Cluster roots, also known as proteoid roots, are

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s that form clusters of closely spaced short lateral rootlets. They may form a two- to five-centimetre-thick mat just beneath the leaf litter. They enhance nutrient uptake, possibly by chemically modifying the soil environment to improve nutrient solubilisation. As a result, plants with proteoid roots can grow in soil that is very low in nutrients, such as the

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

-deficient native soils of

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. They were first described by

Adolf Engler Heinrich Gustav Adolf Engler (25 March 1844 – 10 October 1930) was a German botanist. He is notable for his work on alpha taxonomy, plant taxonomy and phytogeography, such as ''Die natürlichen Pflanzenfamilien'' (''The Natural Plant Families'' ...

in 1894, after he discovered them on plants of the family

Proteaceae The Proteaceae form a family of flowering plants predominantly distributed in the Southern Hemisphere. The family comprises 83 genera with about 1,660 known species. Together with the Platanaceae and Nelumbonaceae, they make up the order Pro ...

growing in Leipzig Botanic Gardens. In 1960, Helen Purnell examined 44 species from ten Proteaceae

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

, finding proteoid roots in every genus except ''

Persoonia ''Persoonia'', commonly known as geebungs or snottygobbles, is a genus of about one hundred species of flowering plants in the family Proteaceae. Plants in the genus ''Persoonia'' are shrubs or small trees usually with smooth bark, simple leav ...

''; she then coined the name "proteoid roots" in reference to the plant family in which it was known to occur. Proteoid roots are now known to occur in 27 different Proteaceae genera, plus around 30 species from other families, including

Betulaceae Betulaceae, the birch family, includes six genera of deciduous nut-bearing trees and shrubs, including the birches, alders, hazels, hornbeams, hazel-hornbeam, and hop-hornbeams numbering a total of 167 species. They are mostly natives of the t ...

Casuarinaceae The Casuarinaceae are a family of dicotyledonous flowering plants placed in the order Fagales, consisting of four genera and 91 species of trees and shrubs native to eastern Africa, Australia, Southeast Asia, Malesia, Papuasia, and the Pacifi ...

, Eleagnaceae,

Leguminosae The Fabaceae or Leguminosae,International Code of Nomenc ...

Moraceae The Moraceae — often called the mulberry family or fig family — are a family of flowering plants comprising about 38 genera and over 1100 species. Most are widespread in tropical and subtropical regions, less so in temperate climates; however ...

and

Myricaceae The Myricaceae are a small family of dicotyledonous shrubs and small trees in the order Fagales. There are three genera in the family, although some botanists separate many species from ''Myrica'' into a fourth genus ''Morella''. About 55 specie ...

. Similar structures also occur in species of

Cyperaceae The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" genus ''Carex'' w ...

and

Restionaceae The Restionaceae, also called restiads and restios, are a family (biology), family of flowering plants native to the Southern Hemisphere; they vary from a few centimeters to 3 meters in height. Following the APG IV system, APG IV (2016): the fami ...

, but their

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

is yet to be studied. Two forms are recognised: ''simple'' cluster roots form rootlets only along a root; ''compound'' cluster roots form the primary rootlets, and also form secondary rootlets on the primary rootlets. Some Proteaceae, such as ''

Banksia ''Banksia'' is a genus of around 170 species in the plant family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes, and fruiting "cones" and heads. ''Banksias'' range i ...

'' and ''

Grevillea ''Grevillea'', commonly known as spider flowers, is a genus of about 360 species of evergreen flowering plants in the family Proteaceae. Plants in the genus ''Grevillea'' are shrubs, rarely trees, with the leaves arranged alternately along the b ...

'', are valued by the

horticulture Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...

and

floriculture Floriculture, or flower farming, is a branch of horticulture concerned with the cultivation of flowering and ornamental plants for gardens and for floristry, comprising the floral industry. The development of new varieties by plant breeding is ...

industries. In cultivation, only slow-release low-phosphorus

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

s should be used, as higher levels cause phosphorus toxicity and sometimes

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

deficiency, leading to plant death. Crop management should minimise root disturbance, and weed control should be via slashing or contact herbicides. Many plants with proteoid roots have economic value. Cultivated crops with proteoid roots include ''

Lupinus ''Lupinus'', commonly known as lupin, lupine, or regionally bluebonnet etc., is a genus of plants in the legume family Fabaceae. The genus includes over 199 species, with centers of diversity in North and South America. Smaller centers occur ...

'' and ''

Macadamia ''Macadamia'' is a genus of four species of trees in the flowering plant family Proteaceae. They are indigenous to Australia, native to northeastern New South Wales and central and southeastern Queensland specifically. Two species of the genus ...

''.

References

{{reflist 5. Lambers, H. & Poot, P. (eds) 2003. Structure and Functioning of Cluster Roots and Plant Responses to Phosphate Deficiency. Kluwer Academic Publishers, Dordrecht. 6. Shane, M.W. & Lambers, H. 2005. Cluster roots: A curiosity in context. Plant Soil 274: 99-123. https://doi.org/10.1007%2Fs11104-004-2725-7 7. Lambers, H., Shane, M.W., Cramer, M.D., Pearse, S.J., & Veneklaas, E.J. 2006. Root structure and functioning for efficient acquisition of phosphorus: matching morphological and physiological traits. Ann. Bot. 98: 693-713. http://aob.oxfordjournals.org/cgi/content/abstract/98/4/693 Plant nutrition Plant roots