An F-actin mega-cable supports the migration of the sperm nucleus during the fertilization of the polarity-inverted central cell of Agave inaequidens

Abstract

AbstractAsparagaceae’s large embryo sacs display a central cell nucleus polarized toward the chalaza, which means the sperm nucleus that fuses it during double fertilization migrates a 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 F-actin structures different from the short-range aster-like ones observed in Arabidopsis. Here, we analyzed the F-actin dynamics of Agave inaequidens, a typical Asparagaceae, before, during, and after central cell fertilization. Several parallel F-actin cables emerging from the nucleus within the central cell, enclosing the vacuole, and reaching the micropylar pole 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 the central cell one. Once karyogamy finished, the mega-cable disassembled, but new F-actin structures formed during the endosperm development. 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.