Root-root interactions: extending our perspective to be more inclusive of the range of theories in ecology and agriculture using in-vivo analyses

Abstract

Background: There is a large body of literature on competitive interactions among plants, but many studies have only focused on above-ground interactions and little is known about root-root dynamics between interacting plants. The perspective on possible mechanisms that explain the outcome of root-root interactions has recently been extended to include non-resource-driven mechanisms (as well as resource-driven mechanisms) of root competition and positive interactions such as facilitation. These approaches have often suffered from being static, partly due to the lack of appropriate methodologies for in-situ non-destructive root characterization.

Scope: Recent studies show that interactive effects of plant neighbourhood interactions follow non-linear and non-additive paths that are hard to explain. Common outcomes such as accumulation of roots mainly in the topsoil cannot be explained solely by competition theory but require a more inclusive theoretical, as well as an improved methodological framework. This will include the question of whether we can apply the same conceptual framework to crop versus natural species.

Conclusions: The development of non-invasive methods to dynamically study root-root interactions in vivo will provide the necessary tools to study a more inclusive conceptual framework for root-root interactions. By following the dynamics of root-root interactions through time in a whole range of scenarios and systems, using a wide variety of non-invasive methods, (such as fluorescent protein which now allows us to separately identify the roots of several individuals within soil), we will be much better equipped to answer some of the key questions in root physiology, ecology and agronomy.

Keywords: Root; competition; facilitation; green fluorescent protein; interaction; interspecific; intraspecific; nuclear magnetic resonance; positron emission tomography; red fluorescent protein; resource; rhizotrons.

Fig. 1.

Conceptual figure on root–root interactions and fundamental and realized niches. Adding effects of facilitation on realized niches to our overall perspective on root–root interactions. Hutchinson's fundamental and realized niche concept (1957) has been seen as where competition, predation, disease and parasitism and recruitment limitation limit the realized niche compared with the possible fundamental niche for a species. Recently, Bruno et al. (2003) have broadened the concept to include the possibility that positive interactions happening during facilitation could actually increase the size of the realized niche compared with the fundamental niche. Adapted from Bruno et al. (2003), based on Hutchinson (1957).

Fig. 2.

Outcomes of root–root interactions with intraspecific and interspecific scenarios. (A) Outcome scenarios according to competition theory whereby growing with neighbours leads to avoidance of competition and spatial segregation, including spatial (or other) niche separation leading to niche complementarity in interspecific situations. (B) Evidence from direct studies of root–root interactions that cannot solely be explained by competition theory. In intraspecific scenarios, roots of different individuals have been found to be attracted to one another despite a nutrient patch being positioned elsewhere (Cahill et al., 2010), or more lateral roots developed if individuals were not related to one another suggesting kin recognition (Dudley and File, 2008). In interspecific scenarios, an accumulation of roots in the topsoil has often been found, or a simultaneous accumulation in topsoil and spatial segregation (Lehmann et al., 1998). We hypothesize that this is partly due to the roots of non-N₂-fixing neighbours foraging closer to the N₂-fixing species and that this form of facilitation may explain some but not all of the interaction outcomes found in direct studies. More inclusive use of a range of theories as we as new non-destructive techniques will provide much-needed help explaining surprising outcomes often found.

Fig. 5.

Quantifying root–root interactions between plant individuals using MRI. MRI images of root distribution of soya grown with maize (A–C, interspecific) or with another soya individual as a neighbour (D–F, intraspecific) at 14 d after transplanting seedlings into pots. (A, B) and (D, E) show two different 3-D side views; (C) and (F) show 2-D top views (projection in the axial plane) with specification of three concentric rooting zones (see Rascher et al., 2011 and fig. 7 therein for more details of the quantification method and MRI specifications). In this example, for the soya–maize interaction 9.3, 34.1 and 56.6 % or the total roots were found in concentric circles 1, 2 and 3, respectively (1 being the inner circle). For the soya–soya interaction we found 20.3, 30.2 and 49.5 % of the roots in concentric circles 1, 2 and 3, respectively.

Fig. 3.

Quantifying root–root interactions between different species using fluorescent markers. (A, B) Images of seedlings (4 d after sowing). From left to right in each: transgenic maize expressing green fluorescent protein (GFP), transgenic wheat expressing red fluorescent protein (RFP) and rapeseed wild-type (WT) grown on blotting paper. (C, D) Images of a rhizotron filled with topsoil containing three plants (12 d after sowing). The arrow shows the wild-type non-transformed root of Colza rape which is then visible only in conventional images. From left to right in each: maize expressing GFP, wheat expressing RFP and rapeseed WT. Pictures were taken either with a conventional camera (Faget et al., 2009) (A, C) or with a modified camera that allowed the green and red fluorescence emission wave lengths to be captured. The images of GFP and RFP signals were then merged together (B, D). Plant material: the maize genotype ETH-M72 was genetically transformed to include the gene that encodes green fluorescent protein (gfp) (ETH-M72GFP). The wheat genotype RFP BOBWHITE 6h was genetically transformed to include the gene coding for the red fluorescent protein (rfp). The variety of Colza rape (Brassica napus ‘Heros’) was non-transformed and used as the wild type.

Fig. 4.

Analysing root–root interactions of plants grown in soil-filled rhizotrons. (A) Representative image of nine barley (Hordeum vulgare ‘Barke’) plants grown for 4 weeks after sowing in rhizotrons filled with black peat soil. The rhizotron was set to an inclination angle of 43° (with the transparent side facing downwards) to force roots to grow along the transparent plate of the rhizotron and make them visible and accessible for cameras (for more details, see Nagel et al., 2012). (B) A higher resolution image showing an area of interest (as indicated in A) with ×2.5 magnification.

Fig. 6.

Quantifying root–root interactions between plant individuals using MRI–PET coregistration (after Jahnke et al., 2009). (A) Photograph of two maize plants ( and 2) growing in a soil-filled pot. (B–D) Co-registered MRI (grey) and PET (colour) root images of the 18-d (B, C) and 19-d-old (D) plants. Radioactive ¹¹CO₂ was administered either to the shoots of both plants (B) or only to the shoot of plant 2 (C). The PET and MRI data in (B) and (C) were obtained on the 18-d-old plants and the MRI images are identical. On the following day, plant 1 shoot was radiolabelled and a new MRI image was obtained (D). Scale bar = 1 cm.