Plant Gene and Traits 2024, Vol.15, No.4, 162-173 http://genbreedpublisher.com/index.php/pgt 165 3 Mechanisms of Glufosinate Tolerance 3.1 Mode of action of glufosinate Glufosinate is a broad-spectrum herbicide that inhibits glutamine synthetase (GS), an enzyme critical for nitrogen metabolism in plants. By inhibiting GS, glufosinate causes an accumulation of ammonia in plant tissues, leading to cellular toxicity and plant death (James et al., 2018). The herbicide's effectiveness is due to its ability to disrupt the synthesis of glutamine from glutamate and ammonia, which is essential for plant growth and development (Bao et al., 2022). 3.2 Genetic basis of tolerance The genetic basis of glufosinate tolerance in weedy rice and other crops often involves the introduction or overexpression of genes that encode for modified or additional copies of GS. For instance, in maize, the gene ZmGHT1, which encodes an aminotransferase, has been identified as a key player in conferring glufosinate tolerance. This gene likely participates in ammonia elimination involving GS activity, thereby reducing the toxic effects of glufosinate (Bao et al., 2022). Similarly, in transgenic rice, the overexpression of GS genes such as OsGS1;1 and OsGS2 has been shown to enhance tolerance to glufosinate by maintaining higher GS activity and lower ammonia levels (James et al., 2018). Additionally, the bar gene, which encodes phosphinothricin acetyltransferase, is commonly used in transgenic crops to confer glufosinate resistance by detoxifying the herbicide (Zhang et al., 2003; Yu et al., 2023). 3.3 Physiological responses to glufosinate Physiological responses to glufosinate in tolerant plants include enhanced GS activity and reduced ammonia accumulation. For example, transgenic rice lines overexpressing OsGS1;1 and OsGS2 showed higher fresh weight, chlorophyll content, and relative water content under stress conditions, indicating improved physiological resilience (James et al., 2018). In maize, plants with the ZmGHT1 gene exhibited lower ammonia content and higher GS activity after glufosinate treatment, suggesting a robust mechanism for ammonia detoxification (Bao et al., 2022). Furthermore, studies on weedy rice have shown that certain accessions exhibit reduced sensitivity to glufosinate, which may be due to inherent genetic variations that enhance their physiological tolerance (Shrestha et al., 2019). Collectively, these mechanisms highlight the complex interplay between genetic modifications and physiological adaptations that enable certain plants to tolerate glufosinate, providing valuable insights for breeding and biotechnological interventions aimed at developing herbicide-resistant crops. 4 Identification of Glufosinate-Tolerant Rice 4.1 Screening and selection techniques Screening and selection of glufosinate-tolerant rice involve several methodologies to ensure accurate identification and effective breeding. One common approach is the use of herbicide application to identify resistant individuals. For instance, in maize, a screening process involved spraying 854 inbred lines and 25 620 seedlings with glufosinate, identifying a single tolerant plant (Bao et al., 2022). Similarly, in rice, transgenic lines are often developed using gene transfer techniques such as particle bombardment, followed by selection using agents like bialaphos or hygromycin B to confirm resistance (Jiang et al., 2000; Kim et al., 2007). Field trials are also essential, where transgenic rice lines are grown alongside weedy rice to observe natural outcrossing and resistance transfer (Figure 4) (Zhang et al., 2003; Chen et al., 2004; Busconi et al., 2014). 4.2 Genotypic and phenotypic characterization Genotypic and phenotypic characterization of glufosinate-tolerant weedy rice is crucial for understanding the underlying genetic mechanisms and the expression of resistance traits. Molecular techniques such as Southern blot analysis and polymerase chain reaction (PCR) are employed to confirm the integration and expression of resistance genes like pat or bar in the rice genome (Jiang et al., 2000; Chen et al., 2004; Lu et al., 2014). Phenotypic traits such as plant height, maturity, seed dormancy, and seed production are evaluated to assess the impact of resistance genes on the overall fitness and agronomic performance of hybrids (Oard et al., 2000; Song et al., 2011). For example, hybrids between transgenic rice and weedy rice often exhibit traits like increased height and delayed maturity, which are indicative of successful gene transfer (Zhang et al., 2003; Zhang et al., 2018).
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