Plant 142

Phaseolus vulgaris L. (Faaceae)


Common bean

Phaseolus vulgaris is a highly variable bean grown for its unripe pods and ripe, dry seed, and used under many different names including flageolet, French, green, haricot, kidney, navy and pinto beans. Phaseolus is an American genus of some 60 species, which includes other internationally important foods such as Lima (Phaseolus lunatus) and runner (Phaseolus coccineus) beans.

Phaseolus vulgaris was domesticated in the Americas more than seven millennia ago, and was an essential component of traditional pre-Columbian agricultural systems. For example, Phaseolus vulgaris is one element of the 'Three Sisters' system (together with maize and pumpkins) used by native North Americans. Phaseolus vulgaris was domesticated twice, in geographically separate regions: in the Oaxaca Valley in Mesoamerica; and southern Bolivia and northern Argentina in the Andes. Overall, the genetic variability of beans derived from the Andean domestication is considerably lower than that found in the Mesoamerican domestication. The experiment had been run twice independently in the bean, making it an excellent model for understanding the processes involved in plant domestication.

Bean size has increased during domestication, and can be used as a marker for separating wild and domesticated beans in archaeological contexts. Bean pods also have another useful morphological marker of domestication; domesticated beans do not split open to release their seeds. In wild beans, the pods break open and twist as the fibre-rich, inner layer of the pod wall shrinks. In domesticated beans, the inner layer is reduced so the pods rarely split open. Modern 'stringless' beans, which are cultivated for their immature pods, lack the inner fibres altogether.

Hundreds of known cultivated forms create a kaleidoscope of colourful beans. In the United States, yellow beans (specifically a strain called 'Enola') became controversial in the early twenty-first century as a biopiracy cause célèbre. A plant breeder collected some yellow beans from Mexico and was then awarded a patent for the import and sale of yellow beans in the United States in 1999. Genetic data showed the 'Enola' bean was part of the Andean genepool, and that it was identical to a group of Mexican yellow beans. A long dispute about the rights and wrongs of exploiting genetic resources was not settled until the patent was finally rejected in 2009.

Despite their widespread use, beans, especially kidney beans, are well known as being toxic unless properly cooked. The toxic compound is a lectin called phytohaemagglutinin, which is reduced by prolonged boiling.

Further reading

Bitocchi E et al. 2013. Molecular analysis of the parallel domestication of the common bean (Phaseolus vulgaris) in Mesoamerica and the Andes. New Phytologist 197: 300-313.

Kwak M. and Gepts P. 2009. Structure of genetic diversity in the two major gene pools of common bean (Phaseolus vulgaris L., Fabaceae). Theoretical and Applied Genetics 118: 979-992.

Pallottini L et al. 2004. The genetic anatomy of a patented yellow bean. Crop Science 44: 968-977.

Stephen Harris