Zuehl, L. | Search bioRxiv

bioRxiv2022

Differential expression analysis of sexual and apomictic Boechera uncovers FAS4 as crucial for gametogenesis

During sexual reproduction of higher plants, seed formation is initiated by double fertilization of egg and central cell. In contrast, pseudogamous apomicts form embryos asexually by parthenogenesis of the egg, but initiation of endosperm development still depends on central cell fertilization. It can be envisioned that these differences are determined during gametogenesis and specification of gametophytic cells. To deepen the understanding of the transcriptional basis underlying sexual and apomictic reproduction, we applied tissue type-specific RNA-Seq. We compared expression in reproductive tissues of different Boechera accessions at distinct developmental stages. This confirmed previous evidence for an enrichment of RNA helicases at onset of reproductive development. We further identified a small number of members of this gene family as differentially expressed in female reproductive ovule tissues harbouring mature gametophytes from apomictic and sexual accessions. This included homologues of A. thaliana FASCIATED STEM 4 (FAS4) and of ENHANCED SILENCING PHENOTYPE 3 (ESP3), which have previously been identified as potential candidates for gametogenesis and apomixis, respectively. Unlike in A. thaliana, for either of them additional homologues or copies of related genes are present in Boechera, indicating complex evolutionary histories. As the expression patterns implied potential roles of FAS4 during gametogenesis, we first studied A. thaliana lines carrying mutant alleles. Indeed, we observed defects during male and female gametogenesis and severely reduced transmission efficiencies through both parents. In conclusion, our study identifies FAS4 as crucial for plant reproduction and suggests the potential for sub-functionalization of additional homologous genes in Boechera to shape reproductive development.

plant biology↗

bioRxiv2018

Differential gene expression indicates involvement of F-box proteins and E3 ligases in sexual versus apomictic germline specification in Boechera.

Germline specification is the first step during sexual and apomictic plant reproduction. This takes place in a specialized domain of the reproductive flower tissues, the nucellus of the ovule. In each case, a sporophytic cell is determined to initiate germline development. These cells, the megaspore mother cell (MMC) or apomictic initial cell (AIC) in sexual plants and apomicts, respectively, differ in their developmental fate. While the MMC undergoes meiosis, the AIC aborts or omits meiosis to form the female gametophyte. Although these distinct developmental processes have long been described, little is known about the gene regulatory basis involved.\n\nTo elucidate gene regulatory networks underlying sexual and apomictic germline specification, we conducted tissue-specific transcriptional profiling using laser-assisted microdissection and RNA-Seq. We compared the transcriptomes of the nucellar tissues harbouring the MMC or AIC between different accessions of Boechera. The six accessions we used represented four species and two ploidy levels, allowing us to distinguish between differences in gene expression caused by the genetic background or the reproductive mode.\n\nComparative data analysis revealed widely overlapping gene expression patterns in apomictic versus sexual Boechera accessions. Nevertheless, 45 significantly differentially expressed genes were identified, which potentially play a role for determination of sexual versus apomictic reproductive mode. Interestingly, based on annotations, these include F-box proteins and E3 ligases that might relate to genes previously described as regulators important for sexual or apomictic reproduction. Thus, our findings provide new insight into the transcriptional basis of sexual and apomictic germline specification.\n\nOne sentence summary\n\nA comprehensive tissue type-specific transcriptional analysis using laser-assisted microdissection combined with RNA-Seq identifies 45 genes consistently differentially expressed during germline specification in different sexual versus apomictic Boechera accessions, indicating roles of protein degradation related to cell cycle, transcriptional and post-transcriptional regulatory processes, and stress response for apomixis.

plant biology↗