Rice: The Model Monocot Plant
Among the cereal crops, rice (Oryza sativa) has several attributes that make it the model monocot plant. Rice has a DNA content smaller than that of any crop plant (C=450Mbp; Arumanagathan and Earle, 1991), about three times the size of the Arabidopsis thaliana genome. The small genome of rice includes a large percentage (ca. 75%) of single copy DNA (Deshpande and Ranjekar, 1980; McCouch et al., 1988). A vast reservoir of germplasm (> 200,000 accessions) of both domestic and wild rices is available for genetic and breeding research. Rice has proven to be the most readily transformable cereal crop (Zhang et al., 1988; Hiei et al., 1994).
In the last 10 years, two high-density molecular linkage maps of rice containing more than 2300 markers have been developed in the US and Japan, making the marker density in the rice genome, on average, one marker per cM (200-300 kb) (Causse et al., 1994; Shomura et al., 1994). Over 100,000 expressed sequence tags (ESTs) have been deposited in the public database and many of them are mapped on the molecular maps. In the Rice Genome Program of Japan, a YAC library has been fingerprinted and ordered with mapped markers currently covering 52% of the rice genome. Several BAC libraries with an average insert size more than 100 kb have been described and are being used in physical mapping and map-based cloning of important genes (Wang et al., 1995, Zhang and Wing, 1997). More excitingly, an international collaboration on sequencing the rice genome was launched in 1998 with participating centers and labs from Japan, the US and European Union. It is anticipated that the complete rice genomic sequence will be publicly available in the end of 2002.
Recent work on wheat, rye, barley, maize, sorghum, millet, and rice indicates that grass genomes have similar genetic maps over large blocks of the chromosomes. By selecting tightly linked single copy markers in wheat, wheat geneticists will be able to screen the homologous region in rice for their candidate homologue. The syntenic relations can be exploited in other directions as well. For example, mapping data can be taken from maize where there is extensive work in both transmission and molecular genetics to predict the location of a homologue in rice. Therefore, all the information generated from rice can be directly or indirectly applicable to other cereals.
The fungus Magnaporthe grisea is the causal agent of rice blast. Aside from rice, this fungus can also attack more than fifty other species of grasses and sedges. Rice blast cause disease at seedling and adult stages on the leaves, nodes, and panicles. On the leaves, lesions are typically spindle-shaped-wide in the center and pointed toward either end. Large lesions usually develop a greyish center, with a brown margin on older lesions (see below). Under conducive conditions, lesions on the leaves of susceptible lines expand rapidly and tend to coalesce, leading to complete drying of infected leaves. The instability of the fungus and the high variability in its pathogenicity make its control and management difficult. The disease occurs in most rice growing areas worldwide, costing farmers a loss of nearly $5 billion a year (Moffat, 1994).
M. grisea is becoming an excellent model organism for studying fungal phytopathogenicity and host-parasite interactions. It is a haploid, filamentous Ascomycete with a relatively small genome of ~40 Mb contained in 7 chromosomes (Orbach, 1996; Talbot et al., 1993). Unlike many phytopathogenic fungi such as the mildews and rusts, the rice blast fungus can be cultured on defined media, facilitating biochemical and molecular analyses. Significantly, early stages of the infection process including germination, appressorium formation and penetration can be studied ex planta Tools for molecular genetic manipulation are well developed in the last decade. A typical transformation using 107 protoplasts yields 10-50 stable transformants per microgram of added DNA. Random insertion of transforming DNA has also been successfully used to tag genes involved in conidiation and pathogenicity. Many genomic resources such as ESTs, BACs, physical map and genomic sequence are now public available. The whole genome will be sequenced within 2 years. For more information on the rice blast, please visit these two websites: