**4. Serro Azul as a source of disease resistance**

and the water absorption rate [23]. Further examination of the genes involved in cooking time

It is well known that the seed coat is the structure that protects the seeds from pathogens and insects, and the glossiness seems to have an important role in such protection. Moreover, seeds with glossiness might have enhanced antioxidant properties due to a higher concentration of specific secondary metabolites in the seed coat, therefore, having an impact in

Usually, in the case of landraces, where local selection has been performed, it is more frequent to find common bean accessions that show glossy seed coat [24] (checking the list of genotypes) than in breeding programs. In the case of the landrace Serro Azul, both variants Serro Azul Brilhante (glossy) and Serro Azul Fosco (opaque) have been cultivated [11, 13]. Morphological and biochemical findings are hereafter discussed to show advantages of the

**3.3. Biochemical nature of seed coat glossiness and its implications for human health**

The seed coat glossiness has been studied to be mainly conditioned by the *Asp* gene but also influenced by the *J* locus, especially with the dominant allele [14]. A number of studies have also been conducted to better understand the biochemical implications of the expression of

Classical work has suggested that *J* is essential for the synthesis of proanthocyanidins or condensed tannins [14]. Therefore, the recessive *jj* genotypes have been found to be absent in condensed tannins [25], contrary to the *J\_* genotypes, that are able to synthesize such compounds. Based on the genetic maps that identified the RAPD marker as linked to *J*, a recent study has shown that *J* is linked to a region containing *MYB123* [26], similar to TT2 in *Arabidopsis thaliana* (AT5G35550.1) [27] and *Glycine max* [26], which acts as a key determinant in the proanthocy-

Proanthocyanidins are oligomers or polymers formed by the condensations of flavan-3-ols units such as catechins and epicatechins [28, 29]. In common bean, condensed tannins are mainly composed of catechin monomers [30]. As secondary metabolites, they play important

On the other hand, the *Asp* locus is said to be the main gene involved in seed glossiness. Some line of evidence has shown that *Asp* affects the accumulation of anthocyanins due to a structural change that it promotes on the seed coat. Therefore, genotypes with glossy seed coat (*Asp\_*) accumulate more anthocyanins than dull seed coats (*asp asp*) [20]. Anthocyanins have been investigated for their roles in humans such as in anti-inflammatory, lipid peroxidation

Therefore, although glossiness has been generally neglected by consumers and, as a result, by selection programs, it might have positive implication for human health. Moreover, the indication that glossiness is not necessarily associated with higher cooking time (since there is a lack of correlation between water absorption and cooking time) as shown by Garcia et al.

and membrane strengthening processes [33, 34], as well as in preventing cancer [35].

roles as antioxidants, anticarcinogenic and anti-inflammatory [28, 31, 32].

is necessary, though.

184 Rediscovery of Landraces as a Resource for the Future

human health [12].

such genes on the seed coat.

seed glossiness for aspects related to human health.

anidin accumulation of a developing seed [27].

One of the most important aspects of a breeding program is to find genotypes that are tolerant or even resistant to diseases. The cultivation of common bean is majorly affected by diseases such as common bacterial blight caused by *Xanthomonas axonopodis* pv. *phaseoli*, the angular leaf spot caused by the fungus *Pseudocercospora griseola* and anthracnose by the fungus

**Figure 3.** Screening for anthracnose resistance with races 65 and 73 (*Colletotrichum lindemuthianum*) on SAB and SAF plants. Controls: Rosinha G2 (susceptible) and G2333 (resistant).

*Colletotrichum lindemuthianum*. The genetic architecture and mechanisms of resistance to such diseases have been studied at genomic scales, identifying specific genes, pathways and QTL associated to each one (see [16, 36, 37]). As landraces are usually genetically structured and locally adapted, they might be the source of new alleles for disease resistance, which could be added to the disease resistance breeding programs.

The evident difference between SAF and SAB needs further examination. It raises questions such as if the anthracnose resistance is somehow influenced by the glossiness of SAB. After all, genes related to anthracnose resistance are also located in chromosome 7 [40], but a specific study linking *Asp* to such genes is not yet available. However, it could be simply a new source of resistance originated from a mutation or to a combination of specific alleles conferring resistance. These are only speculations that need experimental clarification. The available genomic technology and the bioinformatic tools for constructing genetic maps with high resolution might be helpful in

Genetic Variation of Landraces of Common Bean Varying for Seed Coat Glossiness and Disease…

http://dx.doi.org/10.5772/intechopen.73425

187

**5. Analysis and conservation of the genetic diversity of landraces**

The genetic diversity of common bean landraces, cultivars and wild accessions has been investigated in multiple studies, mainly based on morphological and molecular markers in the last two decades (from 1995 to 2017). In the 1990s, studies have shown high genetic diversity based on morphological, enzyme and DNA-based markers (RAPD and microsatellites) [41–43]. After 2000, several studies involving amplified fragment length polymorphic (AFLP) [44, 45] and microsatellite [24, 46, 47] markers have been conducted. After 2010, with the advances of the sequencing technology, numerous papers have addressed the molecular diversity of common bean based on SNP markers [4, 48–51]. In general, most of the studies revealed that common beans are divided into two main gene pools, the Mesoamerican and Andean. In the case of Brazil, where common beans were introduced and have been cultivated mainly in small farming systems, varieties of both pools have been encountered. However, Burle et al. [24] investigated the genetic diversity of almost 300 landraces cultivated in Brazil and demonstrated that almost 80% of the genotypes have a Mesoamerican background, based on a population structure analysis with microsatellite

In the case of Serro Azul, we also examined the molecular diversity of a sample of plants from both the variants (Serro Azul Brilhante and Serro Azul Fosco) by using the AFLP markers. Selective amplifications were done with four primer combinations (*EcoRI-A*/*MseI-CC*; *EcoRI-AC*/*MseI-CC*; *EcoRI-AT*/*MseI-CA*; *EcoRI-T*/*MseI-CA*). The AFLP gels revealed a considerable variation within and among SAB and SAF plants (**Figure 4**). An UPGMA tree was designed using Bionumerics software (**Figure 5**). Our data revealed a good separation among SAB and SAF samples, although some mixture was detected. In general, the SAB subgroup presented 75% of similarity with the SAF subgroup. Two plants of the SAF, however, were grouped within the SAB subgroup. From the analysis of the UPGMA tree, it is possible to infer that SAB plants were derived from SAF, which is consistent with the prior observation that SAB seeds were observed within 1 kg of SAF seeds harvested at a farm in Cunha, SP. Hereby, the high variability at the morphological levels can be verified at the molecular level, revealing a landrace with considerable genetic diversity to be explored. The development of the SAF × SAB population, for instance, revealed a high variation for seed size, color and water uptake [12]. Moreover, we showed that the parental lines have noticeable differences for disease

answering those questions.

polymorphisms.

resistance.

Here, we present new findings obtained with experiments conducted with Serro Azul Brilhante and Serro Azul Fosco, as regards their variation for anthracnose resistance. The resistance degree to *C. lindemuthianum* was evaluated using the method of detached leaves [38], with modifications. Seeds of each parental line (SAB and SAF) and control standards from the literature (cultivars Rosinha G2 and G2333 are highly susceptible and resistant, respectively) were germinated and transferred to pots with substrate (Plantmax) in a greenhouse, and irrigated properly until the establishment of the first trifoliate (around 21 days). One leaf was collected from each plant and immediately placed in a suspension of 1.2 × 10<sup>6</sup> conidia mL−1 of *C. lindemuthianum* races 65 and 73 [39]. After 1 min, the leaves were placed in Petri dishes containing two layers of moistened filter paper. The plates were then incubated in a BOD type chamber and maintained in a photoperiod of 12 h at 21°C (± 2°C) for 7 days. For analysis, plant resistance was inferred according to the standards proposed by CIAT, using a scale from 1 (resistant—no symptoms) to 9 (susceptible—evident necrosis).

The results revealed an interesting difference between the parents SAF and SAB, used to constitute the segregating populations. The detached leaf method clearly showed that SAB was highly resistant to both races studied (65 and 73), while SAF showed high susceptibility to the *Colletotrichum* races, especially race 65 (**Figure 3**). The scores for disease resistance were significantly different among the two parents (SAB and SAF) for the race 65, but the average score with the race 73 also suggested higher degree susceptibility in SAF (**Table 1**). It is interesting to notice that the score for SAF was higher than the standard, which is used as a control for anthracnose susceptibility (Rosinha G2). In a similar manner, SAB was even more resistant than the standard resistant line G2333 (**Table 1**).


**Table 1.** Average scores of *Colletotrichum lindemuthianum* infection in leaves of the variants Serro Azul Brilhante (SAB) and Serro Azul Fosco (SAF), compared to G2333 (resistant control) and Rosinha G2 (susceptible control).

The evident difference between SAF and SAB needs further examination. It raises questions such as if the anthracnose resistance is somehow influenced by the glossiness of SAB. After all, genes related to anthracnose resistance are also located in chromosome 7 [40], but a specific study linking *Asp* to such genes is not yet available. However, it could be simply a new source of resistance originated from a mutation or to a combination of specific alleles conferring resistance. These are only speculations that need experimental clarification. The available genomic technology and the bioinformatic tools for constructing genetic maps with high resolution might be helpful in answering those questions.
