Pollination Plasticity

Pollinators are great -- that is, unless they also feed upon the plant they are pollinating. In the arid regions of western North America, Nicotiana attenuata, sometimes referred to as coyote tobacco, has this very problem. 

Blooming at night, its white flowers are heavily scented, which attracts its pollinator, a species of hawkmoth. Female hawkmoths do a little bit more than just grab a sip of nectar. Their larvae feed on members of the tobacco family and, as anyone with tomatoes can tell you, they have a voracious apatite. Visiting female moths use the meal break as a chance to lay their eggs. However, this does not have to be a death sentence for the plant. Researchers noticed a strange thing about N. attenuata plants that had feeding damage from hawkmoth caterpillars. Their flowers seemed to change.

And change they did. Coyote tobacco plants with caterpillars will start to produce flowers that open during the day, instead of at night. The plants also stopped producing a scent. What's more, the flowers didn't open very far either. What iss the reason for these drastic changes? Are the plants stressed out from the caterpillar attack?

Not exactly. In fact, the answer is quite remarkable. As it it turns out, plants with caterpillars munching on them were intentionally shifting their entire reproductive strategy to avoid the larvae of their intended pollinators. Flowers that open during the day no longer attracted the attention of moths, which reduces the number of new eggs being laid. Instead, the flowers became quite attractive to hummingbirds. Hummingbirds are quite effective as pollinators and their offspring don't eat the plants that their parents feed on. 

So, how does the plant know when its being fed upon? Chemicals in the saliva of the caterpillar trigger a chemical response within the plant that tells it to start ramping up defenses (of which nicotine is one). This signaling cascade also tells the plant to start producing day opening flowers instead of night opening flowers. It just goes to show you how a little attention to detail can uncover some amazing aspects of the world around us. 

Photo Credit: Danny Kessler, MPI chemische ├ľkologie

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Bacterial Enduced Shield


Legumes are famous the world over for their nitrogen fixing capabilities. These hardy plants can live in soils that would otherwise not support much of anything. As such, nitrogen fixation is one reason that the legumes have found themselves as a focus of agriculture. However, this ability is not solely the plants doing nor has it evolved to benefit humans. Legumes owe their ability to turn a gas into food to a symbiotic relationship with special soil bacteria known collectively as "rhizobia." The legumes produce special root structures called "nodules" to house these bacteria. In return for nitrogen, the bacteria receive carbohydrates and other organic compounds. The nature of this relationship may seem pretty straight forward but, as with anything in nature, the closer we look the more interesting things get. As it turns out, rhizobia also play a role in plant defense.

When a team of researchers began raising Crotalaria, a genus of legume native to Africa, they noticed something strange. Plants that were not inoculated with rhizobia didn't produce nodules nor were they producing any of the alkaloid chemicals that defend them from herbivores. Even adding artificial nitrogen to the soil didn't stimulate the plants to produce their chemical cocktails. Something was going on here and it would seem that the missing bacteria were the key to the puzzle. 

Indeed, only after the plants were inoculated with their native rhizobia did they begin producing nodules and eventually the defensive alkaloid compounds. Could it be that the bacteria produce these chemicals for the plant? 

Not quite. As it turns out, the area of biosynthesis for these defense compounds happens to be in the root nodules that house the rhizobia. The rhizobia trigger the production of the nodules, which in turn triggers the production of the alkaloids. From there, the plant can export them to above-ground structures as a means of defense. The bacteria are simply a key that unlocks a genetic pathway for defense. Seeing as the alkaloids are, in part, made from nitrogenous molecules, this is not too surprising. There is no sense in trying to make these compounds if the chemical ingredients aren't there. This research serves as further evidence of how complex the microbiome can be. 

Photo Credit: Dick Culbert - Wikimedia Commons

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