Ontogenesis and secretion mechanism of Morinda citrifolia L. (Rubiaceae) colleters

Abstract

Colleters are secretory structures present in more than 60 families of angiosperms, whose function is protecting the leaf primordium and meristem against biotic and abiotic damage. The colleters are developed from protoderm and ground meristem and the secretory process begins during cell differentiation. The secretion is a complex phenomenon of synthesis, separation and isolation of substances, as well as release or extracellular elimination. This research describes the ontogenesis of Morinda citrifolia L. colleters and the mechanism of exsudate externalization. For this purpose, various microscopy techniques, including scanning electron microscopy, light microscopy (including histochemical tests) and confocal scanning laser microscopy were used. Standard type colleters are attached on the stipule adaxial surface. During development, the colleters pass through four developmental stages defined by structural and anatomical changes: undifferentiated, pre-secretory, secretory and senescent. In the undifferentiated stage, the secretory structures are characterized by small protuberances. In the pre-secretory stage, the secretory epithelium begins its differentiation. In the following stages, the colleters present a parenchymatic central axis covered by palisade epidermal cells. Secretion occurs by the rupture of the cuticle. At the end of the secretory process, the colleters begin the senescent stage, characterized by deformation, color change and cuticle wrinkling. This work evidences the section passage mechanism by cuticle rupture, accumulating knowledge in the still poorly understood process of molecules passages by the external periclinal wall in Rubiaceae colleters.