Programmed cell death is the cause of central cell degeneration and single fertilization in Marathrum schiedeanum (Cham.) Tul (Podostemaceae)

The Podostemaceae family is exceptional among angiosperms because of the absence of double fertilization, a remarkable characteristic that appears to extend to the entire family. A recent investigation of some members of the family, including Dalzellia zeylanica (Tristichoideae), Griffithella zeylanica, Polypleurum stylosum, Zeylanidium lichenoides, and Zeylanidium maheshwarii (Podostemoideae), revealed that the central cell in all of them is degenerated when the pollen tube arrives at the synergid. In the present work, we studied the ontogenetic development of the female gametophyte. We also conducted a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and an ultrastructural transmission electron microscopy (TEM) analysis of Marathrum schiedeanum (Podostemoideae) to determine the causes of central cell degeneration. The results show that in the early stage, the female gametophyte was 4-celled and 4-nucleated, but the central cell degenerated very quickly before the start of anthesis. The reduced mature female gametophyte is 3-nucleate, 3-celled. TUNEL staining and ultrastructural TEM analysis indicated that developmental programmed cell death (PCD) mediated the degeneration of the central cell. The central cell displayed a significant quantity of vacuoles containing some cellular material, and the compact chromatin was present in the nucleus and DNA fragmentation occurred. The factors that trigger PCD in the central cell are unknown, but the test results confirm that pollination is not necessary for it to occur. The death of the central cell before pollination prevents the possibility of double fertilization and consequently results in failure to form the endosperm.