Odontarrhena is a large genus of flowering plants in the family Brassicaceae (order Brassicales), closely related to and long confused with the genus Alyssum. The two genera were historically merged, but morphological study and molecular (DNA) analysis have firmly re-established Odontarrhena as a distinct, monophyletic clade within the tribe Alysseae.
Plants in the genus share a habit similar to Alyssum: low-growing herbs bearing small yellow flowers. The inflorescences are typically compound, subumbellate racemes. A key diagnostic character is the suborbicular silicle (pouch-like fruit) with only one seed per cell, and fruit valves that are at most slightly inflated — features that distinguish the genus from the broader Alyssum aggregate. There is a single ovule per loculus.
The genus contains approximately 80–96 accepted species (the exact count varies between Plants of the World Online and BrassiBase), and GBIF records 83 descendant taxa. It was first formally described by the botanist Carl Anton von Meyer in Flora Altaica (edited by Carl Friedrich von Ledebour), Volume 3, page 58, published in 1831.
One of the most scientifically significant characteristics of Odontarrhena is the prevalence of nickel hyperaccumulation. About 48 member species are known to accumulate nickel (Ni) from soil to very high concentrations in their tissues — in roots, stems, leaves, and flowers. This property was first documented in the Italian endemic Odontarrhena bertolonii (synonym Alyssum bertolonii) by Minguzzi and Vergnano in 1948. The hyperaccumulating species belong entirely to the Odontarrhena section, and their ability to concentrate heavy metals has attracted research interest in the field of phytoremediation — the use of plants to extract contaminants from soil.
Etymology
The genus name Odontarrhena is derived from two Greek words: odous (ὀδούς), meaning "tooth," and arrhen (ἄρρην), meaning "male." The name was applied by Carl Anton von Meyer when he described the genus in 1831.
Taxonomy Notes
Odontarrhena was historically merged into the broader genus Alyssum and treated as a section within it. Recent morphological and molecular evidence — including studies by Warwick et al. (2008), Cecchi et al. (2010), Rešetnik et al. (2013), and Li et al. (2015) — demonstrated that Alyssum and Odontarrhena represent separate, clearly monophyletic clades within tribe Alysseae of family Brassicaceae, warranting distinct generic status. The United States Department of Agriculture and Agricultural Research Service recognise the genus name but continue to list it as a synonym of Alyssum L. in some resources.
Ecology
Approximately 48 species of Odontarrhena are nickel (Ni) hyperaccumulators, meaning they can accumulate nickel from the soil to concentrations far exceeding those tolerated by most plants. The metal is sequestered throughout the plant — in roots, stems, leaves, and flowers. This adaptation is associated with soils derived from ultramafic (serpentine) rock, which are naturally high in heavy metals and low in nutrients. The hyperaccumulation trait has made Odontarrhena species subjects of research into phytoremediation, the technique of using plants to extract or stabilise soil contaminants.
