PnTgs1-like expression during reproductive development supports a role for RNA methyltransferases in the aposporous pathway
Résumé
Background:In flowering plants, apomixis (asexual reproduction via seeds) is widely believed to result from failureof key regulators of the sexual female reproductive pathway. In the past few years, both differential display andRNA-seq comparative approaches involving reproductive organs of sexual plants and their apomictic counterparts haveyielded extensive lists of candidate genes. Nevertheless, only a limited number of these genes have been functionallycharacterized, with few clues consequently available for understanding the molecular control of apomixis. We havepreviously identified several cDNA fragments with high similarity to genes involved in RNA biology and with differentialamplification between sexual and apomicticPaspalum notatumplants. Here, we report the characterization of one ofthese candidates, namely, N69 encoding a protein of the S-adenosyl-L-methionine-dependent methyltransferasessuperfamily. The purpose of this work was to extend the N69 cDNA sequence and to characterize its expression atdifferent developmental stages in both sexual and apomictic individuals.Results:Molecular characterization of the N69 cDNA revealed homology with genes encoding proteins similar to yeastand mammalian trimethylguanosine synthase/PRIP-interacting proteins. These proteins play a dual role as ERK2-controlledtranscriptional coactivators and mediators of sn(o)RNA and telomerase RNA cap trimethylation, and participate inmammals and yeast development. The N69-extended sequence was consequently renamedPnTgs1-like.ExpressionofPnTgs1-likeduring reproductive development was significantly higher in floral organs of sexual genotypes comparedwith apomicts. This difference was not detected in vegetative tissues. In addition, expression levels in reproductivetissues of several genotypes were negatively correlated with facultative apomixis rates. Moreover,in situhybridizationobservations revealed thatPnTgs1-likeexpression is relatively higher in ovules of sexual plants throughout development,from premeiosis to maturity. Tissues where differential expression is detected include nucellar cells, the site ofaposporous initials differentiation in apomictic genotypes.Conclusions:Our results indicate thatPnTgs1-like(formerly N69) encodes a trimethylguanosine synthase-likeprotein whose function in mammals and yeast is critical for development, including reproduction. Our findingsalso suggest a pivotal role for this candidate gene in nucellar cell fate, as its diminished expression is correlated withinitiation of the apomictic pathway in plants.
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