Rice Blast Fungus Attacks Leaves and Roots Differently

Rice blast, the causal agent of which is Magnaporthe grisea, is one of the most destructive plant pathogens known. It has been studied in extraordinary detail from almost all conceivable standpoints (Ziegler, Leong & Teng 1994, Tharreau et al. 2000, Talbot 2003). The genome has been sequenced, and the infection process on leaves is known to comprise a series of developmental steps involving over 30 genes. The fungus is, however, a member of the same family as the soil-borne take-all pathogen of cereals,Gaeumannomyces graminis, which invades through roots. The development of M. grisea on roots has now been investigated in detail using strains tagged with a green fluorescent protein (GFP) and chlorazole black E staining (Sesma & Osbourn 2004). It was discovered that on roots the fungus infection developed in a different way to the process on leaves. The melanized appressoria typical of foliar infections and which build up astonishing turgor pressures in order to penetrate leaf surfaces were not developed. Instead, when growing on the root surface, much simpler penetration structures are developed, which take the form of hyphal swellings and in turn give rise to hyphal pegs. The authors refer to these swellings as ‘hyphopodia’, a term better subsumed under ‘appressoria’ as the structures are functionally similar (Mibey & Hawksworth 1995), but it many be more helpful to use the terms ‘functional ’ and ‘nonfunctional ’ appressoria. Whatever label is used for the non-functional swellings, the excitement is that these are unlike the appressoria formed on leaves, but do recall the structures produced by the root-infecting G. graminis. Further, sclerotium-likes structures developed on the root surface and inside the outermost layers of root tissue; these recall the growth-cessation structures familiar from G. graminis. Once inside roots, GEF-tagging shows that M. grisea can extend into the aerial parts of the plant. The knowledge that the fungus can invade through roots is of significance for the development of control strategies which have traditionally been focussed on aerial infections. Sesma & Osbourn (2004), who are based at The Sainsbury Laboratory in Norwich, also generated strains of M. oryzae which were melanin-deficient; these could not form functional appressoria and were unable to penetrate leaves, but could still produce the smaller non-functional appressoria on roots and successfully infect them. The genetic basis for the differences were investigated using an FOW1 homologue linked with infection processes in Fusarium oxysporum, a characteristic root-invader. When the gene was deleted, there was a reduction in root browning. In addition, the authors searched for evidence of gene-forgene resistance with the host using different rice cultivars, and found some evidence of specific disease resistance to the rice blast fungus in roots. This is a fascinating story, particularly well-illustrated by colour photographs and also superb confocal microscopy images, and is sure to be something lecturers in plant pathology will wish to include in future courses.