The stem, composed of nodes and internodes, is the axis of shoots commonly seen among seed plants and is the most important target for regulating plant height in breeding practice. It is produced from the shoot apical meristem as a phytomer unit, along with the adjacent leaves and axillary buds. However, compared to other major organs such as leaves, roots, and flowers, our understanding of stem development has lagged far behind. Katsutoshi Tsuda, Akiteru Maeno, and Kenichi Nonomura at the National Institute of Genetics, Research Organization of Information and Systems, have defined the rice stem structure and described the developmental process using various approaches including micro CT. They also established a heat-shock inducible clonal analysis system and analyzed how cell fate establishment occurs for each organ during the development of the phytomer, which consists of the leaf, stem, and axillary bud. They found that cell fate establishment proceeds in a stepwise manner. First, cell fates for the phytomer are established prior to its initiation. Then, the emergence of cells that are destined for node splits of leaf and stem fates. Subsequently, the fate for the axillary bud fate is established. Finally, cell fate determination to the internodes occurs in a limited number of cells. This study clarified the key events and their chronological sequence in stem development, which had been unknown for a long time. It is expected to provide a basis for further research at the molecular level, which will lead to the improvement of stem traits for the design of ideal crops in the future.
Figure: The temporal order of cell fate acquisition in the rice stem. The fate of the flag leaf phytomer is established 10 to 5 days prior to its initiation. Fates for non-elongating domains such as foot II, node I, and the pulvinus start being established earlier than elongating domains. Internode II is the last part of the stem whose cell fate determination starts. It is likely to originate from a very limited number of cells between node I and foot II (inset). Black gradient lines at the bottom left show the start (left) and end (right) of cell fate establishment. Stages at which the founder cells are fully present are indicated by dots at the end of black lines.
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