Maintenance of grafting‐induced epigenetic variations in the asexual progeny of Brassica oleracea and B. juncea chimera
Grafting‐induced variations have been observed in many plant species, but the heritability of variation in progeny is not well understood. In our study, adventitious shoots from the C cell lineage of shoot apical meristem (SAM) grafting chimera TCC (where the origin of the outmost, middle and innermost cell layers, respectively, of SAM is designated by ‘T’ for tuber mustard and ‘C’ for red cabbage) were induced and identified as r‐CCC (r = regenerated). To investigate the maintenance of grafting variations during cell propagation and regeneration, different generations of asexual progeny (r‐CCCn, n = generation) were established through successive regeneration of axillary shoots from r‐CCC. The fourth generation of r‐CCC (r‐CCC4) was selected to perform whole genome bisulfite sequencing for comparative analysis of hetero‐grafting‐induced global methylation changes relative to r‐s‐CCC4 (s = self‐grafting). Increased CHH methylation levels and proportions were observed in r‐CCC4, with substantial changes occurring in the repeat elements. Small RNA sequencing revealed 1135 specific small interfering RNA (siRNA) tags that were typically expressed in r‐CCC, r‐CCC2 and r‐CCC4. Notably, 65% of these specific siRNAs were associated with repeat elements, termed RE siRNAs. Subsequent analysis revealed that the CHH methylation of RE siRNA‐overlapping regions was mainly hypermethylation in r‐CCC4, indicating that they were responsible for directing and maintaining grafting‐induced CHH methylation. Moreover, the expression of 13 differentially methylated genes (DMGs) correlated with the phenotypic variation, showing differential expression levels between r‐CCC4 and r‐s‐CCC4. These DMGs were predominantly CG hypermethylated, their methylation modifications corresponded to the transcription of relative methyltransferase.
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