The MIDASIN and NOTCHLESS genes are essential for female gametophyte development in Arabidopsis thaliana

Female gametophyte development in Arabidopsis thaliana follows a well-defined program that involves many fundamental cellular processes. In this study, we report the involvement of the Arabidopsis thaliana MIDASIN1 (AtMDN1) gene during female gametogenesis through the phenotypic characterization of plants heterozygous for an insertional mdn1 mutant allele. The MDN1 yeast ortholog has previously been shown to encode a non-ribosomal protein involved in the maturation and assembly of the 60S ribosomal subunit. Heterozygous MDN1/mdn1 plants were semisterile and mdn1 allele transmission through the female gametophyte was severely affected. Development of mdn1 female gametophyte was considerably delayed compared to their wild-type siblings. However, delayed mdn1 female gametophytes were able to reach maturity and a delayed pollination experiment showed that a small proportion of the female gametophytes were functional. We also report that the Arabidopsis NOTCHLESS (AtNLE) gene is also required for female gametogenesis. The NLE protein has been previously shown to interact with MDN1 and to be also involved in 60S subunit biogenesis. The introduction of an AtNLE-RNA interference construct in Arabidopsis led to semisterility defects. Defective female gametophytes were mostly arrested at the one-nucleate (FG1) developmental stage. These data suggest that the activity of both AtMDN1 and AtNLE is essential for female gametogenesis progression.     

Underexpression of the plant <Emphasis Type="Italic">NOTCHLESS</Emphasis> gene,encoding a WD-repeat protein,causes pleitropic phenotype during plant development

WD-repeat proteins are involved in a breadth of cellular processes. While the WD-repeat protein encoding gene NOTCHLESS has been involved in the regulation of the Notch signaling pathway in Drosophila, its yeast homolog Rsa4p was shown to participate in 60S ribosomal subunit biogenesis. The plant homolog ScNLE was previously characterized in Solanum chacoense (ScNLE) as being involved in seed development. However, expression data and reduced size of ScNLE underexpressing plants suggested in addition a role during shoot development. We here report the detailed phenotypic characterization of ScNLE underexpressing plants during shoot development. ScNLE was shown to be expressed in actively dividing cells of the shoot apex. Consistent with this, ScNLE underexpression caused pleiotropic defects such as a reduction in aerial organ size, a reduction in some organ numbers, delayed flowering, and an increase in stomatal index. Analysis of adaxial epidermal cells revealed that both cell number and cell size were reduced in mature leaves of ScNLE underexpressing lines. Two-hybrid screens with the Nle domain and the WD-repeat domain of ScNLE allowed the isolation of homologs of yeast MIDASIN and NSA2 genes, the products of which are involved in 60S ribosomal subunit biogenesis in yeast. A ScNLE-GFP chimeric protein was localized in both the cytoplasm and nucleus. These data altogether suggest that ScNLE likely plays a role in 60S ribosomal subunit biogenesis, which is essential for proper cellular growth and proliferation during plant development.

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