Research uses fragment of wild plant DNA to combat wheat blast

A fragment of DNA originating from a wild relative of wheat, called an element of 2NS/2AS translocation, may be responsible for the largest advance to combat grain blast. Scientists from Embrapa Trigo (RS) and the International Maize and Wheat Improvement Center (CIMMYT) achieved promising results with the technique. “In plants with 2NS/2AS, the severity of blast is up to 50% lower compared to those that do not have translocation”, says researcher CIMMYT Pawan Singh.

The so-called 2NS/2AS translocation is a fragment of a chromosome removed of Aegilops ventricosa, wild species related to the cultivated plant. The first cultivar developed containing 2NS was VPM1, in France, at the end of 1960s. After that, this cultivar was crossed with others and the 2NS/2AS translocation conquered the world, mainly through crosses carried out at CIMMYT. As a research base, the international institution has with four blast nurseries in Bolivia and Bangladesh, where they are evaluated more than 15 thousand lines of wheat per year to check sources of resistance genetics to the disease.

Researchers consider 2NS/2AS the beginning of the search for markers genes related to blast resistance. This excerpt, associated with other which have yet to be identified, could be the key to a resistant cultivar to the disease, according to scientists. This is a long job. As comparison, for rice, the crop in which blast appeared, have already been identified more than 100 disease resistance genes in wheat, only nine resistance genes have been detected so far.

The main disease of wheat in the tropics

Blast is the main disease that limits wheat growth tropical in the world, but new knowledge in genetic improvement points out the ways to resolve the problem. 

It is considered the most recent disease of economic importance detected in wheat around the world. Caused by a fungus, Pyricularia oryzae Triticum (Magnaporthe oryzae pathotype Triticum) that attacks leaves and ears, blast causes damage that can compromise up to 100% of the crop. O fungus is an old acquaintance of rice, since the first reports of rice blast culture dates back to around 1.600 in China. In wheat, the fungus that causes blast was identified for the first time in Brazil, in 1985, in crops in the Paraná, but the disease quickly spread to other Brazilian states. Today, in addition to Brazil, blast is already known in Bolivia, Paraguay, Argentina, Bangladesh and Zambia. 

Research

Selection for resistance with 2NS/2AS has two lines of research that go hand in hand: use of molecular markers and phenotyping. The use of molecular markers aims to improve the selection of the DNA fragment that will be translocated to the plant; Phenotyping, on the other hand, consists of observing the plants with 2NS/2AS that were inoculated with blast fungus, or even field trials, where the strains with the best defense response are evaluated the disease. According to Embrapa Trigo researcher Gisele Torres, the presence of 2NS/2AS in the plant is not guarantee of blast resistance: “We need to check the presence of 2NS/2AS with the molecular marker and then evaluate the plant's reaction to the fungus. They are complementary techniques in the search for the best result,” she considers. 

In Brazil, CIMMYT has the support of several partner companies to evaluate the performance of the lines regarding blast. One of them is the OR Seed Improvement, in which research on the 2NS/2AS translocation began in 2002, aiming at the introduction of resistance genes to leaf rust, rust linear and wheat stem rust, linked to translocation and which add resistance to a wide range of fungal races, being used in combinations with other durable resistance genes. According to OR researcher Camila Turra, crosses with 2NS/2AS express plant defense against blast in cob. However, field selection in experimental areas with irrigation and disease pressure is fundamental, seeking to combine genetic inheritance with phenotypic expression. “Today the selection for 2NS/2AS is 100% of the program genetic improvement, seeking the development of cultivars with broad cultivation adaptability, that is, cultivars that can be sown in the most several wheat-growing regions wherever a blast epidemic could occur. occur,” says Turra. 

At Biotrigo Genética, after publicizing the effect of the 2NS/2AS segment on blast, researchers sought to identify lineages with this segment, directing the use of technology at intersections and increasing its frequency in the wheat breeding. "I have no doubt that 2NS/2AS is a milestone in solution to the blast problem, but we still need to progress in research to identify other resistance genes”, comments researcher Paulo Kuhnem. "While research evolves in search of greater levels of resistance, aspects related to complementary chemical management, such as application of fungicides, also need to evolve", explains Kuhnem, highlighting that blast infects the rachis (main axis of the cob), where the fungicides still cannot satisfactorily provide protection efficiently.

In 2013, Embrapa Trigo brought 1581 lines from CIMMYT to evaluate the resistance to blast in the field, with subsequent use of markers molecular tests to confirm the presence of 2NS/2AS and experiments in nursing homes vegetation with pre-selected strains. “In the final selection, only eight lines with 2NS/2AS showed good resistance to blast and performed well satisfactory agronomic performance in our field conditions in the Brazilian Cerrado. So far, we have selected a candidate strain to reach the market as cultivar, which in addition to resistance to blast also presented excellent quality of flour for baking”, says the Embrapa researcher, Vanoli Fronza.

In 2021, Embrapa Cerrados (DF) researcher Angelo Sussel evaluated another 12 CIMMYT strains with 2NS/2AS and verified the incidence of, at most, 20% of blast in wheat dryland, without any application of fungicides. However, he remembers that the challenge for the last two wheat harvests has been the lack of water, causing losses even greater than the potential damage caused by blast: “Management practices, such as adjustments in sowing time, more genetic tolerance and efficient use of fungicides, have ensured protection against blast above 80% in crops. But, just like the blast fungus, wheat it needs water to survive”, argues the researcher.

In search of durable resistance

Although the 2NS/2AS translocation represents a milestone for research, in the fight against blast worldwide, caution is needed to include technology in breeding programs, with possible breaking of resistance in cultivars already reported in the scientific community: “We cannot be dependent only on 2NS/2AS to control wheat blast. The nature of the fungus is adaptation. There are no guarantees that, in the future, the resistance conferred by the sequence 2NS/2AS cannot be broken and this creates serious problems even for the food security in the country, if all cultivars made available to producers have been developed from this single alternative as source of resistance to blast”, explains Embrapa researcher, João Leodato Maciel. He also highlights that the main studies related to the 2NS/2AS sequence have indicated that resistance is associated only with wheat ear blast, requiring further studies on the effect of 2NS/2AS on resistance in cases of incidence of blast on leaves. 

According to Pawan Singh, other possible sources of genetic resistance, but the selection process requires time and investment. “We are working with the most modern biological techniques molecular to help in the genetic improvement of wheat, aiming to achieve great diversity of wheat producing regions in the world and guarantee the food security of the population”, says Singh. 

In the world, according to Kuhnem, many research works have sought wheat parental species aiming to identify new sources of resistance to blast, but, to date, none have shown the effect achieved with the 2NS/2AS. 

“2NS/2AS technology is today the best we have to live with blast. Combining translocation with chemical control and sowing time most suitable for each cultivar, we achieved enormous progress in triticulture tropical. The use of new technologies has allowed the area to increase in Goiás and Minas Gerais and now we will also be able to expand to other states. AND, when we can put 2NS/2AS together on the same cultivar with other sources of resistance, let's take another big step in coexistence with wheat blast”, concludes Fronza.

Epidemics can worsen hunger in poor countries 

According to information released by the International Center for Corn and Wheat Improvement (CIMMYT), a blast epidemic affected 3 million hectares of wheat in South America in the 1990s. In 1996, the first blast outbreak in Bolivia resulted in almost 80% production losses. The following year, the disease once again devastated crops, causing 100% loss of productivity, which was responsible for the sharp drop in wheat area in subsequent years in Bolivia. In Paraguay, where the first epidemic occurred in 2002, production losses of more than 70% were recorded.

In Brazil, losses due to wheat blast occurred in 2009 and 2012, when damage above 40% compromised crops in the heading phase of wheat in PR, MS and SP. Isolated cases of blast occurrence in RS have been detected, but epidemics of the disease have never been recorded. In 2019, blast caused losses in MG, GO and DF with crops that had losses of up to 100%, but what caught our attention was the fungus infection earlier: the conditions Climate conditions at the beginning of the crop cycle led to the incidence of blast still in the tillering of the plants, raising the alert so that, with conditions favorable environmental conditions, anticipate control of the disease while still on the leaves and not only on the cob, as research had guided until then.

“Even without epidemics in 2022, temperature fluctuations allowed record blast lesions on leaves in practically all regions wheat crops in the country. In the South, blast lesions are often confused with brown spot symptoms (Bipolaris sorokiniana), but the control of leaf spots end up masking possible blast damage”, says the researcher at OR Melhoramento de Sementes, Camila Turra.

The arrival of wheat blast in Asia surprised researchers in international scientific community. The disease, historically restricted to Latin America, was registered in Asia for the first time in 2016, resulting in an epidemic in Bangladesh's crops that decimated around 15 thousand hectares, with a 51% crop failure. As fungus spores can travel with the wind, there is a risk of blast if spread to neighboring countries such as China, India, Nepal and Pakistan. In 2018, the disease was recorded in Zambia, in southern Africa, but without losses significant. An important route of dissemination of the fungus is trade international, a way for contaminated grains and seeds in areas endemic diseases can spread to other countries.

In general, the development of the disease is favored by high temperatures and humidity, conditions present in countries with tropical climates and subtropical. In North America and Europe, there have not yet been reported records of the disease, as the colder climate can limit the survival of the fungus and even the infection process in the plant. However, in a scenario of climate change and possible mutations of the fungus, the global threat that blast represents cannot be ruled out.

According to researcher Pawan Singh, head of the wheat phytopathology department at CIMMYT, only in South Asia, it is estimated that more than 7 million hectares are vulnerable to blast: “If the fungus compromises just 10% of a wheat crop in this region, there will be 260 million dollars in losses. A tragedy that could compromise the food security in poor countries that do not have the resources to import of wheat”.

Control Challenge

Embrapa Trigo researcher, João Leodato Maciel, explains that the disease is considered a global threat due to the condition that most wheat cultivars used in the world do not have in their genealogy sources of resistance to blast. Furthermore, resistance to diseases, especially wheat blast, has a complex nature: “Although wheat is one of the most studied around the world, different genes are involved in disease resistance. In the case of blast, some ensure adult plant resistance, others only resistance in the initial development of the crop, genes that prevent lesions on leaves or spikes. It is a complex interaction, which varies in each cultivar or environment, in addition to the constant mutation of the fungus to break these resistances.”

Even though advances in the efficiency of chemical control, associated with crop management, has evolved a lot in recent years, resistance genetics of wheat cultivars is still the safest and most economical way to contain blast. 

Several wheat research companies in Brazil have invested in breeding programs focused on blast resistance. Currently, with based on information provided by breeders for publication Technical Information for Wheat and Triticale - 2022 Harvest, there are 31 cultivars indicated as moderately resistant to blast. However, according to results from the Cooperative trials for Resistance to Wheat Ear Blast, many of these cultivars have presented reactions of greater susceptibility to the disease when tested in field tests, which may indicate the presence of variants of the fungus, with possible breaking resistance.