Exploring the sister cells of embryo sac: developmental and functional attributes

Synergids are metabolically dynamic cells of the egg apparatus and represent an important component of the female gametophyte. Besides directing the growth of the pollen tube towards the micropylar end of the embryo sac, these ephemeral structures make room for the pollen tube cytoplasm. The nature of chemotrophic substances that direct the growth of the pollen tube, the mechanism of degeneration of one of the synergids before fertilization and the molecular aspects of synergid morphogenesis have been studied in detail. Research carried out on model systems such as Arabidopsis, Brassica, Capsella, Triticum and Torenia has expanded our understanding of the molecular regulation of the pollen tube journey, its guidance and navigation in the pistil. Recently, the critical role of the central cell in fertilization and prevention of polytubey has also been thoroughly investigated. Interesting aspects that lead to degeneration of synergids, and the factors governing degeneration, including molecular aspects, have produced a paradigm shift in the understanding of these intriguing units. Sophisticated confocal microscopy, live cell imaging, and molecular tools have helped in furthering our knowledge of the functioning of synergids. Recent research using high throughput techniques has deciphered the role of various genes that regulate and govern the release of chemotropic substances, cell-to-cell interaction and synergid cell degeneration. Moreover, with the diversity displayed in form and function of organs in the angiosperms, and the switching of roles of the cells of egg apparatus, new insights have been provided into the involvement of synergids both pre- and post-fertilization. The present review provides a comprehensive account of synergids, their role in fertilization and the post fertilization events that have emerged using interdisciplinary approaches in recent years. We also discuss the variations observed in degeneration of synergids and the mechanisms that have been unraveled recently. Study of the dynamism exhibited by synergids reveals newer roles of these in fertilization. How synergids in angiosperm taxa where genetic transformation/alteration is carried out will respond to pollen stimuli is still unknown. Since environmental factors such as light and temperature have a significant impact on synergids and fertilization, it would be rewarding to study the role of chemo-attractants and other factors in elucidating the functional roles of synergids. Further research into developing adequate protocols for manipulating synergid functions is certainly required. This research has enormous potential in the advancement of basic science and has potential applications in agriculture, horticulture, and bioprospecting.