Fertilization-independent Cell-fusion between the Synergid and Central Cell in the Polycomb Mutant

In flowering plants, fertilization of the central cell gives rise to an embryo-nourishing endosperm. Recently, we reported that the endosperm absorbs the adjacent synergid cell through a cell-fusion, terminating the pollen tube guidance by a rapid inactivation of the synergid cell. Although this synergid-endosperm fusion (SE fusion) initiates soon after fertilization, it was still unknown whether the triggers of SE fusion are stimuli during fertilization or other seed developmental processes. To further dissect out the SE fusion process, we investigated the SE fusion in an Arabidopsis mutant defective for MULTICOPY SUPPRESSOR OF IRA1 (MSI1), a subunit of the polycomb repressive complex 2 (PRC2). The mutant msi1 develops autonomous endosperm without fertilization. Time-lapse imaging revealed a rapid efflux of the synergid contents during the autonomous endosperm development, indicating that the initiation of SE fusion is under the control of some of the events triggered by fertilization of the central cell distinct from the discharge of pollen tube contents and plasma membrane fusion.