9. Concluding remarks
The basic principles of plant regeneration have been known for a number of decades but actual molecular mechanisms behind those principles have been only started to be elucidated. Current advances and developments in microscopic, molecular and computational technologies have been used to generate new data and relevant information. These advances are opening new questions and invite us to rethink classical concepts in plant biology such as pluripotency, cell fate specification and even the actual meaning of what we consider a stem cell in plants.
In the last years, remarkable advances in the knowledge of plant regeneration have been made, and a number of players and mechanism in plant regeneration have been elucidated. However, it is still unknown the reason behind the wide variety of regenerative mechanisms found in plants and the molecular signatures underlying regenerative capacity of certain cell-types, which, in turn, might influence different regenerative capacities between organs and among species. Particularly intriguing is callus formation. As callus is achieved by exogenous hormonal supplementation and it was not thought to be part of endogenous plant regenerative mechanisms, it is unclear why it showed a specific tissue organization and genetic program. More recently, callus has been shown to be formed upon wounding and as part of endogenous regenerative processes. Further studies might shed light on molecular connections and possibly common roles of certain cell-types shared between hormone-induced callus and endogenouslyformed callus. Overall, regeneration associates with cells able to change their identity or transdifferentiate, and with apparently low level of differentiation. Thus, it appears that although regeneration uses stem cell regulating factors, it does not require stem cells undergoing asymmetric divisions but rather cells not committed into a specific cell fate or differentiation program.
