Climate change is here to stay. From the farmer's point of view, the main setback is the increase in climate risks, particularly prolonged periods of drought, as occurred in the 2021/22 harvest. The germination, development and final yield of crops can be greatly harmed by abiotic stresses, with immeasurable losses for all links in agribusiness, for the consumer, for society and for governments. A study published on 5/5/2022 showed that 80% of the cultivated area in the world will face serious water stress problems throughout the current century (http://bitly.ws/qIeo).
One of the main tools that farmers will be able to count on in the future, to face climate change, will be the use of cultivars tolerant to drought and high temperatures. Starting from already cultivated plants, with high productivity and other desirable characteristics, and using classical breeding or advanced biotechnology tools, scientists are introducing into them the ability to obtain adequate productivity, in an environment of strong water restriction. In some cases, tolerance to higher temperatures, which usually occur in periods of absence of rain, is also present in drought-tolerant cultivars.
Early attempts to develop drought-tolerant crops using biotechnology involved genes from plants adapted to desert environments. These genes give plants a greater capacity to synthesize osmolytes, which help maintain turgidity, or enzymes involved in the elimination of reactive oxygen species (ROS), which accumulate in plants under stress conditions. These are oxygen radicals, energetically more reactive than molecular oxygen, that is, they are more easily able to react with other substances, and can generate a sequence of oxidation reactions that lead to the deterioration of membranes and biological macromolecules such as proteins and DNA, causing harm to the body. Using this approach can result in plants with drought tolerance, but with lower productivity than when grown under conditions of adequate water supply. The challenge was set for scientists: tolerate drought, but not lose productivity in the absence of water stress.
20 years ago, a private company (Bioceres), a university (Universidad Nacional del Litoral - UNL) and a public agency (CONICET) from Argentina joined forces to obtain a drought-tolerant wheat variety. It all started with the identification of a gene present in sunflower, called HaHB-4, which, transposed to a model plant used by scientists (Arabidopsis thaliana), demonstrated to be responsible for drought tolerance. Argentine scientists used a different strategy than previously described, working with genes responsible for signaling cascades and regulation of gene expression, building on previous observations that regulatory genes allow maintaining or even increasing crop yields, even under adverse conditions.
That's why they chose the gene HaHB-4, which is a transcription factor that modulates the expression of several hundred genes and provides drought tolerance. Furthermore, the action of this gene is not related to the early closure of stomata, a target that was discarded after the failure of the first attempts to obtain genotypes with drought tolerance. The scientific article that describes the acquisition can be found at http://bitly.ws/qHwe.
The hormonal action of ethylene in plants plays an important role in decreasing the yield of crops grown under conditions of abiotic stress. For this reason, a particularly efficient version of the gene HaHB-4 was used in order to not only reduce the synthesis of ethylene, but also make plants more insensitive to its effects.
The first varieties of wheat transformed with the tolerance gene were field tested in 2008. The best strains were selected in 2012, with test results showing that the technology improved productivity in adverse years when yields are generally low.
The authors of the study exemplify that a transgenic line yielded 6% more and had a water use efficiency 9,4% higher than the control, on average across the evaluated environments. Differences in grain yield between cultivars were explained by the 8% improvement in the number of grains per square meter, and were more pronounced under stressed conditions (on average 16%) than under non-stressed conditions (on average 3%). , reaching a maximum of 97% in one of the driest environments. In transgenic plants, the increase in the number of grains per square meter was accompanied by increases in the number of spikelets per spike, tillers per plant and fertile florets per plant.
Once the final events were selected in each harvest, additional regulatory studies were initiated, as marketing approvals at each production or consumption site require extensive biosafety data in order to demonstrate their safety to human, animal and to the environment.
In this regard, it is very important to highlight three fundamental aspects regarding the safety of consuming products containing the gene HaHB4, and not just wheat. Above all, we have to consider that the gene has been present in sunflower since the beginning of consumption of this plant, whether by humans or animals, without any health problem associated with it having ever been reported. Secondly, the HaHB4 acts as a transcriptional regulator of endogenous pathways, which make up the natural physiological processes of plants. Therefore, no proteins or metabolites other than those already naturally present in non-transgenic varieties are found. Last but not least: being a transcription factor, the gene is expressed at extremely low levels, making its presence in food a negligible safety risk.
Remembering that the equivalence of chemical composition between transgenic genotypes and their conventional counterparts is a measure required by food safety regulatory authorities for approval. The results of the equivalence of the chemical composition of transgenic and conventional wheat are described in http://bitly.ws/qHNq.
The new wheat variety is targeted for production and consumption in Latin America, as this region is a net importer of wheat, having been patented in Argentina and 14 other countries, including Brazil, which imports more than 80% of its wheat arriving from abroad. In the 2021/22 harvest, Argentina cultivated around 50.000 ha of drought-tolerant wheat, most of which was used for seed production to expand the area in the next harvests.
Different approaches have been used in an attempt to obtain drought-tolerant cultivars, including the expression of osmoprotectants, chaperones, transporters, membrane proteins and enzymes, but no products from these techniques have reached the market.
In Brazil, the Embrapa Soja team develops studies with the incorporation of different transcription factors in soybean genotypes, including the Arabidopsis gene AtAREB1, which acts as a regulator of plant responses to water stress. The results showed that seedlings of transgenic genotypes had a higher survival rate under severe water deficit, greater water use efficiency and greater yield stability compared to the conventional background, when tested in the field under water deficit situations. The studies are described in http://bitly.ws/qW5Y.
The same team of Argentine scientists that developed drought-tolerant wheat also developed a soybean cultivar with similar characteristics. The gene HaHB-4 was introduced into soybean cultivars, generating transgenic genotypes that, on average from 27 experiments conducted in Argentina, produced 4% more than the original, non-transgenic cultivar. When the results are broken down by environment, the advantages of the drought-tolerant genotype become more evident because, in drought conditions, the tolerant lines produced 8,6% more, being 10,5% higher when the temperature was higher and 5,1. XNUMX% with lower temperature (details in http://bitly.ws/qHPC).
The technology was patented in 11 countries, including Brazil. The equivalence of chemical composition between transgenic, drought-tolerant, and conventional soybeans can be accessed at http://bitly.ws/qHMo. With the approval of HB4 soybeans by China, which occurred at the end of April 2022 (http://bitly.ws/qHQ3), a strong increase in its commercial cultivation in Argentina is expected in the next harvest, since in the 2021/22 harvest the cultivated area was 21.000 hectares, almost entirely for seed production.
In the United States, drought-tolerant corn hybrids were developed, both through classical breeding and through the use of transgenesis, with an estimate that 22% of the area was cultivated with drought-tolerant genotypes in 2016. In the case of Drought- Guard and PT-Perkebunan the approach used was the introduction of a chaperone, originally present in Bacillus subtilis, which allows it to tolerate certain degrees of water stress.
Meanwhile, according to research carried out by the USDA, drought-tolerant corn genotypes currently cultivated yield slightly higher yields than non-tolerant ones, under the usual drought conditions that farmers experience in their fields. Additionally, some studies suggest that they are not effective in severe droughts.
USDA Agricultural Resource Management Survey (ARMS) data for 2016 shows that drought-tolerant corn yields were, on average, 4% higher than non-tolerant corn yields, with no difference was statistically significant, according to a USDA report (http://bitly.ws/qHSA). Dr. McFadden, author of the report, points out that drought-tolerant corn genotypes were not designed to produce significantly higher yields than conventional varieties under drought-free growing conditions that prevail in most of the major corn-producing areas of the world. USA.
Researchers from the Federal University of Alagoas launched the cultivar RB0442 onto the market, developed through classical breeding, with the characteristics of high productivity and good tolerance to drought, with great adaptation to the production conditions of northeastern Brazil. The results indicated average productivity gains of 14% and a 3,5% increase in total recoverable sugar content (ATR). Under water stress conditions, the productivity gain was 24,5%, in addition to an 11,5% increase in ATR.
A team from Embrapa incorporated the gene DREB2 in sugarcane, obtaining genotypes that, in a greenhouse, demonstrated tolerance to water stress, associated with higher levels of sucrose, considered a strong indication of the possibility of developing transgenic varieties tolerant to drought (http://bitly.ws/qHUX).
With the deepening of climate change underway, the unpredictability of the climate and the occurrence of extreme events – such as drought – with increasing frequency and intensity, represent the greatest challenge for agriculture in the immediate and long-term future. The development of drought-tolerant genotypes will be an excellent alternative for farmers to face the problem, without forgetting other technologies, especially soil and crop management and the use of irrigation. This integration will be essential to guarantee food security on planet Earth in the medium and long term.
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