Hybrid vigor or heterosis refers to the superior performance of F 1 hybrid plants over their parents. Heterosis is particularly important in the production systems of major crops. Recent studies have suggested that epigenetic regulation such as DNA methylation is involved in heterosis, but the molecular mechanism of heterosis is still unclear. To address the epigenetic contribution to heterosis in Arabidopsis thaliana, we used mutant genes that have roles in DNA methylation. Hybrids between C24 and Columbia-0 (Col) without RNA polymerase IV (Pol IV) or methyltransferase I (MET1) function did not reduce the level of biomass heterosis (as evaluated by rosette diameter). Hybrids with a mutation in decrease in dna methylation 1 (ddm1) showed a decreased heterosis level. Vegetative heterosis in the ddm1 mutant hybrid was reduced but not eliminated; a complete reduction could result if there was a change in methylation at all loci critical for generating the level of heterosis, whereas if only a proportion of the loci have methylation changes there may only be a partial reduction in heterosis.heterosis | hybrid vigor | Arabidopsis | DNA methylation | DDM1

Level IV, therapeutic case series.

Brassica rapa L. is an important vegetable and oilseed crop. We investigated the distribution of the histone mark tri-methylation of H3K27 (H3K27me3) in B. rapa and its role in the control of gene expression at two stages of development (2-day cotyledons and 14-day leaves) and among paralogs in the triplicated genome. H3K27me3 has a similar distribution in two inbred lines, while there was variation of H3K27me3 sites between tissues. Sites that are specific to 2-day cotyledons have increased transcriptional activity, and low levels of H3K27me3 in the gene body region. In 14-day leaves, levels of H3K27me3 were associated with decreased gene expression. In the triplicated genome, H3K27me3 is associated with paralogs that have tissue-specific expression. Even though B. rapa and Arabidopsis thaliana are not closely related within the Brassicaceae, there is conservation of H3K27me3-marked sites in the two species. Both B. rapa and A. thaliana require vernalization for floral initiation with FLC being the major controlling locus. In all four BrFLC paralogs, low-temperature treatment increases H3K27me3 at the proximal nucleation site reducing BrFLC expression. Following return to normal temperature growth conditions, H3K27me3 spreads along all four BrFLC paralogs providing stable repression of the gene.

There is a wide variation of flowering time among lines of Brassica rapa L. Most B. rapa leafy (Chinese cabbage etc.) or root (turnip) vegetables require prolonged cold exposure for flowering, known as vernalization. Premature bolting caused by low temperature leads to a reduction in the yield/quality of these B. rapa vegetables. Therefore, high bolting resistance is an important breeding trait, and understanding the molecular mechanism of vernalization is necessary to achieve this goal. In this study, we demonstrated that BrFRIb functions as an activator of BrFLC in B. rapa. We showed a positive correlation between the steady state expression levels of the sum of the BrFLC paralogs and the days to flowering after four weeks of cold treatment, suggesting that this is an indicator of the vernalization requirement. We indicate that BrFLCs are repressed by the accumulation of H3K27me3 and that the spreading of H3K27me3 promotes stable FLC repression. However, there was no clear relationship between the level of H3K27me3 in the BrFLC and the vernalization requirement. We also showed that if there was a high vernalization requirement, the rate of repression of BrFLC1 expression following prolonged cold treatments was lower.

Brassica rapa L. is an important agricultural crop that requires a period of prolonged cold for flowering. This process is known as vernalization. Studies have shown that long noncoding RNAs (lncRNAs) play important roles in abiotic stress responses and several cold-responsive noncoding RNAs have been suggested to be involved in vernalization. We examined the transcriptome of the Chinese cabbage inbred line ( B. rapa L. var. pekinensis ) RJKB-T24, and identified 1,444 long intergenic noncoding RNAs (lincRNAs), 551 natural antisense transcripts (NATs), and 93 intronic noncoding RNAs (incRNAs); 549 of the 2,088 lncRNAs significantly altered their expression in response to four weeks of cold treatment. Most differentially expressed lncRNAs did not lead to a change of expression levels in mRNAs covering or near lncRNAs, suggesting that the transcriptional responses to four weeks of cold treatment in lncRNA and mRNA are independent. However, some differentially expressed mRNAs had NATs with expression altered in the same direction. These genes were categorized as having an abiotic stress response, suggesting that the paired-expression may play a role in the transcriptional response to vernalization or cold treatment. We also identified short-term cold treatment induced NATs in BrFLC and BrMAF genes, which are involved in vernalization. The lncRNAs we identified differed from those reported in Arabidopsis thaliana , suggesting the role of lncRNAs in vernalization differ between these two species.