Abstract
Apomixis, an asexual mode of reproduction through seeds, has immense scope for crop improvement due to its ability to fix hybrid vigor. In C. ciliaris, a predominantly apomictically reproducing range grass, apomixis is genetically controlled by an apospory-specific-genomic-region (ASGR) which is enriched with retrotransposons. Earlier studies showed insertional polymorphisms of a few ASGR-specific retrotransposons between apomictic and sexual plants of C. ciliaris. REs are mainly regulated at the transcriptional level through cytosine methylation. To understand the possible association of ASGR-specific retrotransposon to apomixis, the extent and pattern of differential methylation of Gy163 RE and its impact on transcription were investigated in two genotypes each of apomictic and sexual plants of C. ciliaris. We observed that Gy163 encodes for an integrase domain of RE Ty3-Gypsy, is differentially methylated between reproductive tissues of apomictic and sexual plants. However, leaf tissues did not exhibit differential methylation between apomictic and sexual plants. Among the three contexts (CG, CHG, and CHH) of cytosine methylation, the maximum variation was observed in CHH context in reproductive (at aposporous initial and mature embryo sac stages) tissues of apomictic plants implicating RdDM pathway in methylation of Gy163. Quantitative PCR analysis showed that Gy163 transcripts are expressed more in the reproductive tissues of apomictic plants compared to that in the sexual plants, which was negatively correlated with the methylation level. Thus, the study helps in understanding the role of RE present in ASGR in epigenetic regulation of apomictic mode of reproduction in C. ciliaris.
Highlights
Apomixis is an asexual mode of reproduction through seeds which bypasses meiotic division and fertilization of egg cell to produce progenies that are the replica of the mother plant (Kumar, 2017)
Protein domain analysis of the conserved monomorphic region Gy163 has chromodomain downstream to the integrase domain (Gy163) revealed that it encodes for an integrase enzyme of RE which is specific to Ty3-gypsy retroelements (Metaviridae)
Apomixis has become an attractive trait to produce genetically uniform populations, and it is highly desired for crop improvement (Toenniessen, 2001; Kumar, 2017)
Summary
Apomixis is an asexual mode of reproduction through seeds which bypasses meiotic division and fertilization of egg cell to produce progenies that are the replica of the mother plant (Kumar, 2017). The traditional breeding approach for incorporating apomixis in crop plants through interspecific hybridization resulted in the production of unviable germplasm (Bicknell and Koltunow, 2004). Different models have been proposed for explaining the sexual and apomictic modes of reproduction, yet successful transfer of apomixis to a crop plant is still awaited (Hörandl and Hojsgaard, 2012; Kumar et al, 2019). Recent evidence shows that female gametophyte development and seed formation are controlled by epigenetic mechanisms that distinguish sexual and apomictic development (RodríguezLeal et al, 2015). It is proposed that apomixis might be regulated through epigenetic mechanisms (Rodrigues and Koltunow, 2005; Curtis and Grossniklaus, 2008; Podio et al, 2012, 2014; Zappacosta et al, 2014; Kumar et al, 2017b; Bocchini et al, 2018; Albertini et al, 2019). Many reports showed that mutation in ARGONAUTE 9 (AGO9), which is involved in methylation of retroelements (REs) in the female germline, results in apomixis like trait in maize and Arabidopsis (Olmedo-Monfil et al, 2010; Singh et al, 2011)
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