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Plant mutualism: a little help from the prokaryotes

June 8, 2012

Plant Mutualism: Plant-Bacteria Mutualism

Soil mediums are full of bacteria that fix various nutrients useful to plants. Nitrogen is very important for plant growth, arguably the most important nutrient for overall growth, needed in relatively large amounts for protein synthesis and amino acids. Some of it is taken up from the soil as organic nitrogen from the decay of other organisms and some of it is supplied by the nitrogen fixing bacteria in soil (scott). Some plants have developed a symbiotic relationship with nitrogen fixing bacteria, creating nodules which house and support the bacteria in a structure on the root known as a nodule. The number of plants that have developed the ability to attract symbiotic bacteria and form nodules is very limited, found only in two genera and a few other seperate species but includes very important agricultural crops such as beans, peas, peanuts, and clover (Scott). Although few realtive an individual plant may form nodules with many different nitrogen fixing bacteria which are rich in diversity (Kiers).

The root nodules of a 4-week-old Medicago italica inoculated with Sinorhizobium meliloti.

These symbiotic bacteria are referred to as rhizobia. When rhizobia come in close contact with the roots of a plant that can form nodules extended epidermal cells known as root hairs curl around the rhizobia. A passageway is created inside the cell known as an infection thread and the cell wall degrades allowing the rhizobia to enter the interior cells of the root and form a nodule (Scott). A coevolution for both plants and rhizobia has taken place. While plants that form nodules still have immune response reactions to rhizobia they have developed ways to discriminate against rhizobial microorganisms suppressing further infection but forming nodules. Rhizobia too have adapted to be able to form a symbiosis with nodule forming plants by developing enough resistence to the plants defenses to grow in a formed nodule (Soto).

Some plants have also formed a mutualistic relationship with nitrogen fixing bacteria around the roots that rely heavily on carbon fixed by the plant. A study on soils inhabited by sea grasses that form this relationship shows that the population levels of nitrogen fixing bacteria increase around the grasses and higher productivity of nitrogen fixation occurs (Welsh). These factors show correlation with the amount of photosynthetic activity of plants, with a greater amount of productivity going hand in hand with bacteria population levels. This is significant because studies have shown that the nitrogen dissolved around the seagrasses are not sufficient to support their growth and that little nitrogen is reabsorbed from the falling leaves of seagrasses. This varies between different species and habitat (Welsh).

Freshly dug peanuts, a well known legume.

Plants that form nodules for nitrogen fixing bacteria benefit from the reaction, but to maintain the benefit and receive an optimal output from bacteria they need to impose a check so that rhizobia do not adapt to giving minimal benefit while consuming resources from the plant. Selection against rhizobia to perform well is known as a sanction by the plant. Experimentation shows that bacteria deprived of the ability to fix the nitrogen suffer decreased reproduction rates and that decreased oxygen supply may be a mechanism for imposing the sanction against them (Kiers, Heath). Nodule forming plants may also recognize less efficient or poorly performing rhizobia and prevent them from colonizing. This is likely the more cost effective for a given plant than imposing sanctions after forming a nodule (Heath). This is ecologically significant as sanctions and discrimination against inefficient rhizobia create a selection in which more mutualistic rhizobia will persist and reproduce more than they would just as free living organisms. This gives more mutualistic rhizobia an edge against others who will suffer sanctions against them or are not allowed to colonize plants at all.

What do these interactions and benefits mean for the plant and it’s own success? Nodule forming plants are more photosynthetically active and roots show increased root growth under stimulation of nodules (Coleman). This may be very important for plants where nitrogen is the most limiting nutrient. However there is a cost to this symbiosis as the rhizobia require a much larger carbon input compared to processes of acquiring nitrogen from compounds taken up by the plant. Therefore if compounds containing nitrogen are abundant and available it may be unnecessarily costly for a plant to be supporting nodules (Lambers).


Scott. Physiology and Behaviour of Plants.Chichester: John Wiley and Sons Ltd, 2008. Print.

Welsh D.T. (2002) Nitrogen fixation in seagrass meadows: Regulation, plant-bacteria interactions and significance to primary productivity. Ecology Letters. Vol 3. Issue 1. pp. 58-71. 2002.

Soto M.J. et al. (2009) Mutualism versus pathogensis: the give-and-tank in plant-bacteria interactions. Cellular Microbiology. Vol 11. Issue 3. pp 381-388. 2009.

Kiers T.E. et al. (2003) Host sanctions and the legume-rhizobium mutualism. Nature. Vol. 425 pp. 78-81. 2003.

Heath K.D., Tiffin P. (2009) Stabilizing Mechanisms in a Legume–Rhizobium Mutualism. The Author(s). Journal compilation. 63-3. pp. 652-662. 2009.

Coleman, Crossley, Jr., Hendrix. Fundamentals of Soil Ecology. 2nd edition. Georgia: Elsevier Academic Press, 2004. Print.

Lambers, Chapin III, Pons. Plant Physiological Ecology. New York: Springer-Verlag, 2004. Print.


Fig. 1 – User: Medicago italica root nodules 2. 2009. Photograph. Web. 20 Jul. 2009.

Fig. 2 – User: Peanut 9417. 2002. Photograph. Web. 12 Feb. 2004.

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