What Seduces Megaspore?

    Before actually entering into details, let us take you to secondary school where you might have studied the process of megaspore formation (megasporogenesis). During megasporogenesis megaspore mother cell (MMC) undergoes meiosis to produce four haploid cells called ‘megaspores’. Depending upon the pattern of cell plate and haploid cell formation it can be monosporic, bisporic or tetrasporic. 70% of angiospermic plants are monosporic (also called polygonum type), where out of four uninucleate megaspore, three degenerates resulting to a single functional megaspore (FM). It is well established that the functional megaspore then undergoes further process to form female gametophyte. However, the genetic basis and molecular mechanisms that determine the specification and fate of haploid cells remain unknown.

Megasporogenesis | Photo: www.502biosixth.blogspot.in

Demesa- Arevalo and Vielle-Calzada of Langebio Cinvestav, Mexico in their recent research paper published in ‘The Plant Cell’ shown that the classical arabinogalactan protein (AGP18) plays an active role in regulating the selection and survival of megaspore in Arabidopsis. They performed a molecular analysis of the genomic regulatory region that drives AGP18 expression and determines its pattern of protein localization during megaspore formation. Their results suggest that AGP18 is transcriptionally regulated i) during early ovule development in integumentary cells (sporophytic phase ) at the time of the sporophyte-to-gametophyte transition and ii) in haploid gametophytic cells at the time of female gametophyte cellularization.

They determined the pattern of AGP18 localization in developing ovules to establish the role of AGP18 during gametophytic development. They observed that:

Female Gametophyte | Diagram: www.2classnotes.com

i) in premiotic ovules, AGP18 was distributed uniformly in sporophytic cells but absent in MMC.

ii) At the intracellular level, AGP18 was localised in the plasma membrane and also within the cytoplasm of sporophytic nucellar cells adjacent to MMC.

iii) After Meiosis II, AGP 18 was initially expressed in cells adjacent to functional megaspore (FM). In nucellar cells the AGP18 was present in cellular edges adjacent to FM.

iv) In fully differentiated ovules, they found that AGP1 was localised within the female gametophyte, in the central cells and the egg apparatus, but absent in the antipodals.

In a previous report by the same team (Acosta-Garcia and Vielle-Calzada) in 2004, using in situ hybridization (ISH) they showed that AGP18 mRNA is localised in premiotic cells of the developing ovule and the localization of AGP18 mRNA precedes the localization of corresponding protein in the gametophytic lineage. With the above mentioned localization of AGP18 and their previous studies they summarizes that AGP18 gene expression initiates in a cluster of sporophytic cells at stages encompassing megasporogenesis and FM differentiation.

They also established that supernumerary cells in ovules over-expressing AGP18 are of meiotic region and can acquire an FM identity.

Further, immunolocalization experiments showed that AGP18 is required for selection of a viable megaspore but not sufficient for its specification.

They concludes that these findings represent a significant step in establishing the reproductive mechanism prevailing in megasporogenesis and can also open the possibilities evolutionary studies that AGPs could have played in flowering plants’ sexual reproduction.