Abstract
Evidence is presented for the role of a mitochondrial ribosomal (mitoribosomal) L18 protein in cell division, differentiation, and seed development after the characterization of a recessive mutant, heart stopper (hes). The hes mutant produced uncellularized endosperm and embryos arrested at the late globular stage. The mutant embryos differentiated partially on rescue medium with some forming callus. HES (At1g08845) encodes a mitochondrially targeted member of a highly diverged L18 ribosomal protein family. The substitution of a conserved amino residue in the hes mutant potentially perturbs mitoribosomal function via altered binding of 5S rRNA and/or influences the stability of the 50S ribosomal subunit, affecting mRNA binding and translation. Consistent with this, marker genes for mitochondrial dysfunction were up-regulated in the mutant. The slow growth of the endosperm and embryo indicates a defect in cell cycle progression, which is evidenced by the down-regulation of cell cycle genes. The down-regulation of other genes such as EMBRYO DEFECTIVE genes links the mitochondria to the regulation of many aspects of seed development. HES expression is developmentally regulated, being preferentially expressed in tissues with active cell division and differentiation, including developing embryos and the root tips. The divergence of the L18 family, the tissue type restricted expression of HES, and the failure of other L18 members to complement the hes phenotype suggest that the L18 proteins are involved in modulating development. This is likely via heterogeneous mitoribosomes containing different L18 members, which may result in differential mitochondrial functions in response to different physiological situations during development.
Highlights
During plant development, cell division, growth, and differentiation are precisely coordinated and regulated to generate tissues that contribute to the formation of vegetative and reproductive organs
Ethyl methanesulfonate mutagenesis of the Arabidopsis Landsberg erecta (Ler) ecotype resulted in an M2-generation plant exhibiting small seeds that had aborted in the early stages of seed development at silique maturity (Fig. 1A-D)
These results suggest that the mutation is strictly inherited in a sporophytic recessive manner
Summary
Cell division, growth, and differentiation are precisely coordinated and regulated to generate tissues that contribute to the formation of vegetative and reproductive organs. The triploid endosperm progenitor nucleus undergoes nine rounds of division without cytokinesis, resulting in a multinucleate syncytium. This is followed by cellularization, and the endosperm is later consumed by the developing embryo (Sørensen et al, 2002). Over 400 independent embryo-lethal mutants, arrested at various stages of embryogenesis in Arabidopsis, have been generated in mutagenesis screens, providing a significant resource to investigate the molecular events underlying embryo and endosperm development (Meinke et al, 2008). Characterization of some mutants has uncovered genes involved in the embryo patterning and endosperm cellularization Most of these are poorly characterized for specific molecular and developmental functions.
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