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Multicellular organisms show the ability to replace damage cells, tissues and even whole organs through regeneration mechanisms. Plants show a remarkable regenerative potential. While the basic principles of plant regeneration have been known for a number of decades, the molecular and cellular mechanisms underlying such principles are currently starting to emerge. Some of these mechanisms point to the existence of highly reprogrammable cells. Developmental plasticity is a hallmark for stem cells, and stem cells are responsible for the generation of distinctive cell types forming plants. In the last years, a number of players and molecular mechanism regulating stem cell maintenance have been described, and some of them have also been involved in regenerative processes. These discoveries in plant stem cell regulation and regeneration invite us to rethink several of the classical concepts in plant biology such as cell fate specification and even the actual meaning of what we consider stem cells in plants. In this review we will cover some of these discoveries, focusing on the role of the plant stem cell function and regulation during cell and organ regeneration.
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.