The English name of Douke is Fabaceae, or Leguminosae. The name “Fabaceae” comes from the already non-existent genus Faba, which is now included in Vicia. The term “faba” comes from Latin and seems to refer only to “bean”. Leguminosae is a more remote but still valid name. The fruits of these plants called legumes.

Leguminosae are the third largest family of land plants in the number of species, after the Orchidaceae and Asteraceae, with about 770 genera and about 19 500 known species, which account for about 7% of all flowering plant species (Magallón and Sanderson, 2001; Judd et al., 2010; LPWG, 2013a; Christenhusz and Byng, 2016). The five largest genera are Astragalus (more than 3 000 species), Acacia (more than 1 000 species), Indigofera (about 700 species), Crotalaria (about 700 species), and Mimosa (about 400 species), representing approximately a quarter of all legume species.

Economically, the importance of Leguminosae is second only to Gramineae plants. For example, it is estimated that the world legume export more than doubled from 1990 to 2012, from 6.6×10⁶ t to 13.4×10⁶ t, and the value of legume export was estimated to be $9.5 billion in 2012 (FAO: http://www.fao.org/pulses-2016/en/). The General Assembly of the United Nations designated 2016 as the international legume year to increase awareness of the nutritional value of legumes, food security, importance of sustainable agriculture, and reduction of biodiversity loss and climate change. Legumes are important food crops that are sources of high nutrient proteins and micronutrients that can greatly benefit health and livelihood, especially in developing countries (Yahara et al., 2013). Since the beginning of agriculture, Leguminosae have been domesticated with Gramineae in different parts of the world and have played a key role in their early agricultural development (Gepts et al., 2005; Hancock, 2012). Many plants in Leguminosae have been a staple food for humans for thousands of years, and their use is closely associated with human evolution (Dimitri, 1987). In today's world, plants in Leguminosae have an important role in the manufacturing of wood, oil, resin, varnish, paint, dyes, pharmaceutical, etc., also an indispensable part of garden greening. Leguminosae plants are also uniquely important as feed and green manure in temperate and tropical regions.

Leguminosae plants are widely distributed and are important ecological components in almost all biological communities worldwide, even distributed in the most extreme habitats (Schrire et al., 2005). They constitute an important element of species diversity and abundance in tropical forests of Africa, South America, and Asia (Banda et al., 2016), and throughout the tropics they dominate dry forests and tropical savannahs, the same situation occurs in Mediterranean, desert, and temperate areas and high latitude and high altitude areas. Leguminosae plants can be large trees, perennial or annual herbaceous plants, annual or perennial curly climbing plants, shrubs, woody vine plants, and less frequently aquatic plants. The symmetry of Leguminosae flowers span the full range from radially symmetric to bilaterally symmetric and asymmetric, and these flowers are adapted to a wide range of pollinators such as insects, birds, and bats. The ability of most legumes to symbiotically fix atmospheric nitrogen with soil rhizobia is perhaps the best-known ecological feature of this family, however, not all Leguminosae are associated with nitrogen fixing bacteria. In summary, this family is exceptionally diverse in morphology, physiology, and ecology and represents one of the most striking examples of plant evolutionary diversification. All these characteristics have led biologists of Leguminosae to maintain great interest in the biology, family diversity and evolution, evolution of functional characteristics, and ecology and biogeography of families (Stirton and Zarucchi, 1989; Lavin et al., 2004; Champagne et al., 2007; Simon et al., 2009; Doyle, 2011; Simon and Pennington, 2012; Koenen et al., 2013; Moncrieff et al., 2014; Dugas et al., 2015).

1 Evolution of Leguminosae

Fabales comprises about 7.3% of dicotyledonous species, and much of this diversity is contained within only one of the four families included in this order: Fabaceae. This clade also includes the Polygalaceae, Surianaceae, and Quillajaceae. Fabales origin dates back to 94~89 million years ago, whereas the beginning of diversification occurred about 79~74 million years ago (Angiosperm Phylogeny Website. Version 7, May 2006. http://www.mobot.org/MOBOT/Research/APweb/orders/fabalesweb.htm#Fabaceae managed by Stevens, P. F). In fact, Leguminosae diversified early in the tertiary, with other families belonging to flowering plants becoming a ubiquitous part of modern terrestrial biota (Herendeen et al., 1992; Lewis et al., 2005).

Leguminosae plants have a rich and diverse fossil record, especially in the tertiary, during which time flower, fruit, leaf, wood, and pollen fossils were found in many locations (Crepet and Taylor, 1985; Crepet and Taylor, 1986; Herendeen, 2001). Whereas the earliest fossils determined to belong to the family Leguminosae appeared in the late Paleocene (approximately 56 million years ago) (Herendeen and Wing, 2001; Wing et al., 2004). Cesalpinioideae, Papilionoideae and Mimosoideae are three subfamilies traditionally considered to represent members of the Leguminosae family, as well as members of a large number of evolutionary clades within these subfamilies--for example, genistoides, found to have arisen in a subsequent period, starting 55~50 million years ago (Herendeen et al., 1992). In fact, from the middle Eocene to the late fossil record, various taxa representing the major lineages of the Leguminosae have been found, indicating that most of the taxa of the modern Leguminosae are already present and that extensive diversity emerged during this period (Herendeen et al., 1992).

Thus, the Leguminosae started to diversify about 60 million years ago, while the most important clades diverged about 50 million years ago, with the major Cesalpinioideae clade diverging between 56 and 34 million years ago, and the Mimosoideae basal group diverging (44±2.6) million years ago (Lavin et al., 2005; Bruneau et al., 2008). The division between the Mimosoideae and Faboideae dates from 59 million to 34 million years ago with the Faboideae basal group (5 860±20) million years ago (Wikström et al., 2001). Despite the relatively late stage of species diversification within each genus, it remains possible to determine the age of divergence of some taxa within the Faboideae. Astragalus, for example, was isolated from Oxytropis approximately 16 million to 12 million years ago, and in addition, the isolation of aneuploid species of Neoastragalus began 4 million years ago. Inga is another genus of the Papilionoideae with approximately 350 species that appears to have diverged over the past 2 million years (Wojciechowski et al., 1993; Wojciechowski, 2003; Wojciechowski, 2005; Wojciechowski et al., 2006).

Based on fossil and phylogenetic evidence, it has been well documented that plants in Leguminosae originally evolved during the Paleogene period along the arid or semi-arid regions of the Tethys seaway (Schrire et al., 2005). However, others argue that it cannot yet be excluded that Africa (or even the Americas) is the origin of this population (Doyle and Luckow, 2003; Pan et al., 2010).

The current hypothesis regarding the evolution of genes required for root nodules is that leguminous plants acquire them through other pathways after polyploid events (Yokota and Hayashi, 2011). There are several different pathways that are considered to be the way to supply the replication genes needed by root nodules. The main donors of these pathways are arbuscular mycorrhiza symbiotic genes, pollen tube forming genes and hemoglobin genes, while SYMRK gene involved in plant-bacteria recognition is one of the main symbiotic genes of arbuscular mycorrhiza pathway and nodulation pathway (Markmann et al., 2008). The growth of pollen tubes is similar to that of invasive lines, which exhibit polar growth, similar to the polar growth of pollen tubes towards the ovule. Both pathways require the same type of pectin degrading cell wall enzyme (Rodríguez-Llorente et al., 2004). Enzymes in root nodules require a large amount of ATP during nitrogen fixation, but are also very sensitive to free oxygen. Therefore, to address this contradiction, these plants express a hemoglobin called leghaemoglobin, which is believed to have been obtained after polyploid events (Downie, 2005).

2 Construction of the Leguminosae Classification System

The phylogeny development of leguminous plants has always been the research object of research groups around the world. These groups used morphology, DNA data (Chloroplast intron trnL, chloroplast genes rbcL and matK, or ribosomal spacer ITS), and branch chain analysis to investigate the relationship between the different pedigrees of the family (Lavin et al., 1990; Käss and Wink, 1996; Sanderson and Wojciechowski, 1996; Käss and Wink, 1997; Bruneau et al., 2008; Cardoso et al., 2013; Félicien et al., 2018).

Early scholars classified Leguminosae mainly on the basis of their morphology, and this traditional classification method continues to be used for a long time, typically the 3-subfamily classification system. But with the continuous development of science and technology, the rise of molecular systematics, and the disadvantages of the traditional classification system gradually manifest, at which point scholars realize that the classification system of the 3-subfamilies is unjustified, and a new classification system is urgently needed to replace the original system. The Legume Phylogeny Working Group (LPWG), established in 2010, is aimed at the study of novel classification systems, and in 2017, a 6-subfamily classification system was officially proposed, which has been used until now.

3 The 3-subfamily Classification System for Leguminosae

In contrast to some other large angiosperm families, such as Gramineae (Grass Phylogeny Working Group, 2001; Grass Phylogeny Working Group, 2012) and Asteraceae (Panero and Funk, 2002), the subfamily of Leguminosae has been a widely used central level. It has traditionally been used to divide into three subfamilies, the Mimosoideae subfamily, the Caesalpinioideae subfamily, and the Papilionoideae subfamily, based on flower morphology (Hutchinson, 1964; Cronquist, 1981). The first nomenclature of these 3 subfamilies was in 1825, when Candolle subdivided the Leguminosae into four suborders (= subfamilies), with the exception of the fourth “subfamily”, Swartzieae, in which the remaining 3 “subfamilies” are subfamilies today (Candolle, 1825). Bentham (1865) elaborated on this 3-subfamily classification system, which became the basis for the classification of the Leguminosae for the subsequent 140 years (Taubert, 1891; Lewis et al., 2005).

Hutchinson raised the 3 subfamilies to the family level (Hutchinson, 1926; Hutchinson, 1964), but in Volume I of Advances in Legume Systematics, these three groups were considered subfamilies (Polhill and Raven, 1981). Regardless of rank, these 3 taxa have been used since the 19th century as standards for identifying and classifying plants in Leguminosae, are widely taught in botany, phytography, and taxonomic courses, and have been used by agronomists, horticulturalists, and ecologists around the world.

4 Analysis of the Problems Associated with the Classification System of the 3 Subfamilies

The classification system of the 3 subfamilies is based essentially on a set of striking floral features, especially the petal arrangement pattern (Caesalpinioideae in rising imbricate; Mimosoideae in valvate; Papilionoideae in descending imbricate) and the symmetry of the flowers (Caesarpinioideae as uncertain symmetry, Figure 1; Figure 2; Figure 3; Mimosoidea as radiative symmetry, Figure 4; Papilionoideae are bilaterally symmetrical, Figure 5; Figure 6; Figure 7). Although some characteristics of these flowers may be useful in defining the Papilionoideae and Mimosoideae, they have great limitations in defining the traditional Caesarpinioideae (Tucker, 2003; Bruneau et al., 2014). Moreover, even for the Papilionoideae and Mimosoideae, most of the characteristics of these flowers are now considered homologous (Pennington et al., 2000). For example, a single species or clade characterized by radiatively symmetric flowers evolved independently many times on the basis of the Papilionoideae (Pennington et al., 2000; Cardoso et al., 2012b; Cardoso et al., 2013a; Ramos et al., 2016) (Figure 5; Figure 6; Figure7). Similarly, although radial symmetry is the most striking feature of the Mimosoideae, closely related lineages distributed across the MCC (Mimosoideae-Caesalpinieae-Cassieae) clade also have flowers of radial symmetry (Figure 3).

Figure 1 A-F, Cercidoideae; G, Duparquetioideae; H-L, Dialioideae. A, Cercis siliquastrum; B, Bauhinia galpinii; C, Bauhinia divaricata; D, Piliostigma thonningii; E, Griffonia physocarpa; F, Schnella cupreonitens; G, Duparquetia orchidacea; H, Zenia insignis; I, Apuleia leiocarpa; J, Poeppigia procera; K, Distemonanthus benthamianus; L, Kalappia celebica (Adopted from LPWG, 2017)

Figure 2 Detarioideae. A, Goniorrhachis marginata; B, Hymenaea stigonocarpa; C, Daniellia ogea; D, Peltogyne chrysopis; E, Brodriguesia santosii; F, Brownea longipedicellata; G, Amherstia nobilis; H, Brachycylix vageleri; I, Cryptosepalum tetraphyllum; J, Paramacrolobium coeruleum; K, Gilbertiodendron quinquejugum; L, Aphanocalyx pteridophyllus (Adopted from LPWG, 2017)

Figure 3 Caesalpinioideae I. A, Gleditsia amorphoides; B, Pterogyne nitens; C, Batesia floribunda; D, Moldenhawera blanchetiana; E, Cassia fistula; F, Tachigali rugosa; G, Arapatiella psilophylla; H, Caesalpinia cassioides; I, Arquita grandiflora; J, Delonix floribunda; K, Campsiandra comosa; L, Dimorphandra pennigera (Adopted from LPWG, 2017)

Figure 4 Caesalpinioideae II. A, Chidlowia sanguinea; B, Entada chrysostachys; C, Gagnebina commersoniana; D, Lemurodendron capuronii; E, Neptunia plena; F, Mimosa benthamii; G, Acacia dealbata; H, Senegalia sakalava; I, Inga calantha; J, Inga grazielae; K, Macrosamanea amplissima; L, Albizia grandibracteata (Adopted from LPWG, 2017)

Figure 5 Papilionoideae I. A, Castanospermum australe; B, Petaladenium urceoliferum; C, Pterodon abruptus; D, Swartzia acutifolia; E, Trischidium molle; F, Exostyles venusta; G, Harleyodendron unifoliolatum; H, Haplormosia monophylla; I, Ormosia lewisii; J, Harpalyce lanata; K, Leptolobium brachystachyum; L, Camoensia brevicalyx (Adopted from LPWG, 2017)

Figure 6 Papilionoideae II. A, Uleanthus erythrinoides; B, Cadia purpurea; C, Sophora cf. microphylla; D, Virgilia divaricata; E, Cyclopia pubescens; F, Lupinus weberbaueri; G, Dalea botterii; H, Errazurizia megacarpa; I, Zornia reticulata; J, Poiretia tetraphylla; K, Pterocarpus amazonum; L, Baphia leptobotrys (Adopted from LPWG, 2017)

Figure 7 Papilionoideae III. A, Chorizema glycinifolium; B, Bossiaea walkeri; C, Mucuna gigantea; D, Chadsia longidentata; E, Canavalia brasiliensis; F, Erythrina velutina; G, Gliricidia robusta; H, Poissonia weberbaueri; I, Anthyllis montana; J, Astragalus uniflorus; K, Trifolium rubens; L, Pisum sativum subsp. Biflorum (Adopted from LPWG, 2017)

Leguminosae plants as a monophyletic group were strongly supported in all molecular phylogenetic analyses regardless of taxonomic unit or gene sampling (LPWG, 2013a). Despite uncertainty regarding the close species of the Leguminosae family (Dickison, 1981; APG III, 2009; Bello et al., 2009), its monophyly and uniqueness have never been questioned morphologically since the family was established (Lewis et al., 2005; Bello et al., 2012). With few exceptions, the most striking feature of this family is that the gynophore usually consists of a single carpel, the ovary is epistatic, 1-chamber, often sessile or absent at its base, with lateral membranous loci along abdominal sutures, the ovules 2 to several, hanging or rising, arranged in 2 rows of reciprocal rows (Lewis et al., 2005). However, systematists of Leguminosae have long been aware of the differences between current subfamily classifications and emerging phylogenetic outcomes (Irwin, 1981; Käss and Wink, 1996; Doyle et al., 1997). The most famous is the paraphyletic group Caesalpinioideae, many of which are not monophyletic with subfamilies, implying that the phylogenetic structure of these clades is not directly reflected in current classifications (Lewis et al., 2005).

Therefore, it is also becoming imperative that the legume taxonomic community needs a new classification system for Leguminosae to replace the traditional 3-subfamily classification system.

5 The 6-subfamily Classification System of Leguminosae

Because of the relative enthusiasm of the legume taxonomic community for the branch taxonomic system, in 2010, the Legume Phylogeny Working Group (LPWG) was established, an international group dedicated to advancing legume phylogenetic research, with the primary task of presenting a compendium and program arrangements for legume phylogenetic research and the core questions to be addressed. The LPWG convened the 6th International Conference on Leguminosae in Johannesburg, South Africa, in 2013, with the theme “New Classification System for Leguminosae”, whereupon a re-revision of the legume classification system was officially put to the forefront (LPWG, 2013).

Until 2017, LPWG constructed a Leguminosae phylogenetic tree of the most thoroughly sampled (about 91% of the genera and 20% of the species) to date based on chloroplast matK sequence data, and combined with morphological evidence, proposed a new classification system of six subfamilies, in which only the traditional Papilionoideae (Figure 5; Figure 6; Figure 7) were retained. The traditional Mimosoidea no longer exists but becomes a branch of the new Caesalpinioideae (Figure 3; Figure 4), the mimosoid clade; In addition to the taxa retained in the new Caesarapinioideae, the remaining four clades of the original Caesarapinioideae were newly assigned to four subfamilies: namely, Cercidoideae (Figure 1A~F), Detarioideae Burmeist (Figure 2), Dialioideae (Figure 1H~L), and Duparquetioideae (Figure 1G) (LPWG, 2017).

Needless to say that the new classification system will have an important impact on the study of legume taxonomy, genomics, developmental biology and evolutionary biology, and as the research continues, the classification system of Leguminosae will become more refined.

6 Prospect

The importance of Leguminosae is self-evident, both in terms of economic value, as well as academic research value. Moreover, Leguminosae plants almost cover the form, growth, development and other dimensions of most plants, such as the flower form, distribution range, etc., which is one of the best species in plant diversity research. Studies about the legume classification system, which is transitioning from 3 to 6 subfamilies, have also been constantly enquired and fumbled, becoming more and more reasonable, but there are still many controversies and unanswered questions. However, science is constantly advancing, and the study of legume taxonomy will be deepened and more rationalized, which is not only of great significance to the legumes themselves, but also of great reference value to the study of plant evolutionary diversity.

Authors’ contributions

LJH was responsible for the literature collection and writing of the paper; ZJ was the project leader and was responsible for revising and finalizing the paper; YSY was responsible for paper translation; WZR and LCC were responsible for revision and proofreading. All authors read and approved the final manuscript.

Acknowledgements

This study was supported by Funding for Cuixi Innovation Research & Development Project of Cuixi Academy of Biotechnology, Zhuji.

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