Heritable variation and small RNAs in the progeny of chimeras of Brassica juncea and Brassica oleracea

Chimeras have been used to study the transmission of genetic material and the resulting genetic variation. In this study, two chimeras, TCC and TTC (where the origin of the outer, middle, and inner cell layers, respectively, of the shoot apical meristem is designated by a ‘T’ for tuber mustard and ‘C’ for red cabbage), as well as their asexual and sexual progeny, were used to analyse the mechanism and the inheritance of the variation induced by grafting. Asexual TCC progeny were obtained by adventitious shoot regeneration, while TTC sexual progeny were produced by self-crossing. This study observed similar morphological variations in both the asexual and sexual progeny, including changes in leaf shape and the pattern of shoot apical meristem termination. The leaf shape variation was stable, while the rate of shoot apical meristem termination in the TTC progenies decreased from 74.52% to 3.01% after three successive rounds of self-crossing. Specific red cabbage small RNAs were found in the asexually regenerated plants (rTTT) that were not present in TTT, indicating that small RNAs might be transmitted from red cabbage to tuber mustard during grafting. Moreover, in parallel with the variations in phenotype observed in the progeny, some conserved miRNAs were differentially expressed in rTTT and TTT, which correlated with changes in expression of their target genes. These results suggest that the change in small RNA expression induced by grafting may be an important factor for introducing graft-induced genetic variations, providing a basis for further investigating the mechanism of graft-induced genetic variation through epigenetics.