Forever young: stem cell and plant regeneration one century after Haberlandt 1921.

Plants are characterized by a post-embryonic mode of organ development, which results in a need for these photoautotrophic organisms to regenerate lost parts in the course of their life cycle. This capacity depends on the presence of "pluripotent stem cells," which are part of the meristems within the plant body. One hundred years ago, the botanist Gottlieb Haberlandt (1854-1945) published experiments showing wounding-induced callus formation, which led ultimately to plant regeneration in tissue culture and thence to the techniques of "plant biotechnology," with practical applications for mankind. Here, we recount Haberlandt's discovery within the context of his long research life and his most influential book Physiologische Pflanzenanatomie. In the second part, we describe and analyze a plant tissue-culture regeneration system using sterile, dark-grown sunflower (Helianthus annuus) seedlings as experimental material. We document that excised hook segments, which contain a "stem cell niche," can regenerate entire miniature H. annuus-plantlets that, raised in a light/dark regime, develop flowers. Finally, we discuss molecular data relevant to plant regeneration with reference to phytohormones and conclude that, one century after Haberlandt, 1921, the exact biochemical/genetic mechanisms responsible for the capability of stem cells to remain "forever young" are, although already complex, really just beginning to become known.