Romanova, Marina , Jernstedt, Judy .
Do shoot apical cells determine everything in some ferns?.
Although there is not complete agreement about the composition of the fern shoot apical meristem (SAM), the contemporary view emphasizes its multicellular structure, rather than an exclusive role for the single apical cell (AC) in organo- and histogenesis. After investigation of Ceratopteris richardii SAM dynamics associated with morphogenesis, we dare to question the current view, at least in this diminutive species. All leaf apical cells (LACs) (except the first, which appears in embryogenesis simultaneously with the shoot AC) originate by three unequal divisions in each immediate derivative of the AC. For the first 6-9 simple leaves of the young sporophyte, subsequent division of the AC is correlated with cell proliferation in the incipient leaf primordiun (LP); the AC does not undergo a further division until the LP has emerged. At this stage, leaves have a unibundle leaf trace (LT) and the shoot vascular system is protostelic. Later, after a developmental pause, the SAM changes its morphogenetic tempo. LPs are arrested until displaced some distance from the AC, while the AC continues to segment and creates an increasing “pool” of incipient LPs, resulting in an elongated adult sporophyte apex. These LPs eventually develop into compound leaves with multibundle leaf traces (LTs), and the shoot vascular system becomes dictyostelic. Ontogenetic changes in stelar type might be explained by the loss of meristematic identity and competence to differentiate into procambium by the central cells of the elongated adult apex. Plasmodesmata (PD) density in the AC and its immediate derivatives in both juvenile and adult SAMs is about ten-fold higher than reported for dicot SAMs. Abrupt decrease of PD density in the 4th merophyte, where LACs and, hence, LPs are established, supports our conclusion that only the AC and its immediate derivatives, the latter being constantly displaced during shoot morphogenesis, might be considered undifferentiated.
1 - University of California, Davis, Agronomy and Range Science, One Shields Avenue, Davis, California, 95616-8515, US
shoot apical meristem
shoot apical cell
leaf apical cell
Presentation Type: Paper
Location: Superior B (Cliff Lodge)
Date: Monday, August 2nd, 2004
Time: 2:15 PM