Pollen tube enters ovule through degeneated synergid?
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In flowering plant reproduction, pollen tube reception is the signaling system that results in pollen tube discharge, synergid degeneration, and successful delivery of male gametes (two sperm cells) to the site where they can fuse with female gametes (egg cell and central cell). Some molecules required for this complex and essential signaling exchange have been identified; however, fundamental questions about the nature of the interactions between the pollen tube and the synergid cells remain to be clarified. Here, we monitor pollen tube arrival, pollen tube discharge, and synergid degeneration in Arabidopsis (Arabidopsis thaliana) wild type and in male and female gametophytic mutants that disrupt development and function of the gametophytes. By combining assays used previously to study these interactions and an assay that facilitates simultaneous analysis of pollen tube discharge and synergid degeneration, we find that synergid degeneration could be initiated without pollen tube discharge. Our data support the hypothesis that pollen tube-synergid contact, or signaling via secreted molecules, initiates receptive synergid degeneration. We also find that when pollen tubes successfully burst, they always discharge into a degenerated synergid. In addition to this pollen tube-dependent promotion of synergid degeneration, we also show that a basal developmental pathway mediates synergid degeneration in the absence of pollination. Our results are consistent with the model that a complex set of interactions between the pollen tube and synergid cells promote receptive synergid degeneration.
Flowering plant reproduction requires that development of the male and female gametophytes is coordinated so that gametes can be brought together to fuse and initiate seed development. A critical phase of this coordination occurs when the two gametophytes encounter each other to complete double fertilization: each gametophyte directly influences the development and function of the other. The male gametophyte develops in the anther and differentiates into a pollen grain with two sperm cells in its cytoplasm (Berger and Twell, 2011). The female gametophyte develops within the ovule and is more complex, comprising seven cells in Arabidopsis (Arabidopsis thaliana), six of which are haploid (two synergids, an egg, and three antipodals) and one homodiploid central cell (Drews and Yadegari, 2002). When a pollen grain lands on a receptive stigma, it germinates a highly polar pollen tube that extends without cell division through stigma and stylar tissue before entering an ovule micropyle (Palanivelu and Tsukamoto, 2012). The first female gametophytic cell encountered by the pollen tube in the ovule is one of the two synergids, and it is clear that synergids directly control pollen tube functions: they secrete attractant proteins that guide the pollen tube into the micropyle (Takeuchi and Higashiyama, 2012) and induce pollen tube discharge (Kessler and Grossniklaus, 2011). After pollen tube arrival, one of the two synergid cells degenerates (called the receptive synergid) and receives the pollen tube contents to allow sperm to access the female gametes, the egg, and the central cell (Christensen et al., 1997; Drews and Yadegari, 2002; Sandaklie-Nikolova et al., 2007; Hamamura et al., 2011, 2014).
=> POLLEN TUBE ENTERS OVULE THROUGH DEGENERATED SYNERGIDS
=> POLLEN TUBE ACTUALLY IT IS A PART OF THE MALE GAMETOPHYTE IN PLANTS .IT IS LONG TUBE LIKE STRUCTURE THAT CARRIES THE MALE GAMETE FROM STIGMA TO THE OVULES ✔️✔️✌️