ABSTRACT
Reproduction in flowering plants is closely related to the megagametophyte, since the megagametophyte is involved in pollen tube reception and contains the two female gametes-egg cell and central cell. Previous conventional light microscopy methods have shown that female sterility in perfect flowers of Maytenus obtusifolia is associated with the occurrence of sterile ovules whose megagametophytes have hypertrophied synergids. Here, using transmission electron microscopy and cytochemical methods, we compare the megagametophytes in fertile and sterile ovules from perfect and pistillate flowers, and investigate the cellular events that result in the degradation of the megagametophyte cells from sterile ovules. In fertile ovules of perfect and pistillate flowers, mature megagametophytes have two synergids, egg cell and central cell. In fertile ovules, the synergids present an extensive rough endoplasmic reticulum (RER) profile, large populations of mitochondria, when compared to egg cells, vesicles, Golgi bodies, plastids and a nucleus with heterochromatin. Besides that, the egg cell has a small population of organelles and the central cell exhibits cytoplasm with free ribosomes, RER, vesicles originating from the RER, Golgi bodies and oil inclusions. In mature megagametophytes from sterile ovules of perfect and pistillate flowers, massive autophagy occurs by tonoplast rupture promoting hydrolase release, leading to protoplast and cell wall degradation-typical evidence of programmed cell death (PCD). Therefore, female sterility in the majority of M. obtusifolia sterile ovules is the result of PCD by massive autophagy in the megagametophyte cells. In a few other sterile ovules, sterility is due to the delayed or the absence of megagametophyte development.
Subject(s)
Apoptosis/physiology , Maytenus/chemistry , Ovule/chemistryABSTRACT
Perfect flowers of Maytenus obtusifolia have partial sterility of pollen grains, resulting in collapsed and developed free microspores. However, the cellular events resulting in partial male sterility have not been determined. In pistillate flowers of this species, male sterility is related to the premature programmed cell death (PCD) in tapetum and sporogenic cells. The process occurs through autophagy via macroautophagy and massive autophagy and is associated with sporophytic cytoplasmic male sterility (CMS). Here, we characterised the development of pollen grains and investigated the cellular events that result in tapetum cells and free microspores PCD in perfect flowers, using light and transmission electron microscopy combined with the TUNEL (Terminal deoxynucleotidyl transferase mediated dUDP end-Labeling) assay and the ZIO (Zinc iodide-osmium tetroxide) method. Pollen grain development in perfect flowers was divided into eight developmental stages based on the characteristics of the pollen grains. Tapetum cells undergo PCD at the free microspore stage, through a macroautophagic process, by formation of autophagosomes and by autophagosomes giving rise to lytic vacuoles at maturity. In the final stage of PCD, massive autophagy occurs by rupture of the tonoplast. The development of viable and inviable microspores diverges at the vacuolated microspore stage, when PCD occurs in some free microspores, causing interruption of pollen development through necrosis. These events result in the observed partial male sterility. Viable microspores undergo mitosis and develop into tricellular pollen grains. Male sterility in hermaphrodite individuals is here interpreted as gametophytic CMS.
