(Inside Science) -- Plants that glow with the aid of a genetically implanted firefly protein are now shedding new light on the hidden world of plant roots. This research could help breed crops better at resisting problems such as drought and salinity, scientists added.
"I liken the plant to an iceberg, where most of it is hidden under the surface," said study co-author José Dinneny, a plant biologist at the Carnegie Institution for Science in Stanford, California.
The networks of roots that plants use to absorb water and nutrients can encompass a space larger than the part of the plant visible above ground. The nature of these root systems can help plants adapt to challenging environments such as deserts -- for instance, mesquite trees can develop tap roots capable of digging more than 50 yards deep to reach water.
As important as roots are, much about them remains a mystery. Scientists can study deep and hidden roots by excavating them, but such work is laborious, time-consuming, and prevents researchers from seeing how growing roots behave.
The new technique enables scientists to grow genetically modified glowing plants in special vessels. The investigators can then observe roots growing under a variety of different circumstances.
"We can now see that iceberg in all its glory," said Dinneny.
The system is called GLO-Roots, for Growth and Luminescence Observatory for Roots. First researchers genetically engineered plants to generate a firefly enzyme called luciferase, which made all of the plants glow, including their roots. They generate light, but not enough to be detected by the naked eye.
"Our plants are not suitable for lighting your home," Dinneny said.
Next, the plants were grown in custom-designed vessels called rhizotrons that hold a thin amount of soil between two sheets of transparent plastic. Black vinyl sheets can be placed over the rhizotrons to protect roots from outside light. Light-sensitive cameras could then detect the faint glow the roots gave off to help scientists distinguish the roots from the soil.
"It was amazing to see the complexity and beauty of living root systems in soil," Dinneny said.
The researchers found the thin, flat nature of the rhizotrons did not impede plant growth. Plants routinely endure obstacles to their growth, including rocks, compacted soil and other plants, Dinneny said.
Multiple versions of luciferase exist and each emits light of different wavelengths. The researchers can design plants so these luciferases are made only when certain plant genes are active. The genes govern certain root decisions, so the light given off provides a hint about how plants determine which direction to grow and when to branch.
The researchers used GLO-Roots to simultaneously track the growth speed and direction of dozens of root tips and connect this behavior to the amount of water in the soil around the roots. Simulated droughts caused roots to grow deeper, presumably to hunt untapped water.
These findings could help scientists understand how plants can make the most out of the soil and the associated factors related to the bacteria and chemicals in it, which could lead to better crops.
"Our method will allow researchers to test the effects of different bacterial strains and chemical environments to understand the relationship between these treatments, root growth and plant health," said Dinneny.
Although scientists have recently developed transparent soil to help gaze at roots, the material is expensive, making it difficult for researchers to use regularly, especially when it comes to plants with large root networks, Dinneny said. Still, he noted that transparent soil does help researchers analyze fine details of roots.
Plant geneticist Julia Bailey-Serres at the University of California, Riverside, who did not take part in this research, cautioned that "one challenge of the GLO-Roots system is that it works for very thin roots." This system might not work "with the larger, denser roots of corn and rice," she said.
The scientists are now developing a robotic system to automatically carry plants and their rhizotrons from the greenhouse to the imaging system. They detailed their findings online August 19 in the journal eLife.