Quorum sensing: an understudied control in plant nutrient availability

It is a well known fact that many bacteria like to live in groups. Moreover, just as human societies strive to facilitate group living, (through grocery stores that provide food for otherwise unsustainably crowded cities, and municipal waste centers to ensure that our concentrated living spaces are kept clean enough to remain livable), bacterial populations employ a variety of strategies that enhance collective life.

The bacterial analog to human group-living strategies is called quorum sensing.  Quorum sensing, or QS, is really an umbrella term used to describe a range of bacterial behaviors that enhance group life. QS behaviors are considered common to all bacteria and have likely been tracking bacterial evolution since the first true cells of Precambrian earth crawled out of their prebiotic soup. Here I’ll focus on a particular group of bacteria that employ some very ecologically important QS strategies.

Bacteria are broadly divided into several distinct phyla that occupy a range of earth habitats and employ an enormous variety of survival strategies. Proteobacteria are one such major group that is particularly interesting to me because of their posited dominance in the zone of plant-nutrient uptake known as the rhizosphere. The significance of proteobacteria in rhizosphere microbial populations has really only been examined in temperate forests, and hopefully some of my own work will bring a tropical perspective to our understanding of rhizosphere community composition.

It turns out that proteobacteria release a specific signal molecule known as N-acyl-homoserine lactone, or (AHL) in order to alert other proteobacteria of their existence. Thus proteobacteria are able to “quorum sense” their environment and receive information of the relative density of their fellow quorum members. Moreover, the release of AHLs operates via a positive feedback mechanism- higher concentrations of AHLs attract more bacteria, resulting in even higher concentrations of AHLs.

Bacteria don’t employ quorum sensing simply because they enjoy each other’s company. In fact, there are almost certainly trade offs associated with group life, including higher levels of predation, lower oxygen availability and buildups of toxic waste products. However, the benefits that can be obtained by living in groups are also significant. Soil bacteria produce extracellular enzymes that release soluble, digestible molecules into their environment. A greater concentration of bacteria results in a greater concentration of free enzymes and thus a more nutrient-rich environment.  In the rhizosphere, high concentrations of bacteria can also exert positive feedbacks on rhizosphere priming, the mechanism by which plants release sugars into the soil to nourish the bacterial community.

Though bacterial QS behavior has been well studied for decades, most research has been in the field of disease ecology and few studies have experimentally demonstrated QS to be an important phenomenon in soils. One recent study tackled this problem through a detailed examination of bacterial populations in rhizosphere soil, using controlled pot experiments. The authors found that AHL, the QS signal specific to proteobacteria, was 10 times more concentrated in rhizosphere soils compared to bulk soil. Similarly, bacterial densities in the rhizosphere were about 10 times higher. Furthermore, the researchers discovered a tight correlation between QS and enzyme activity. In particular, enzymes involved in acquiring nitrogen were closely linked to QS expression. Nitrogen is considered the major plant-limiting nutrient in temperate ecosystems, and it is believed that one of the primary motivations for a plant to “prime” the soil around its roots is to access soluble nitrogen from microbial decomposers.

I knew almost nothing about quorum sensing before reading this paper. As always, I found myself overwhelmed by the nuanced mechanisms that bacteria employ in order to explore and shape their environment, coupled with the interactions of plants that can manipulate this complex community for their own nutrition. Quorum sensing could well be an important control in rhizosphere nitrogen availability, and will have to be studied in more detail to fully assess its role in controlling ecosystem productivity.

Reference
DeAngelis, K.M., Lindow, S.E. & Firestone, M.K. Bacterial quorum sensing and nitrogen cycling in rhizosphere soil. FEMS Microbiology Ecology 66, 197-207 (2008).

Protozoa drive growth enhancing hormone release in the rhizosphere: where biochemistry meets ecology

Though numbering far fewer in the soil than the bacteria they prey on, protozoa are an indispensible link in the transfer of nutrients through the food web that drives forest productivity. These single celled, eukaryotic “bactivores” concentrate themselves in regions of high bacterial activity, notably in the vicinity of plant roots. I’ve previously discussed the “microbial loop theory”, a paradigm for understanding plant nutrient acquisition in terms of the interactions between root exudates, protozoan predators and bacterial prey. To summarize briefly, plant roots exude sugary compounds to “prime” the surrounding soil, making it a highly suitable habitat for bacterial populations. Protozoans naturally move in, too. As quickly as bacteria decompose organic matter to recycle nutrients for their own growth and metabolism, protozoans eat bacteria and excrete those very same nutrients in a form readily available for plants. This “microbial loop” of nutrients is essentially an ecological fertilization system built on a very simple predator-prey model.

Given the advantage plant obtain by maintaining a large and healthy microbial (bacterial + protozoan) community, what strategies can plants employ to ensure that they are supporting the largest and best community possible? (Note that best, from the plants perspective, means the community that mineralizes the most plant-available nutrients in the rhizosphere.) A first obvious strategy for a growing plant would be to release more food- to exude more sugary carbon from its fine root tips. But another, possibly more important step precedes this, and it has to do with root architecture.

Most plants begin their foray into the earth as a seedling, by sending a long, primary taproot straight down like a sledgehammer. Lateral roots begin branching off this main taproot slightly later, and from these lateral roots networks of fine roots, or root hairs, spread out like tiny fingers to penetrate the smallest nooks and crannies in the soil matrix. It is these root hairs which become the site of almost all nutrient and water acquisition and can end up covering an enormous surface area in a mature plant. And it is in the narrow band around these root hairs known as the rhizosphere that a microbial food web has evolved to provide those nutrients.

But plants don’t just grow root hairs everywhere. That would be a waste of energy. Root growth is highly plastic and sensitive to environmental parameters such as soil moisture and nutrient availability. If, for instance, a calcium deposit exists several inches from a primary lateral root, root hairs will likely develop in the direction of that deposit to access as many nutrients as possible. How can plants regulate their growth so precisely in order to ensure themselves the best chance of survival?

It turns out that a complex set of biochemical pathways drive plant growth, and that these pathways can be switched “on” or “off” according to the presence or absence of growth hormones. Auxins are a class of hormones that are particularly important in mediating the growth of plastic stem cells in response to the environment. They are largely responsible for phototropism, the phenomenom that anyone with a windowsill plant has observed, that plants tend to concentrate their above-ground growth in the direction of the most sunlight. Belowground, auxins are largely responsible for root branching and the selective production of root hairs.

At this point you might be wondering why I’ve diverged from my original topic (the microbial loop) to discussing the biochemistry of plant growth. Well, recent research suggests that these two subjects may be even more intricately linked than previously imagined. Growth hormones such as auxins are responsible for the production of fine roots, and by the same token responsible for the maintenance of a rhizosphere in which microbial communities thrive. Though they are hardly aware of it, microbes desperately need auxins to ensure the continued maintenance of the roots they depend upon as a primary food source. A recent study conducted by rhizosphere ecologists (there aren’t very many of them, in case you were wondering) in Germany has found that protozoa selectively “graze” on certain bacteria in the rhizosphere while largely ignoring others. Which bacteria do they choose to ignore? The ones that produce auxins that promote root growth. By selectively removing amoebae, a key bacterial predator, from experimental plant roots, the researchers found a marked decrease in plant auxin concentrations compared to treatments that contained amoebae. Soils with amoebae predators maintained plants with higher auxin concentrations and increased root branching.

It is becoming clear that the interspecies interactions that plants, protozoa and bacteria all depend on may be far more nuanced than we previously understood. Future research to characterize the specific players in this complex web would allow scientists to develop a more holistic picture of exactly who and what is driving plant growth and ecosystem nutrient cycling.

Krome et al. 2010. Soil bacteria and protozoa affect root branching via effects on the auxin and cytokinin balance in plants. Plant Soil: 328, 191-201.