Molecular Plant Breeding 2024, Vol.15, No.6, 391-402 http://genbreedpublisher.com/index.php/mpb 397 programs to improve rubber yield and stress resilience (Francisco et al., 2021). Furthermore, the high-quality chromosome-level genome sequence analysis of wild rubber tree species revealed significant genetic markers associated with latex yield. The study identified 155 trait marker associations and 326 candidate genes, including genes related to sugar transport, metabolism, and ethylene biosynthesis. These findings provide valuable resources for genome-assisted breeding, aimed at improving rubber yield through targeted gene enhancement (Figure 3) (Cheng et al., 2023). Figure 3 Assembly of rubber tree genome (Adopted from Cheng et al., 2023) Image caption: (a) Rubber tree yield collected by tapping. Left, low-yield wild accession MT/VB/25A 57/8; Right, high-yield Wickham clone Reyan7-33-97. (b) Circos plot of repeat sequences and expression patterns in the rubber genome. The curves connect the colinear blocks between different chromosomes. The heatmap illustrates the gene expression levels (Log2FPKM) in young leaves, young bark, old leaves, old bark and latex of rubber (from inside to outside). The red and green colours indicate high and low expression levels, respectively. The density of line charts represents gene density, transposable element (TE), short tandem repeat (STR) and simple sequence repeat (SSR) across 18 chromosomes in the rubber tree (from inside to outside). The colour density indicates a high density of interactions. Distinct chromosomes are separated by dotted lines. (c) Genome-wide all-by-all highest-throughput chromosome conformation capture (Hi-C) interaction heatmap for the 18 linkage groups in Hevea brasiliensis. Heat map showing the density of the Hi-C interactions between chromosomes. (d) Heatmap of the DEGs (differentially expressed genes) expression levels in leaf, bark tissues and latex. In total, 4192 DEGs were identified to be differentially expressed (Adopted from Cheng et al., 2023) 5.2 Poplar species The species of poplar have been extensively studied due to their rapid growth and wide adaptability, making them ideal candidates for genetic improvement to enhance biomass production and stress resistance. Although poplar research is not directly related to rubber production, the genetic findings from these studies can be applied to other tree species, includingEucommia ulmoides. Poplar research primarily focuses on unraveling the genetic basis of traits such as growth rate, wood quality, and stress resistance. Techniques such as genome-wide association studies (GWAS) and transcriptome analysis have been used to identify key genes and regulatory networks involved in these processes. For example, studies have highlighted the roles of specific transcription factors and hormone signaling pathways in regulating poplar growth and development. These findings can provide a theoretical basis for genetic improvement strategies in Eucommia
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