REVIEW ARTICLE Genomic advances will herald new insights into the Brassica: Leptosphaeria maculans pathosystem A. Hayward 1 *, J. McLanders 1 *, E. Campbell 1 , D. Edwards 2 & J. Batley 1 1 ARC Centre of Excellence for Integrative Legume Research and School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD, Australia 2 Australian Centre for Plant Functional Genomics, School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD, Australia INTRODUCTION Brassica is a genus of plants in the mustard family (Brassica- ceae), which is comprised of approximately 3350 species in 340 genera and includes some of the most important crops worldwide. A number of the species are grown as vegetable crops or ornamentals, and the oil derived from the seed is useful for nutritional and industrial purposes. There are six cultivated Brassica species: B. napus (canola / rapeseed / oilseed rape), B. rapa (Chinese cabbage / turnip rape), B. oleracea (including broccoli, cabbage and cauliflower), B. juncea (Indian mustard), B. nigra (black mustard) and B. carinata (Ethiopian mustard). B. napus is the evolutionary result of an interspecific hybridisation between B. rapa (AA genome, 2n = 20) and B. oleracea (CC genome, 2n = 18), generating an amphidiploid genome AACC (2n =4x = 38). Similarly, B. carinata (BBCC) and B. juncea (AABB) are the conse- quence of interspecific hybridisation. Evidence suggests very little recombination occurred between the progenitor genomes of B. juncea, B. carinata and B. napus during hybridisation (Snowdon et al. 2002). Worldwide, the 2010–2011 total oilseed production is esti- mated to be 440 million metric tons, with rapeseed contrib- uting 60.4 million metric tons (USDA 2010). From this, rapeseed is estimated to produce over 34 million metric tons of meal and 23 million metric tons of oil. This production can, however, be limited by the virulent fungal pathogen Lep- tosphaeria maculans, the causal agent of blackleg disease. As the most damaging disease to Australian canola, black- leg (also termed stem canker or phoma disease) virtually eliminated canola production throughout Australia in the early 1970s (Howlett 2004; Wang et al. 2009). L. maculans is a Dothideomycete that undergoes phases of biotrophic, nec- rotrophic and saprobic growth during the infection life cycle. During the saprobic phase, pseudothecia are produced and survive on the stubble (short, stiff stalks of plant material left in the field after harvest) of infected rapeseed (Howlett et al. 2001). Sexual ascospores differentiate inside these pseudothe- cia, which then act as the major source of future infection. Both ascospores and asexual conidia are able to infect via stomata and / or wounds on the surface of cotyledons and young leaves. Following infection, the fungus multiplies asex- ually for a very brief necrotrophic phase, causing leaf spots (Rouxel & Balesdent 2005). The fungus then becomes endo- phytic and spreads intercellularly in the leaf as a biotroph. Subsequent symptomless growth into the petiole between the xylem parenchyma and cortex, or within the xylem vessels, extends the fungal colonisation into the stem cortex, where a number of individuals may exist quiescently for many months (Hammond et al. 1985; Howlett et al. 2001; Rouxel & Balesdent 2005). At the end of the growing season, the fungus becomes necrotrophic, with symptoms being dry Keywords Blackleg; Brassica; canola; disease resistance; gene-for-gene interaction; Leptosphaeria maculans; R gene. Correspondence J. Batley, School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD 4072, Australia. E-mail: J.Batley@uq.edu.au Editor A. Weber *Joint first authors. Received: 3 February 2011; Accepted: 8 April 2011 doi:10.1111/j.1438-8677.2011.00481.x ABSTRACT The study of the relationship between plants and phytopathogenic fungi is one of the most rapidly moving fields in the plant sciences, the findings of which have con- tributed to the development of new strategies and technologies to protect crops. Plants employ sophisticated mechanisms to perceive and appropriately defend themselves against pathogens. A good example of plant and pathogen evolution is the gene-for-gene interaction between the fungal pathogen Leptosphaeria maculans, the causal agent of blackleg disease, and Brassica crops. This interaction has been studied at the genetic and physiological level due to its agro-economic importance. The newly available genome sequence for Brassica spp. and L. maculans will pro- vide the resources to study the co-evolution of this plant and pathogen. Particularly, an understanding of the co-evolution of genes responsible for virulence and resis- tance will lead to improved plant protection strategies for Brassica canola and pro- vide a model to understand plant–pathogen interactions in other major crops. This review summarises the research-to-date in the study of the Brassica–L. maculans gene-for-gene interaction, with a focus on the genetics of resistance in Brassica and the wealth of information to be gained from genome sequencing efforts. Plant Biology ISSN 1435-8603 Plant Biology ª 2011 German Botanical Society and The Royal Botanical Society of the Netherlands 1