Classical and molecular genetics of Bremia lactucae, cause of lettuce downy mildew

Abstract

Lettuce downy mildew caused by Bremia lactucae has long been a model for understanding biotrophic oomycete-plant interactions. Initial research involved physiological and cytological studies that have been reviewed earlier. This review provides an overview of the genetic and molecular analyses that have occurred in the past 25 years as well as perspectives on future directions. The interaction between B. lactucae and lettuce (Lactuca sativa) is determined by an extensively characterized gene-forgene relationship. Resistance genes have been cloned from L. sativa that encode proteins similar to resistance proteins isolated from other plant species. Avirulence genes have yet to be cloned from B. lactucae, although candidate sequences have been identified on the basis of motifs present in secreted avirulence proteins characterized from other oomycetes. Bremia lactucae has a minimum of 7 or 8 chromosome pairs ranging in size from 3 to at least 8 Mb and a set of linear polymorphic molecules that range in size between 0.3 and 1.6 Mb and are inherited in a non-Mendelian manner. Several methods indicated the genome size of B. lactucae to be ca. 50 Mb, although this is probably an underestimate, comprising approximately equal fractions of highly repeated sequences, intermediate repeats, and lowcopy sequences. The genome of B. lactucae still awaits sequencing. To date, several EST libraries have been sequenced to provide an incomplete view of the gene space. Bremia lactucae has yet to be transformed, but regulatory sequences from it form components of transformation vectors used for other oomycetes. Molecular technology has now advanced to the point where rapid progress is likely in determining the molecular basis of specificity, mating type, and fungicide insensitivity.

Natural history of Arabidopsis thaliana and oomycete symbioses

“…Much of the infrastructure is falling into place, behind major advances in the molecular genetics of downy mildew resistance in A. thaliana over the past two decades, and the discovery of large numbers of oomycete effectors providing raw material for another complementary round of investigation in the downy mildew parasites of A. thaliana and crops such as lettuce (Michelmore and Wong 2008;Tör 2008;Win et al 2007). In parallel, the infrastructure for A. thaliana field biology is beginning to emerge, which has exciting potential for advancements in understanding the evolutionary context of a range of developmental and physiological processes in plants.…”

“…Similarly, non-specific elicitors have come in recent years to be referred to as PAMPs (pathogen associated molecular patterns), an abbreviated term adopted from the research literature of innate immunity in animals. Many ATR (A. thaliana recognized) effectors were predicted in HpA following the genetic identification of numerous R-genes conferring downy mildew resistance in A. thaliana and classical genetic analysis of avirulence in Bremia lactuca (Holub and Beynon 1997;Michelmore and Wong 2008). However, confirmation of a 'gene-forgene' correspondence has only recently been confirmed for five cloned RPP genes, followed swiftly by the molecular identification of the first two ATR effectors (Table 1; Fig.…”

“…For instance, molecular genetics led to the discovery of numerous pathogen receptorlike genes that provide the molecular basis for downy mildew resistance in A. thaliana (Holub 2001(Holub , 2007. These major discoveries in turn have been instrumental in the early development of marker-assisted selection for disease resistance in crops (Aarts et al 1998a;Botella et al 1997;Michelmore and Wong 2008;Shen et al 2002;Speulman et al 1998).…”

Comparative Large-Scale Analysis of Interactions between Several Crop Species and the Effector Repertoires from Multiple Pathovars of Pseudomonas and Ralstonia

“…Nineteen cultivars of lettuce and 19 cultivars of tomato were selected as representing a large number of known resistance specificities, many of which had been introgressed from wild species (Farrara et al, 1987;Laterrot, 1987;Williams and St. Clair, 1993;Grube et al, 2000;Van Deynze et al, 2007;Michelmore and Wong, 2008). These accessions, therefore, were expected to express diverse clusters of R genes.…”

Resistência de linhagens de alface-crespa às raças de Bremia lactucae identificadas no Estado de São Paulo

“…Destacase que para o caso do IL, só na cultivar suscetível se apresentaram diferenças significativas entre as raças e a mistura (fator A), e que existem diferenças de resposta entre os nove genótipos (fator B), quando inoculadas com todas as raças, exceto quando inoculadas com a raça SPBl:12 e a mistura das nove raças (fator A , L 8 dependendo da linhagem e da raça/mistura inoculada, apresentaram alto nível de resistência (0-4,6% de PE), resistência incompleta (6,0-15,1% de PE), e a linhagem L 5 apresentou resposta heterogênea de resistência e suscetibilidade à raça SPBl:07 (27,9% de PE). Possivelmente, a resistência incompleta, identificada nestas linhagens, esteja relacionada com o fato de que um gene de efeito principal possa, sob certas condições ambientais e/ou genéticas, ter seu efeito reduzido, tornando-se de efeito secundário, como se tem identificado no caso dos genes Dm6, Dm7 e R18, cuja expressão tem sido incompleta em certas cultivares de alface (8,14). …”