Abstract
The embryogenic transition of somatic cells requires an extensive reprogramming of the cell transcriptome. Relevantly, the extensive modulation of the genes that have a regulatory function, in particular the genes encoding the transcription factors (TFs) and miRNAs, have been indicated as controlling somatic embryogenesis (SE) that is induced in vitro in the somatic cells of plants. Identifying the regulatory relationships between the TFs and miRNAs during SE induction is of central importance for understanding the complex regulatory interplay that fine-tunes a cell transcriptome during the embryogenic transition. Hence, here, we analysed the regulatory relationships between AGL15 (AGAMOUS-LIKE 15) TF and miR156 in an embryogenic culture of Arabidopsis. Both AGL15 and miR156 control SE induction and AGL15 has been reported to target the MIR156 genes in planta. The results showed that AGL15 contributes to the regulation of miR156 in an embryogenic culture at two levels that involve the activation of the MIR156 transcription and the containment of the abundance of mature miR156 by repressing the miRNA biogenesis genes DCL1 (DICER-LIKE1), SERRATE and HEN1 (HUA-ENHANCER1). To repress the miRNA biogenesis genes AGL15 seems to co-operate with the TOPLESS co-repressors (TPL and TPR1-4), which are components of the SIN3/HDAC silencing complex. The impact of TSA (trichostatin A), an inhibitor of the HDAC histone deacetylases, on the expression of the miRNA biogenesis genes together with the ChIP results implies that histone deacetylation is involved in the AGL15-mediated repression of miRNA processing. The results indicate that HDAC6 and HDAC19 histone deacetylases might co-operate with AGL15 in silencing the complex that controls the abundance of miR156 during embryogenic induction. This study provides new evidence about the histone acetylation-mediated control of the miRNA pathways during the embryogenic reprogramming of plant somatic cells and the essential role of AGL15 in this regulatory mechanism.
Highlights
The molecular mechanisms that underlie the phenomenon of the toti-/pluripotency of somatic cells remain at the centre of developmental biology
We showed that to repress the miRNA biogenesis genes AGL15 seems to co-operate with other components of the SIN3/HDAC silencing complex including the TOPLESS corepressors and HDAC6 and HDAC19 histone deacetylases
We hypothesized that similar histone acetylation-related mechanisms operate during the AGL15-mediated repression of the miRNA biogenesis genes during embryogenic induction. In support of this assumption, we found that the repressive effect of AGL15 on the DCL1, HEN1, and SERRATE genes is modulated by TSA, which is an HDAC inhibitor (Figure 4) and that AGL15 significantly decreased the acetylation level of the H3 histone associated with the DCL1 and SERRATE promoters in the embryogenic culture (Figure 7)
Summary
The molecular mechanisms that underlie the phenomenon of the toti-/pluripotency of somatic cells remain at the centre of developmental biology. Identifying the regulatory relationships between the TFs and miRNAs is of central importance for understanding the complex network that fine tunes the transcriptomes of the somatic cells during the induction of SE. The binding of AGL15 to numerous hormone-related loci in the Arabidopsis genome implies that AGL15 controls embryogenic induction by regulating gibberellic acid and ethylene metabolism and auxin signalling [13,14]. In addition to directly regulating the hormone-related genes AGL15 indirectly affects hormone metabolism and signalling via regulatory interactions with other TFs that have been indicated as having essential functions in SE, including LEAFY COTYLEDON2, FUSCA3 and BABY BOOM (reviewed in [18,19,20])
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