Historical Background

Most of the essential elements for all life forms enter the biosphere through bacteria or through the roots of plants (Nissen 1991), so the study of the resource acquisition by roots is of much interest and importance. Today the study of plant roots is a meeting point between ecology, physiology, developmental and molecular biology - a vibrant and challenging field.

In the study of the roots’ special adaptations for nutrient uptake much emphasis is placed on mycorrhizas and nitrogen-fixing nodule symbioses, whilst significantly less attention is paid to another root adaptation, namely the cluster root.

Nearly 100 years ago, in 1894, Adolf Engler described an unusual root morphology of plants in the family Proteaceae growing in Leipzig Botanic Gardens. The roots were extensively branched and covered with long densely grouped hairs. But it wasn’t until 1960 that Purnell coined the term ‘proteoid roots’ to describe a root with “dense clusters of rootlets of limited growth”. She observed this structure in all but one (Persoonia) genera of the family Proteaceae, so the name ‘proteoid root’ stems from the plant family in which it was first discovered.

Up today, proteoid roots have been reported in Betulaceae, Casuarinaceae, Eleagnaceae, Moraceae, Leguminosae (Fabaceae) and Myricaceae families, all of which can symbiotically fix atmospheric N₂, apart from Proteaceae that cannot (Dinkelaker et al, 1995). Because of that more recent term ‘cluster roots’ appeared to describe an entire root (or a part of it) from any species that forms one or more clusters of rootlets along its length.

It should be mentioned that the terms proteoid root, cluster root and root cluster have all been used in the literature to describe either the proteoid root axis or the clusters of rootlets themselves, so that caution should be taken when comparing studies because the definitions may vary.

Proteoid Root Morphology

Although there is considerable variation in cluster root morphology between species, the general structure is as follows: along the axis of primary or secondary root discrete clusters of closely spaced rootlets develop. These rootlets emerge in contiguous rows and grow to reach a similar length. This is a rare but not unique example of determinant growth in plant roots (determinacy in lateral roots also occurs in non-proteoid plants such as maize).

Clusters can form singly, as is commonly observed in Lupinus albus, or as complex or compound systems, as in mat-forming members of Proteaceae.

Fig. 1: Schematic representation of Cluster Roots

A- the simple cluster root, common in non-Proteaceae and in some members of Proteaceae. B – a complex cluster root with a second cluster emerging from within the first one. C – the compound cluster root, common in mat-forming members of Proteaceae. (After Skene 1998)

Proteoid roots of :

Lupinus albus (Fabaceae)

Hakea sp. (Proteaceae)

Gervilea sp. (Proteaceae)

Rootlet final length can range from 1 to 30 mm, depending on the species, and the length of a cluster along a proteoid root axis also varies between species (Dinkelaker et al., 1995).

Cluster Root Function

It was Purnell (1960) who for the first time suggested that proteoid roots were involved in nutrient uptake. All species with proteoid roots can grow in soils with poorly available nutrients, and most do not form mycorrhizal symbioses. Species with cluster roots are also often pioneers in primary and secondary succession, and some are used in land reclamation )Skene, 1998). So what is the significance of cluster root structure in nutrient uptake by plant?

Proliferation of rootlets in a cluster presents a massive increase in root surface area for contact with soil, which is thought to be advantageous in nutrient uptake. For example, a mature Hakea oblique proteoid root cluster has a surface area (excluding root hairs) 25 times greater than that of an equivalent mass of axial root.

Coupled with the proliferation of surface area, proteoid root clusters chemically modify the surrounding soil by exuding compounds. These compounds include carboxylate organic anions, acid phosphatases, phenolics, mucilages, and water, and they facilitate the mobilization of nutrients from soil. Organic anions, especially citrate, mobilize P by chelating soil minerals such as Fe, Al, and Ca, all of which bind P. Acid phosphatases are enzymes that hydrolyze organic forms of P.

The mechanism by which exudation of organic acids occurs is not yet known. It has been shown experimentally that the composition of organic anions within the tissue doesn’t reflect that of the exudates and that the rates of exudation do not reflect tissue concentration, indicating that the mechanism has specificity and is not driven solely by a concentration gradients of organic anions between the root tissue and the soil.

Interestingly, exudation in cluster roots represents a physiological clustering, akin to the morphological clustering observed in their structure. It occurs as an ‘exudative burst’, a remarkable phenomenon wherein the rootlets exude little or no material until fully grown, but then, over 2-3 days, exude large amounts of citrate and malate. Following this, the level of exudation drops back to almost zero (Dinkelaker et al. 1995). This action is supposed to be cyclical, and there are some speculations about the nature of such adaptation: first, it may prevent soil bacteria from completely metabolizing the exudates before it can enhance nutrient (P) uptake; second, since phosphate has a low diffusion coefficient, a rapid, concentrated event would be more effective than a long-term but lower level of exudation (for review see Skene 1998). It is clear that the ‘exudative burst’ is a consequence of an alteration in the whole plant metabolism, the details of which remain largely unknown.

Developmental Aspects of Cluster Formation

Cluster roots develop under low P or Fe conditions, and their formation is inhibited by mineral abundance.

The weird phenomenon is that for almost all proteoid-root-forming species, those that respond to P stress do not produce proteoid roots under Fe stress and vice versa, at least for the conditions under which they have been grown (Watt and Evans, 1999). The nature of this remains to be solved.

It has been showed that clusters tend to form in nutrient rich [artificial] layers of organic poor soils, such as sand dunes, suggesting than the mechanism of proteoid root initiation depends not only on internal control within the plant, but can also respond to a local concentrations of organic matter in the soil adjacent to the root.

For detailed description of cluster formation see Skene, 2000.

Further Cluster Roots Research

Root developmental biology

Cluster roots offer an ideal experimental system to study basic elements of root development and cell cycle control. They are predictable in terms of initiation (Skene 2000) so that it becomes possible to investigate the root initiation process from its beginning, which remained mostly vague until now.

Physiology

Physiological studies on cluster roots are focused on the aspects of exudation. The use of cluster roots provides a useful system in which to investigate control of exudation, because of its unique temporal and spatial context. Here the questions such as: what controls the release of exudates from plant roots and can such controls be manipulated to improve the ability of non-cluster-forming plants to acquire nutrients – might find their answers.

Ecophysiology

The studies of plant-bacterial interactions are of great interest in the context of proteoid roots. The rhizosphere of the cluster root undergoes dramatic chemical change under the exudative burst, and the microflora within it may also change during these events. Following the rhizosphere microbial population of proteoid roots in the course of exudative burst (before, during and after) may give a novel insights into microbial population response to changing resourses.

As it was mentioned above, species with cluster roots are often pioneers in primary and secondary succession, and important in soil stability. Their role in plant communities deserves further study. The activity of cluster roots in mobilizing phosphate and other nutrients may also impact other species, especially in terms of competition and nutrient cycling.

Evolutionary Studies

The evolution of the cluster roots and their phylogenetic distribution is interesting: the diversity of response, together with the increasing number of families in which proteoid roots have been observed, suggests that proteoid roots have evolved independently . Among the Proteaceae, all species so far examined, with the exception of one genus, Persoonia, produce cluster roots. This family is indigenous to South Africa, South America and Australia, suggesting an ancient Gondwanan distribution of cluster roots. All cluster-forming species except Proteaceae form nodule symbioses with nitrogen fixing bacteria, mycorrhizal or actinorhizal symbioses. The association of proteoid root formation with other nutrient uptake adaptations may be of common or separate evolutionary origin. With the molecular phylogeny techniques available today such questions can be solved.

Biotechnology

What are the opportunities of genetic manipulation in order to facilitate cluster root production in species within which they are not presently produced? Unlike mycorhizas and nitrogen fixing nodules, cluster roots aren’t dependent on other organisms for their development. The broad phylogenetic distribution and the conservative developmental and physiological characteristics across all speciessuggest that there may be only a limited number of changes needed to lead to the formation of these structures. Thus, the task of identifying the genes underlying cluster roots development and subsequent engineering of other species to produce them seems rather possible. And the ability to improve phosphate and iron uptake without the addition of fertilizers makes the cluster root a valuable biotechnological target.

To summarize , cluster roots are considered today , along with mycorrhizas, actinorhizas and nitrogen-fixing nodules, one of the major adaptations of plant roots to nutrient acquisition. This adaptation, which combines changes of both developmental and physiological patterns of plant roots deserves further study, which can lead to far reaching horizons in many fields of plant science.

Literature and References:

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

Nissen P. (1991) Uptake Mechanisms. In Plant Roots: The Hidden Half

Skene K.R.(1998), Cluster roots: some ecological considerations. Journal of Ecology 86, 1062-1066.

Skene K.R. (2000) Pattern Formation in Cluster Roots:Some Developmental and Evolutionary Considerations. Annals of Botany 85:901-908

Skene K.R. (2001) Cluster Roots: Model Experimental Tools for Key Biological Problems. Journal of Experimental Botany 52:479-485

Watt M. and Evans J.R. (1999), Proteoid Proteoid Roots. Physiology and Development. Plant Physiology, 121: 317–323, www.plantphysiol.org

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