When we think of plants, we often imagine solitary beings, rooted in place and working to maximize their own growth and reproduction, but plants are in a constant fight with their neighbors for light and space.
If plants, that are normally growing in full sunlight, are getting shaded by other plants or leaves, they invest their resources in, for example, elongating their petioles (the part that connects the leaf to the stem) to grow out of the shade as fast as possible. This strong response is known as shade avoidance response. While competition for light seems very obvious to us, we often forget that the world below ground is equally complex as above.
The fight for resources continues in the dark. Roots provide plants with water and nutrients. However, the soil is a complex network of roots from all kind of plants. Surprisingly, plants are actually capable of recognizing and responding to their neighbors in complex ways, including through the recognition of kin. It seems like plants are not as individualistic as we once thought.
Kin recognition is a phenomenon that has long been studied in animals, but it is only in recent years that researchers have begun to explore its occurrence in plants. Plants, it turns out, are able to sense and respond to chemical signals released by their neighbors, and use these signals to distinguish between close relatives and unrelated individuals. Between friends and foes, so to speak.
So, how does a plant know when it is competing against the roots of a neighboring plant or its own?
Research has shown that roots are able to discriminate between non-related roots and kin roots. Furthermore, roots can recognize roots of their own to avoid intra-planta competition. Plants show less competitive behavior if they recognize their own kin. Research has not yet fully uncovered how roots identify kin and non-kin neighbors. Evidence suggests that kin recognition works on the molecular level through the release and detection of chemical signals. These molecules are called root exudates.
Root exudates are fluids that have been secreted by the roots. These can contain for example amino acids, sugars, organic acids, vitamins, sterols, ions, or other chemicals. Root exudates are not only important for kin recognition, they are essential for healthy plant growth. For example, some root exudates can inhibit the growth of harmful microorganisms in the soil. The metabolite allantoin is an example of a root exudate. It plays a major role in kin recognition in rice. But it is also involved in the assimilation, transport, and storage of nitrogen in plants. At the same time, allantoin has been shown to increase growth of soil bacteria and fungi, which are beneficial for the plant. If rice is grown in a kin vs. non-kin environment, the concentration of allantoin around the roots increased, showing an increase in competition.
Another important group of molecules involved in root kin recognition are strigolactones. These plant hormones are also released by the roots and can act as signals to nearby plants, indicating the presence of a potential neighbor. But strigolactones fulfill many roles in a plant such as the germination of seeds of parasitic plants, and they are also involved in the branching and growth of roots.
When a plant detects the chemical signature of a close relative, it is able to adjust its growth and behavior in response. The response is often dependent on the species involved in kin recognition. For example: while roots of black walnut trees and tomatoes tend to avoid each other, woodland strawberry roots seem to be attracted to ground ivy roots, while roots of ground ivy tend to avoid strawberries.
Kin recognition is not only about avoiding (or not avoiding) non-related kins. Plants have been shown to direct more nutrients in the direction of roots of their close relatives and thus support their growth. This phenomenon was studied very recently by Bento et al. (2022). The scientists investigated how kin recognition affects biomass allocation in Arabidopsis thaliana (thale cress) under different nutrient conditions and at different stages of plant growth.
The researchers grew A. thaliana in either nutrient-rich or nutrient-poor conditions, and monitored how the plants allocated biomass in response to the presence of accessions (different genetic variants of the species). They found that when grown in nutrient-rich conditions, A. thaliana plants showed a preference for allocating biomass to their own genetic variant over a different accession. However, when grown in nutrient-poor conditions, this preference was not observed. Interestingly, the researchers also found that the biomass allocation response varied depending on the stage of plant growth. Younger plants showed a stronger preference for allocating biomass to their own genetic variant, while older plants showed a weaker response. This study shows the complexity of plant behavior and indicates why some researches tend to label plants as ‘social’.
There is no denying that root kin recognition represents an important and fascinating area of research and there are many open questions. Let's wait and see what more we are going to find out about the ‘nearly social’ behavior of plants.
Sources:
Depuydt, S. (2014). Arguments for and against self and non-self root recognition in plants. Frontiers in plant science, 5, 614.
Pezzola, E., Pandolfi, C., & Mancuso, S. (2020). Resource availability affects kin selection in two cultivars of Pisum sativum. Plant Growth Regulation, 90(2), 321-329.
Goddard, E. L., Varga, S., John, E. A., & Soulsbury, C. D. (2020). Evidence for root kin recognition in the clonal plant species Glechoma hederacea. Frontiers in Ecology and Evolution, 354.
Biedrzycki, M. L., Jilany, T. A., Dudley, S. A., & Bais, H. P. (2010). Root exudates mediate kin recognition in plants. Communicative & integrative biology, 3(1), 28-35.
Bento, T. S., Moffett, M. B., Centeno, D. C., Scrocco, A. P. D., Fox, A., & Palmer, A. G. (2022). Biomass allocation in response to accession recognition in Arabidopsis thaliana depends on nutrient availability and plant age. Plant Signaling & Behavior, 2004025.
Bais, H. P. (2018). We are family: kin recognition in crop plants. New Phytologist, 220(2), 357-359.
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