Cluster Roots and Their Functional Equivalents: Ecological and Agronomic Significance

Cluster roots were first described for Banksia species (Proteaceae) in the botanical gardens in Leipzig as roots that were ‘very much branched’ (Engler 1894). A detailed description of cluster roots in non-mycorrhizal Proteaceae waited until Purnell (1960), who coined the term ‘proteoid’ roots, in genera of Australian Proteaceae. It has since been discovered that they are widespread, though not universal, in Proteaceae. They also occur in several Fabaceae as well as many actinorhizal species, and hence the term cluster roots is now commonly used (Shane and Lambers 2005). Some species produce both cluster roots and mycorrhizas. Cluster roots are short-lived (3-4 weeks) structures that release large amounts of phosphorus (P)-mobilising carboxylates, often in an exudative burst. In addition to P, they also mobilise manganese (Mn), and leaf [Mn] can be used as a proxy for carboxylate concentrations in the rhizosphere of different functional plant types (Lambers et al. 2015).

Morphologically rather different, but functionally very similar structures occur in several other families. For example, dauciform roots in Cyperaceae, capillaroid roots in Restionaceae, vellozioid roots in Velloziaceae, and sand-binding roots in Fabaceae, Haemodoraceae and many other families (Shane and Lambers 2005). Some Fabaceae, e.g., Cicer arietinum (chickpea) exhibit no obvious structures, yet also release large amounts of carboxylates (Pang et al. 2018b).

Cluster roots and functionally equivalent structures are more costly than mycorrhizas, but very effective in soils where the P availability is very low, either because total soil [P] is low or because a large fraction of soil P is sorbed to soil particles. These specialised structures occur in a wide range of species that are of agronomic significance. Within species, e.g., chickpea, carboxylate-releasing genotypes can be selected for by screening for leaf [Mn] (Pang et al. 2018a).

Engler A 1894 In: Die natürlichen Pflanzenfamilien, III Teil 1 Haelfte Eds. A Engler and K Prantl, pp 119-156. Verlag von Wilhelm Engelmann, Leipzig.

Lambers H, Hayes PE, Laliberté E, Oliveira RS, et al. 2015 Trends Plant Sci 20:83-90

Pang J, Ruchi B, Zhao H, Bansal R, et al. 2018a New Phytol in press.

Pang J, Zhao H, Bansal R, Bohuon E, et al. 2018b Plant Cell Environ in press

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Shane MW & Lambers H 2005 Plant Soil 274:101-125