Researcher Angela Mehta in the greenhouse. (Photo: Cláudio Bezerra)
- The cultivar used in the study was BRSMG Curinga, launched in 2005 and replaced in the market after losing resistance to blast over the years.
- Study identified group of proteins potentially involved in susceptibility to the disease and selected some corresponding genes to do the CRISPR knockout.
- The research resulted in two rice strains resistant to blast, presenting mutations for the two target genes.
- The technology is still in the testing phase, but it has the potential to reach the market in a few years.
(September 4, 2024) - Gene editing, using the CRISPR/Cas9 technique, is being used to confer blast resistance to the BRSMG Curinga rice cultivar, in a research by Embrapa Genetic Resources and Biotechnology (DF). Launched in 2005, BRSMG Curinga was replaced in the market after becoming susceptible to this disease, caused by the fungus Magnaporthe oryzae, over the years. The technology, the result of the knockout of two target genes related to blast, is still in the testing phase, but has the potential to reach the market in a few years.
According to researcher Angela Mehta, cultivars such as BRSMG Curinga, launched on the market as resistant or tolerant to blast, become susceptible in a few years, as the variability of the fungus is very high. This is a problem for both rainfed and irrigated rice. The study compared a susceptible rice genotype with a resistant genotype in search of proteins that, after infection by the pathogen, become more abundant in the susceptible plant.
"We identified a group of proteins potentially involved in this susceptibility and selected some corresponding genes to do the CRISPR knockout," Mehta said. The term "knockout" means to remove the function of a certain gene, which stops producing the functional protein. For the researcher, this work also shows the importance of prospecting for genes of agronomic interest by omics techniques (a set of molecular tools that help in understanding the different biological molecules that give functionalities to an organism, such as genomics, transcriptomics, proteomics, and metabolomics).
Three knockout genes were identified, two of which were validated during the sandwich doctorate of Fabiano Távora, supervised by Mehta, carried out at the Center for International Cooperation in Agronomic Research for Development (Cirad), in France.
Knockout
When knocking out these genes in the Kitaake rice variety, considered a model, Távora found that the plant became a little more resistant to blast compared to the unedited variety. "When he returned from France, we used one of these genes as a target, and selected two others to make a combination of targets for the knockout of two genes at the same time, in two distinct constructs in the BRSMG Curinga cultivar," explains Mehta. The study resulted in two rice lines resistant to blast (one for each construct), presenting mutations for the two target genes.
According to the researcher, knockouts of two genes were performed at the same time to enhance resistance and obtain a better response. She points out that these results, so far, have been obtained in a greenhouse, challenging the strains to an isolate of the fungus. "The next step is to challenge them with other isolates to see if this resistance will continue," he adds.
Mehta also notes that BRSMG Curinga was chosen because it is a transformable genotype and because it has agronomic characteristics closer to the producer's interest when compared to model cultivars, such as Nipponbare and Kitaake. "These varieties do not have the level of improvement for the uplands of Curinga, which has been adapted to the conditions of the Brazilian Cerrado", he points out.
For the researcher, the result of the research offers an important source of genetic resistance to blast, different from what is available today, for incorporation into varieties and elite strains and even into strains of populations of recurrent selection of the rice breeding program.
The study is being developed in partnership with researchers Raquel Mello and Adriano Castro, from Embrapa Rice and Beans (GO), in a project led by Angela Mehta, funded by Embrapa, with the support of an Embrapa-Monsanto project, led by researcher Márcia Chaves, from Embrapa Temperate Climate (RS).
Blast
Blast, caused by the fungus Magnaporthe oryzae, is considered the most destructive disease of rice and occurs throughout the Brazilian territory. The losses are variable, being greater in upland rice, in the Midwest Region, and can compromise production by up to 100% in years of epidemic attacks. It also occurs in several grasses common in rice and wheat fields.
The main sources of primary contamination are infected seeds and crop residues. Secondary infection, on the other hand, has as its source the sporulative lesions of the infected leaves. All phases of the disease cycle are highly influenced by climatic factors. In general, high temperatures (from 25 °C to 28 °C) and humidity above 90% are required.
Symptoms on the leaves begin with the formation of small brown necrotic lesions, which increase in size and become elliptical, with brown margins and a gray or whitish center. Under favorable conditions, the lesions cause the death of the leaves and, often, the entire plant.
The characteristic symptoms in the nodes are brown lesions that can reach the regions of the stem near the attacked nodes. Infection of the node at the base of the panicle (rice plant inflorescence) is known as neck blast, as a brown lesion surrounds the nodal region and causes the plant to strangle.
It may present hatching (anomaly that occurs when the grains do not develop correctly). The panicles turn whitish, and are easily identified in the field. Several parts of the panicle, such as rachis, primary and secondary branches, and pedicels, are also infected.
Control
Currently, the damage caused by blast can be reduced by the use of resistant cultivars, cultural practices and fungicides, used in an integrated way in crop management with adequate soil preparation; balanced fertilization, avoiding the exaggerated vegetative growth of the plant; use of seeds of good phytosanitary and physiological quality; planting done in a minimum period of time and started in the opposite direction of the wind; incorporation of crop residues; uniform planting depth; and recommended sowing density for the cultivar or planting system.
Weed control is also important; destruction of voluntary and diseased plants; good soil leveling; maintenance of the appropriate level of irrigation water during the plant cycle; adequate sizing of irrigation and drainage systems; change of cultivars sown every three or four years; planting at the beginning of the rainy season; and use of fungicides applied in seed treatment and spraying of the aerial part.
Protection against panicle blast is done preventively, through spraying with systemic fungicides: one application should be carried out at the end of the rubberizing period and the other with up to 5% panicle emission.
