Approaches for improving crop resistance to soilborne fungal diseases through biotechnology using Sclerotinia sclerotiorum as a case study

Genetic engineering of crop plants with enhanced disease resistance has offered considerable promise and experimental power, however, with varying degrees of success. Traditional breeding has been very successful, though not in all cases. While the technology for gene manipulation in virtually any crop plant has been available for several years, field success has been hampered by our overall lack of understanding of the essential determinants mediating disease. Two key questions regarding molecular breeding will be addressed: (i) what genes or conceptual approaches can be used that have a realistic chance to be effective? and (ii) can we extrapolate useful information from model plants? Arabidopsis has served as an invaluable model system in many aspects of plant biology, including plant pathology and plant stress physiology, with many insights viewed to be directly applicable to crop plants. In addition, Arabidopsis has several experimental advantages: the genome has been sequenced, microarray chips are available, and there are a multitude of well characterised mutants. In addition, reverse genetics will continue as a powerful tool to examine gene function in Arabidopsis. The pros and cons of Arabidopsis application will be discussed. Sclerotinia sclerotiorum will serve as an example for approaches to disease control for soilborne fungal pathogens. The idea of interfering with fungal compatibility determinants coupled with biotechnology approaches will be described. S. sclerotiorum is an extremely broad host range, economically important, necrotrophic fungal plant pathogen. Diseases caused in economically important plants by S. sclerotiorum occur worldwide, cause considerable damage, have proven difficult to control (culturally or chemically) and host resistance to this fungus has been inadequate. A primary determinant contributing to the pathogenic success of this fungus is the ability to form sclerotia. The sclerotium of S. sclerotiorum is a multicellular, highly pigmented, rigid, asexual, resting or overwintering structure composed of condensed vegetative hyphal cells, which become interwoven and aggregate together, and it is capable of surviving years in soil. The importance of sclerotia for the pathogenic success of this fungus is underscored by the fact that sclerotia are the primary survival structures of this fungus upon which all other developmental phases of the fungus depend. Thus, sclerotia are an attractive target for intervention with the persistence of this pathogen. Effective pathogenesis by this fungus requires the secretion of oxalic acid, a primary pathogenicity determinant. Since this necrotrophic fungus requires host cell death for pathogenic success, we examined whether or not modulation of programmed cell death would impact the plant response to this aggressive pathogen. In animals, programmed cell death or its morphological equivalent, apoptosis, is genetically controlled cellular suicide. Multicellular organisms eliminate redundant, damaged or aged cells by this gene-directed cell death process. It is a complex process that is essential for development, maintenance of cellular homeostasis and for defence against environmental insults such as pathogen attack. Taking a trans-kingdom approach, transgenic crop plants that express animal anti-apoptotic genes have been generated. These genes all suppress apoptotic death in animal cells. We have shown that expression of these genes in tobacco and tomato abrogate disease development in plants infected with S. sclerotiorum. Plants with null mutations in these transgenes did not protect against pathogens. These data suggest that disease development requires host cell death pathways, thus differing from traditional concepts associated with necrotrophy. Transgenic plants also displayed tolerance or resistance to several abiotic stresses (heat, cold, salt and drought). Functional plant homologues of these mammalian genes are being identified. Taken together, our data suggest that modulation of host cell death is crucial in dictating the outcome of several fungal-plant interactions. The complete genome of S. sclerotiorum has been sequenced. The assembled sequence encodes a 39Mb genome size with >8 fold coverage. The generation of an optical map and our collaboration with the ‘Botrytis community’ is expected to yield new insight into fungal biology via comparative genomics.