Acetolactate synthase (ALS)-targeting herbicides are among the most widely used weed-control chemicals globally. Mutations in the ALS gene can confer herbicide resistance in crops, thereby allowing selective elimination of weeds without harming crops. Herbicide-resistant ALS alleles were initially discovered in weeds and subsequently developed through artificial mutagenesis techniques. With the advancement of CRISPR/Cas technologies, various genome-editing tools are now available to introduce these resistant alleles, as well as novel variants, into diverse crop species. Moreover, emerging methodologies, such as directed evolution, enable the generation and screening of large populations of random ALS mutants. Consequently, ALS has become one of the most extensively targeted genes in plant gene evolution. This paper provides a comprehensive overview of both conventional and recently developed strategies for ALS evolution, with particular emphasis on CRISPR/Cas-based genome editing and directed evolution. Future perspectives on technological application are also discussed. By advancing our understanding of herbicide-resistant ALS allele development for crop improvement, these methodologies may also pave the way for their application to the evolution of other agronomically important genes.

Rice is one of the most important food crops in the world. Weeds seriously affect the rice yield and grain quality. In recent years, there are tremendous progresses in the research and application of herbicide-resistant genes in rice worldwide. This article reviews the working mechanisms of six herbicides (glyphosate, glufosinate, acetolactate synthase inhibitor herbicides, acetyl-CoA carboxylase inhibitor herbicides, hydroxyhenylpyruvate dioxygenase (HPPD) inhibitor herbicides and dinitroaniline herbicides), the resistance mutations of the corresponding herbicide-target genes, and the herbicide detoxification mechanisms by non-target genes. Examples are provided on herbicide-resistant rice materials obtained by transformation of exogenous resistance genes, by artificial mutagenesis and mutant screening, and by modifying the target genes through gene editing. This paper also introduces the current application of herbicide-resistant rice, points out problems that may be caused by utilization of herbicide resistant rice and solutions to the problems, and discusses the future prospects for the development of herbicide-resistant rice.

The breeding and large-scale application of hybrid rice contribute significantly to the food supply worldwide. Currently, hybrid seed production uses cytoplasmic male sterile (CMS) lines or photoperiod/thermo-sensitive genic male sterile (PTGMS) lines as female parent. Despite huge successes, both systems have intrinsic problems. CMS systems are mainly restricted by the narrow restorer resources that make it difficult to breed superior hybrids, while PTGMS systems are limited by conditional sterility of the male sterile lines that makes the propagation of both PTGMS seeds and hybrid seeds vulnerable to unpredictable climate changes. Recessive nuclear male sterile (NMS) lines insensitive to environmental conditions are widely distributed and are ideal for hybrid rice breeding and production, but the lack of effective ways to propagate the pure NMS lines in a large scale renders it impossible to use them for hybrid rice production. The development of “the third-generation hybrid rice technology” enables efficient propagation of the pure NMS lines in commercial scale. This paper discusses the establishment of “the third-generation hybrid rice technology” and further innovations. This new technology breaks the limitations of CMS and PTGMS systems and will bring a big leap forward in hybrid rice production.

Herbicide resistance in crop plants is valuable for integrated weed management in agriculture. Herbicide resistant rice, in particular, is important to management of weedy rice, a close relative of cultivated rice and a noxious weed prevalent in rice fields that remains challenging to farmers worldwide. Herbicide resistant plants can be obtained through transgenic approach or by mutagenesis of regular plant and screening of mutants with elevated resistance to herbicide. In this study, we conducted ethyl methyl sulfonate mutagenesis (EMS) to elite indica cultivar Huanghuazhan (HHZ) and screened for mutants resistant to imazapic, a herbicide that can inhibit the acetolactate synthase (ALS) in plants. We obtained three mutants of OsALS gene that have not been reported previously in rice. One of the mutants, with Trp₅₄₈ changed to Met (W₅₄₈M), was analyzed in more details in this study. This mutation had no negative effect on the plant physiology and morphology as well as rice yield. Compared with the imidazolinone-resistant mutant S₆₂₇N (Ser₆₂₇ changed to Asn) that has been deployed for Clearfield rice development, W₅₄₈M mutant showed high levels of resistance to a broad spectrum of five families of ALS-inhibiting herbicides, in addition to a higher level of resistance to herbicides of the imidazolinone family. The herbicide-resistance was stably inherited by crossing into other rice lines. Thus, the W₅₄₈M mutation provides a valuable resource for breeding of herbicide resistant rice and weed management.