Bioscience Methods 2024, Vol.15, No.6, 289-301 http://bioscipublisher.com/index.php/bm 295 Figure 2 A Somatic chromosome number of the haploids (2n = x = 17). b Ploidy levels obtained from 3-week-old first leaf samples from haploid plants by flow cytometric analysis. c Ploidy levels obtained from 3-week-old first leaf samples from a mixture of haploid and diploid plants by flow cytometric analysis. d A haploid plant (left) and diploid plant (right) of E. ulmoides (Adopted from Li et al., 2020) 6.2 Breeding efforts inHevea brasiliensis Breeding efforts in Hevea brasiliensis have traditionally focused on improving rubber yield, disease resistance, and stress tolerance. Unlike E. ulmoides, H. brasiliensis synthesizes cis-polyisoprene via the mevalonate pathway. The genetic basis for this pathway has been well-studied, and breeding programs have leveraged this knowledge to develop high-yielding and disease-resistant clones. The genome of H. brasiliensis has been extensively mapped, revealing key genes involved in rubber biosynthesis and stress responses. These genes have been targeted in breeding programs to enhance the overall productivity and resilience of rubber trees. Comparative studies have shown that while E. ulmoides relies on the MEP pathway, H. brasiliensis has evolved a distinct set of genes for cis-polyisoprene synthesis, providing a rich genetic resource for breeding efforts (Wuyun et al., 2017). 6.3 Comparative outcomes of biosynthesis pathway modifications The comparative outcomes of biosynthesis pathway modifications in Eucommia ulmoides and Hevea brasiliensis highlight the distinct evolutionary paths these species have taken. E. ulmoides, with its reliance on the MEP pathway, offers a unique model for studying rubber biosynthesis. Genetic modifications targeting the MEP pathway in E. ulmoides could lead to significant improvements in rubber yield and quality, leveraging its unique genetic makeup (Liu et al., 2018; Li et al., 2020). On the other hand, H. brasiliensis, with its well-characterized mevalonate pathway, continues to benefit from traditional breeding and genetic engineering efforts aimed at enhancing cis-polyisoprene production. The independent expansion of farnesyl diphosphate synthases (FPSs) and rubber elongation factor/small rubber particle protein gene families in E. ulmoides compared to H. brasiliensis underscores the divergent evolutionary strategies these species have adopted for rubber biosynthesis (Wuyun et al., 2017). This comparative analysis not only provides insights into the fundamental biology of rubber production but also informs targeted genetic and breeding strategies to optimize rubber yield and quality in both species. 7 Environmental and Ecological Factors Influencing Rubber Biosynthesis 7.1 Influence of climatic conditions Climatic conditions play a crucial role in the biosynthesis of rubber in both Eucommia ulmoides and Hevea brasiliensis. The rubber tree (Hevea brasiliensis) thrives in tropical climates with high humidity and consistent rainfall, which are essential for optimal latex production. These conditions help maintain the physiological processes necessary for rubber biosynthesis, including the activity of enzymes like farnesyl pyrophosphate synthase (FPS) that are critical for the formation of polyisoprenoids (Chuntai et al., 2017). On the other hand, Eucommia ulmoides, also known as the hardy rubber tree, is more adaptable to a wider range of climatic conditions, including temperate zones. This adaptability is partly due to its unique biosynthetic pathway that relies on the methylerythritol-phosphate (MEP) pathway rather than the mevalonate pathway, which is predominant in Hevea brasiliensis (Wuyun et al., 2017; Li et al., 2020).
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