Background: In flowering plants, apomixis (asexual reproduction via seeds) is widely believed to result from failure
of key regulators of the sexual female reproductive pathway. In the past few years, both differential display and
RNA-seq comparative approaches involving reproductive organs of sexual plants and their apomictic counterparts have
yielded extensive lists of candidate genes. Nevertheless, only a limited number of these genes have been functionally
characterized, with few clues consequently available for understanding the molecular control of apomixis. We have
previously identified several cDNA fragments with high similarity to genes involved in RNA biology and with differential
amplification between sexual and apomictic Paspalum notatum plants. Here, we report the characterization of one of
these candidates, namely, N69 encoding a protein of the S-adenosyl-L-methionine-dependent methyltransferases
superfamily. The purpose of this work was to extend the N69 cDNA sequence and to characterize its expression at
different developmental stages in both sexual and apomictic individuals.
Results: Molecular characterization of the N69 cDNA revealed homology with genes encoding proteins similar to yeast
and mammalian trimethylguanosine synthase/PRIP-interacting proteins. These proteins play a dual role as ERK2-controlled
transcriptional coactivators and mediators of sn(o)RNA and telomerase RNA cap trimethylation, and participate in
mammals and yeast development. The N69-extended sequence was consequently renamed PnTgs1-like. Expression
of PnTgs1-like during reproductive development was significantly higher in floral organs of sexual genotypes compared
with apomicts. This difference was not detected in vegetative tissues. In addition, expression levels in reproductive
tissues of several genotypes were negatively correlated with facultative apomixis rates. Moreover, in situ hybridization
observations revealed that PnTgs1-like expression 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 of
aposporous initials differentiation in apomictic genotypes.
Conclusions: Our results indicate that PnTgs1-like (formerly N69) encodes a trimethylguanosine synthase-like
protein whose function in mammals and yeast is critical for development, including reproduction. Our findings
also suggest a pivotal role for this candidate gene in nucellar cell fate, as its diminished expression is correlated with
initiation of the apomictic pathway in plants.