| Summary: | The ability of somatic cells to lose their differentiated state and attain a pluripotent or even totipotent state under specific conditions is of great interest to those wishing to understand how most cells maintain their differentiated state throughout their lifetimes. Animal somatic cell acquisition of pluripotency has been of particular interest because of the potential for personalized regenerative therapies, as well as the possibility that creating a source of pluripotent cells from somatic tissue would overcome ethical concerns with direct stem cell harvesting. While the ability to induce the loss of differentiation and acquisition of pluripotency is a recent development in animals (Takahashi K et al. 2006) plant scientists have been generating masses of cells with the ability to fully regenerate any plant organ since the 1930s (White 1939b). In plants, certain somatic cells can be induced to proliferate in culture and acquire regenerative competence through exposure to exogenous hormones. This cultured tissue is commonly known as callus. While the inducing hormones are well known the molecular mechanisms underlying regenerative callus formation not fully understood. We have discovered that, in the model plant Arabidopsis thaliana, loss of LIKE HETERCHROMATIN PROTEIN 1 ( LHP1) not only increases the formation of callus but also enhances the rate at which these calli acquire regenerative competency. LHP1 in plants encodes a central component of the Polycomb complex that is responsible for epigenetic repression of target gene expression. This suggests that Polycomb regulation acts to repress genes involved in callus formation and maturation. Consistent with this idea, we were able to confirm that loss of other key Polycomb complex genes also leads to increased callus formation and maturation. We further showed that inhibition of Polycomb activity in these mutants eliminated the requirement for active cell division and initial hormone treatment prior to the acquisition of regenerative competency. This suggest that a key step in the acquisition of regenerative competence in calli is the relief of Polycomb repression, and that one of the functions of both hormones and cell division is to allow for the removal of Polycomb regulation from specific genes. Lastly we describe the production of callus from a novel tissue layer in one of our LHP1 mutants. The production of this alternate callus most likely is the result of a secondary mutation segregating in that particular LHP1 mutant line.
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