Identification of protoplast-isolation responsive microRNAs in Citrus reticulata Blanco by high-throughput sequencing

MicroRNA biogenesis and activity in plant cell dedifferentiation stimulated by cell wall removal

BACKGROUND

Despite the frequent use of protoplast-to-plant system in in vitro cultures of plants, the molecular mechanisms regulating the first and most limiting stages of this process, i.e., protoplast dedifferentiation and the first divisions leading to the formation of a microcallus, have not been elucidated.

RESULTS

In this study, we investigated the function of miRNAs in the dedifferentiation of A. thaliana mesophyll cells in a process stimulated by the enzymatic removal of the cell wall. Leaf cells, protoplasts and CDPs (cells derived from protoplasts) cultured for 24, 72 and 120 h (first cell division). In protoplasts, a strong decrease in the amount of AGO1 in both the nucleus and the cytoplasm, as well as dicing bodies (DBs), which are considered to be sites of miRNA biogenesis, was shown. However during CDPs division, the amounts of AGO1 and DBs strongly increased. MicroRNA transcriptome studies demonstrated that lower amount of differentially expressed miRNAs are present in protoplasts than in CDPs cultured for 120 h. Then analysis of differentially expressed miRNAs, selected pri-miRNA and mRNA targets were performed.

CONCLUSION

This result indicates that miRNA function is not a major regulation of gene expression in the initial but in later steps of dedifferentiation during CDPs divisions. miRNAs participate in organogenesis, oxidative stress, nutrient deficiencies and cell cycle regulation in protoplasts and CDPs. The important role played by miRNAs in the process of dedifferentiation of mesophyll cells was confirmed by the increased mortality and reduced cell division of CDPs derived from mutants with defective miRNA biogenesis and miR319b expression.

Genome-wide identification of microRNAs involved in the somatic embryogenesis of Eucalyptus

MicroRNAs (miRNAs) are small noncoding RNAs (18-24 nt) and function in many biological processes in plants. Although Eucalyptus trees are widely planted across the world, our understanding of the miRNA regulation in the somatic embryogenesis (SE) of Eucalyptus is still poor. Here we reported, for the first time, the miRNA profiles of differentiated and dedifferentiated tissues of two Eucalyptus species and identified miRNAs involved in SE of Eucalyptus. Stem and tissue culture-induced callus were obtained from the subculture seedlings of E. camaldulensis and E. grandis x urophylla and were used as differentiated and dedifferentiated samples, respectively. Small RNA sequencing generated 304.2 million clean reads for the Eucalyptus samples (n = 3) and identified 888 miRNA precursors (197 known and 691 novel) for Eucalyptus. These miRNAs were mainly distributed in chromosomes Chr03, Chr05, and Chr08 and can produce 46 miRNA clusters. Then, we identified 327 and 343 differentially expressed miRNAs (DEmiRs) in the dedifferentiation process of E. camaldulensis and E. grandis x urophylla, respectively. DEmiRs shared by the two Eucalyptus species might be involved in the development of embryonic callus, such as MIR156, MIR159, MIR160, MIR164, MIR166, MIR169, MIR171, MIR399, and MIR482. Notably, we identified 81 upregulated and 67 downregulated miRNAs specific to E. camaldulensis, which might be associated with the high embryogenic potential. Target prediction and functional analysis showed that they might be involved in longevity regulating and plant hormone signal transduction pathways. Further, using the gene expression profiles, we observed the negative regulation of miRNA-target pairs, such as MIR160~ARF18, MIR396~GRF6, MIR166~ATHB15/HD-ZIP, and MIR156/MIR157~SPL1. Interestingly, transcription factors such as WRKY, MYB, GAMYB, TCP4, and PIL1 were found to be regulated by the DEmiRs. The genes encoding PIL1 and RPS21C, regulated by upregulated miRNAs (e.g., egd-N-miR63-5p, egd-N-miR63-5p, and MIR169,) were downregulated exclusively in the dedifferentiation of E. camaldulensis. This is the first time to study the miRNA regulation in the dedifferentiation process of Eucalyptus and it will provide a valuable resource for future studies. More importantly, it will improve our understanding of miRNA regulation during the somatic embryogenesis of Eucalyptus and benefit the Eucalyptus breeding program.

An Arabidopsis protoplast isolation method reduces cytosolic acidification and activation of the chloroplast stress sensor SENSITIVE TO FREEZING 2

Chloroplasts adapt to freezing and other abiotic stresses in part by modifying their membranes. One key-remodeling enzyme is SENSITIVE TO FREEZING2 (SFR2). SFR2 is unusual because it does not respond to initial cold stress or cold acclimation, instead it responds during freezing conditions in Arabidopsis. This response has been shown to be sensitive to cytosolic acidification. The unique lipid products of SFR2 have also been detected in response to non-freezing stresses, but what causes SFR2 to respond in these stresses is unknown. Here, we investigate protoplast isolation as a representative of wounding stress. We show that SFR2 oligogalactolipid products accumulate during protoplast isolation. Notably, we show that protoplast cytosol is acidified during isolation. Modification of the buffers reduces oligogalactolipid accumulation, while prolonged incubation in the isolated state increases it. We conclude that SFR2 activation during protoplast isolation correlates with cytosolic acidification, implying that all SFR2 activation may be dependent on cytosolic acidification. We also conclude that protoplasts can be more gently isolated, reducing their stress.