**2. The diverse common bean germplasm as a potential for discovering new drought-responsive traits**

Common bean originates from Mexico and was separated in two ecologically and geographically different Mesoamerican and Andean gene pools [19, 20]. Mesoamerican bean genotypes can be distinguished by longer flowering times and small seeds, while Andean genotypes have large and colorful seeds [21, 22]. Phylogenetic studies and evaluation of common bean genotypes collected from different regions, ranging from the Americas, Africa [23, 24] and Europe [25–28], have confirmed independent domestication events specific to each of the gene-pools [20, 29].

Common bean was introduced in Europe centuries ago by independent domestication events from both major centers of origin [25–28]. Our phylogenetic studies shed more light on the understanding of dissemination pathways and the evolution of this species in central Europe and have been focused on the germplasm from the Central European, South East European and Balkan region [27, 28, 30]. Evaluations of genetic diversity and the population structure of 167 historical and current accessions with the different geographical origin (Slovenia, Austria, Bosnia and Herzegovina, Croatia, Macedonia and Serbia) have revealed great allelic polymorphism in 14 SSR markers. The strong predominance of Andean genotypes in Slovenia and several Western Balkan countries indicates their introduction from the Mediterranean basin and countries such as Italy. On the other hand, a high proportion of Mesoamerican genotypes in the present Austrian germplasm (44%) could indicate their introgression from western and northern European countries driven by historical events.

Cultivation in diverse local environments and climate areas, ranging from lowlands to high altitude regions in equatorial and more temperate climate conditions have contributed to the high diversity of common bean in terms of growth type, seed properties and maturity time [31]. Consequently, the diverse common bean germplasm represents an important trait pool for searching for new traits such as abiotic stress tolerance traits [31]. The screening process is largely based on phenotyping genotypes exposed to a form of drought, with possible subsequent rehydration in comparison to irrigated conditions. It is usually performed in different locations, over several seasons, either in the field or in controlled greenhouse conditions. Screening commonly includes phenotyping of phenological traits such timing of flowering and maturation, anatomical and morphological traits describing plant fitness and yield-associated traits as a measure of the effect of drought on yield output.

In the past decades, considerable efforts have been made to characterize the American germplasm for different traits including drought responsive traits [5]. The identified drought resistance sources of race Durango have become a cornerstone for research of complex drought tolerance mechanism and introgression of traits into the cultivars.

Another example is Central and East European common bean germplasm that consists of thousands of collected genotypes deposited in the national and regional gene banks and preserving this variability is an important step in preventing gene erosion,

*Drought - Detection and Solutions*

depending on the timing, intensity and duration of drought, as well as on the stage of development of the exposed plant. Exposure to drought stress can be fairly constant throughout the season or may affect plants in a specific stage of their life cycle, thus delaying early plant development, vegetative growth, flowering and/ or maturation [6]. In the most arid areas, annual losses of common bean yield can

Plants have, during evolution, acquired different modes of adaptation to harsh environmental conditions, including drought stress [5, 8, 9]. Some plants escape the latter by early maturation and, in consequence, more rapid development of seeds, thus completing their life cycle before the onset of prolonged and severe drought. Others have developed morphological, anatomical or physiological adaptations that enable them to maintain high water potential during drought (drought avoidance), or mechanisms directed to survival in the presence of low water potential (drought tolerance). Adaptations leading to drought avoidance include increased density of roots and their deeper propagation that enables more effective water absorption from the soil, and decreased leaf area and stomata closure that control the limitation of water loss through transpiration [10]. Drought tolerance is based on tissue- and cell-specific physiological and molecular adaptations such as synthesis of

An outstanding characteristic of common bean germplasm is its particularly high diversity. In the second part of this chapter, the general picture obtained by commonly used screening methodology is presented. This includes phenotyping of phenological, anatomical and morphological, as well as yield associated traits, illustrating this diversity with the aim of pointing out that common bean responses to drought can differ greatly between specific genotypes. An example of this approach is that phenotyping of American genotypes in the last decades led to the discovery of important drought resistance sources from the Mesoamerican gene pool, largely belonging to the race Durango, thus forming the basis for numerous subsequent studies [5]. For such reasons, continuous efforts are devoted to screening common bean germplasm for more drought resistant genotypes that exhibit different, and potentially complementary, drought resistance traits that are and will be used to study and better understand the mechanisms of resistance and for the breeding of

The plant response to drought results from complex and diverse adaptation. It has, therefore, to be studied on levels ranging from morphological and physiological changes observed in organs to the intricate responses on the gene expression and regulation levels and to biochemical responses on the level of cells and organelles [12]. In the present chapter we therefore focus further on the physiology of the response of common bean to drought, followed by a survey of research on the influence of drought on its transcriptome, proteome and post-translational

In the last part of this chapter the genetic level is considered, disclosing drought

Further, identification of genes that are expressed differently under drought conditions from that in well-watered plants, especially if contrasting tolerant and sensitive genotypes, may also lead to the discovery of specific markers that can be used in breeding. Similarly, protein markers can be discovered by proteome

response as a complex quantitative trait controlled by a number of major and minor genes clustered on specific loci, as well as several genomics and molecular approaches that have been utilized for their study [14]. For common bean genotyping and for subsequent mapping of quantitative trait loci (QTLs) a large variety of genetic markers, from simple sequence repeat (SSR) to single-nucleotide polymorphism (SNP) markers, are now available, enabling use of this approach in identify-

ing molecular markers of tolerance to drought.

exceed 60%, rising to 80% at the height of the drought [5–7].

osmoprotective proteins like dehydrins and chaperons [11–13].

**110**

new varieties.

modifications.

as well as supporting breeding programs with genotypes showing different environmental adaptations. Characterization and evaluation of this germplasm are an ongoing process and have confirmed the very broad genetic diversity of common bean in Eastern Europe. Our recent proceedings have resulted in formation of a core collection having applicative value for direct breeding purposes [32]. Screening for representative genotypes for core collection included initial evaluation of basic multi-crop passport descriptors (e.g., geographic origin, biological status, and ancestral data), phenotypic seed characteristics and phaseolin type, as well as assessment of genetic structure by genotyping with genetic markers. The resulting core collection encompasses 63 accessions representing the global genetic diversity and 14 standard genotypes with desirable traits from the East European region (unpublished data) and was evaluated under field conditions as well as for the presence of genetic markers associated with traits of interest and biochemical analysis. Core collection was further evaluated for agronomic traits in field conditions (response to abiotic stress), genetic markers for desirable traits and nutritional traits of importance (multi-elemental composition, fats, proteins, and phytic acid). These results enabled selection of superior genotypes in core collection for further breeding applications.
