The genetic basis of the commercial common bean classes is narrow as compared to worldwide germplasm, which in contrast shows a wide diversity of seed and pod traits, plant growth habit, phenological traits, flowering time, photoperiod sensitiv- ity, adaptation to different soil types, wide range of resistance to diseases and stress, and different nutritional seed quality. The genetic variation of common bean germ- plasm has been widely used by breeders to further enhance the crop since the late nineteenth century and early twentieth century. However, so far a large part of the variation observed in gene pools, races and wild relatives has not been used in breed- ing. The major limitation to its utilization can be attributed to the lack of adaptation
of germplasm to new ecological niches, the presence of undesired traits such as seed shattering and the time-consuming analysis of progenies.
Due to the economic value of common bean, several breeding programmes are presently in progress throughout the world. Breeders freely crossed between Mesoamerican and Andean gene pools, as well as among races, although intergene pool crosses have had only limited success, suggesting an ongoing process of spe- ciation (González, Rodiño, Santalla, & De Ron, 2009; Koinange & Gepts, 1992). Breeding can also involve gene introgression from additional gene pools. Indeed, the secondary and the tertiary gene pools of common bean, covering a range of envi- ronments from cool moist highlands to hot semi-arid regions, could be an important resource for the genetic improvement of common bean, which will increasingly suffer from the increase of temperatures and moisture, and from drought periods, as a conse- quence of climatic changes (Beebe, Rao, Mukankusi, & Buruchara, 2012).
Several species of Phaseolus can be hybridized to common bean. The species belonging to its secondary gene pool, such as P. coccineus, P. polyanthus and P. cos-
taricensis Freytag & Debouck, can freely be crossed with each other without embryo rescue, particularly when common bean is used as the female parent. P. coccineus has been more commonly used in wide crosses with P. vulgaris, especially for traits such as cold temperature tolerance, root rot and bean yellow mosaic virus resistance. However, hybrid progenies may be partially sterile, preventing the recovery of desired stable traits. The tertiary gene pool of common bean comprises P. acutifolius and
P. parvifolius Freytag; crosses of common bean with these two species are successful, but require embryo rescue, and backcrosses to the recurrent common bean parent are often required to restore hybrid fertility. Genes for disease resistance have been suc- cessful moved from P. acutifolius to common bean. Crosses with other species, such as P. lunatus, P. filiformis Benth. and P. angustissimus A. Gray have been attempted without producing viable hybrid progenies, so these species could be considered the quaternary gene pool of common bean (Singh, 2001).
Early maturity, adaptation to higher altitude, upright plant type, high pod qual- ity and seed yield, and some resistances to diseases such as viruses and rust, insect pests, and drought and abiotic constraints such as deficiency of nitrogen, phospho- rus and zinc or tolerance to aluminium and manganese toxicity have been bred into common bean cultivars. Most, if not all, commonly used crop breeding methods have been employed with common bean (Beaver & Osorno, 2009). Differences in genetic distance among gene pools, races and species dictate specific breeding methods and strategies. The results and the efficiency of the different methods applied have been the object of some detailed reviews (Graham & Ranalli, 1997; Kelly, 2010; Singh, 2001). Challenges such as drought, root rot, heat, depleted soils, excessive rainfall and new and old pests and diseases pose new breeding targets and require increased efforts to address them. To overcome some of the inherent difficulties faced by con- ventional plant breeding, new biotechnology tools have been developed and are grow- ing in importance and use. Molecular approaches, such as marker-assisted selection (MAS), can support breeders facilitating and accelerating the transfer of desired traits. A detailed report on implementation and adoption of MAS in common bean breed- ing is provided by Miklas, Kelly, Beebe, and Blair (2006), who reported highlighted
examples of MAS success in gene pyramiding, rapid and simpler detection, and selec- tion of resistance genes. Slower progress has been obtained in the improvement of nitrogen fixation, insect resistance and tolerance to abiotic stresses. Moreover, pro- gress in increasing seed yield potential has been only moderately successful, because multiple constraints limit bean productivity (Beaver & Osorno, 2009).
In terms of consumer preferences, the most desirable traits are those related to the technical and nutritional quality of dry seeds, such as ease of cooking, soft coat tex- ture, good taste and protein content. Cooking time is certainly one of the factors that limit the home consumption of dry bean. Some studies showed that it is an oligogenic trait with high genetic variation but also significantly affected by the growing loca- tion. The recent identification of quantitative trait loci (QTLs), which define the loca- tion of genes governing this target trait, is the first step in future breeding programmes (Garcia et al., 2012).
It should be keep in mind that common bean is most produced and consumed in developing countries, where yield is often affected by deficiencies and toxicities of minerals in soil. This is the case of aluminium toxicity that negatively affects the yield in acid soils of tropic regions. Studies conducted at CIAT have shown that some accessions of P. coccineus are more resistant than common bean to aluminium tox- icity. Butare et al. (2012) crossed an Al-sensitive common bean with an Al-resistant
P. coccineus accession, obtaining recombinant inbred lines, among which were prom- ising resistant common bean genotypes.
Finally, since each region has different agro-techniques, pedo-climatic conditions, biotic and abiotic constraints, and consumer preferences, breeding programmes must be tailored to the needs of farmers and consumers who will use the new cultivars.