W1150: Exotic Germplasm Conversion and Breeding Common Bean (Phaseolus vulgaris L.) for Resistance to Abiotic and Biotic Stresses and to Enhance Nutritional Value
Statement of Issues and JustificationSTATEMENT OF THE PROBLEM. Dry bean (Phaseolus vulgaris L.), introduced by the Native Americans from Mexico, Central America, and South America, has been grown in the U.S. for millennia. However, compared to corn, soybean, and wheat, the average national yield of dry bean is low (=1700 lbs/acre). Several abiotic and biotic stresses can severely limit both dry and snap (vegetable form) bean production. Furthermore, several diseases can simultaneously reduce dry and snap bean yield and quality within and across different production regions and yield losses can range from 10-90%, depending on the diseases involved. For example, in the Western U.S., Beet curly top virus (BCTV), Bean common mosaic virus (BCMV), Fusarium root rot (caused by Fusarium solani f.sp. phaseoli), and white mold (caused by Sclerotinia sclerotiorum) can simultaneously attack susceptible cultivars in the same field. Similarly, in Michigan, Minnesota, North Dakota, and Wisconsin, anthracnose (caused by Colletotrichum lindemuthianum), bacterial brown spot [caused by Pseudomonas syringae pv. syringae (Psp)], BCMV, common bacterial blight [caused by Xanthomonas campstris pv. phaseoli (Xcp) and X. campestris pv. phaseoli var. fuscans (Xcpf)], halo blight (caused by Pseudomonas syringae pv. phaseolicola), root rots, rust (caused by Uromyces appendiculatus), and white mold can be sympatric and cause severe yield losses. The recent introduction of soybean rust into the southeastern U.S. has serious potential implications for dry and snap bean as they are susceptible to the soybean rust pathogens, Phakopsora pachyrhizi and P. meibomiae. Furthermore, many of these diseases are seed-borne, are caused by pathogenetically variable organisms, and their chemical control can be expensive or not practical. Fungicides increase production costs and can result in environmental and human health hazards if improperly used.
The genetic base of dry bean cultivars within most market classes in the U.S. is narrow (McClean et al., 1993; Miklas, 2000; Silbernagel and Hannan, 1992; Sonnante et al., 1994). As only a very small number of wild beans have been domesticated (Gepts et al., 1986), useful traits such as resistance to bruchids (Zabrotes subfasciatus) are not found in cultivars (Schoonhoven et al., 1983) supporting the evidence that a large reduction in genetic diversity occurred early during domestication (Gepts et al., 1986; Koenig et al., 1990). Furthermore, resistance to heat, drought, and diseases such as Bean golden yellow mosaic virus (BGYMV), common bacterial blight, and white mold are inadequate in most cultivars grown in the U.S.
This interdisciplinary, multi-state, collaborative W-150 project proposal comprises several complementary sub-projects (see Appendix Table 1). Collaboration among participants in these sub-projects is designed to achieve our overall goals and objectives of developing high yielding cultivars with enhanced culinary and nutritional qualities and resistance to major abiotic and biotic stresses. These cultivars will help reduce production costs and pesticide use, increase yield and competitiveness of the U.S. bean growers, and sustain production for domestic consumption and export. Researchers participating in each sub-project have complementary expertise and represent two or more institutions. They have jointly prepared their sub-project documents and are committed to collaborating with each other to achieve the overall objectives. For simplicity, these projects are grouped into: (1) exotic germplasm conversion, (2) abiotic stresses, (3) biotic stresses, and (4) nutritional value. Details of each sub-project listed in Appendix Table 1 will be provided upon request.
Exotic Germplasm Conversion. Most tropical and subtropical dry bean germplasm are highly sensitive to long days in the U.S. mainland bean growing environments. This sensitivity inhibits flower and seed formation. Consequently, photoperiod-sensitive tropical germplasm must be converted to temperate adaptation before it can be used to broaden the genetic base and improve cultivars in the U.S. for yield, enhanced culinary and nutritional qualities, and resistance to major biotic and abiotic stresses.
Abiotic Stresses. Although drought, high temperatures (>30 degrees C.), and soil mineral deficiencies and toxicities can adversely affect bean production in the U.S., the primary emphasis in this project will be on breeding for resistance to high temperatures. Evaluation for other abiotic stresses will be limited to only the most promising breeding lines and cultivars through regional and national nurseries. High temperatures reduce yield, quality, and geographic adaptation of cultivars. Production areas in the U.S. occasionally experience seasonal heat waves during flowering, resulting in blossom drop and a split time period for pod set. Heat stress also increases pollen sterility and distally located ovules often fail to develop, reducing seed number per pod in dry bean and pod quality in snap bean.
Biotic Stresses. Most cultivars are highly susceptible to numerous diseases caused by bacterial, fungal, and viral pathogens. In this project, we will focus on common and halo bacterial blights, Fusarium root rot and wilt (caused by F. oxysporum f. sp. phaseoli), geminiviruses such as beet curly top (in the Western U.S., Larson and Miklas, 2004) and bean golden mosaic (in southern Florida and Puerto Rico; Blair et al., 1995), rust (east of the Continental Divide), and white mold. Yield losses from these diseases can vary from 10% to 90%. In the case of white mold, the fungus produces sclerotia that can survive in soil for years and can be spread from field to field by internally infected seeds, sclerotia mixed with seed, contaminated soil on equipment, and irrigation runoff water, in addition to wind-blown ascospores. Common and halo bacterial blights are seed-borne, thus restricting free movement of seed from one production region to another and limiting seed production in other regions. Moreover, halo blight and rust are caused by variable pathogens, such that cultivars that are resistant in one location or year may be susceptible in another location or year. Cultivars that contain single gene resistance are vulnerable to new pathogen race. Chemical control for most of these diseases is difficult and sometimes impractical because the pesticides are expensive, ineffective, and damaging to the environment.
Nutritional Value. Dry beans have been widely shown to have significant health benefits, however they are under-utilized in the diets of people worldwide. Health benefits from eating beans are numerous and include the provisions of low fat and high protein, calories, dietary fibers, vitamins, iron, potassium, calcium, iron, zinc, phosphorus, and other essential nutrients. Several of the most severe chronic health problems (e.g., certain types of cancer, Type 2 diabetes, and cardiovascular diseases) in the U.S. could be prevented or alleviated by eating more beans (Andersen et al., 1984; Tietyen-Clark, 1986). Therefore, identification and promotion of the health-benefits of dry beans are extremely important. Moreover, a greater emphasis must be placed on providing more diversified bio-fortified bean-based foods and convenience snacks with improved nutrition and consumer appeal.
JUSTIFICATION. Exotic Germplasm Conversion. An important limitation to the genetic improvement of bean cultivars in the U.S. is the narrow genetic diversity currently available to researchers. Most of the tropical and sub-tropical germplasm are highly photoperiod sensitive such that they may not flower and produce seed on the U.S. mainland. Conversion of useful exotic germplasm for adaptation to the U.S. bean growing environments, and introgression and pyramiding of favorable alleles and QTL for resistance to major abiotic and biotic stresses, and enhanced yield and nutritional value are essential for broadening the genetic base of cultivars. Also, it will permit the introduction of new market classes of beans to the U.S. farmers. The study of the genetic and molecular basis of the principal domestication traits will speed the introgression and pyramiding of favorable alleles and QTL especially from wild bean.
Abiotic Stresses. Resistance to high temperatures and drought occurs in a few tropical cultivars, although tepary bean (P. acutifolius) possesses much higher levels of heat and drought resistance (Thomas et al., 1983). Resistance to heat and drought stresses in bean are heritable (Schneider et al., 1997; Shonnard and Gepts, 1994; Singh, 1995). Thus, introgressing and pyramiding heat (and drought) resistance from other Phaseolus species into dry and snap bean cultivars will increase yield and yield stability, and extend adaptation to marginal environments.
Biotic Stresses. We have limited information regarding the nature of the viruses that cause curly top disease in the Western U.S. Concurrent pathogen surveys are also needed for other geminiviruses, Fusarium root rot and wilt, common and halo bacterial blights, rust, and white mold. Knowledge of pathogen diversity is critical for the identification and deployment of appropriate specific resistance genes and QTL to combat these diseases on a regional and national basis. There is a need to carry out comparative studies of Agrobacterium-mediated inoculation (agroinoculation) and vector-based inoculation methods for geminiviruses to assure that resistant cultivars developed using agroinoculation in the lab and greenhouse are also resistant in the field under vector pressure. Similarly, for white mold, yearly testing of resistance donor and improved germplasm using different screening methods in greenhouse, laboratory, and multiple field environments is essential to identify both disease avoidance mechanisms and true resistance. In the past, W-150 researchers have developed germplasm and cultivars resistant to anthracnose, BCMV, BGYMV, Fusarium root rot and wilt, rust, and white mold. However, in some cases the genetic base of resistance is narrow, vulnerable to new pathogen strains, and/or is inadequate to reduce crop losses. At present, only one or two major Andean resistance genes for anthracnose and rust have been identified and deployed (Kelly and Vallejo, 2004; Pastor-Corrales, 2003). There is an urgent need to identify and introgress additional Andean resistance genes for these diseases caused by highly variable and virulent pathogens. Similarly, introgression and pyramiding of high levels of resistance from other Phaseolus species for bean golden mosaic, white mold, and other diseases are essential. Development of common bacterial blight resistant breeding lines from crosses of common bean with tepary and runner beans was a major milestone (Miklas et al., 1994; Singh and Muñoz, 1999). Similarly, identification of molecular markers (BC420, SAP6, SU91) linked with each of the three major QTL imparting resistance to common bacterial blight has facilitated breeding for resistance (Kelly et al., 2003; Yu et al., 2000). Nonetheless, molecular markers for other major and minor QTL for resistance to anthracnose, BCMV, common and halo bacterial blights, rust, white mold, and other diseases must be identified to facilitate cultivar development.
Nutritional Value. The potential for beans to prevent and alleviate chronic diseases is not well known. Research to identify bean components that mitigate chronic diseases is essential as are outreach efforts to promote awareness and acceptance of beans as a healthy food. Therefore, both bean genetics and food science must be utilized to reduce constraints that contribute to low per capita consumption of beans in the U.S. The number of convenience entrees and snack items prepared from beans should be increased considerably. Also, avoidance of beans because of flatulence continues to be a major constraint and must be addressed in the future.
Need for a Multi-State Collaborative Project. For conversion of useful tropical and sub-tropical germplasm that are poorly adapted to the U.S. bean growing environments it is necessary to make adapted x exotic crosses and backcrosses during short-days in the tropics (e.g., Mayaguez, PR) (or in the greenhouse during the winter months). Furthermore, crossing and backcrossing in the tropics should be alternated by selection for photoperiod insensitivity on the mainland during the summer months. The use of winter nurseries in Puerto Rico complements and expedites germplasm conversion, and the mainland U.S. breeding for other constraints.
Anthracnose, curly top, halo blight, rust, and other diseases caused by variable pathogens, require testing using different screening methods and multi-location field and greenhouse environments. Also, for white mold field and greenhouse data from a single location often do not permit the identification of avoidance and physiological resistance with any degree of assurance. It is therefore essential to continue to characterize and monitor genetic variability of bacterial, fungal, and viral pathogens causing major bean diseases in the U.S. Also, it is imperative to determine the reaction of useful germplasm to the pathogenic diversity to identify complementary resistance genes and mechanisms for broadening the genetic base and development of improved cultivars. Introgression and pyramiding of favorable alleles and QTL from across races, gene pools, and related wild and cultivated Phaseolus species into cultivars is often achieved only through a tiered breeding approach (Kelly et al., 1998; Singh, 2001; White and Singh, 1991). Most researchers often work at one or two tiers, and depend on other collaborators for complementary germplasm and information to accomplish development of successful cultivars (Kelly et al., 1998). Inter-disciplinary and inter-institutional collaborative research must also continue to find alternative recombination and selection methods and identify and use molecular markers to facilitate efficient introgression and pyramiding of favorable alleles and QTL into improved cultivars for a diverse cropping system. Thus, to develop germplasm and cultivars with multiple-disease resistance and abiotic stresses, researchers with limited expertise and facilities share responsibilities and exchange segregating populations and breeding lines to complement screening and selection in contrasting field environments, laboratories, and greenhouses regionally and nationally.
Because exotic germplasm is increasingly being used to broaden the genetic base and breed cultivars for higher yield, enhanced nutritional quality, and resistance to abiotic and biotic stresses, it is essential to evaluate new breeding lines and cultivars across production regions for broad adaptation and stability of performance. Regional and national germplasm development and testing are also important because only one field crop/year is feasible in the continental U.S. Although not described separately, multi-location testing is accomplished through the national Cooperative Dry Bean Nursery (CDBN), Bean Rust Nursery (BRN), White Mold Nursery (WMN), Midwest Regional Performance Nursery (MRPN), and Western Regional Bean Trial (WRBT). These nurseries are essential to identify high yielding broadly adapted cultivars and breeding lines with durable resistance, and to detect pathogen diversity in the shortest time possible. These nurseries therefore form an integral part and sound basis for strong collaborative efforts within the W-150 project. No single state or institution can conduct all the research necessary to develop improved cultivars for sustainable production, consumption, and export. This is especially true when most programs are grossly under-funded and have inadequate resources and personnel to carryout a relevant and efficient breeding program for their own state. Certain expertise is available in a few states (e.g., nutritionists and pathologists), but there are some states (e.g. Arizona, Montana, New Mexico, and Wyoming) that do not have dry or snap bean breeding programs. As in the past, researchers in these states also will have access to new breeding lines and cultivars of all market classes through the W-150 project. Moreover, research and outreach efforts of agronomists, breeders, geneticists, food scientists, human nutritionists, and plant pathologists must be coordinated to improve domestic consumption and export. No single research institution is capable of providing the breadth of expertise necessary to achieve regional and national impact. Thus, additional resources and multi-state regional and national collaboration are essential to solve major abiotic and biotic stresses, yield, and food quality problems that currently limit the yield potential and domestic consumption and export of dry and snap bean. This comprehensive, multidisciplinary, and multi-state collaborative project is essential to convert useful exotic germplasm; and to maintain and exchange pathogens, parental stocks and improved breeding lines and cultivars, and research data. The collaborative project provides a broad range of selection environments whereby researchers can share and complement responsibilities that could not be achieved by any single state or institution alone. Moreover, a coordinated, multidisciplinary effort will allow the most efficient use of genetic resources, avoid duplication of research, and maximize efforts to increase bean production, consumption, and export.
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