Distribution and Importance of Medicago Species
The genus Medicago (family Leguminosae) contains a number of species, which, following breeding efforts over 2000 years, have become the world’s major forage legumes. Due to their ability to fix nitrogen, the various species have been cultivated for use as green-manuring agents and as a forage crop of high nutritional value to pigs, cattle, sheep, and poultry. At various times, Medicago has also been used as a source of fiber for paper production and as a salad or vegetable garnish for human consumption, while the seeds have been extracted for edible oils and dye-stuffs. The most commonly encountered Medicago species are listed and briefly described in Table Common names, distribution, and uses of Medicago species. Within the individual species there is considerable genetic variability, which has ensured the distribution of the plants in a wide variety of environments. This large genetic pool has allowed the plant breeders to incorporate desirable traits into commercial cultivars. An example of this has been the development of cold-hardy alfalfa cultivars by crossing M. falcata with M. sativa ().
Table Common names, distribution, and uses of Medicago species
|Medicago species||Common name||Description (cold tolerance +/-)|
|arabica||Spotted/ southern burclover||Annual|
|falcata||Yellow flowered alfalfa, sickle medic||Perennial
( + )
|Annual / perennial
( + )
|orbicularis||Button clover/ large disk medic||Annual|
|scutellata||Snail medic||Annual / perennial
Although seven Medicago species are described in Table Common names, distribution, and uses of Medicago species, in terms of total area under cultivation, M. sativa is by far the predominant forage legume crop in the world today. Originally cultivated in Persia (Iran), M. sativa L., commonly referred to as alfalfa in the USA or lucerne in Europe, has been distributed through Europe, Asia, and the Americas by invading armies who planted the crop as forage for their horses. Although grown throughout Europe and Asia, it is in North America that alfalfa has become a major crop, initially in the southwestern states and later with the introduction of winter-hardy varieties into the northern states and Canada.
Propagation of Medicago sativa
Alfalfa is a cross-pollinating auto-tetraploid, and this gives rise to considerable genetic variability even within a given cultivar such that isogenic lines of alfalfa are not available. Alfalfa is either grown mixed with grasses or other legumes for pasture use or monocultured in rows or ridges on well-ploughed land. The seed is scarified or soaked in water prior to sowing, and may also be inoculated with competent strains of Rhizobium meliloti to promote nodulation. Monocultured “stands” of alfalfa are harvested by mowing after the first flowers have opened and the crop may then be cut again as growth is re-established throughout the season. In the USA, highest yields are obtained when the plants are grown under irrigation in hot southern areas. The harvested hay may either be baled, used in silage, or pelleted before use as animal fodder. For human consumption either the fresh foliage or, more usually, young sprouts are used.
A recent development in the agronomic use of the alfalfa has arisen from its potential for genetic transformation, which is unusual among the legumes. Both Agrobacterium tumefaciens- and A. rhizogenes-mediated transformations of alfalfa have been established, though subsequent out-breeding may be required to introduce the transgene into commercially useful varieties as the transformable cultivars are of limited use in the field. Programmes to develop improved disease-resistance traits and produce compounds of commercial importance are already underway in transgenic plants. As will be discussed later, this development opens up a range of exciting possibilities for the production of secondary products in alfalfa.
Medicinal Uses of Medicago Species
The Greek herbalist Dioscorides, writing in the first century A.D., described the use of M. sativa “which being dried is mixed for its sweet savor’s sake to salt sauces” and also discussed its merits as a salad vegetable but made no mention of its medical uses (Gunther 1934). Similarly, the Medicago genus in general does not seem to have been extensively used in folk medicine, though the antibacterial properties of aqueous extracts have been mentioned in several articles. In particular, extracts from the leaves of M. lupulina and M. sativa have been used as aids to digestion, as laxatives, and have also been recommended for the treatment of peptic ulcers and for urinary and bowel problems. Alfalfa leaf preparations have been variously described as being estrogenic, diuretic, and useful in the treatment of scurvy. Seed extracts have apparently been used in India in poultices for boils, and a water infusion is reported to assist in weight gain. In view of the estrogenic coumestans and anti-inflamatory flavonoids and saponins present in this genus, many of these claims may have a chemical basis.
Conclusions and New Developments
Medicago species comprise a commercially important genus which contains an interesting range of biologically active secondary metabolites. In the past a good deal of attention has been given to the flavonoid content of these plants, but it is also clear that the saponins and betaines warrant further study. This is particularly true of the in vitro studies, which have concentrated on the isoflavonoids. The potential for gene manipulation of Medicago presents novel possibilities for enhancing the production of existing metabolites and of engineering whole new pathways of secondary product biosynthesis. It has been suggested that alfalfa could be engineered to produce alternative stereoisomers of pterocarpan phytoalexins using late biosynthetic enzymes from other legumes. The positive isomer of medicarpin is more active as a fungicide than the negative isomeric form which is normally synthesized in alfalfa. It has been proposed to introduce the last two enzymes of pterocarpan biosynthesis from a species such as peanut which makes the positive isomers into alfalfa, thereby enhancing disease resistance. Alternative objectives of the genetic engineers may be to use antisense technology to inhibit coumestan biosynthesis and reduce the estrogenic potential of Medicago species, or to disable mainstream isoflavonoid biosynthesis and determine the effect of reducing phytoalexin accumulation on disease resistance. Alternative strategies might include the manipulation of genes encoding branch point enzymes such that medically useful flavonoids could be produced at much higher concentrations than those seen normally.