Fabaceae
Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and agriculturally important family of flowering plants. It includes trees, shrubs, and perennial or annual herbaceous plants, which are easily recognized by their fruit and their compound, stipulate leaves. The family is widely distributed, and is the third-largest land plant family in number of species, behind only the Orchidaceae and Asteraceae, with about 765 genera and nearly 20,000 known species.
The five largest genera of the family are Astragalus, Acacia, Indigofera, Crotalaria, and Mimosa, which constitute about a quarter of all legume species. The known legume species amount to about 7% of flowering plant species. Fabaceae is the most common family found in tropical rainforests and dry forests of the Americas and Africa.
Recent molecular and morphological evidence supports the fact that the Fabaceae is a single monophyletic family. This conclusion has been supported not only by the degree of interrelation shown by different groups within the family compared with that found among the Leguminosae and their closest relations, but also by all the recent phylogenetic studies based on DNA sequences. These studies confirm that the Fabaceae are a monophyletic group that is closely related to the families Polygalaceae, Surianaceae and Quillajaceae and that they belong to the order Fabales.
Along with the cereals, some fruits and tropical roots, a number of Leguminosae have been a staple human food for millennia and their use is closely related to human evolution.
The family Fabaceae includes a number of plants that are common in agriculture including Glycine max, Phaseolus, Pisum sativum, Cicer arietinum, Vicia faba, Medicago sativa, Arachis hypogaea, Ceratonia siliqua, Tamarindus indica, Trigonella foenum-graecum, and Glycyrrhiza glabra. A number of species are also weedy pests in different parts of the world, including Cytisus scoparius, Robinia pseudoacacia '', Ulex europaeus, Pueraria montana, and a number of Lupinus'' species.
Etymology
The name 'Fabaceae' comes from the defunct genus Faba, now included in Vicia. The term "faba" comes from Latin, and appears to simply mean "bean". Leguminosae is an older name still considered valid, and refers to the fruit of these plants, which are called legumes.Description
Fabaceae range in habit from giant trees to small annual herbs, with the majority being herbaceous perennials. Plants have indeterminate inflorescences, which are sometimes reduced to a single flower. The flowers have a short hypanthium and a single carpel with a short gynophore, and after fertilization produce fruits that are legumes.Growth habit
The Fabaceae have a wide variety of growth forms, including trees, shrubs, herbaceous plants, and even vines or lianas. The herbaceous plants can be annuals, biennials, or perennials, without basal or terminal leaf aggregations. Many Legumes have tendrils. They are upright plants, epiphytes, or vines. The latter support themselves by means of shoots that twist around a support or through cauline or foliar tendrils. Plants can be heliophytes, mesophytes, or xerophytes.Leaves
The leaves are usually alternate and compound. Most often they are even- or odd-pinnately compound, often trifoliate and rarely palmately compound, in the Mimosoideae and the Caesalpinioideae commonly bipinnate. They always have stipules, which can be leaf-like, thornlike or be rather inconspicuous. Leaf margins are entire or, occasionally, serrate. Both the leaves and the leaflets often have wrinkled pulvini to permit nastic movements. In some species, leaflets have evolved into tendrils.Many species have leaves with structures that attract ants which protect the plant from herbivore insects. Extrafloral nectaries are common among the Mimosoideae and the Caesalpinioideae, and are also found in some Faboideae. In some Acacia, the modified hollow stipules are inhabited by ants and are known as domatia.
Roots
Many Fabaceae host bacteria in their roots within structures called root nodules. These bacteria, known as rhizobia, have the ability to take nitrogen gas out of the air and convert it to a form of nitrogen that is usable to the host plant. This process is called nitrogen fixation. The legume, acting as a host, and rhizobia, acting as a provider of usable nitrate, form a symbiotic relationship. Members of the Phaseoleae genus Apios form tubers, which can be edible.Flowers
The flowers often have five generally fused sepals and five free petals. They are generally hermaphroditic and have a short hypanthium, usually cup-shaped. There are normally ten stamens and one elongated superior ovary, with a curved style. They are usually arranged in indeterminate inflorescences. Fabaceae are typically entomophilous plants, and the flowers are usually showy to attract pollinators.In the Caesalpinioideae, the flowers are often zygomorphic, as in Cercis, or nearly symmetrical with five equal petals, as in Bauhinia. The upper petal is the innermost one, unlike in the Faboideae. Some species, like some in the genus Senna, have asymmetric flowers, with one of the lower petals larger than the opposing one, and the style bent to one side. The calyx, corolla, or stamens can be showy in this group.
In the Mimosoideae, the flowers are actinomorphic and arranged in globose inflorescences. The petals are small and the stamens, which can be more than just 10, have long, coloured filaments, which are the showiest part of the flower. All of the flowers in an inflorescence open at once.
In the Faboideae, the flowers are zygomorphic, and have a specialized structure. The upper petal, called the banner or standard, is large and envelops the rest of the petals in bud, often reflexing when the flower blooms. The two adjacent petals, the wings, surround the two bottom petals. The two bottom petals are fused together at the apex, forming a boat-like structure called the keel. The stamens are always ten in number, and their filaments can be fused in various configurations, often in a group of nine stamens plus one separate stamen. Various genes in the CYCLOIDEA /DICHOTOMA family are expressed in the upper petal; in some species, such as Cadia, these genes are expressed throughout the flower, producing a radially symmetrical flower.
Fruit
The ovary most typically develops into a legume. A legume is a simple dry fruit that usually dehisces on two sides. A common name for this type of fruit is a "pod", although that can also be applied to a few other fruit types. A few species have evolved samarae, loments, follicles, indehiscent legumes, achenes, drupes, and berries from the basic legume fruit.Physiology and biochemistry
The Fabaceae are rarely cyanogenic. Where they are, the cyanogenic compounds are derived from tyrosine, phenylalanine or leucine. They frequently contain alkaloids. Proanthocyanidins can be present either as cyanidin or delphinidine or both at the same time. Flavonoids such as kaempferol, quercitin and myricetin are often present. Ellagic acid has never been found in any of the genera or species analysed. Sugars are transported within the plants in the form of sucrose. C3 photosynthesis has been found in a wide variety of genera. The family has also evolved a unique chemistry. Many legumes contain toxic and indigestible substances, antinutrients, which may be removed through various processing methods. Pterocarpans are a class of molecules found only in the Fabaceae. Forisome proteins are found in the sieve tubes of Fabaceae; uniquely they are not dependent on ADT.Evolution, phylogeny and taxonomy
Evolution
The order Fabales contains around 7.3% of eudicot species and the greatest part of this diversity is contained in just one of the four families that the order contains: Fabaceae. This clade also includes the families Polygalaceae, Surianaceae and Quillajaceae and its origins date back 94 to 89 million years, although it started its diversification 79 to 74 million years ago. The Fabaceae diversified during the Paleogene to become a ubiquitous part of the modern earth's biota, along with many other families belonging to the flowering plants.The Fabaceae have an abundant and diverse fossil record, especially for the Tertiary period. Fossils of flowers, fruit, leaves, wood and pollen from this period have been found in numerous locations. The earliest fossils that can be definitively assigned to the Fabaceae appeared in the early Palaeocene. Representatives of the 3 sub-families traditionally recognised as being members of the Fabaceae – Cesalpinioideae, Papilionoideae and Mimosoideaeas well as members of the large clades within these sub-familiessuch as the genistoideshave been found in periods later, starting between 55 and 50 million years ago. In fact, a wide variety of taxa representing the main lineages in the Fabaceae have been found in the fossil record dating from the middle to the late Eocene, suggesting that the majority of the modern Fabaceae groups were already present and that a broad diversification occurred during this period. Therefore, the Fabaceae started their diversification approximately 60 million years ago and the most important clades separated 50 million years ago. The age of the main Cesalpinioideae clades have been estimated as between 56 and 34 million years and the basal group of the Mimosoideae as 44 ± 2.6 million years. The division between Mimosoideae and Faboideae is dated as occurring between 59 and 34 million years ago and the basal group of the Faboideae as 58.6 ± 0.2 million years ago. It has been possible to date the divergence of some of the groups within the Faboideae, even though diversification within each genus was relatively recent. For instance, Astragalus separated from the Oxytropis 16 to 12 million years ago. In addition, the separation of the aneuploid species of Neoastragalus started 4 million years ago. Inga, another genus of the Papilionoideae with approximately 350 species, seems to have diverged in the last 2 million years.
It has been suggested, based on fossil and phylogenetic evidence, that legumes originally evolved in arid and/or semi-arid regions along the Tethys seaway during the Palaeogene Period. However, others contend that Africa cannot yet be ruled out as the origin of the family.
The current hypothesis about the evolution of the genes needed for nodulation is that they were recruited from other pathways after a polyploidy event. Several different pathways have been implicated as donating duplicated genes to the pathways need for nodulation. The main donors to the pathway were the genes associated with the arbuscular mycorrhiza symbiosis genes, the pollen tube formation genes and the haemoglobin genes. One of the main genes shown to be shared between the arbuscular mycorrhiza pathway and the nodulation pathway is SYMRK and it is involved in the plant-bacterial recognition. The pollen tube growth is similar to the infection thread development in that infection threads grow in a polar manner that is similar to a pollen tubes polar growth towards the ovules. Both pathways include the same type of enzymes, pectin-degrading cell wall enzymes. The enzymes needed to reduce nitrogen, nitrogenases, require a substantial input of ATP but at the same time are sensitive to free oxygen. To meet the requirements of this paradoxical situation, the plants express a type of haemoglobin called leghaemoglobin that is believed to be recruited after a duplication event. These three genetic pathways are believed to be part of a gene duplication event then recruited to work in nodulation.