CASE STUDY 3.1  Cluster (proteoid) roots

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M. Watt

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Figure 1  Cluster roots in Banksia serrata growing on Hawkesbury Sandstone hillslopes in the Sydney region. (a) Roots that have grown across a dead eucalypt leaf extract nutrients remaining in the decaying leaf. (b) Clusters of fine rootlets at the tips of roots increase the surface area for nutrient extraction from surrounding soil. Scale bar = 100µm (Scanning electron micrograph courtesy S. Gould)

Cluster roots (Figure 1) are found worldwide in species from nutrient-deficient soils (Dinkelaker et al. 1995). In these soils, cluster roots enhance uptake of nutrients, especially phosphate, and help prevent soil erosion. Many species which develop cluster roots, including members of the Proteaceae where they were first described by Purnell (1960), are native to Australia. Other families such as the Casuarinaceae, Cyperaceae, Mimosaceae and Restionaceae also have heavily branched root systems (Lamont 1993); similarities in function between capillaroid roots of the Restionaceae and cluster roots of the Proteaceae might be expected as they are both major Gondwanan families. Significantly, few species with cluster roots are mycorrhizal, implying that root clusters fulfil a similar role to mycorrhizal fungi.

Australian soils generally contain low concentrations of plant-available phosphate, much of it bound with iron–aluminium silicates into insoluble forms or concentrated in the remains of decaying plant matter. Because very little of this phosphate is soluble, most roots extract it only slowly. Plants with cluster roots gain access to fixed and organic phosphate through an increase in surface area and release of phosphate-solubilising exudates (Section 3.3). Hence plants with cluster roots grow faster on phosphate-fixing soils than species without clusters, supporting claims that cluster roots are an adaptation to phosphate deficiency.

Cluster roots have a distinct morphology. Intense proliferation of closely spaced, lateral ‘rootlets’ occurs along part of a root axis to form the visually striking structures. Root hairs develop along each rootlet and result in a further increase in surface area compared to regions where cluster roots have not developed (e.g. 26 times in Leucadendron laureolum; Lamont et al. 1984).

Factors driving cluster root formation in relation to overall morphology of a root system remain a matter for conjecture. In the Proteaceae, clusters generally form on basal laterals so that they are abundant near the soil surface where most nutrients are found. For example, Banksia serrata produces a persistent, dense root mat capable of intercepting nutrients from leaf litter and binding the protecting underlying soil from erosion (Figure 1). New clusters differentiate on the surface of this mat after fires and are well placed to capture nutrients. In contrast, Banksia prionotes forms ephemeral clusters which export large amounts of nutrients during winter (Jeschke and Pate 1995). Lupinus albus has more random clusters which appear on up to 50% of roots (Figure 2).

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Figure 2 Basal roots of a two-week-old Lupinus albus plant grown in nutrient culture with 1 µM phosphate. Proteoid roots have emerged along the primary lateral roots (arrowhead) and the oldest proteoid rootlets have reached a determinate length of 5 mm. As rootlets approach their final length, they exude citrate for 2-3 d. (mm scale on left side) (Photograph courtesy M. Watt, Research School of Biological Sciences, ANU)

Rootlets not only represent an increase in surface area but also exude protons and organic acids, solubilising phosphate and making it available for uptake. Exudates from cluster roots represent up to 10–23% of the total weight of an L. albus plant, suggesting that they constitute a major sink for photoassimilates. However, not all this additional carbon comes from photosynthesis because approximately 30% of the carbon demand of clusters is met by dark CO2 fixation via phosphoenolpyruvate carboxylase (Johnson et al. 1994). Because cluster roots form on roots of L. albus even when phosphate supply is adequate, growth of L. albus in soils with low phosphate availability is not restricted by an additional carbon ‘drain’ to roots. On the other hand, the great many species which produce cluster roots in response to environmental cues like phosphate deficiency (Dinkelaker et al. 1989) or seasons (e.g. Banksia prionotes) might experience a carbon penalty to support these roots.

Cluster roots on L. albus are efficient with respect to carbon consumption by generating citrate on cue. Most of the citrate exuded by clusters is released during a two to three day period when the cluster is young (Watt et al. 1997). A large root surface area in clusters works in concert with this burst of exudation to solubilise phosphate before it is re-fixed to clay surfaces or diffuses away (Gardner et al. 1981).

Form and function are thus coordinated in time and space, so that cluster roots can mine a pocket of phosphate-rich soil which would otherwise not yield its nutrients. Cluster roots are an elegant adaptation of root structure and biochemistry to nutrient-poor soils.

References

Dinkelaker, B., Hengeler, C. and Marschner, H. (1995). ‘Distribution and function of proteoid roots and other root clusters’, Botanica Acta, 108, 183–200.

Dinkelaker, B., Römheld, V. and Marschner, H. (1989). ‘Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.)’, Plant, Cell and Environment, 12, 285–292.

Gardner, W.K., Parbery, D.G. and Barber, D.A. (1981). ‘Proteoid root morphology and function in Lupinus albus’, Plant and Soil, 60, 143–147.

Jeschke, W.D. and Pate, J.S. (1995). ‘Mineral nutrition and transport in xylem and phloem of Banksia prionotes (Proteaceae)’, Journal of Experimental Botany, 46, 895–905.

Johnson, J.F., Allan, D.L. and Vance, C.P. (1994). ‘Phosphorus stress induced proteoid roots show altered metabolism in Lupinus albus’, Plant Physiology, 104, 657–665.

Lamont, B.B. (1993). ‘Why are hairy root clusters so abundant in the most nutrient impoverished soils of Australia?’, Plant and Soil, 156, 269–272.

Lamont, B.B., Brown, G. and Mitchell, D.T. (1984). ‘Structure, environmental effects on their formation, and function of proteoid roots in Leucadendron laureolum (Proteaceae)’, New Phytologist, 97, 381–390.

Purnell, H.M. (1960). ‘Studies of the family Proteaceae 1. Anatomy and morphology of the roots of some Victorian species’, Australian Journal of Botany, 8, 38–50.

Watt, M., Millar, A.H., Day, D.A. and Evans, J.R. (1997). ‘Organic acid efflux from the proteoid roots of phosphorus limited Lupinus albus’, Abstracts of papers presented at the 41st ASBMB and 37th ASPP Annual Conferences, Melbourne, 29 September to 2 October, P1-103, ASBMB Inc.: Kent Town, South Australia.

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