Primed histone demethylation regulates shoot regenerative competency.

Acquisition of pluripotency by somatic cells is a striking process that enables multicellular organisms to regenerate organs. This process includes silencing of genes to erase original tissue memory and priming of additional cell type specification genes, which are then poised for activation by external signal inputs. Here, through analysis of genome-wide histone modifications and gene expression profiles, we show that a gene priming mechanism involving LYSINE-SPECIFIC DEMETHYLASE 1-LIKE 3 (LDL3) specifically eliminates H3K4me2 during formation of the intermediate pluripotent cell mass known as callus derived from Arabidopsis root cells. While LDL3-mediated H3K4me2 removal does not immediately affect gene expression, it does facilitate the later activation of genes that act to form shoot progenitors when external cues lead to shoot induction. These results give insights into the role of H3K4 methylation in plants, and into the primed state that provides plant cells with high regenerative competency.

To regenerate or not to regenerate: factors that drive plant regeneration.

Plants have a remarkable regenerative capacity, but it varies widely among species and tissue types. Whether plant cells/tissues initiate regeneration largely depends on the extent to which they are constrained to their original tissue fate. Once cells start the regeneration program, they acquire a new fate, form meristems, and develop into organs. During these processes, the cells must continuously overcome various barriers to the progression of the regeneration program until the organ (or whole plant) is complete. Recent studies have revealed key factors and signals affecting cell fate during plant regeneration. Here, we review recent research on: (i) environmental signal inputs and physical stimuli that act as initial triggers of regeneration; (ii) epigenetic and transcriptional cellular responses to those triggers leading to cellular reprograming; and (iii) molecules that direct the formation and development of the new stem cell niche. We also discuss differences and similarities between regeneration and normal development.

Characterization of somatic embryogenesis initiated from the Arabidopsis shoot apex.

Somatic embryogenesis is one of the best examples of the remarkable developmental plasticity of plants, in which committed somatic cells can dedifferentiate and acquire the ability to form an embryo and regenerate an entire plant. In Arabidopsis thaliana, the shoot apices of young seedlings have been reported as an alternative tissue source for somatic embryos (SEs) besides the widely studied zygotic embryos taken from siliques. Although SE induction from shoots demonstrates the plasticity of plants more clearly than the embryo-to-embryo induction system, the underlying developmental and molecular mechanisms involved are unknown. Here we characterized SE formation from shoot apex explants by establishing a system for time-lapse observation of explants during SE induction. We also established a method to distinguish SE-forming and non-SE-forming explants prior to anatomical SE formation, enabling us to identify distinct transcriptome profiles of these two explants at SE initiation. We show that embryonic fate commitment takes place at day 3 of SE induction and the SE arises directly, not through callus formation, from the base of leaf primordia just beside the shoot apical meristem (SAM), where auxin accumulates and shoot-root polarity is formed. The expression domain of a couple of key developmental genes for the SAM transiently expands at this stage. Our data demonstrate that SE-forming and non-SE-forming explants share mostly the same transcripts except for a limited number of embryonic genes and root genes that might trigger the SE-initiation program. Thus, SE-forming explants possess a mixed identity (SAM, root and embryo) at the time of SE specification.