Plants with Benefits

Maintaining relationships, even great ones, can be challenging.

Some plants juggle multiple relationships ALL of the time.

Think of an organism that does something helpful for a plant. Bees, hummingbirds, and other pollinators may come to mind. But plants also recruit toucans, black bears, and other seed dispersers as well as predatory insects such as ants and wasps for defense.

And then there are microbes.

My goal for this research paper was to show how one microbe in particular, nitrogen-fixing bacteria called rhizobia, can alter a plants’ entire network of relationships—from the bottom-up.

Rhizobia live inside nodules on plant roots in a tight-knit symbiotic relationship, exchanging fertilizer for sugars from photosynthesis. Plants that form this relationship benefit immensely from the nitrogen, but providing sugar in return can tax the plant.

Plants like dip n' dots too. #rhizobia #nitrogenfixation #biopsu

A post shared by Adrienne Godschalx (@agodschalx) on

Plants also use their homemade sugar to secrete extrafloral nectar. Nectar typically attracts pollinators, but in the case of extrafloral nectar, plants produce nectar to attract ants, which patrol their sugar source like bodyguards. By evicting intruding bugs attempting to feed on the host plant, extrafloral nectar can be an effective indirect plant defense…

…as long as the ants show up to do their part.

But the key result from our paper is that ants are less attracted to plants that have nitrogen-fixing rhizobia in root nodules belowground. Keep in mind- the ants and bacteria do not interact directly. What connects ant to bacteria?

The plant between them.

Plant chemistry changes when plants form symbiosis with rhizobia. Plants with nitrogen-fixers make more of the nitrogen-based traits, protein and cyanogenesis. Surprise. But these plants also secrete less sugary nectar, therefore attracting fewer ants.

Even in the plant world, some relationships can be more demanding than others.

How do rhizobia cause plants to compromise their ant relationships?

It could be that rhizobia demand so much sugar to keep the nitrogen flowing that the plant’s excess sugar supply is exhausted, leaving little to serve as ant lures. Alternatively, why would plants that get a constant supply of nitrogen to make cyanide need to attract ants anyways?

Either way, we now know rhizobia can change plant relationships with ants.

But why would that matter?
Ants are everywhere- so are rhizobia. Both play important roles in how ecosystems function, but the fact that they can indirectly affect one another may have strong and widely overlooked impacts on plant ecology.

© Adrienne Godschalx (adrg@pdx.edu) August 19, 2015

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