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When a root becomes a shoot, how plant organs change identity
As it grows, a plant produces stems and roots, organs that develop according to different patterns. Researchers from INRA, UPMC and ENS have shown, in the model plant Arabidopsis thaliana, that roots can be transformed into shoots, and vice versa, in a conversion phenomenon involving stem cells. Their findings, published in the journal Development dated March 28, 2017, could lead to novel methods for the vegetative multiplication of plant cultivars.
After germination, the organs of a plant derive from undifferentiated cells nested in structures called meristems that sustain indefinite proliferation. As they divide and grow in size, these cells leave the meristem and, together, form different types of organs: roots originate from meristems in the lower part of the embryo, shoots emerge from apical meristems. The molecular mechanisms involved in the very first stages of organ differentiation and the factors that determine organ identity remain poorly characterized, although they are essential to plant biology.
From root to shoot, a conversion only possible at certain developmental stages
Combining cytological, molecular and genomic approaches, researchers from INRA, UPMC and ENS have shown that Arabidopsis thaliana lateral root meristems can be rapidly converted into buds following treatment with a plant hormone called cytokinin that controls organogenesis. This transformation is particularly efficient when the lateral root meristems are treated at a precise early stage of development.
The identity of young organs is remarkably plastic
The scientists have shown that the root-to-shoot conversion starts with a slowing down of the cell divisions at the heart of the root meristem, followed by the recruitment of its stem cells for the formation of the bud that eventually replaces the root. These major cellular events occur together with changes in gene expression, as genes involved in the development of the shoot are turned on, and those participating to root development are turned off.
The researchers have also demonstrated that organ identity remains uncertain for a few days immediately following conversion. Depending on growth conditions, the organ can continue its development as a shoot or can become a root again, thereby stressing the great plasticity of plant organogenesis programmes.
Altogether, these results clearly show that the conversion of a lateral root meristem into a shoot meristem occurs through a fast and coordinated change of cell type, not requiring proliferation, a phenomenon known as transdifferentiation. The resulting shoots can then be rooted in soil and produce seeds.
Since the 1950s, in vitro culture methods have enabled the propagation of plant materials towards commercial or scientific goals. Conversion is an atypical way to form new organs that could be fine-tuned into novel methods to streamline the vegetative multiplication of valuable plant cultivars.
Olga Rosspopoff, Liudmila Chelysheva, Julie Saffar, Lena Lecorgne, Delphine Gey, Erwann Caillieux, Vincent Colot, François Roudier, Pierre Hilson, Richard Berthomé, Marco Da Costa, and Philippe Rech. Direct conversion of root primordium into shoot meristem relies on timing of stem cell niche development. Development, March 28, 2017.