Maize Genomics and Genetics 2025, Vol.16, No.4, 229-238 http://cropscipublisher.com/index.php/mgg 230 The objective of this study is to use base editing technology to modify some hereditary point mutations in the genes of corn itself, making corn more resistant to herbicides. This not only enhances the weeding effect but also reduces the usage of herbicides, which is conducive to cultivating more environmentally friendly and better-performing corn varieties. In the future, as long as the base editing technology continues to be optimized and the policy allows it, it is very likely to play a greater role in corn breeding. 2 Principles of Base Editing Technology in Plants 2.1 Mechanism of cytosine and adenine base editors in plants The key to achieving base substitution lies in a set of specially designed proteins. Scientists combined the functionally inactivated CRISPR-Cas9 (which could be a "cut" version or not cut at all) with deaminases to create a base editing tool. In plants, there are mainly two types of such tools: one is the cytosine base editor (CBE), and the other is the adenine base editor (ABE). CBE converts cytosine (C) into uracil (U), and when cells replicate DNA, they mistake uracil for thymine (T). The result is that C becomes T. ABE, on the other hand, changes adenine (A) to inosine (I), and inosine is read as guanine (G), thus completing the transition from A to G. These alterations do not occur at random locations but are carried to that site of the target gene by a specific sgRNA (single-guide RNA) (Mishra et al., 2019; Hillary and Ceasar, 2024). 2.2 Advantages over traditional genome editing (e.g., CRISPR-Cas9) The traditional CRISPR-Cas9 mainly achieves editing by "cutting" the double strands of DNA (Figure 1). The problem is that this kind of breakage needs to be repaired by the cells themselves, and whether it is repaired well or not is beyond the control of researchers. Non-homologous end joins (NHEJ) are prone to errors, and insertions or deletions often occur. However, another repair method - homologous recombination (HDR) - is very inefficient. In contrast, the base editor's approach is much more "gentle". It does not cut DNA and does not require additional templates; it only makes individual base changes at the target position. This method is precise, predictable and highly efficient, and is particularly suitable for traits determined by a single base (Bharat et al., 2020; Molla et al., 2021). 2.3 Potential for targeted point mutations without double-strand breaks One of the greatest highlights of base editing is that it can directly change one base to another without causing a double-strand break, and this change is irreversible. In this way, the risk is much smaller, but the efficiency is not compromised. For instance, in corn, point mutations resistant to herbicides can be achieved in this way. Because it does not require DNA interruption, this technology has a particular advantage when dealing with some traits where details are very crucial. It has been regarded as a very practical tool for crop breeding, which can help cultivate new varieties with clear genetic improvement goals (Azameti and Dauda, 2021; Min et al., 2022). 3 Target Genes for Herbicide Resistance in Maize 3.1 Key enzymes in herbicide action pathways (e.g., ALS, EPSPS) Why can corn tolerate some herbicides? The core lies in just a few enzymes. Enzymes like acetyllactate synthase (ALS, also known as AHAS) and EPSPS are precisely the "targets" of certain herbicides. If these enzymes are inhibited, for instance, once ALS is suppressed by sulfonylurea or imidazolinone herbicides, corn cannot synthesize branched-chain amino acids and will eventually die. However, as long as specific mutations occur in the ALS gene, corn can be made resistant (Zhu et al., 2000). Another enzyme, EPSPS, is a key point in the action of glyphosate and is very important in the process of synthesizing aromatic amino acids. Studies have shown that if EPSPS undergo mutations or are expressed in large quantities, corn can tolerate glyphosate (Liu et al., 2023). In addition, P450 enzymes (such as CYP81A9) are also crucial. They can metabolize some herbicides, and herbicides like metosulfuron can be "detoxified" by them (Zhang et al., 2024). In addition, enzymes such as glutamine synthase or certain aminotransferases (for example, ZmGHT1) have also been found to be associated with resistance to glufosinate-ammonium (Bao et al., 2022). 3.2 Known resistance-conferring point mutations in maize genes Some point mutations can also make corn resistant to herbicides, and research in this area has become relatively clear. Mutations in the ALS gene are quite typical. For instance, when a base at a certain site in ZmALS1 and
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