New insights on the fertile megagametophyte and female sterility obtained from Maytenus obtusifolia Mart. (Celastraceae) plants under TEM

Abstract

Maytenus obtusifolia Mart. is an abundant species in restingas (sandy coastal plains), mainly in Rio de Janeiro state, Brazil. Populations in restinga of Maricá are gynodioecious formed by female individuals with pistillate flowers and hermaphrodites with perfect flowers. Pistillate flowers bear a developed gynoecium and staminodes. Perfect flowers have gynoecium with varied development and fertile stamens. Hermaphrodite plants have lower fruiting rate than female plants which was associated to the presence of sterile ovules with hypertrophied synergids and egg cell. The aim of this study was to compare fertile and sterile megagametophytes under TEM. Preanthesis buds were collected at restinga of Maricá/RJ, fixed in Karnovsky, post‐fixed in osmium tetroxide, embedded in Araldite, sectioned, contrasted and observed in Tecnai Spirit TEM. In fertile ovules of pistillate and perfect flowers, the egg cell presents cytoplasm dense and polarized, with vacuole facing the micropilar pole. Synergids have thin walls with evident fibrillar apparatus; polarized cytoplasm, vacuome facing the chalazal pole; mitochondria; plastids; nucleus; linear RER concentrically arranged; well developed dictyosomes with enlarged borders forming secretory vesicles. The central cell has thin walls with slightly disorganized cellulose fibrils; cytoplasm arranged in the cell periphery; developed vacuome occupying almost the entire cell; polar nucleus; ribosomes; vesicles with associated ribosomes, derived from RER; dictyosomes; oil bodies and mitochondria. In sterile ovules of perfect flowers, synergids undergo a vacuolization process, with subsequent fusion of small vacuoles; nucleus with fragmented or degenerated nuclear envelope; mitochondria and RER with signs of degeneration; organelles debris and plastids. In these hypertrophied synergids and the egg cell, disruption of tonoplast promote the cell lyses. Vacuole membranes and organelles debris are found inside synergids and central cell. Egg cell cytoplasm has fragments of nucleus. This study reveals that female sterility in perfect flowers seems to be caused by a vacuolization process followed by tonoplast disruption suggesting the occurrence of massive autophagy in megagametophyte cells.