Urticaceae is a botanical family well known for its urticating trichomes that cause allergic reactions when touching the human skin.  The members of the family are anemophilous, which means that pollen grain transfer from staminate flowers to the stigma of pistillate flowers of the same individual occurs through the wind, without the participation of any biotic vector. They are also characterized by monoecious or dioecious sexual expression, and staminate flowers can develop a pistil called pistillode which is aborted during development. In most cases, the term pistillode refers to an aborted nonfunctional gynoecium in the mature flower that can be developed or rudimentary; the developed pistillode can perform various functions in pollination. Morphology studies based on microscopy help us to understand the floral mechanisms acting on pollen dispersal by the wind. Thus the objective of the present study was to investigate by light and scanning microscopy the morphology and functioning of staminate flowers of four species belonging to four Urticaceae genera: Boehmeria cylindrica (L.) Sw., Laportea aestuans (L.) Chew, Myriocarpa stipitata Benth. and Urera baccifera (L.) Gaudich. ex Wedd., with emphasis on the pistillode. Flower buds in a stage preceding anthesis and developed flowers were collected, fixed in buffered formalin or in Karnovsky solution, dissected with the aid of a Leica MZ 75 stereomicroscopic magnifying glass and processed for surface (scanning electron microscopy – SEM) and anatomical observations (light microscopy – LM). For SEM examination the dissected material was dried to the critical point, mounted on metal holders, placed on a carbon adhesive tape, sputtered with gold (exposure of 340 seconds) and observed with a Jeol JSM-6610LV scanning electron miroscope at 25 kV. For the LM exam the material was embedded in historesin and sectioned crosswise and lengthwise with a rotary microtome (2 to 4 μm thick sections). Serial sections were stained with 0.05% Toluidine Blue in phosphate buffer, pH 4.4, mounted on synthetic resin and observed with a light microscope. Photomicrographs were obtained with a Leica DM 4500 B photomicroscope coupled to a Leica DFC 320 digital camera. The pistillode arises as a single central protuberance after the initiation of four or five sepal primordia together with the primordia of lateral stamens. The elongation of the pistillode is rapid, exceeding the height of the stamens during the intermediate phases of flower development (figure 1). At the end of development, stamens and pistillode exhibit the same height since the stamens take on an inflex position in the flower bud. The pistillode does not contain ovules, exhibits an epidermis with voluminous and mucilaginous cells and a mesophyll consisting of aerenchyma in Laportea aestuans and Urera baccifera (figure 2), empty in Boehmeria cylindrica (figure 3), and of parenchyma in Myriocarpa stipitata (figure 4). Our results, added to those reported in the literature, indicate that the pistillode, together with the inflexed stamens, whose anthers are embedded in the sepals, acts on the explosive mechanism of release of the pollen to be transported by the wind. The pistillode, inflated by the accumulation of air in the mesophyll during development, presses the anthers, which separate from the sepals and move in the direction opposite to the flower, releasing pollen that can be agglutinated by the mucilage produced in the epidermis of the pistillode. We believe that this floral mechanism involving pistillode, stamens and sepals, optimizes anemophily, so that the pollen can reach farther distances, avoiding self-pollination and guaranteeing a greater genetic variability for these species.

Acknowledgements: This study was supported by São Paulo Research Foundation (Fapesp – process numbers: 2013/19459-3 and 2014/07453-3).

Figures:

Figure 1: Electron micrograph of the inflated and widened pistillode of Urera baccifera (SEM). Note that the pistillode is much larger than the stamens. Symbols: p = pistillode, st = stamens. Scale: 50 μm.

Figure 2: Longitudinal anatomical section of the pistillode of Laportea aestuans showing the outer epidermis consisting of voluminous and mucilaginous cells (arrowhead) and the mesophyll filled with aerenchyma (arrowhead) (LM). Symbols: a = anther, p = pistillode. Scale: 100 μm.

Figure 3: Crosswise anatomical section of a flower bud of Boehmeria cylindrica (LM). The pistillode consists of an epidermis with voluminous and mucilaginous cells (arrowhead) and an empty interior (arrow). Scale: 100 μm.

Figure 4: Longitudinal anatomical section of a flower bud of Myriocarpa stipitata (LM). The pistillode consists of epidermis with voluminous and mucilaginous cells and is filled with various layers of smaller and rounded parenchyma cells (arrowhead). Scale: 50 μm.

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EMC Abstracts - https://emc-proceedings.com/abstract/light-and-electron-microscopy-analyses-elucidate-the-structure-and-function-of-the-pistillode-in-urticaceae-species/