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

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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 ( August 19, 2015

How to Catch a Bee

During my days as a summer camp unit director, one of my campers brought me a flower. When I looked closer, I realized this 11-year-old was not interested in the flower, but the bee and spider on top of the flower, head to head. I assumed they were fighting, but they were both pretty still, and it dawned on me that the bee was a goner, locked in the “jaws” of the spider, whom I figured had won the fight. I was still wrong, and the reality of this interaction is even cooler than I had imagined:

The spider hunts by hiding in flowers that attract their prey, the bees.

The unsuspecting pollinator. #entomology #ecology #biology #tritrophicinteractions #pollination…almost.

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I posted my spider-bee-flower pic on Instagram recently, which received more interest than I had expected. I even had comments asking about the evolution of this flower-sitting ambush spider, so I looked it up, and I stumbled upon a cool chemical ecology story.

The first thing I learned is that, when given a choice, both the bees and this group of spiders, crab spiders, pick the same flowers. By covering the flower with saran-wrap and watching the spiders’ flower choice change, scientists were able to figure out that spiders are choosing their flowers by following chemical signals in the air, aka smells. This means these spiders are adapted to smelling out flowers that are more likely to attract their dinner.

Do bees fall for this trap? Bees not only fall for this ambush, but are more attracted to flowers with spiders than to safe, spider-free flowers.  Why would a bee fly closer to a purple flower that has a white, hungry spider in the middle?

Clearly, bees see differently from us. As I was perfecting my Instagram post, I was struck by my black and white Instagram filter, which showed the flower and spider as same shade of white (Figure 1). Even without extensive training in bee sensory biology, I figured there must be some sort of visual trick at play.

Figure 1. Screenshot of my Instagram post with 100% of the color saturation removed.

Too bad there is no UV filter on insta, because bees can see ultraviolet. Flowers take advantage of the bees’ visible spectrum in UV and often attract bees with target-like patterns, using dark UV spots in the middle. Darker UV target patterns can mean more pollination, so this trait is selected for in both flowers and bees.

So why are bees attracted to the crab spider flowers? Crab spiders have a layer of transparent cells covering spider skin cells that can change color! There are a few types of this pigment- the ommochrome pigment, which either allows spiders to or yellowish to red, or allows white spiders to have UV fluorescent patterns. With a UV pattern on their backs, flowers with spiders look like an extra dark flower target and attract bees more effectively than flowers without spiders.

Of course, natural selection goes both ways. Native bees in Australia fly close to, but can recognize and veer away from native crab spiders, whereas introduced honeybees have not adapted to recognize this danger.

Plants interact with insects and their predators. Scientists use the term “tritrophic interactions” to describe three trophic levels, or links in the food chain, interacting and affecting one another. As my thrilled camper and curious Instagram “fans” could pick up on, tritrophic interactions are fascinating! From an applied science perspective, knowing the intricacies of tritrophic interactions is essential to fully understand the side effects of potential global solutions in food security, and pest management, and conserving biodiversity.


Note: All the papers I cite in this post are by Dr. Astrid M. Heiling, who has many other fantastic papers. Check her work out!