Kin recognition by roots occurs in cycads and probably in conifers

Root exudate signals in plant-plant interactions

Plant-to-plant signalling is a key mediator of interactions among plant species. Plants can perceive and respond to chemical cues emitted from their neighbours, altering survival and performance, impacting plant coexistence and community assembly. An increasing number of studies indicate root exudates as key players in plant-to-plant signalling. Root exudates mediate root detection and behaviour, kin recognition, flowering and production, driving inter- and intra-specific facilitation in cropping systems and mixed-species plantations. Altered interactions may be attributed to the signalling components within root exudates. Root ethylene, strigolactones, jasmonic acid, (-)-loliolide and allantoin are signalling chemicals that convey information on local conditions in plant-plant interactions. These root-secreted signalling chemicals appear ubiquitous in plants and trigger a series of belowground responses inter- and intra-specifically, involving molecular events in biosynthesis, secretion and action. The secretion of root signals, mainly mediated by ATP-binding cassette transporters, is critical. Root-secreted signalling chemicals and their molecular mechanisms are rapidly revealing a multitude of fascinating plant-plant interactions. However, many root signals, particularly species-specific signals and their underlying mechanisms, remain to be uncovered due to methodological limitations and root-soil interactions. A thorough understanding of root-secreted chemical signals and their mechanisms will offer many ecological implications and potential applications for sustainable agriculture.

Towards a framework for collective behavior in growth-driven systems, based on plant-inspired allotropic pairwise interactions

A variety of biological systems are not motile, but sessile in nature, relying on growth as the main driver of their movement. Groups of such growing organisms can form complex structures, such as the functional architecture of growing axons, or the adaptive structure of plant root systems. These processes are not yet understood, however the decentralized growth dynamics bear similarities to the collective behavior observed in groups of motile organisms, such as flocks of birds or schools of fish. Equivalent growth mechanisms make these systems amenable to a theoretical framework inspired by tropic responses of plants, where growth is considered implicitly as the driver of the observed bending towards a stimulus. We introduce two new concepts related to plant tropisms: point tropism, the response of a plant to a nearby point signal source, and allotropism, the growth-driven response of plant organs to neighboring plants. We first analytically and numerically investigate the 2D dynamics of single organs responding to point signals fixed in space. Building on this we study pairs of organs interacting via allotropism, i.e. each organ senses signals emitted at the tip of their neighbor and responds accordingly. In the case of local sensing we find a rich state-space. We describe the different states, as well as the sharp transitions between them. We also find that the form of the state-space depends on initial conditions. This work sets the stage towards a theoretical framework for the investigation and understanding of systems of interacting growth-driven individuals.

Kin recognition in rice (Oryza sativa) lines

Kin recognition is an important mediator of interactions within individuals of a species. Despite increasing evidence of kin recognition in natural plant populations, relatively little is known about kin recognition in crop species where numerous cultivars have been generated by artificial selection. We identified rice (Oryza sativa) cultivars with the ability for kin recognition from two sets of indica-inbred and indica-hybrid lines at different levels of genetic relatedness. We then assessed this ability among kin and nonkin and tested potential mechanisms in a series of controlled experiments and field trails. Rice cultivars with the ability for kin recognition were capable of detecting the presence of kin and nonkin and responded to them by altering root behavior and biomass allocation, particularly for grain yield. Furthermore, we assessed the role of root exudates and found a root-secreted nitrogen-rich allantoin component to be responsible for kin recognition in rice lines. Kin recognition in rice lines mediated by root exudates occurs in a cultivar-dependent manner. Rice cultivars with the ability for kin recognition may increase grain yield in the presence of kin. Such an improvement of grain yield by kin recognition of cultivar mixtures offers many implications and applications in rice production.

Root-mediated sex recognition in a dioecious tree

Recent studies have demonstrated that plants can determine the identity of neighbouring roots (e.g., self and non-self, kin and non-kin), but whether they can discriminate by sex remains an open question. Here, we predict that dioecious plants can modulate their root performance in response to local root conditions related to sex. Female and male Populus cathayana cuttings were planted in a greenhouse in root-owner (one individual without a root neighbour) or root-sharer pairs (two individuals with roots neighbouring each other) with equal amounts of nutrients and space per plant in three combinations (females-females, males-males or females-males); root morphology, biomass and allocation were investigated. P. cathayana root-sharers altered their root growth in same-sex but not in different-sex combinations. Females enhanced root growth and allocation but decreased root proliferation (greater diameter with reduced branching and specific root length) in the presence of a female root neighbour, while males reduced root growth but increased root morphological proliferation in contact with another male. Therefore, the effect of a neighbour of the same sex differed from that of a neighbour of the opposite sex, which suggests that these plants can recognize the sexual identity of their neighbours.

Kin recognition is a nutrient-dependent inducible phenomenon

Recognition and response to prospective competitors are crucial variables that must be considered in resource distribution and utilization in plant communities. Associated behaviors are largely mediated through the exchange of low-molecular weight exudates. These cues can significantly alter the root system architecture (RSA) between neighboring plants and are routinely sensitive enough to distinguish between plants of the same or different accessions, a phenomenon known as kin recognition (KR). Such refined discrimination of identity, based on the composition and detection of patterns of exudate signals is remarkable and provides insight into the chemical ecology of plant-plant interactions. The discovery that KR occurs in Arabidopsis thaliana provides a model system to resolve many of the mechanistic questions associated with this process. We hypothesized that the low-molecular weight cues which direct changes to the RSA during KR was driven by nutrient availability. Here we present evidence in support of a nutrient-inducible model for KR. Our findings underscore how exudate production and detection are influenced by nutrient availability as well as how this information is integrated into 'decisions' about competition and root system architecture which may have broader impacts on community composition.

Arguments for and against self and non-self root recognition in plants

Root-root interaction research gained more and more attention over the past few years. Roots are pivotal for plant survival because they ensure uptake of water and nutrients. Therefore, detection of adjacent roots might lead to competitive advantages. Several lines of experimental evidence suggest that roots have ways to discriminate non-related roots, kin, and-importantly-that they can sense self/non-self roots to avoid intra-plant competition. In this mini-review, the existence of self/non-self recognition in plant roots will be discussed and the current knowledge on the mechanisms that could be involved will be summarized. Although the process of identity recognition is still not completely understood, interesting data are available and emerging new technologies will certainly aid to better understand this research field that can have an important biological, ecological, and agricultural impact.