When it comes to how some people like to live, the phrase “good fences make good neighbors” rings true. But for plants, an above-ground fence isn’t much help in solving territory disputes between neighbors—after all, their roots can still become intertwined in the underlying soil. So plants have adapted various strategies for interacting with the roots of their neighbors to stake out subterranean territory and maximize their own growth. The roots of rice plants, Fang et.al. have concluded in a new Early Edition PNAS paper, grow toward other roots which contain the same genes as their own, but grow away from roots with a different genotype. The avoidance and attraction is mediated by a different mechanism than how root shoots react to solid, non-plant obstacles, the team found.
It’s clear to any observer of plants and their growth that flora growing in close proximity to each other can have drastic effects on each other’s growth patterns. Partially, these effects are carried out through indirect interactions revolving around the competition for resources, like water and sunlight. If one plant overshadows the other, for example, it gains a clear advantage. But evidence has also emerged over time that some of the interactions are direct forms of molecular communication that occur between the roots of plants. Studying the interactions between root shoots, however, has been limited by the difficulty in obtaining real-time measurements of root growth.
To overcome this challenge, the team—mostly based in the Duke University lab of biologist Philip Benfey—turned to a gel-based plant growth system previously developed by the lab. By growing rice plants in the gel matrix, the scientists could follow root growth in real-time, capture images of the roots from all angles and reconstruct the organization on a computer program for further analysis. (A video of a separate system developed by the group, allowing real time observation of gene expression in growing roots, can be found on YouTube).
Fang et. al. grew three rice varieties with different genotypes—Azucena, Caiapo, and IR64—in different combinations in the gel. For all three of the rice varieties, plants grown alone developed the largest root systems and having any neighbors decreased the size of the root system. In addition, all three kinds of rice grew smaller root systems when they were planted in smaller containers.
Then, the researchers began pairing different combinations of the rice plants together. Roots showed a greater extent of overlap, they found, when the two plants were of the same variety. To exclude the possibility that the plants were communicating above-ground, the researchers separated the portions of the plant above the soil, only letting them interact below-ground. The results still held true: when rice plants are of the same variety, they send roots toward each other, but when they’re different varieties, their roots avoid each other. After growing plants of different sizes and at varied distances from each other, Fang et. al. concluded that the interactions are occurring at the tips of the roots.
The team went on to examine how root systems change when they encounter physical barriers in the soil—a different response than how they react to a neighbor’s encroaching roots. When a root first encounters a barrier, they found, it spurs the plant to increase the size of its root system (presumably in an attempt to break through the barrier or grow around it). But when an increasing number of root tips touch the barrier, the size of the individual roots begins to decline.
“Our data suggest that rice roots are able to recognize and identify objects in their vicinity through two mechanisms,” the authors write in the conclusion of their paper. “Genotype-specific root recognition likely mediated by signaling through root tips, and physical object recognition via size-dependent root tip contact.”
Additional work is needed to examine whether the root responses seen in rice are typical of other plants and what chemical messengers and molecular pathways are responsible for translating the behaviors throughout the entire plant root system.
Genotypic recognition and spatial responses by rice roots. Suqin Fang, Randy T. Clark, Ying Zheng, Anjali A. Iyer-Pascuzzi, Joshua S. Weitz, Leon V. Kochian, Herbert Edelsbrunner, Hong Liao, and Philip N. Benfey. PNAS. Published online before print January 29, 2013, doi:10.1073/pnas.1222821110