Pollen from Arabidopsis thaliana and other Brassicaceae are functionally omniaperturate

Abstract

The polarity of pollen germination in A. thaliana is externally induced, not linked to aperture location. The biomechanical force for breaking nonaperture walls is found in localized swelling of intine pectins. As such, the pollen from A. thaliana, and likely many Brassicaceae family members, are functionally omniaperturate. This new mechanism for germination between extant apertures raises questions about exine porosity and the diversity of mechanisms across taxa.

Pollen wall degradation in the Brassicaceae permits cell emergence after pollination

“…To determine whether pollen grains of other species would be similarly sensitive to reactive oxygen species (ROS), we treated pollen grains from three obligate aperture users ( Brassica rapa , Barbarea vulgaris , and Barbarea orthoceras ), and three “break‐out” species able to germinate through inter‐aperture walls ( A. thaliana , Lunaria annua , and Hesperis matronalis ; Edlund et al, 2016). By fluorescence microscopy, only species with natural inter‐aperture germination were detectably sensitive to experimental redox wall degradation (Fig.…”

“…To emerge from durable exine walls after pollination, the delicate cells inside move through openings (apertures) in the exine of various shape and number (Edlund et al, 2004), aiming especially for the aperture closest to the stigma (Wolters‐Arts et al, 1998). But apertures are not the only way out; many pollen grains are inaperturate (Furness, 2007), and in some flowers of the Brassicaceae family, including Arabidopsis thaliana , the pollen tube can breach inter‐aperture exine for direct access to the stigma, regardless of the positions of the grain's three large apertures (Hoedemaekers et al, 2015; Edlund et al, 2016).…”

“…When pollen is dusted onto a stigma, contact is sometimes at a grain's aperture, but often a grain lies so that the shortest path from its interior to the stigma is across a wall. When this happens in A. thaliana , tube cells can take that shortest path; they aim directly through the wall at the pollen–stigma interface, toward the source of stigma fluid (Edlund et al, 2016). As a desiccated grain hydrates, the pectin‐rich intine underlying the contact point with the stigma swells, straining the exine (Hoedemaekers et al, 2015).…”

“…As a desiccated grain hydrates, the pectin‐rich intine underlying the contact point with the stigma swells, straining the exine (Hoedemaekers et al, 2015). We previously used atomic force microscopy to find that the inter‐aperture exine is an order of magnitude stiffer than the material covering the aperture (Edlund et al, 2016), but because these properties were measured in pollen growth medium, not on the stigma, it remained to be seen (1) whether the wall at the break‐out site might be weakened in situ by more than simple mechanical strain and (2) if it were weakened, then by what biochemistry.…”

3D Manipulation and Imaging of Plant Cells using Acoustically Activated Microbubbles

“…Plant cells, such as the pollen grains and pollen tubes, play a key role in plant sexual reproduction, and serve as model systems for cellular morphogenesis. Pollen, also known as the male gametophyte of flowering plants, carries and transports the two sperm cells within its cytoplasm.…”