Plant Gene and Trait 2024, Vol.15, No.6, 314-322 http://genbreedpublisher.com/index.php/pgt 315 process is very important for plants and is a key step in sulfur utilization and cysteine synthesis. These enzymes are also called O-acetylserine (thiol) lyases (OASTLs). They require a small molecule called pyridoxal phosphate to help them, and they are also very functional. For example, in Arabidopsis thaliana, scientists have discovered three cysteine synthase genes (AtcysC1, AtcysD1 and AtcysD2). The activities of these three genes and their locations in the cell are different (Yamaguchi et al., 2000). In rice, four cysteine synthase genes (rcs1, rcs2, rcs3 and rcs4) have also been found. These genes respond differently to environmental changes such as sulfur, nitrogen and light. Cysteine synthase proteins have some important structural regions, such as the PXXSVKDR sequence, which are critical for their catalytic function and regulation (Nakamura et al., 1999; Hesseet al., 2005). 2.2 The role of cysteine in plant metabolism Cysteine is very important in plants. It is a precursor of many important molecules, such as glutathione, vitamins and some coenzymes. Cysteine is an essential material for protein synthesis and can also be used as a source of sulfur in the synthesis of methionine and other sulfur-containing compounds (Tan et al., 2019). The synthesis process of cysteine is closely related to the ability of plants to cope with various stresses (such as drought and disease). Genes such as the β-substituted alanine synthase (BSAS) family, including OASTLs, can link cysteine metabolism and stress signaling pathways, which can help plants survive in difficult environments (Tahir and Dijkwel, 2016). In rice, the gene GRA78 encoding a putative S-sulfocysteine synthase is involved in chloroplast development and is very sensitive to temperature changes in the early seedling stage, playing an important role (Zhou et al., 2020). 2.3 Evolutionary perspective of cysteine synthase genes The evolutionary history of cysteine synthase genes shows their changes in order to adapt to different environmental conditions. In cyanobacteria, serine acetyltransferase (SAT) and OASTL genes have been found. This shows that the cysteine biosynthetic pathway appeared very early and formed different evolutionary branches in different types of cyanobacteria (unicellular, filamentous and heterocystous) (Kharwar et al., 2021). In higher plants, such as Arabidopsis and rice, there are many different types of cysteine synthase. This shows that they have been optimizing sulfur utilization and adjusting cysteine synthesis according to different environmental requirements during evolution (Hesse et al., 2005). Phylogenetic analysis shows that these genes are highly conserved and are also subject to evolutionary pressure that promotes functional diversification (Nakamura et al., 1999). 3 Discovery and Characterization of GRA78 3.1 Identification of GRA78 in rice Through forward genetic screening and genome-wide association study (GWAS), researchers found a key gene GRA78 related to rice leaf color changes. At first, people found that different leaf colors were related to certain gene loci, so they speculated that these places might contain genes that control pigment synthesis or regulation. Later, after more detailed positioning and analysis of candidate genes, it was confirmed that GRA78 is a gene mainly responsible for encoding cysteine synthase and is also an important factor in determining leaf color. This discovery was made possible by high-quality rice genome data and comprehensive leaf color phenotype information (Wang et al., 2023). 3.2 Genetic and molecular characteristics of GRA78 The study found that GRA78 is located on chromosome 3 of rice, and there are several exons and introns in the gene. Sequence analysis showed that the protein encoded by GRA78 is very similar to cysteine synthase in other plants, indicating that it plays a similar role in sulfur metabolism and assimilation. By studying mutants, the importance of GRA78 in leaf color formation was further demonstrated. In rice plants that have knocked out this gene, the pigmentation of leaves has changed, with a significant decrease in chlorophyll, and changes in the levels of carotenoids and anthocyanins. Most of these problems are caused by problems with cysteine synthesis, which affects the metabolic pathways responsible for pigment synthesis (Khan et al., 2020).
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