Dedifferentiation of Plant Cells: A Term Covering Multiple Pathways?

The remarkable plasticity of plants is best exemplified by the capacity of their somatic cells to regenerate entire organs or the organism itself. The molecular and cellular events underlying this ability are complex and multifaceted. The initial phase leading to cell cycle reactivation is often called dedifferentiation. This process is triggered either by wounding or exogenous hormone application. In this opinion paper, I propose that the dedifferentiation of mature somatic cells is a two-step process. It involves a transition into a transient senescence-like state induced by stress and/or signals emanating from dying cells. This state entails the loss of genetic information required for cell differentiation, resulting in a critical cellular condition. In the absence of subsequent proliferative signals, dedifferentiating (senescing) cells become committed to programmed cell death. Exogenous and/or endogenous plant hormones, such as auxin and cytokinin, might override this pathway. This rescue step, in most cases, activates cell divisions to replace lost cells/tissues. If cell division is maintained, it may result in callus formation. A callus is not an undifferentiated, homogeneous mass of cells. It is an unorganised tissue with at least some cells having ground-tissue-like molecular identity and high developmental potential. A callus might also form from pre-existing competent cell populations, e.g., pericycle cells, with no senescence-like intermitting state. It is discussed whether this "one-step" callus-formation pathway can be considered dedifferentiation.