Recent progress on plant regeneration

Abstract

For survival from severe natural conditions, plants have evolved powerful regenerative abilities, conferring specific cells with totipotency or pluripotency. The regenerative abilities have been widely exploited in agricultural production, and the commonly used technologies include cuttage, engraft and the propagation through plant tissue culture. In seed plants, regeneration usually results in two different consequences. Firstly, damaged tissues can be repaired by tissue regeneration; Secondly, certain somatic cells can be used as a source to regenerate a whole plant via de novo organogenesis or somatic embryogenesis. In this review, we mainly summarize the recent advances in de novo organogenesis and somatic embryogenesis in seed plants, and intend to provide useful information to the plant scientists, especially those who are interested in the improvement of agricultural applications of plant regeneration. De novo organogenesis refers to the formation of adventitious roots or shoots from the regeneration-competent cells in wounded or detached plant organs. During de novo organogenesis, the regeneration-competent cells, such as procambium, pericycle or other parenchyma cells in the vasculature of various plant tissues, do not experience a dedifferentiation process backward to the embryo-stage state. De novo organogenesis may occur directly from the regeneration-competent cells in cultured explants, or progress indirectly from the non-embryonic callus. Interestingly, non-embryonic callus formation from different plant organs follows a common mechanism and appears to be the ectopic activation of a root development program. Therefore, unlike previously believed, non-embryonic callus consists of a group of root primordium-like cells. During somatic embryogenesis, differentiated cells change their fates to become embryonic cells via dedifferentiation. The somatic embryos can be formed either directly from somatic cells or indirectly from embryonic callus. Plant hormones, genes involved in embryo development and shoot apical meristem maintenance, and some epigenetic factors play key roles in either direct or indirect somatic embryogenesis. The underlying theme of plant regeneration is the cell fate transition upon wounding or stress. In recent years, our knowledge about cell lineage during the fate transition in plant regeneration and molecular mechanism that directs the cell fate transition has been greatly improved. These benefit our understanding of the plant cell flexibility significantly. Wound and stress signals, actions of phytohormones and functions of transcription factors and epigenetic factors were explored in different types of plant regeneration. It becomes clear that wound and stress signals induce phytohormone actions at the earliest stage of regeneration. While auxin is required for de novo root organogenesis and callus formation, cytokinin triggers de novo shoot organogenesis. In somatic embryogenesis, auxin and abscisic acid play key roles in cell dedifferentiation. The phytohormone actions usually result in expressional changes of many key transcriptions factors, which act together or coordinate with epigenetic factors to control changes of transcriptomes in the cells for their fate transition. Despite the very rapidly progresses, many questions still remained unanswered in the regulation of plant regeneration. What is the molecular basis of wound and stress signals? Can any kind of cells in the plant undergo dedifferentiation to form somatic embryo? What is the common molecular basis for cells to acquire the regeneration competence? What are the molecular mechanisms guiding actions of phytohormones, transcription factors and epigenetic factors? What is the cell lineage during fate transition of different plant cells in regeneration? All these questions need to be further addressed in the future.