Secretory Structures in &amp;lt;i&amp;gt;Flourensia campestris&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;F. oolepis&amp;lt;/i&amp;gt;: Ultrastructure, Distribution, and (-)-Hamanasic Acid A Secretion

Mariana P Silva,Leonardo A Piazza,Beatriz G Galati,Ana L Scopel,Graciela M Tourn,Daniela López,Gabriela Zarlavsky,Juan J Cantero

doi:10.4236/ajps.2015.67100

Abstract

In this work, the localization, density, morphology and ultrastructure of secretory structures in aerial organs of Flourensia campestris (FC) and F. oolepis (FO) (Asteraceae) by means of a combination of light, fluorescence, transmission (TEM) and scanning electron microscopy (SEM) were examined. The possible role of secretory structures in the production and secretion of the phytotoxic sesquiterpene (-)-hamanasic acid A ((-)HAA) in both species was also assessed. Capitate glandular trichomes were found in all reproductive organs of FC and FO, and were being reported for the first time. These glandular trichomes, typically associated to edges and veins, were of the same type as those already described for vegetative organs, and were abundant in involucral bracts and corolla of tubulose and ligulate flowers. Their density in reproductive organs of both species was similar (ca. 30/mm2) and lower than that found in leaves (ca. 100/mm2) and stems (ca. 160/mm2 in FC, and up to 650/mm2 in FO). Glandular trichomes in vegetative organs followed a species-specific pattern of distribution. TEM and SEM observations suggest that each species differs in the way in which secretory materials are released to the outside: through cracks or pores in FC, or through a loose cuticle in FO. Similar inspections of the secretory ducts revealed lipophilic vacuoles localized in subepithelial and epithelial cells, in which secretions accumulated before being transferred to the duct. The presence of wall ingrowths in subepithelial cells suggests that granulocrine secretion operates in these species. Secretory ducts varied in density and diameter among the organs in both species, with the combination being maximal in woody stems. (-)HAA was only detected in surface secreted resins of both species, and its concentration (2D-TLC, GC-FID) was intimately associated with the distribution and density of glandular trichomes in each organ (capitula, leaves, and stems with primary or secondary growth). In addition, no (-)HAA was detected internally in the resins collected from secretory ducts. The composition of these resins showed distinctive profiles for FC and FO, and only four from ca. 30 compounds detected (GC/MS) were shared by both species. In addition to the elucidation of ultrastructural traits, distribution and density of secretory structures in aerial organs of FC and FO, present findings suggest a functional role for glandular trichomes in the secretion of the putative phytotoxic allelochemical (-)HAA.

Highlights

The Asteraceae are among the largest families of flowering plants with ca. 1700 genera and 23,000 - 26,000 species in all continents except for Antartica [1], and many species are well known for their different and important uses [2]
Secretory ducts varied in density and diameter among the organs in both species, with the combination being maximal in woody stems. (-)HAA was only detected in surface secreted resins of both species, and its concentration (2D-TLC, GC-FID) was intimately associated with the distribution and density of glandular trichomes in each organ
Light and Fluorescence Microscopy Secretory structures in the capitula of both species were composed by glandular trichomes and secretory ducts (Figure 1A), similar to those observed in leaves (Figures 1B and Figures 1C), and stems (Figure 2 and Figure 3)

Summary

Introduction

The Asteraceae are among the largest families of flowering plants with ca. 1700 genera and 23,000 - 26,000 species in all continents except for Antartica [1], and many species are well known for their different and important uses [2]. In Argentina, it is the largest family with ca. Typically constituted by mixtures of volatile and non-volatile phenolic and/or terpenoids secondary compounds, are usually secreted in specialized structures located either internally or on the surface of the plant [6]. These resins, according to their constituents, are known to play important ecological functions, ameliorating or preventing damage from abiotic and biotic stresses in their natural environments (high irradiance, UVB, dissecation, herbivory, competition), whose properties have been used for the management of harmful diseases, insects, pests and weeds [7]-[9]

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Discussion

Conclusion