Abstract
Asparagaceae’s large embryo sacs display a central cell nucleus polarized toward the chalaza, which means the sperm nucleus that fuses with it during double fertilization migrates an atypical long distance before karyogamy. Because of the size and inverted polarity of the central cell in Asparagaceae, we hypothesize that the second fertilization process is supported by an F-actin machinery different from the short-range F-actin structures observed in Arabidopsis and other plant models. Here, we analyzed the F-actin dynamics of Agave inaequidens, a classical Asparagaceae, before, during, and after the central cell fertilization. Several parallel F-actin cables, spanning from the central cell nucleus to the micropylar pole, and enclosing the vacuole, were observed. As fertilization progressed, a thick F-actin mega-cable traversing the vacuole appeared, connecting the central cell nucleus with the micropylar pole near the egg cell. This mega-cable wrapped the sperm nucleus in transit to fuse with the central cell nucleus. Once karyogamy finished, and the endosperm started to develop, the mega-cable disassembled, but new F-actin structures formed. These observations suggest that Asparagaceae, and probably other plant species with similar embryo sacs, evolved an F-actin machinery specifically adapted to support the migration of the fertilizing sperm nucleus within a large-sized and polarity-inverted central cell.
Highlights
Two sperm cells are released from the pollen tube at the egg apparatus boundary during Angiosperm’s fertilization
To elucidate the mechanism that supports the transport of sperm nuclei during the central cell fertilization in Asparagaceae species, we studied the mature embryo sac of non-pollinated flowers of a so far uncharacterized family member: A. inaequidens
The egg apparatus was located at the opposite side of the embryo sac, composed of an egg (Figures 1A,C) and two synergid cells (Figures 1A,D)
Summary
Two sperm cells are released from the pollen tube at the egg apparatus boundary during Angiosperm’s fertilization. F-Actin Dynamics During Agave Fertilization central cell nucleus is polarized toward the micropylar end (Sprunck and Gross-Hardt, 2011), the distance the second sperm nucleus travels from the plasmogamy site to the central cell nucleus is around 1 μm (Kawashima and Berger, 2015). In Agave tequilana, the distance between the egg cell and the central cell nucleus is about 200times longer than in Arabidopsis (González-Gutiérrez et al, 2014). The latter implies that the second sperm nucleus needs to undertake a longer journey in Asparagaceae. It is plausible that these plant species evolved a specialized long-range transport machinery to support the migration of the sperm nucleus
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