among these plant species.
Surprisingly, the proportion of obligate mycorrhizal plants with LDD strategies was higher than expected, while facultative mycorrhizal and non-mycorrhizal plant species showed the opposite pattern. Therefore, finding mycorrhizal partners does not seem to be limiting for plant establishment. The authors explain these findings based on low symbiotic specificity and to the broad distribution range of fungi (that remain to be tested for many species!). The high proportion of mycorrhizal plants with LDD strategies were mostly ectomycorrhizal plants, due to the fact that they include many tree species that are typically dispersed by wind over longer distances. The authors proposed that the association between ectomycorrhizal plants and the LDD strategy of wind dispersal (in 51% of ectomycorrhizal plant species included in this study) was advantageous for the expansion of ectomycorrhizal plants since many of their mycorrhizal partners can also be dispersed to considerable distances by wind. Dispersal strategies in ectomycorrhizal fungi are poorly known and there is still a lot to investigate on this topic!
On the other hand, the negative correlation between non-mycorrhizal plants and LDD strategies can be more easily explain by the fact that non-mycorrhizal plants are often habitat specialists adapted to extreme environments. Because of their narrow ecological niche, LDD may therefore decrease the probability of finding suitable site to establish, and the cost of producing LDD structures may be disproportionately high for non-mycorrhizal plants.
General evolution patterns between plant mycorrhizal status and seed dispersal need to be further tested in other part of the world. The tropics would be particularly interesting since mycorrhizal plant types may have different proportions of LDD strategies. The balance between escaping from enemies and finding suitable partners and/or habitats make a lot of sense and trade-offs between seed dispersal and plant mycorrhizal type likely occur in other parts of the globe.
In the last decades, plant traits and trait-based ecology have change our way to observed and analyze community patterns and ecosystem functions. We realized that species is a complex unit, where many internal trade-offs compensate for multiple and environmental effects on plants. This certainly made the ground for looking at specific plant features directly related to plant functions. After having extensively explored the aboveground plant traits (see for instance a nice review by Laliberté et al 2018), soil and underground traits are expected to help in explaining the variability of plant responses to any global change. We know that leaves are fundamental for photosynthesis, but nutrients, chemical defenses and pathogens, along with a great ability to interact with other plants are more related to the roots, and also directly linked to mycorrhiza. But how much we know about root traits? Could we integrate them with the evolution at colonizing new environments? And more importantly for us, what has been the role of mycorrhiza in all these processes? Ma and co-workers are bringing us one of the first answers to all these questions.
Ma and coworkers started analyzing how roots (the active and younger ones, first order roots) are globally organized, hypothesizing that they could mirror the aboveground dominant trait paradigm (leaf economic spectrum, see for instance Diaz et al 2016). In other words, whether the size and the cost-effective trade off in roots, similarly to plant leaves, govern the belowground part of plants. They found that this was far to be confirmed, and after analyzing 10 different root traits, they found that the morphological traits, in particular root diameter, along with specific root length, root density tissue and mycorrhizal colonization, what mostly explained the variation analyzed. Thus roots do not follow the rules of the aboveground traits. They found that thinner roots are normally longer, tend to be lighter (with low tissue density) and less colonized by mycorrhiza, see the figures below.
But if root diameter could be a good adaptive trait of plant strategies – thinner and longer roots can access more nutrient pools and thus mycorrhiza are not so needed for nutrition – does it have an evolutionary signal? For answering this they analyze root diameter along the phylogeny and found a general trend of thicker roots related to the more ancient groups (with less apparent mycorrhizal root colonization) (left panel below). But has it also a current signal among biomes? They found that root diameter as well explains certain variation among different biomes for woody plants (right panel). This led the authors to hypothesized a biome-stability relationship hypothesis; more recent and colder environments “push” plants species to stretch their roots and to depend less in mycorrhiza, as less constant favorable conditions to photosynthesis are found outside the tropics.
Overall this inspiring paper, is showing us a different story belowground, opening the door to integrate roots and mycorrhizal strategies into the evolution and diversification of current plant diversity. However, it is worth to remind readers that the story is in its baby steps, only 369 plant species (1200 plant individuals) were used for building this story (last estimated terrestrial diversity of vascular plants are close to 400000 species) and thus distinction among plant mycorrhizal types were not possible to be done, which could help in disentangling the evolutionary pattern/s in the future… In any case, thinner roots seem to be an advantageous strategy for plants in colder new habitats… I wonder now, could we expect an opposite trend in a long warming process?
Díaz, S. et al. (2016). The global spectrum of plant form and function. Nature 529, 167–171.
South American Mycorrhizal Research Network 