Magnaporthe oryzae

Background information

Rice blast, caused by Magnaporthe oryzae, belongs to the phylum Ascomycota, class Pyrenomycetes, order Diaporthales and family Magnaporthaceae (Cannon, 1994). It is a filamentous, haploid, heterothallic fungus with vegetative hyphae. The classification of the Magnaporthe genus within the order Diaporthales suggests that the fungus belongs to a polyphyletic group (Cannon, 1994). The species M. oryzae is adapted to rice and probably, did not co-evolve with other gramineous species because no parallelism has been found between hosts and host specific sub groups (Kato et al., 2000). The importance of rice blast is reflected by its worldwide distribution and destructiveness under favourable conditions. Infection of rice plants involves the landing of airborne conidia on the rice leaf cuticle and elaboration of an appressorium, a specialized infection cell (Howard and Valent, 1996). Appropriate development of appressorium is required for infection. Four to five days after infection, first lesions appear which begin to coalesce after six days (Wanget al., 2005). All foliar tissues, panicles and, recently, roots (Sesma & Osbourn, 2004) are subject to infection.  Yield losses of 10%–30% are typical, although regional epidemics can lead to losses ranging from 70 to 80% when predisposing factors favor epidemic development (Piotti et al. 2005; Odile et al. 2010). The rice plants easily succumb to the disease when young plants hold 3 to 4 leaves and during flowering. The disease is favored by frequent occurrence of cloudy days with humidity ranging from 80%-100%. Sporulation is highly favored by presence of moisture on the infected leaf surfaces and temperatures ranging from 25-30°C. Host resistance is the most economically viable and environmentally sound practice to manage this disease. However, the fungus overcomes plant resistance quickly, and cultivars typically become ineffective within 2–3 years (Ou, 1980; Zeigler et al., 1994). Emergence of virulent pathotypes is attributed to loss of function of avirulence genes as a result of repeat-induced point mutations (Fudal et al., 2009; Van de Wouw et al, 2010), insertions of transposable elements, or deletions of entire genes (Kachroo et al., 1995; Zhou et al., 2007; De Wit et al, 2009; Chuma et al., 2011). Because M. oryzae is highly amenable to genetic and molecular manipulation (Dean et al., 2005), tracking the underlying molecular processes that lead to phytopathogenesis has gained more ground in the recent years. Fundamental insights into host defense and mechanisms that lead to evolution of resistance breaking isolates may prove useful in understanding phytopathogenesis. We are currently focusing our efforts on understanding rice- M. oryzae interaction at high and normal temperatures.

(GO 2012/11)