The Art of Being Connected in the Microbial World
Similar to the skin of humans millions of microbes are living on the surface of a leaf influencing the health and fitness of the plant. Scientists of the Max-Planck-Institute for Plant Breeding in Cologne have discovered that the mixture of those plant colonizing microbes is not random but is instead influenced by its surrounding nature and by the microbes themselves.
Some microbes have the ability to intimately interact with the host plant such as the fungal-like organism scientifically called Albugo laibachii. Albugo specifically seems to have the capacity to link signals from the plant with the microbial community and thereby to control which microorganisms live there. Revealing these sorts of interactions and functionality – deemed network analysis - in the microbial world is not only crucial for plant sciences and ecology. The discovery of specific highly interactive microbes made by the MPI researchers have interdisciplinary consequences for the medical field and will help to better understand host-associated microbes regardless of the host being a plant, insect or human.
Nowadays, online social networks play a dominant role in our everyday life since they have changed how people are connected around the globe. Heaving a great new idea or invention, a person can quickly become connected to a broad range of people, re-structuring the existing network to its advantage or sometimes disadvantage and linking people that have not been in contact before. Such a highly connected influential person in a network is called a ‘hub’.
In the much tinier world of microbial communities, ‘hub’ microbes were identified by Matthew Agler, a postdoc in the lab of Eric Kemen at the MPIPZ in Cologne, who was motivated to understand if the coexistence of certain microbes with each other and with a plant was random or if there was any structure. “We wanted to see if there are microbes which are always present on the plant and what influences their ability to colonize it” explains Agler.
In order to address these questions they sampled the aerial parts of plants from different wild sites during spring and autumn and measured the community composition of three major groups of microbes: fungi, bacteria and fungal like microbes called oomycetes. With this approach it was possible to get an idea how various factors like season, local climate and plant type can influence the microbial community and how robust occurrence of some microbes is under different conditions.
Interestingly, the microbes themselves had an extremely strong impact on the community composition. Whenever Agler and colleagues discovered white rust symptoms caused by Albugo laibachii on the plants they sampled during field excursions, they could see a great difference in microbial composition compared to samples that did not show white rust. Albugo laibachii infected samples had much lower microbial diversity (fewer numbers of species), however, the microbial community seemed to be significantly more stable. “Albugo keeps the bacterial diversity low and stabilizes the community” explains Agler.
Deeper analysis of the leaf samples allowed the scientists to identify more key players that dominate the network. Such key players the scientists called microbial ‘hubs’. Besides Albugo laibachii, a yeast-like fungus and several bacteria were identified as additional ‘hubs’. Interestingly, although Albugo and the yeast ‘hub’ are as unrelated as humans and bacteria, both seem able to communicate or at least transmit signals to the bacterial community leading to a restructuring. A fundamental characteristic of the ‘hub’ Albugo is its ability to intimately interact with the host plant. Specifically, it grows specialized infection structures into the plant cells and grows between cells inside the leaf without causing major harm to the plant. As the scientists speculate, this close interaction might be the key for being a microbial ‘hub’ - Albugo is a mediator to transmit signals between the microbial community and the host plant. Why and how the other ‘hub’ organisms carry out their function, such as the yeast and other bacteria, and how the communication between such distant microbes works remains to be investigated.
The work of Matthew Agler in the lab of Eric Kemen is the beginning of a whole new insight into a plants life. “If we can understand the holobiont, which is the plant and all of the closely associated organisms living with it, and its complexity, it might be possible to use specific microbes and microbial communities to promote beneficial plant microbe interactions to protect plants and therefore ensure food security”. Considering that many pathogens become resistant to agrochemicals used to ensure plant health and these are detrimental to the environment, targeted control of pathogenic microbes by beneficial microbial networks is an important future step to go.
Fundamental concepts identified by the MPI scientists during this work have the potential to be applied to the medical field. It is known that millions of microbes live in association with our body and influence our health and fitness. For example the human gut microbial community has strong influence on digestion and the immune system of the body. To understand which gut microbes are ‘hubs’ and stabilize the microbial community could help to protect us from dangerous food associated pathogens such as E. coli or Salmonella sp.. Re-shaping the community structures following disease manifestation could possibly allow to cure certain diseases and could help to reduce the use of antibiotics or assist antibiotics use in case of resistant microbes.