Transcriptome and metabolome analysis reveals different photosynthetic characteristics of mulberry trees with different ploidy levels

There are abundant mulberry germplasm resources in China. Generally, in order to better utilize these resources, a comprehensive evaluation is conducted, among which the evaluation of photosynthetic capacity is an important aspect. To evaluate the photosynthetic characteristics of mulberry trees with different ploidy levels, which determine the number of chromosome sets in a cell or organism, we compared the microstructural features and transcriptomes of triploid and diploid mulberry trees. Haploid (n) means having one set of chromosomes, diploid (2n) has two sets, triploid (3n) has three sets, and so on. We compared the microstructural features and transcriptomes of triploid and diploid mulberry trees. In this study, the photosynthetic rates (P_n) of 'guisang6hao' (hereinafter referred to as GS6, 2n = 3x = 42) were 1.2 times that of 'guisangyou12'(hereinafter referred to as GSY12, 2n = 2x = 28). Here, GS6 and GSY12 are abbreviations for the two mulberry varieties. The leaf thickness, main vein thickness, epidermal thickness, palisade tissue thickness, spongy tissue thickness and lower epidermal stomatal density of GS6 were greater than those of GSY12. Additionally, the transcriptome characteristics of GS6 and GSY12 were characterised by Illumina Novaseq 6000 and the differentially expressed genes (DEGs) were significantly enriched in photosynthesis. Four differentially expressed metabolites (DEMs) related to photosynthesis were identified, playing key roles in chlorophyll metabolism and carbon fixation in photosynthetic organisms. The above results indicated that an increase in the ploidy level enhanced the photosynthetic capacity and utilisation of light energy, as well as the accumulation of chlorophyll content, of triploid mulberry trees. This study provides a technical and theoretical basis for germplasm innovation and the genetic improvement of mulberry.