Abstract
Stinging trichomes are rare in plants, occurring only in angiosperms, where they are reported for a few genera belonging to six families. Although there is no report of stinging trichomes in Apocynaceae, previous fieldwork collections of Fischeria and Matelea caused us a mild allergic reaction on the skin when we contacted the dense indumentum of the plants. This fact associated with the well-known presence of glandular trichomes with acute apex in both genera raised suspicions that stinging trichomes could be present in the family. Hence, this study aimed to investigate the likely occurrence of stinging trichomes in Fischeria and Matelea. We analyzed vegetative shoots and leaves of Fischeria stellata and Matelea denticulata through the usual procedures of light and scanning electron microscopy. We also performed several histochemical tests to investigate the chemical composition of trichome secretion. We detected that glandular trichomes occur throughout the surface of the leaf and stem. They are multicellular, uniseriate with an apical secretory cell, which has a dilated base and a needle-shaped apex. The secretion is compressed into the acuminate portion of the apical cell by a large vacuole, and crystals are deposited in the cell wall in a subapical position, providing a preferential site of rupture. The secretion, composed of amino acids and/or proteins, is released under mechanical action, causing skin irritation. Based on our detailed morphological and anatomical analyses, and in the functional aspects observed, we concluded that the glandular trichomes in Fischeria and Matelea can indeed be classified as stinging. Thus, Apocynaceae is the seventh family for which this type of trichome has been reported. We also compiled information on stinging trichomes in all families of angiosperms. Their phylogenetic distribution indicates that they have evolved at least 12 times during angiosperm evolution and may represent an evolutionary convergence of plant defense against herbivory.
Highlights
Introduction published maps and institutional affilFlowering plants have several types of internal and external secretory structures for protection against herbivory
The entire surface of the stem and leaves are covered by an indumentum composed of long, multicellular, and uniseriate non-glandular trichomes and short stinging trichomes
The only studies focusing on structural aspects of glandular trichomes in Apocynaceae are from Solereder [8], who mentioned the trichomes of Dischidia as being unicellular and mucilaginous, and from Stevens [11], who described the glandular trichomes of Matelea as being smaller than the non-glandular ones, with a short stalk, an inflated middle portion, and a short apiculum
Summary
Flowering plants have several types of internal and external secretory structures for protection against herbivory. More complex internal structures, such as secretory ducts and cavities, appeared later in the evolutionary history of angiosperms. They have more complex secretory processes and dynamics of interaction with the environment as they are external structures [1,2]. The stinging ones stand out for their type of defense function against herbivory. These trichomes are rare, found in only a few angiosperm families, and their secretion is composed of a myriad of chemical substances [3]
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