Getting to the Root of the Way Plants Communicate
- By Cheryl Dorschner
In the three floors of daylight-filled laboratories and offices of UVM’s Jeffords Hall, plant research and verdant plants thrive. Likewise, the surrounding gardens soak up east- and west-facing sunshine and flourish as outdoor classrooms and proving grounds for theories and hardiness trials.
However, few people realize that deep below this gleaming two-year-old, biological science building, are two large locked rooms of 13 mausoleum-like chambers. Here, heavy doors clank open to reveal fragile sprouts reaching for regulated red or white fluorescent light, their roots stretching into vermiculite, water and bacteria in this controlled environment called the growth chambers.
It’s the root nodules of these sprouts that interest Jeanne Harris, because under the microscope in her lab she sees evidence that those roots exchange molecular signals with soil bacteria to trigger these legumes to form nodules, which the bacteria then infect. She and colleagues want to know what regulates nodule formation and how it evolved.
Her work is part pure scientific curiosity and part possible practical use. Legumes – including alfalfa and soybeans – are important food crops for animals and humans, because they store protein in their seeds and nitrogen in their roots. The nitrogen-laden roots add fertility back into the soil in which they grow, so optimal growing conditions could mean reducing the use of expensive and polluting added fertilizers. Crop production might benefit from understanding how to regulate the growth of nitrogen-fixing nodules or how to space plants to maximize yield. Also, how these plants signal might be a predictor of their response to rapid climate change.
How Plants Get the Fix
“Plants essentially make choices all the time – the only plants that survive are the ones that make good choices,” says Harris, who is an associate professor of plant biology. Legumes’ choices include whether to put their energy into leaf, stem, root or nodule growth or seed production. And of course, plants’ life-long resources come only from where they stand – it’s not like they can move to better digs.
The secret to legumes’ unusual nitrogen-fixing ability is their symbiotic association with soil bacteria. In order for these nodules (Harris points to a pink bump covered in loose white cells on a plant stem, just below a lateral root) to fix nitrogen, they have to feed the bacteria sugar at the expense of their own growth.
“A plant needs to make enough nitrogen – not too much, not too little – because in doing so, the plant has to use its energy to feed that bacteria sugar,” says Harris.
The source of that chemical energy in the first place is light energy. Plants use the red light portions of the spectrum for photosynthesis, stripping out almost all the red light that hits the leaf, but far red light is not harvested and passes through the leaf to plants below. Plants take advantage of that difference and use the ratio of red to far red light to tell them whether they’re shaded or in full sunlight.
So in her USDA Hatch grant experiments, Harris and colleagues regulate red/far red light to test how that affects legumes’ ability to grow nodules. Harris’s findings may indicate that plants shaded by other plants, realize they’re going to get increasingly less light, so they actually respond and predict – at the cellular level – budgeting for less sugar by stopping nodule growth or producing many fewer, Harris explains.
Light is just one aspect that Harris and her research team study. Others include salinity and nitrogen available in soils and root architecture. The signaling network gets even more complicated and fascinating as these factors are layered. For instance, it turns out that red light both stimulates nodule growth and turns off signaling of stress hormones. Curiously, soil nitrogen and salinity regulate root architecture by modulating these same hormones.
“We think that the fact that the same hormones respond to multiple environmental inputs means that they can function to integrate lots of complex signals from the environment and distill it down to very simple instructions: whether or not to make a nodule. It also makes the system very flexible and able to respond sensitively to small changes in the environment."
“Trying to use this research to inform agriculture, helps us make the best use of these plants – for instance, get the best yield by growing them in optimum conditions,” says Harris. “We’re not at that point yet in the research, but I can imagine it going in that direction.”