Cluster roots, also known as proteoid roots, are

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

root In vascular plants, the roots are the plant organ, 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 bel ...

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 Plant litter (also leaf litter, tree litter, soil litter, litterfall, or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that has fallen to the ground. This detritus or dead organic material and its constituen ...

. 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; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

-deficient native soils of

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. 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 plant taxonomy and phytogeography, such as ''Die natürlichen Pflanzenfamilien'' (''The Natural Plant Families''), edited with K ...

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

Proteaceae The Proteaceae form a family (biology), family of flowering plants predominantly distributed in the Southern Hemisphere. The family comprises 83 genus, genera with about 1,660 known species. Australia and South Africa have the greatest concentr ...

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

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, finding proteoid roots in every genus except '' Persoonia''; 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 ...

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 Pacific ...

, Eleagnaceae,

Leguminosae Fabaceae () or Leguminosae,International Code of Nomen ...

Moraceae Moraceae is a family of flowering plants comprising about 48 genera and over 1100 species, and is commonly known as the mulberry or fig family. Most are widespread in tropical and subtropical regions, less so in temperate climates; however, their ...

and

Myricaceae Myricaceae is a small family of dicotyledonous shrubs and small trees in the order Fagales with its type genus ''Myrica'', the sweet gales. There are three genera in the family, although some botanists separate many species from Myrica into a fo ...

. Similar structures also occur in species of

Cyperaceae The Cyperaceae () are a family of graminoid (grass-like), monocotyledonous flowering plants known as wikt:sedge, sedges. The family (biology), family is large; botanists have species description, described some 5,500 known species in about 90 ...

and Restionaceae, but their

physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...

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 of flowering plants in the family Proteaceae. These Australian wildflowers and popular garden plants are easily recognised by their characteristic flower spikes, and woody fruiting "cones" and head ...

'' and '' Grevillea'', are valued by the

horticulture Horticulture (from ) is the art and science of growing fruits, vegetables, flowers, trees, shrubs and ornamental plants. Horticulture is commonly associated with the more professional and technical aspects of plant cultivation on a smaller and mo ...

and

floriculture Floriculture (from ) is the study of the efficient production of the plants that produce showy, colorful flowers and foliage for human enjoyment in human environments. It is a commercially successful branch of horticulture and agriculture found ...

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

fertilizer A fertilizer or fertiliser 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 liming materials or other non-nutrient soil amendments. Man ...

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

iron Iron is a chemical element; it has symbol Fe () 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, forming much of Earth's o ...

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, is a genus of plants in the legume family Fabaceae. The genus includes over 199 species, with centre of diversity, centres of diversity in North America, North and South A ...

'' and ''

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

''.

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 anatomy Plant roots