Can A Plant Be Altruistic?

Although plants have the ability to sense and respond to other plants, their ability to recognize kin and act altruistically has been the subject of few studies. The authors explored kin recognition in Impatiens pallida (yellow jewelweed). By moving their resources into leaves, these plants not only positively affected their own growth, but also negatively affected their competitors' growth. This is the first instance where researchers demonstrated that a plant's response to an aboveground cue is dependent upon the presence of a belowground cue.

The concept of altruism has long been debated in philosophical circles, and more recently, evolutionary biologists have joined the debate. From the perspective of natural selection, altruism may have evolved because any action that improves the likelihood of a relative's survival and reproduction increases the chance of an individual's DNA being passed on.

Social behavior, kin recognition, and altruism are well known in the animal kingdom; however, although plants have the ability to sense and respond to other plants, their ability to recognize kin and act altruistically has been the subject of few studies.

In a paper published in the November issue of the American Journal of Botany, Ph.D. candidate Guillermo Murphy and Dr. Susan Dudley explore kin recognition in Impatiens pallida, commonly known as yellow jewelweed. Yellow jewelweed individuals are often found growing in close proximity to related individuals and are known to respond strongly to aboveground competition, making this species a likely candidate for kin recognition.

Murphy and Dudley measured plants' responses to two potential cues for competition -- changes in light quality (an aboveground cue) and the presence of root neighbors (an underground cue) -- for plants grown with strangers and with relatives. The researchers found that the response of Impatiens plants differed depending on whether the plants grew with relatives or with strangers. This demonstrates that jewelweed is capable of recognizing kin from non-kin and shows an interesting degree of complexity since both types of responses differed from plants growing with no neighbors at all.

Among close relatives, plants did not increase resource allocation to roots or leaves. Rather, they altered their aboveground morphology by increasing stem elongation and branching. This may be an example of the plants cooperating with kin by attempting to acquire needed resources without shading nearby relatives. Yellow jewelweed is found in the understory of forests, where light may be scarce but the soil is usually nutrient-rich. Because light is the limiting factor for plant growth in this environment, a plant competing with its neighbors would be most likely to allocate resources to leaves.

For Impatiens plants grown with strangers, the plants increased their resource allocation to their leaves relative to allocation to stems and roots, an indication of a competitive response. By moving their resources into leaves, these plants not only positively affected their own growth by enhancing their ability to acquire a limited resource but also negatively affected their competitors' growth by shading nearby plants and decreasing the competitor's light acquisition abilities.

However, these differences in response based on the presence of kin or strangers were only observed in those plants grown with root neighbors, indicating that communication among roots may be necessary for plants to recognize kin. Also, changes in allocation of resources toward roots in response to light quality only occurred in plants grown with root neighbors. This is the first instance where researchers demonstrated that a plant's response to an aboveground cue is dependent upon the presence of a belowground cue. This study demonstrates that plants are social organisms. It shows that altruism is possible among plants and that response to both kin and strangers depend on the ecology of the plant species.

Journal reference:
Murphy et al. Kin recognition: Competition and cooperation in Impatiens (Balsaminaceae). American Journal of Botany, 2009; 96 (11): 1990 DOI: 10.3732/ajb.0900006
Adapted from materials provided by American Journal of Botany, via EurekAlert!, a service of AAAS.

Plants Recognize Siblings: ID System In Roots

Plants may not have eyes and ears, but they can recognize their siblings, and researchers at the University of Delaware have discovered how.

The ID system lies in the roots and the chemical cues they secrete.

The finding not only sheds light on the intriguing sensing system in plants, but also may have implications for agriculture and even home gardening.

The study, which is reported in the scientific journal Communicative & Integrative Biology, was led by Harsh Bais, assistant professor of plant and soil sciences at the University of Delaware.

Canadian researchers published in 2007 that sea rocket, a common seashore plant, can recognize its siblings -- plants grown from seeds from the same mother.

Susan Dudley, an evolutionary plant ecologist at McMaster University in Hamilton, Ontario, and her colleagues observed that when siblings are grown next to each other in the soil, they “play nice” and don't send out more roots to compete with one another.

However, the moment one of the plants is thrown in with strangers, it begins competing with them by rapidly growing more roots to take up the water and mineral nutrients in the soil.

Bais, who has conducted a variety of research on plant signaling systems, read Dudley's study and wanted to find the mechanism behind the sibling recognition.

“Plants have no visible sensory markers, and they can't run away from where they are planted,” Bais says. “It then becomes a search for more complex patterns of recognition.”

Working in his laboratory at the Delaware Biotechnology Institute, a major center for life sciences research at UD, Bais and doctoral student Meredith Biedrzycki set up a study with wild populations of Arabidopsis thaliana.

They utilized wild populations to avoid issues with this common laboratory-bred species, which “always has cousins floating around in the lab,” Bais says.

In a series of experiments, young seedlings were exposed to liquid media containing the root secretions or “exudates” from siblings, from strangers (non-siblings), or only their own exudates.

The length of the longest lateral root and of the hypocotyl, the first leaf-like structure that forms on the plant, were measured.

Additionally, in one experiment, the root exudates were inhibited by sodium orthovanadate, which specifically blocks root secretions without imparting adverse growth effects on roots.

The exposure of plants to the root exudates of strangers induced greater lateral root formation than exposure of plants to sibling exudates. Stranger recognition was abolished upon treatment with the secretion inhibitor.

Biedrzycki did the painstaking laboratory research, rotating more than 3,000 plants involved in the study every day for seven consecutive days and documenting the root patterns.

“The research was very painstaking because Arabidopsis roots are nearly translucent when they are young and were also tangled when I removed them from plates, so measuring the roots took a great amount of patience,” Biedrzycki notes.

“This manuscript is very important for my research since the focus of my thesis project is understanding the biochemical mechanism behind root secretions,” she says. “This research has allowed me to probe the natural mechanism of kin recognition and root secretion.”

The study was replicated by Dudley's lab in Canada, with similar results.

Strangers planted next to each other are often shorter, Bais notes, because so much of their energy is directed at root growth.

Because siblings aren't competing against each other, their roots are often much shallower.

Bais says he and his colleagues also have noticed that as sibling plants grow next to each other, their leaves often will touch and intertwine compared to strangers that grow rigidly upright and avoid touching.

The study leaves a lot of unanswered questions that Bais hopes to explore further. How might sibling plants grown in large “monocultures,” such as corn or other major crop plants, be affected? Are they more susceptible to pathogens? And how do they survive without competing?

“It's possible that when kin are grown together, they may balance their nutrient uptake and not be greedy,” Bais speculates.

The research also may have implications for the home gardener.

“Often we'll put plants in the ground next to each other and when they don't do well, we blame the local garden center where we bought them or we attribute their failure to a pathogen,” Bais says. “But maybe there's more to it than that.”

Bais's research was supported by the National Science Foundation (NSF) and by the NSF-Delaware Experimental Program to Stimulate Competitive Research (EPSCoR). The Natural Science and Engineering Research Council of Canada provided research funding to Dudley.

Journal reference:
Meredith L. Biedrzycki, Tafari A. Jilany, Susan A. Dudley and Harsh P. Bais. Root exudates mediate kin recognition in plants. Communicative & Integrative Biology, Volume 3, Issue 1 (January/February 2010) [link]
Adapted from materials provided by University of Delaware. Original article written by Tracey Bryant.