Plant Breeding for Biotic Stress Resistance || Breeding for Resistance to Bacterial Diseases

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<ul><li><p>Chapter 3Breeding for Resistance to BacterialDiseases</p><p>Carlos A. Lopes and Leonardo S. Boiteux</p><p>Abstract The control of bacterial diseases in plants is difficult and usuallyrequires the combination of several complementary management measures. In thiscontext, genetic resistance is considered to be an effective low-cost strategy thatcould easily be adopted by farmers, who acquire this built-in control technologywithin the seeds of a resistant cultivar. To be effective, breeding for diseaseresistance requires deep knowledge of processes involving the interactions amongthe plant, the pathogen, and the environment. The development of bacterialresistant cultivars is a complex task, which comprises multidisciplinary actionsinvolving the complexity of the plant and the diversity of the pathogen as well asan appropriate interaction with the productive chain. In this chapter, we provide anoverview of the advances and perspectives of breeding plants for bacterial diseaseresistance in distinct pathosystems involving field and vegetable crops.</p><p>Keywords Plant breeding Plant diseases Quantitative genetics Biotechnology</p><p>3.1 Introduction</p><p>The control of bacterial diseases in plants is difficult, especially after the estab-lishment of the epidemics. This difficulty reflects the diversity of the inoculumsources, the rapid multiplication of the pathogen following infection, the</p><p>C. A. Lopes (&amp;) L. S. BoiteuxEmbrapa Hortalias, Brasilia, Brazile-mail: carlos.lopes@embrapa.br</p><p>L. S. Boiteuxe-mail: boiteux@cnph.embrapa.br</p><p>R. Fritsche-Neto and A. Borm (eds.), Plant Breeding for Biotic Stress Resistance,DOI: 10.1007/978-3-642-33087-2_3, Springer-Verlag Berlin Heidelberg 2012</p><p>37</p></li><li><p>emergence of variants capable of overcoming or neutralizing certain controlstrategies, the low level of cultivar resistance, and the low availability of registeredchemical or biological products. An effective control strategy requires the inte-gration of preventive measures, among them the use of cultivars with geneticresistance to bacterial diseases, which are of low cost, have small environmentalimpact, and are of ease adoption by farmers.</p><p>3.2 PathogenHost Interaction</p><p>The infection process of phytopathogenic bacteria requires the genetic activationof recognition, contact establishment, host colonization, and infection. Thesecomplex events have been gradually elucidated through the use of modernmolecular biology and genetic techniques. For example, the recognition of the hostby bacteria initially occurs upon the perception and recognition of chemicalsignals, especially those of the phenylpropanoid metabolic pathway, such as plantflavonoids. For bacteria, particularly Agrobacterium tumefaciens, Pseudomonassyringae, and Xanthomonas campestris, the enzyme histidine kinase is the sensormolecule that allows for the pathogen association with the host plants. There arealso signals that allow bacteria to communicate with one another, triggeringprocesses that are generally dependent on the size of the population, which is aphenomenon known as quorum sensing. In this process, bacteria regulate geneexpression by the concentration of the diffusible signaling molecules produced bythem. In gram-negative bacteria, the most well-known signaling molecules are theN-acyl homoserine lactones (AHLs). After recognition, quorum sensing isessential for the colonization of the host, conjugative transfer of plasmids, regu-lation of the type III secretion system (T3SS), and the production of extracellularpolysaccharides (EPS) (Kado 2010).</p><p>Virulence, which is the ability of a pathogen to induce disease to a plant, variesamong isolates. For phytopathogenic bacteria, virulence depends on severalfactors, including population (quorum sensing), the ability to invade and colonizethe host, and withstand plant defense mechanisms. Phytopathogenic bacteria useT3SS to colonize their hosts, extract essential nutrients, and cause disease (Kado2010). Different T3SS secrete a set of distinct virulence factors (effectors). Theseproteins are crucial elements for pathogenesis, as they mediate the breakdown orsuppression of components of the plant cell defense network. For example, inRalstonia solanacearum, the synthesis of the major virulence factor EPS isregulated through the quorum sensing system, which is abundantly produced athigh bacterial cell densities or when the pathogen invades the xylem vessels ofhost plants (Milling et al. 2011). If recognized by the host cellular defense system.The effector might be subsequently converted into an avirulence factor (avr gene)according to the gene-for-gene model.</p><p>38 C. A. Lopes and L. S. Boiteux</p></li><li><p>3.3 Plant Breeding for Disease Resistance</p><p>Plant breeding has made an unquestionable contribution to society and agriculture.Recent evidences of climate change indicate the need for development of cultivarsthat are genetically resistant to pathogens well-adapted to high temperatures, suchas Pectobacterium spp., Dickeya spp., and Ralstonia solanacearum. Furthermore,there is a growing concern related to the serious threat of genetic erosion due tothese environmental changes. For example, it has been predicted that 1622 % ofthe 108 wild potato species of the genus Solanum employed in breeding programsare threatened with extinction by 2055 (Info-Resources Focus 2008).</p><p>The control of plant diseases, including those of bacterial origin, requiresintegrated measures, among which genetic resistance has been considered. Plantbreeding programs are generally designed for long-term objectives and should,therefore, be well-planned with a clear focus, achievable targets, and a broad viewof the agribusiness chain. Although disease resistance has received high priority inmany breeding programs, it is only one of the many desired characteristics that anewly developed cultivar must have. In practical terms, a disease-resistant cultivarmust also have high yield and quality as the most important characteristics formany species. Russel (1978) suggested that plant breeders should primarily beconcerned with avoiding the extreme susceptibility of cultivars to diseases.Therefore, selections performed in the field under natural pathogen pressure tendto increase the chances of obtaining cultivars with resistance to a greater numberof diseases, even those that have not been a direct target, which is a phenomenonknown as non-intentional selection (Heiser 1988).</p><p>Having defined that disease resistance is a priority for breeding programs, thefollowing considerations should be addressed before initiating the selectionprocedures:</p><p>1. Is the target disease really important? The importance of a given disease isnot the only reason to initiate a resistance breeding program. Many plant dis-eases are able to cause damage to crops and result in a loss of productivity orquality. The main rationale to justify a breeding for resistance program must bethat these diseases cannot be controlled in an effective manner using alternativecontrol methods.</p><p>2. Is the pathogen sufficiently known? It is crucial to understand the variability ofthe pathogen to determine the type of resistance that should be incorporated intoelite germplasm. A phytopathologist should be consulted to evaluate the path-ogen variability and to define the appropriate inoculation methodology, whichwould allow a clear distinction between resistant and susceptible genotypes.</p><p>3. Are there sources of resistance? Typically, this information can be obtainedfrom the literature. When these sources exist, the development of resistantcultivars might also be challenged by the increasing difficulty of germplasmexchange. In Brazil, a license to collect specimens is required from the Bra-zilian Institute of Environment and of Renewable Natural Resources (InstitutoBrasileiro do Meio Ambiente e dos Recursos Naturais RenovveisIBAMA)</p><p>3 Breeding for Resistance to Bacterial Diseases 39</p></li><li><p>and the Council for the Management of the Genetic Heritage (Conselho deGesto do Patrimnio GenticoCGEN).</p><p>4. What type of resistance should be sought? In case of pathogens with littlevariation, qualitative resistance (monogenic/oligogenic) is preferred, due to theease of introgression and incorporation. Quantitative resistance (polygenic) hasbeen recommended for highly variable pathogens, which, despite complexintrogression into genotypes of commercial interest, protects the plant againstall the variants of the pathogen.</p><p>5. What is the most appropriate breeding method? The reproductive system ofthe plant species determines the breeding method. Thus, different methods arerequired for either autogamous, allogamous or vegetatively propagated plantspecies as well as for their distinct ploidy levels. The selection of resistancetraits that display high heritability and simple genetic control, mainly of theadditive type, might be achieved through the individual performance or, rather,the performance per se of inbred lines or populations. When the characteristichas low heritability or displays inheritance that is conditioned by genes withnon-additive effects, selection should be based on progeny testing and hybridperformance. Notably, if there is a significant maternal effect, then there will bea difference in the selection of the female parent for a particular crossing.</p><p>3.4 Sources of Disease Resistance</p><p>The sources of disease resistance, including the bacterial diseases, have beenidentified mainly in wild species. Therefore, genetic breeding programs with awide genetic base would take the advantage of having genes of interest availablewhen needed. According to Rick (1986), traditional breeding programs aiming todevelop pure inbred lines with high yield potential and good commercial char-acteristics promotes the narrowing of the genetic base. Fortunately, the geneticresources of the centers of origin and diversity provide an extraordinary source ofuseful alleles, especially with regard to disease resistance. For the tomato alone,the germplasm collection contains more than 60,000 accessions in gene banksworldwide (Ross 1998). In Brazil, more than 4,000 accessions of tomato aremaintained at Embrapa, Agronomic Institute of Campinas (Instituto Agronmicode Campinas), and Federal University of Viosa (Robertson and Labate 2007).</p><p>Plant breeders usually have access to an abundance of academic literature onthe reaction of a plant species germplasm to bacterial diseases. However, partic-ularly for the case of quantitative resistance, it is common to find conflictinginformations, and it is not always easy to identify the most reliable data. Theseconflicts do not reflect an ethical problem, as discrepancies can result fromdifferent data interpretation and from environmental effects. Most discrepancies doreflect weaknesses in the experimental methodology and the selection systemsused. This chapter will discuss how important is the choice of the right selection</p><p>40 C. A. Lopes and L. S. Boiteux</p></li><li><p>methodology in order to allow for a clear distinction between resistant andsusceptible genotypes. The appropriate methodology will enhance experimentalprecision, which allows for the differentiation of more subtle expression levels ofquantitative resistance. An effective selection of disease resistant plants requiresbreeders to have a clear understanding of the pathosystem being studied.Therefore, the knowlegde about the variability of the pathogen (virulence andaggressiveness), the variability of the host plant as a function of the type andinheritance of resistance, and the effects of the environmental conditions on themanifestation of the phenotypic expression of the disease is desirable.</p><p>3.5 Qualitative Resistance Versus Quantitative Resistance</p><p>The development of disease resistant cultivars requires knowledge of the types ofresistance and the genetic mechanisms that regulate the expression of this trait.There are two categories of resistance: complete resistance, which is controlled byone or a few genes, and incomplete or partial resistance, which is conditioned byseveral genes that individually contribute with smaller effects. Several names areused to describe these types of resistance, such as vertical versus horizontal,complete versus incomplete, monogenic versus polygenic, and qualitative versusquantitative (Table 3.1). For standardization, the terms qualitative and quanti-tative will be used herein.</p><p>Qualitative resistance confers a high degree of resistance and is easily incor-porated. It is generally achieved through backcrossing (Borm and Miranda 2009).A major limitation of qualitative resistance is its low durability, especially inpathosystems in which the pathogen has high evolutionary potential, such asbacteria. In addition, this type of resistance is not always available, even amongwild host species. Bacterial pathogens with high evolutionary rates can overcomespecific recognition mechanisms, while non-specific mechanisms ensure thestability of the resistance when faced with new pathogen variants.</p><p>The quantitative resistance in the vast majority of the pathosystems studied todate does not confer a degree of resistance as high as qualitative resistance,although it offers durable and effective protection against several variants of thepathogen.</p><p>Table 3.1 Differences between qualitative and quantitative resistance</p><p>Characteristics Qualitative resistance Quantitative resistance</p><p>Reaction to pathogen variants Specific Non-specificEnvironmental stability Stable UnstableInheritance Monogenic/oligogenic PolygenicLevel of resistance High ModerateEpidemiologically based concept Vertical Horizontal</p><p>3 Breeding for Resistance to Bacterial Diseases 41</p></li><li><p>3.6 Hypersensitivity Reaction and Qualitative Resistance</p><p>Certain bacteria, such as R. solanacearum and A. tumefaciens, are capable ofinfecting wide range of hosts. Other bacteria display specificity, such as Erwiniaamylovora, which attacks apples, pears, and other rosaceous plants, and P. syringaepv. tomato, which only attacks tomato. The contact between the bacteria and a hostplant might result in a compatible interaction, where the bacterial pathogen rapidlymultiply in the intercellular spaces of the host, leading to the expression of diseasesymptoms, or an incompatible interaction, in which the pathogen fail to multiplyand the plant does not display disease symptoms. The incompatible reaction isnormally followed by rapid cellular collapse, which delimits the affected area andimpedes the spread of the bacteria to neighboring tissues. This phenomenon iscalled hypersensitivity reaction (HR), which is closely associated with thequalitative resistance to bacterial diseases. HR occurs when the bacterial pathogeninjects effector proteins into the host plant using a secretion system, such as T3SS.In response to the effector proteins, resistant plants undergo biochemical reactions,resulting in the almost immediate interruption of the infection process, thuspreventing the bacteria from invading other parts of the plant. Recognition of thepathogen by a resistant plant requires the presence o...</p></li></ul>

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