The genetics of Botrytis cinerea resistance in tomato

Botrytis cinerea Pers:Fr(teleomorph: Botryotina fuckeliana (de Bary) Whetzel) is a necrotrophic pathogenic fungus with a wide host range of at least 235 species (Elad et al. 2004). Modern hybrid tomato ( Solanum lycopersicum ) cultivars are susceptible to B. cinerea although some cultivars show a certain level of quantitative resistance. In heated greenhouses, B. cinerea infection in tomato is usually restricted to the stem (Dik and Wubben 2004;Shtienberg et al. 1998). Stem rot usually develops after pruning of lower leaves or side shoots (Verhoeff 1968) and results in a lower yield or premature plant death. Disease control frequently relies on fungicides (Leroux 2004) or biocontrol (Elad and Steward 2004). However, B. cinerea has developed resistance against several of these control measures (Elad and Steward 2004; Leroux 2004). The development of B. cinerea resistant cultivars by introgressing resistance alleles from wild tomato relatives may provide an effective alternative.Previously, quantitative resistance to B. cinerea has been identified in wild relatives of tomato (Egashira et al. 2000; Nicot et al. 2002; tenHave et al 2007; Urbasch 1986). The scope of this thesis was to identify quantitative trait loci (QTL) conferring resistance to B. cinerea originating from S. habrochaites LYC4 and S. neorickii G1.1601. Susceptibility to B. cinerea was assessed using a quantitative stem assay (ten Have et al. 2007). Three QTLs originating from S. habrochaites LYC4 (Chapter 2) and three QTLs originating from S. neorickii G1.1601 (Chapter 4) conferring resistance to B. cinerea were identified using transient (i.e. F _2-3 ) mapping populations. We presumed that not all QTLs conferring resistance to B. cinerea were identified in these two populations and proposed that an introgression line population would be more powerful for detection of QTLs (Chapter 1). For that reason, we developed a S. habrochaites LYC4 introgression line (IL) population (n=30;Chapter 3). Most ILs contained one S. habrochaites introgression in the S. lycopersicum cv. Moneymaker genetic background. Susceptibility of each IL to B. cinerea was assessed in a greenhouse trial (Chapter 3). In addition to the three QTLs identified in the F ₂ population, seven new QTLs were identified using this introgression line population (Chapter 3).None of the QTLs separately conferred a level of resistance comparable to the resistant parent S. habrochaites LYC4 (Chapters 3and 4). In order to obtain B. cinerea resistant tomato, QTLs need to be combined. In these studies, we have observed examples of combined QTLs acting in an additive manner (Chapter 3) and of combined QTLs showing less-than-additive epistatic interactions (Chapter 2). Redundancy of certain QTLs is also likely (Chapters 3and 6). Using the S. habrochaites LYC4 IL population, controlled crosses can readily be made to test which combinations of QTLs will result in the highest level of resistance while introgressing the minimum number of QTLs.We propose that introgressing three QTLs will be sufficient to obtain B. cinerea resistant plants. One resistant BC ₂ S ₁ genotype, identified while confirming the QTLs identified in the F ₂ population (Chapter 2), was selected and bred in. The level of resistance of this inbred (BRC-5) was comparable to the wild parent S. habrochaites LYC4 (Chapter 3). The high level of resistance of BRC-5 is likely caused by the (inter)action of two, possibly three QTLs (Chapters 3and 6).Each of the three S. neorickii G1.1601 QTLs was located at position homologous to the S. habrochaites LYC4 QTLs (Chapter 4). These loci might have evolved from a common ancestor and may be involved in general defense responses (Chapter 4).Variation was observed within the disease assays (Chapters 2, 4 and 6),yet replicate analyses of a genotype led to consistent conclusions. An IL population is an ideal tool for replicate analysis (Chapter 3). The LYC4 IL population can also easily be screened for additional traits, such as agronomic and morphological characteristics (Chapter 5). Genetic variation was observed for most traits. This data not only showed the additional potential use of this population for further breeding research, but also provided indications for a correlation between a lower yield and B. cinerea resistance (Chapter 6).This study identified loci conferring resistance to B. cinerea , which will be used to develop tomato cultivars that are (partially) resistant to B. cinerea . The ILs thatweredeveloped will provide useful tools for further studies to unravel the resistance mechanisms governed by individual QTLs (Chapter 6).