Phaseolus Species

The Phaseolae (family Leguminosae) are grown agronomically as a grain legume for both human and animal nutrition. Of the four species, Phaseolus acutifolius (tepary bean), P. coccineus (scarlet runner bean), P. lunatus (lima and butter bean), and P. vulgaris (known variously as common, field, green, snap, wax or French bean) are grown extensively. Related species, such as P. angularis and P. aureus have recently been reclassified as belonging to the genus Vigna and will not be considered further in this post. All of the Phaseolae originate from southern or central America and are grown for their dried seeds or fleshy pods for human consumption. After harvesting, the vines may also be used as fresh or silaged cattle feed. Of all the bean species, Phaseolus vulgaris is the most important agronomic crop, being a major dietary component in Latin America and Africa. P. vulgaris was first domesticated in 5000 b.c. in central America and was distributed to the rest of the world by the Spanish in the 16th century. The major world producer of P. vulgaris is the USA where after harvest with typical yields of 1.5 t/ha, it is either dried or canned as baked beans. Similarly, P. lunatus is also grown for its beans with its cultivation largely confined to the USA with yields as high as 1.7 t/ha. The beans of Phaseolus acutifolius are used in soups both in central America and Africa. In contrast, P. coccineus is widely grown in cool-temperate climates predominantly for its fleshy pods, though its seeds and tubers are also edible. Beans obtained from all of these Phaseolus species are an excellent source of protein with up to 22% of the dry weight consisting of protein. However, the protein content tends to be low in sulphur-containing amino acids.

Within each of the Phaseolae species a wide variety of phenotypes from twining climbers to free-standing bushes are available as various cultivars. In addition, the beans of each species occur in a range of sizes, colours and shapes, with different areas of the world showing distinct preferences for bean varieties. In particular, Phaseolus vulgaris has been extensively bred for varieties which grow as smaller, denser plants with reduced internode length, suppressed climbing ability, thicker stems and fewer leaves. Common bean grows best under temperate or subtropical conditions (16-21 °C), being sensitive to temperatures above 27 °C or below 10 °C. Plants grow best in well-drained soils (pH 5.2-6.8) with a high organic content and where water is not limiting. The Phaseolae fix their own nitrogen following nodulation with Rhizobium phaseoli which is reported to enhance growth and yield.

The modern cultivars are monocultured as either upright vines or freestanding bushes well suited to mechanical harvesting. The beans are either harvested fresh or from pods dried after cutting or pulling the crop and allowing to desiccate. In either case, harvesting of the beans is highly specialized as the seed can be easily damaged during collection and processing, resulting in reduced value.

The beans of all of the Phaseolae are a rich source of proteinaceous anti-metabolites, such as inhibitors of trypsin and chymotrypsin. Beans also contain biologically active secondary products such as saponins, cyanogenic glucosides and flavonoids, but little reference has been made to their alkaloid content. Preparations from the foliage and beans of Phaseolus lunatus have been used to treat Bright’s disease, diabetes, dropsy and eclampsia. However, the roots of this species are considered poisonous, resulting in nausea and tachycardia. The beans of Phaseolus vulgaris are reported to be effective in the treatment of a wide range of skin, neuromuscular, cardiovascular and abdominal disorders, In no instance has the active principle been isolated and exploited in conventional medicine.

Summary and Conclusions

Phaseolus species contain a diverse range of flavonoids but a limited number of terpene (saponins) and nitrogen-containing (cyanogenic glucosides) secondary products. The antifungal isoflavonoid phytoalexins which accumulate both in plants and cell cultures in response to treatment with biotic and abiotic elicitors have been particularly well studied and their routes of biosynthesis determined. The accumulation of the various phytoalexins, particularly kievitone and phase-ollin, is differentially regulated by factors as diverse as the types of elicitation, the tissue under study, ambient physiological conditions and temporal considerations. Despite being used in a variety of folk medicines, the efficacy of the secondary products of the Phaseolae have not been investigated in detail. The potential benefits resulting from the modification of secondary metabolism in these important grain legumes are more likely to be observed in the fields of human nutrition and crop protection. Clearly a major obstacle to the improvement of Phaseolae species through biotechnology is the lack of practical methods for generating transgenic plants. Though progress in plant regeneration has been made, the stable integration of foreign DNA into regenerants using Agrobacterium-mediated transfer has yet to be reported. Potential objectives for selective genetic engineering of the secondary metabolism of the Phaseolae include the inhibition of cynogenic glucoside formation in P. lunatus, enhancement and modification of hypocholesterolaemic saponin content in P. vulgaris, and the enhancement of the phytoalexin response in all species. Two particular aspects of phytoalexin synthesis and biological activity warrant further attention in bean. Firstly, the regulation of the biosynthetic pathways leads to 5-hydroxyisoflavones (kievitone) and 5-deoxyisoflavones (phaseollin). Several studies have now demonstrated that the accumulation of these two classes of phytoalexin are under separate control and, in view of the differences in the biological activities of the end products, there is potential for modifying the types of phytoalexin produced in response to infection. Secondly, the isoprenylation of the bean phytoalexins gives rise to products with enhanced biocidal activities. The isolation of the genes encoding the prenyltransferases catalyzing these reactions and their transfer into other legumes which do not make prenylated isoflavonoids (e.g. alfalfa, clover) could allow the formation of novel prenylated phytoalexins in the recipient species with enhanced toxicity toward a wide range of pathogens.

Selections from the book: Medicinal and Aromatic Plants VIII (1995).