Abstract
Unlike seed plants, ferns leaves are considered to be structures with delayed determinacy, with a leaf apical meristem similar to the shoot apical meristems. To better understand the meristematic organization during leaf development and determinacy control, we analyzed the cell divisions and expression of Class I KNOX genes in Mickelia scandens, a fern that produces larger leaves with more pinnae in its climbing form than in its terrestrial form. We performed anatomical, in situ hybridization, and qRT-PCR experiments with histone H4 (cell division marker) and Class I KNOX genes. We found that Class I KNOX genes are expressed in shoot apical meristems, leaf apical meristems, and pinnae primordia. During early development, cell divisions occur in the most distal regions of the analyzed structures, including pinnae, and are not restricted to apical cells. Fern leaves and pinnae bear apical meristems that may partially act as indeterminate shoots, supporting the hypothesis of homology between shoots and leaves. Class I KNOX expression is correlated with indeterminacy in the apex and leaf of ferns, suggesting a conserved function for these genes in euphyllophytes with compound leaves.
Highlights
Vascular plant organs are classically defined based on their position; on their tissue organization; and on the presence, position, and activity of their meristems [1,2]
More studies concerning other genes and leaf morphologies are necessary to expand this conclusion to other fern groups, our data indicates the presence of a transient apical meristem in the pinnae of Mickelia scandens without the presence of a prominent apical cell
The widespread reference to a unicellular meristem for ferns by some authors may be the result of many textbooks that describe in detail the single apical cell and its cutting faces, while lacking further information about the other meristematic cells in this group (e.g., [22,38,39,40]), contributing to the propagation of this concept
Summary
Vascular plant organs are classically defined based on their position; on their tissue organization (symmetry axes and vascular tissue); and on the presence, position, and activity of their meristems [1,2] With these criteria, leaves are lateral determinate organs generally with an abaxial-adaxial asymmetry, and these features seem to generally apply well to leaves in seed plants. Shoots are characterized by indeterminacy and are marked by the expression of Class I KNOTTED-LIKE HOMEOBOX (KNOX) genes in the shoot apical meristem (SAM) [3]. This class of genes belongs to the superclass three amino acid loop extension (TALE) of homeodomain proteins [4,5]. Compound leaves can be interpreted as structures with a delayed determinacy during their development, and Class I KNOX facilitates leaflet formation [7,15,16]
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