Legume flowers bear fruit

Abstract

After the grasses, the legume family (which includes peas, beans, and soya) is the plant group of greatest importance to mankind. The legume family is a source of products and services from tropical timber to atmospheric nitrogen fixation. As the botanist E. J. H. Corner (1) put it, “From arctic circle to tropics, desert to pergola, bacteria to plough, field to mouth, and legend to science, Leguminosae invest our lives.” The family is huge, with ≈20,000 species. Legume diversity is so great (and much of it hidden away in relatively unexplored tropical rain forest) that it has been difficult to gain an overview. Recently, a landmark study has covered the whole family (2). The result, a bird's-eye view of the legumes, is a chronicle of how the legumes have played out 60 million years of dazzling “evolutionary jazz,” riffing and improvising on the basic themes provided by the characteristic leguminous leaf, flower, and fruit. Despite the family's vastness, it is not (by angiosperm standards) particularly old. Current estimates suggest that the family underwent a diversity explosion ≈50–60 million years ago, during which most of the major legume clades arose (3, 4). One of the reasons for the early and continued diversification of legumes may be coevolution with pollinating bees (Hymenoptera, superfamily Apoidea), which are by far the most common pollinators of the legumes and appear to be extremely effective at the job. Bees recognize complex shapes readily, and so the pronounced bilateral symmetry (zygomorphy) of many legume flowers, which leads to complex flower outlines and intricate floral mechanisms, may promote exclusive flower visitation of some species by specific pollinators (5). The important study by Feng et al. (6), in this issue of PNAS, now elucidates the molecular architecture underlying the development of zygomorphy in legume flowers and so provides an important tool for understanding the diversification of the family.