Field Crop 2025, Vol.8, No.1, 20-31 http://cropscipublisher.com/index.php/fc 24 3.3 Balancing stress tolerance and yield Indeed, the biggest fear of developing stress-resistant varieties is to lose sight of one thing while focusing on another-resistance is improved, but yield drops. But this time, CRISPR technology really showed us its hand. Take the OsPYL9 gene for example. After editing, it was like a cheat: not only did the drought resistance soar, but even the yield increased (Usman et al., 2020). This is like installing an intelligent regulator for rice, which automatically starts water-saving mode in the dry season and can produce at full capacity in normal times. Scientists are now increasingly finding these key genes that "kill two birds with one stone", for example, some genes control both the opening and closing of stomata and the filling of grains. This ability of precise regulation is much stronger than the traditional breeding method of taking chances. If this trend continues, maybe we can really cultivate super rice that can "guarantee yield in drought and flood" in the future. When it comes to rice breeding, CRISPR/Cas9 technology has come up with new tricks. It can not only modify genes directly, but also operate on regulatory switches (cis-regulatory sequences). Interestingly, this can lead to new stress resistance QTLs, and most importantly, it will not reduce yield (Romero and Gatica-Arias, 2019). In fact, in the early years, people were worried that stress resistance and yield could not be achieved at the same time, but now researchers have found a trick: by fine-tuning the expression levels of those stress response genes, rice can withstand harsh environments and maintain high yields (Zafar et al., 2020). This fine-tuning ability is much smarter than the drastic transformation methods of traditional breeding. 4 Technical and Ethical Considerations 4.1 Precision of CRISPR editing The most powerful thing about CRISPR/Cas9 is its ability to precisely edit anything. Take rice, for example. Scientists have successfully modified several key genes with it. For example, by changing the OsPIN5b gene, the rice ears become significantly longer; by changing the GS3 gene, the rice ears become larger; by adjusting the OsMYB30 gene, the cold tolerance is immediately improved (Zeng et al., 2020). Although the editing efficiency of different genes varies (approximately between 42% and 66%), this ability to precisely target specific genes is a revolutionary breakthrough in the breeding industry (Sami et al., 2021). In the past, improving rice traits depended on luck. Now with this system, you can change any gene you want to change, and the efficiency is many times higher. To make the CRISPR/Cas9 "gene scissors" cut accurately, the key lies in whether the guide RNA (gRNA) is well designed. It's like equipping the scissors with a navigator-the more accurate the gRNA, the less likely it is to cut the wrong place during editing. Interestingly, researchers found that the edited mutant traits were particularly "obedient" in the offspring of rice, and were transmitted completely according to Mendel's laws of inheritance (Zhang et al., 2014). This shows that as long as the gRNA is designed in place, a successful edit can be stably inherited. But to be honest, designing gRNA is not an easy job, and it requires repeated testing and verification. After all, no one wants to see new varieties that have been cultivated with great effort have unexpected traits due to inaccurate editing. Fortunately, the technology is becoming more and more mature, and the success rate of precise editing is also constantly improving. 4.2 Safety measures and control of off-target effects CRISPR/Cas9, this "gene scissors", is indeed precise, but it can occasionally cut the wrong place-this is the troublesome off-target effect. Imagine that you originally wanted to modify gene A, but accidentally modified the similar-looking gene B as well. This accidental editing can affect the expected effect at best, or even produce some harmful traits at worst. To be honest, although this situation is rare, it would be troublesome if it really happened in breeding. Therefore, researchers are now very cautious. They must repeatedly confirm the uniqueness of gRNA before editing, and they must do a whole genome scan after editing to check for omissions. After all, when using CRISPR technology to cultivate new varieties, safety and reliability are the first priority. Scientists now have ways to deal with off-target effects. The first is to start with the "scissors" themselves-using the modified high-fidelity Cas9 protein, which is like adding a locator to the scissors, and the probability of error is much lower. However, the most reliable way is to do a "full body checkup" after editing, such as a large-scale
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