Formation and cell lineage patterns of the shoot apex of maize

Abstract

In maize, glossy (gl) mutants lack the wax layer normally present on the epidermis of young leaves. By insertion mutagenesis, unstable alleles (gl1‐m) have been induced at the Gl1 locus. In the gl1‐m8 strain, somatic reversions to wild‐type frequently result in the formation of large sectors occupying predictable positions in all seedlings' leaves. In studies of 230 gl1‐m8 seedlings with large reverted sectors covering around 50% of the first leaf, four patterns of sectoring were recognized: one large sector ending in all leaves at the main midrib (32.6% of cases); one central sector on leaves 1, 3 and 5 (or 2 and 4), corresponding to lateral stripes of Gl1 tissue on the other leaves (7.9%); a sector decreasing or increasing in successive leaves (9.1%); other types with one sector covering a leaf surface between 33 and 50% (19.1%) or with complex variegations (31.3%). Based on leaf sectoring, the pattern and stability of cell lineages during shoot apex establishment and embryonic activity leading to leaf primordia, are inferred from the genetic state (Gl or gl1‐m) of leaf founder cells present in the apex at the ring of primordia insertion. A genetic experiment indicates that the large somatic reversions considered derived from both the LI and the LII layers of the apex. A large majority of the observed patterns of reversion can be interpreted as due to a single event of transposition. The data are discussed and relevant conclusions proposed in relation to the age of the proembryo at the time of apex formation, the permanent or impermanent state of initial cells of the apex, the polarization of cell divisions and the plane of early apex cell division as a mechanism leading to the bilateral symmetry of the maize seedling. Copyright © 1992, Wiley Blackwell. All rights reserved