Abstract
Heterosis is a complex phenomenon in which hybrids show better phenotypic characteristics than their parents do. Chinese cabbage (Brassica rapa L. spp. pekinensis) is a popular leafy crop species, hybrids of which are widely used in commercial production; however, the molecular basis of heterosis for biomass of Chinese cabbage is poorly understood. We characterized heterosis in a Chinese cabbage F1 hybrid cultivar and its parental lines from the seedling stage to the heading stage; marked heterosis of leaf weight and biomass yield were observed. Small RNA sequencing revealed 63 and 50 differentially expressed microRNAs (DEMs) at the seedling and early-heading stages, respectively. The expression levels of the majority of miRNA clusters in the F1 hybrid were lower than the mid-parent values (MPVs). Using degradome sequencing, we identified 1,819 miRNA target genes. Gene ontology (GO) analyses demonstrated that the target genes of the MPV-DEMs and low parental expression level dominance (ELD) miRNAs were significantly enriched in leaf morphogenesis, leaf development, and leaf shaping. Transcriptome analysis revealed that the expression levels of photosynthesis and chlorophyll synthesis-related MPV-DEGs (differentially expressed genes) were significantly different in the F1 hybrid compared to the parental lines, resulting in increased photosynthesis capacity and chlorophyll content in the former. Furthermore, expression of genes known to regulate leaf development was also observed at the seedling stage. Arabidopsis plants overexpressing BrGRF4.2 and bra-miR396 presented increased and decreased leaf sizes, respectively. These results provide new insight into the regulation of target genes and miRNA expression patterns in leaf size and heterosis for biomass of B. rapa.
Highlights
Heterosis is a biological phenomenon in which hybrids have better phenotypic characteristics than their parents do for traits such as biomass production, grain yield, growth rate, and stress resistance[1,2,3]
Positive Mid-parent heterosis (MPH) and better-parent heterosis (BPH) values for leaf length, leaf width, plant height, and leaf number were observed for Chinese cabbage (Fig. 1c, d)
Positive correlations were observed among gross weight, single-leaf weight, leaf width, and leaf length, while negative correlations were observed between leaf number and the other traits (Supplementary Fig. S1)
Summary
Heterosis is a biological phenomenon in which hybrids have better phenotypic characteristics than their parents do for traits such as biomass production, grain yield, growth rate, and stress resistance[1,2,3]. Many heterotic crops, such as those of hybrid rice, maize, and many vegetable species, have been developed extensively worldwide. Dominance, overdominance, epistasis, Plant microRNAs (miRNAs) regulate gene expression through epigenetic regulation and posttranscriptional mechanisms[10]. Most miRNAs cause target gene degradation through the RNA-induced silencing complex effector[11]. A recent study showed that the expression level of most miRNA clusters in F1 hybrids of Brassica
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