**6. Conclusion**

*Drought - Detection and Solutions*

traits in drought stress [90].

common bean germplasm.

gating RIL populations [89, 90, 95].

such as those belonging to the race Durango [5].

**5.2 Marker-assisted selection and breeding for drought tolerance**

For decades DNA markers have been the most widely used molecular markers

The marker-assisted selection is especially effective in selection for simple and single gene traits, and has been applied for selection for resistance genes for various common bean diseases of viral, bacterial and fungal origin [93, 94]. Selection for quantitative traits such as quantitative resistance or drought tolerance presents a great challenge as it can involve multiple major and minor QTLs controlling the trait [91]. Improved understanding of the complex drought response mechanisms on the level of physiology and molecular biology has enabled identification of potential molecular markers, which could help us distinguish between drought resistant and susceptible genotypes. Among the recently reported markers associated with the drought response in common bean are AQPs whose expression is discussed in the present chapter under Section 4.1. On the other hand several potentially useful molecular markers associated with drought response traits, such as high yield under drought, have been identified using QTL mapping in a segre-

Common strategy of common bean breeding programs for resistance to drought

is selection of best yielding genotypes that are cultivated in drought-exposed conditions [37, 41]. In addition to that application of novel breeding approaches

in crop improvement, due to their abundance and polymorphisms. Markerassisted selection enables precise and effective selection of common bean genotypes with specific traits and can greatly facilitate the selection process in breeding [91, 92]. These markers are potentially very useful in trait selection and breeding applications, and have been utilized in our procedures, to offer additional informative value on the common bean genotypes included in the breeding program (not published). The advantage of such an approach is that a broad range of economically important traits can be covered, including disease resistance, abiotic stress tolerance, high yield, earliness, phosphorus uptake, and root morphology. However, the practical utilization of molecular markers is at the beginning, also due to lacking validation across the genotypes of the diverse

Drought response-associated QTLs in common bean have been reported in association with yield, phenology, canopy biomass and biomass partitioning. A Mesoamerican and Andean inter-gene pool genetic map with high marker coverage was utilized to detect phenological and seed weight QTLs associated with drought tolerance [88], while intra-gene pool Mesoamerican mapping population has been utilized to identify drought-associated QTL for phenological and yield-related traits [89] as well as QTL for photosynthate acquisition, accumulation and remobilization

The translation of reported QTLs into practical use has, however, been limited, due to highly variable common bean germplasm and strong influence of the environmental conditions on the presence of minor QTLs. It would be ideal to perform the validation of the QTL in crop production areas. Establishing controlled and uniform growth conditions for evaluation of a large recombinant inbred line (RIL) population, exceeding hundred genotypes, can however prove difficult and not very practical, so a compromise approach for validation of major QTL could consist of testing a subsample of the most diverse RILs for a selected segregation trait in multiple trials sites [5]. Much of the work in QTL mapping and development of drought-tolerant cultivars has been performed based on the traits of the drought resistance sources of Mesoamerican origin,

**118**

Drought tolerance is gaining importance in the breeding of common bean for higher yields under the changing environmental conditions. Studying drought tolerance is thus important in order to understand the underlying mechanisms and to identify markers that could help distinguish the more tolerant common bean genotypes. A highly diverse common bean germplasm, adapted to various growth and climatic conditions, constitutes a valuable pool of traits including potential drought tolerance traits. On the other hand, the great complexity of the common bean response to drought on physiological and molecular levels presents a great problem for more effective breeding. The challenge for the future will be to integrate the data obtained by various approaches that include screening of the transcriptome, proteome and metabolome, using advanced bioinformatics and systems biology, identifying molecular markers and QTLs and elucidating the underlying pathways.
