Expressed sequence-tag analysis of ovaries of Brachiaria brizantha reveals genes associated with the early steps of embryo sac differentiation of apomictic plants

Abstract

In apomixis, asexual mode of plant reproduction through seeds, an unreduced megagametophyte is formed due to circumvented or altered meiosis. The embryo develops autonomously from the unreduced egg cell, independently of fertilization. Brachiaria is a genus of tropical forage grasses that reproduces sexually or by apomixis. A limited number of studies have reported the sequencing of apomixis-related genes and a few Brachiaria sequences have been deposited at genebank databases. This work shows sequencing and expression analyses of expressed sequence-tags (ESTs) of Brachiaria genus and points to transcripts from ovaries with preferential expression at megasporogenesis in apomictic plants. From the 11 differentially expressed sequences from immature ovaries of sexual and apomictic Brachiaria brizantha obtained from macroarray analysis, 9 were preferentially detected in ovaries of apomicts, as confirmed by RT-qPCR. A putative involvement in early steps of Panicum-type embryo sac differentiation of four sequences from B. brizantha ovaries: BbrizHelic, BbrizRan, BbrizSec13 and BbrizSti1 is suggested. Two of these, BbrizSti1 and BbrizHelic, with similarity to a gene coding to stress induced protein and a helicase, respectively, are preferentially expressed in the early stages of apomictic ovaries development, especially in the nucellus, in a stage previous to the differentiation of aposporous initials, as verified by in situ hybridization.

The Genetic Control of Apomixis: Asexual Seed Formation

“…This approach is complicated by the nature of asexual reproduction that prevents the generation of isogenic lines that differ solely with regard to reproductive mode. Nevertheless, transcriptome comparisons between apomictic and closely related sexual individuals from reproductive tissues at different developmental stages have uncovered pathways potentially central to apomixis including protein degradation, transcription, stress response, and cell-to-cell signaling (Albertini et al 2005;Laspina et al 2008;Sharbel et al 2010;Silveira et al 2012;Okada et al 2013).…”

“…In the aposporous grass Brachiaria brizantha, low-depth expressed sequence tag (EST) libraries made from ovaries of sexual and apomictic individuals revealed ESTs that show similarity to genes with known involvement in female embryo sac development (Silveira et al 2012). Further characterization of some of these ESTs identified genes that are expressed in the region where aposporous initial cells appear.…”

“…Further characterization of some of these ESTs identified genes that are expressed in the region where aposporous initial cells appear. These genes include a helicase (BbrizHelic), a MADS-box transcription factor (BbrizAGL6), and a stressinduced protein (BbrizSti1), suggesting that stress-related genes may play a role in the induction of aposporous initial cells (Silveira et al 2012;Guimarães et al 2013). In aposporous Hieracium praealtum, stress-like genes are also expressed in individual aposporous initial cells and early stage embryo sacs that contain two to four nuclei .…”

Diplosporous development in Boehmeria tricuspis: Insights from de novo transcriptome assembly and comprehensive expression profiling

“…Different experimental techniques such as map-based cloning approaches, differential display, subtractive hybridisation, and mimicking apomixis in sexual model plants have been used to investigate the genetic factors regulating apomixis and have led to the identification of many candidate genes28. These genes participate in many processes potentially central to apomixis, including protein degradation, transcription, stress response, and cell-to-cell signalling1427293031. Recent research has highlighted the influence of plant hormones (such as auxin and cytokinin323334) and epigenetic regulation353637 on asexual embryogenesis, suggesting their involvement in apomixis induction and regulation.…”

Egg Apparatus in Sexual and Apomictic Species of Taraxacum: Structural and Immunocytochemical Aspects of Synergid Cells

“…The role of the synergid cells remains an open question in species whose embryo and endosperm development is independent of any contribution from pollen, but there has been some progress recently in, for example, Brachiaria brizantha, where synergids were found to be involved in autonomous embryo development (Alves et al, 2007;Silveira et al, 2012). In this species, molecular analysis of the temporal and spatial expression of certain cDNA sequences by in situ hybridization in ovaries of apomictic and sexual plants indicated that synergids may play a crucial role not only at the moment of fertilization but also in the autonomous development of an unreduced egg cell.…”