Molecular Plant Breeding 2024, Vol.15, No.6, 391-402 http://genbreedpublisher.com/index.php/mpb 396 through genetic modifications that enhance the biosynthesis pathways of these metabolites. The small rubber particle protein (SRPP) gene is a key player in the biosynthesis of Eucommia ulmoides gum. Studies have shown that the promoter activity of the EuSRPP gene is regulated by methyl jasmonate (MeJA), gibberellin (GA3), and drought pathways, suggesting that manipulating these pathways can increase gum content (Zhao et al., 2023b). Moreover, the overexpression of genes involved in the biosynthetic pathways of osmoprotectants, such as betaine aldehyde dehydrogenase (BADH) and flavodoxin (Fld), can enhance stress resistance and productivity, thereby indirectly promoting the production of valuable metabolites (Niazian et al., 2021). By utilizing the aforementioned gene improvement strategies, the productivity, stress resistance, and metabolite yield of Eucommia ulmoides can be significantly enhanced, thus increasing its commercial feasibility and application value. Figure 2 Effect of NaCl treatment on the germination and root growth of WT and transgenic lines (P1 and P2) (Adopted from Chen et al., 2022a) Image caption: (a) The germination rate of the seeds from WT and transgenic lines (P1 and P2) under 0 or 50 mM NaCl treatment. Bar = 1 cm. (b) The germination rate. (c) Phenotypes of seedlings from WT and transgenic lines (P1 and P2) treated with 0 or 50 mM NaCl treatment. Bar = 1 cm. (d) The length of root. (e) Phenotypes of seedlings from WT and transgenic lines (P1 and P2) treated with 0 or 100 mM NaCl treatment at 6 d and 8 d. Bar = 1 cm. (f) The MDA content. (g) The PRO content. Expression level of AtHKT1 (h) and AtSOS1 (i) in WT and P1 treated with NaCl. (** p< 0.01, one way ANOVA) (Adopted from Chen et al., 2022a) 5 Case Studies of Genetic Modification in Related Species 5.1 Rubber tree (Hevea brasiliensis) The rubber tree (Hevea brasiliensis) is an important source of natural rubber and has become a central subject of numerous genetic studies aimed at improving its yield and growth characteristics. With the development of genomic tools, such as genome-wide association studies (GWAS) and marker-assisted selection (MAS), deeper insights into the genetic basis of these traits have been provided. For instance, a study integrating GWAS and RNA-sequencing (RNA-Seq) methodologies identified key quantitative trait loci (QTLs) and gene networks associated with rubber tree growth under water stress conditions. This multiomics approach has enhanced our understanding of the molecular mechanisms underlying growth and has potential applications in breeding
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