Plant Gene and Trait 2024, Vol.15, No.5, 243-252 http://genbreedpublisher.com/index.php/pgt 244 enhancing the industrial and medicinal value of E. ulmoides, and providing insights into future directions for the genetic improvement of this economically important species. 2 Current State of Eucommia ulmoides Research 2.1 Summary of available genomic information and tools for E. ulmoides .Eucommia ulmoides, as a tree species with significant economic value, the research progress on its whole genome information is crucial for its genetic improvement. A comprehensive genetic linkage map has been developed using various molecular markers, including sequence-related amplified polymorphism, amplified fragment length polymorphism, inter-simple sequence repeat, and simple sequence repeat markers. This map encompasses 706 markers distributed across 25 linkage groups, covering approximately 89% of the estimated E. ulmoides genome with an average marker interval of 3.1 cM. This genetic linkage map is instrumental for identifying quantitative trait loci (QTL) associated with growth-related traits, thereby facilitating marker-assisted selection and further genomic studies in E. ulmoides (Li et al., 2014). 2.2 Traditional breeding efforts: overview of the breeding programs and their limitations Traditional breeding programs for Eucommia ulmoides have primarily focused on improving phenotypic traits to meet the increasing demand for this versatile species. These programs have involved the selection of preferable genotypes and cultivation areas based on the significant effects of genotype, site, and genotype × environment interactions on phenotypic traits. However, the site effect has been found to account for a larger proportion of the variance in most traits (Deng et al., 2021), followed by genotype and genotype × environment interactions. This indicates that while traditional breeding can improve traits by selecting optimal genotypes or cultivation areas, it is limited by the significant environmental influence on trait performance. Additionally, the need for trait performance stability and the discriminating ability of genotypes in different cultivation areas further complicates traditional breeding efforts (Deng et al., 2022). 2.3 Early transgenic efforts Initial attempts to genetically modify Eucommia ulmoides have been relatively limited compared to traditional breeding efforts. Early transgenic research has focused on understanding the genetic basis of key traits and developing tools for genetic manipulation. The construction of a genetic linkage map and the identification of QTLs for growth-related traits represent foundational steps towards more advanced genetic modification techniques. These efforts aim to overcome the limitations of traditional breeding by enabling more precise and targeted improvements in E. ulmoides traits. However, detailed reports on successful transgenic modifications and their outcomes are still sparse, indicating the nascent stage of transgenic research in this species (Li et al., 2014). 3 Target Traits for Industrial Improvement 3.1 Rubber production: genetic targets for increasing rubber yield and quality Eucommia ulmoides, known for its production of trans-1,4-polyisoprene (Eu-rubber), has been the focus of several studies aiming to enhance rubber yield and quality through genetic modifications. The high-quality haploid genome assembly of E. ulmoides has provided significant insights into the rubber biosynthesis pathway, particularly the methylerythritol-phosphate (MEP) pathway, which is predominant in this species (Li et al., 2020). Additionally, the identification of long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) involved in rubber biosynthesis has opened new avenues for genetic engineering. Specific lncRNAs and miRNAs have been found to regulate key genes in the rubber biosynthesis pathway, suggesting potential targets for increasing rubber yield. For instance, after processing the assembled transcripts through a rigorous procedure, Liu et al. (2018a) identified 29 103 lncRNAs (Figure 1) and found that these lncRNAs are involved in 12 protein-coding genes as well as 95 DE genes related to Eu-rubber biosynthesis. Moreover, the application of growth regulators has been shown to significantly enhance TPI productivity, indicating that both genetic and agronomic approaches can be combined for optimal results (Liu et al., 2018b). 3.2 Disease resistance: enhancing resistance to pests and diseases Improving disease resistance in E. ulmoides is crucial for maintaining healthy plantations and ensuring consistent rubber production. The high-quality genome assembly of E. ulmoides has identified numerous protein-coding
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