Academic journal article Genetics

Arabidopsis SHORT INTEGUMENTS 2 Is a Mitochondrial DAR GTPase

Academic journal article Genetics

Arabidopsis SHORT INTEGUMENTS 2 Is a Mitochondrial DAR GTPase

Article excerpt

ABSTRACT

The Arabidopsis short integuments 2-1 (sin2-1) mutant produces ovules with short integuments due to early cessation of cell division in these structures. SIN2 was isolated and encodes a putative GTPase sharing features found in the novel DAR GTPase family. DAR proteins share a signature DAR motif and a unique arrangement of the four conserved GTPase G motifs. We found that DAR GTPases are present in all examined prokaryotes and eukaryotes and that they have diversified into four paralogous lineages in higher eukaryotes. Eukaryotic members of the SIN2 clade of DAR GTPases have been found to localize to mitochondria and are related to eubacterial proteins that facilitate essential steps in biogenesis of the large ribosomal subunit. We propose a similar role for SIN2 in mitochondria. A sin2 insertional allele has ovule effects similar to sin2-1, but more pronounced pleiotropic effects on vegetative and floral development. The diverse developmental effects of the mitochondrial SIN2 GTPase support a mitochondrial role in the regulation of multiple developmental pathways.

ARABIDOPSIS ovules are a useful model system for studying developmental mechanisms. Arabidopsis ovules initiate on the inner surface of immature carpels as relatively featureless finger-like primordia. As they elongate, three different regions become specialized and undergo morphogenesis and cellular differentiation (ROBINSON-BEERS et al. 1992). The distal-most region, the nucellus, is the site of meiosis and embryo sac formation. The central chalazal region is the site of the most visible morphogenic changes as it gives rise to two appendages, the inner and outer integuments. The basal region elongates through division and coordinated expansion of cells forming the funiculus, a supporting stalk. During this process, the developing ovule becomes bilaterally symmetrical as a result of differential growth that causes the funiculus to curve toward the base of the carpel and the outer integument to curve toward the carpel apex. At maturity, both integuments have grown to enclose the nucellus and form a terminal micropylar opening (Figure 1A). Mutations altering ovule morphogenesis may also disrupt other plant developmental pathways and the relative morphological simplicity of ovules can facilitate overall understanding of the underlying biochemical or molecular processes.

Numerous genes affecting growth and patterning of ovules have been identified via mutagenesis and cloning (SCHNEITZ 1999; SKINNER et al. 2004). Several of the genes regulating Arabidopsis ovule development manifest their effects through alterations in the pattern or progress of cell division. These genes encode proteins with a variety of biochemical functions (SKINNER et al. 2004). For example, mutations in AINTEGUMENTA (ANT) and INNER NO OUTER (INO), encoding AP2 and YABBY-domain transcription factors, respectively, result in a complete absence of both integuments or of only the outer integument (GAISER et al. 1995; ELLIOTT et al. 1996; KLUCHER et al. 1996; BAKER et al. 1997; VILLANUEVA et al. 1999). TSO1 encodes a novel nuclear protein required for proper orientation of cell elongation and cytokinesis during floral organ and integument development (HAUSER et al. 1998, 2000; SONG et al. 2000). Severe mutations in the HUELLENLOS (HLL) gene, encoding a mitochondrial L14 ribosomal subunit, lead to an arrest in ovule growth and degeneration of the apical regions of the primordia, revealing a role for mitochondrial activity in the process of ovule growth (SCHNEITZ et al. 1998; SKINNER et al. 2001). Among other floral effects, reduced activity of the putative protein kinase TOUSLED (TSL) causes short outer and protruding inner integuments (ROE et al. 1997a,b, 1993). The variety of protein classes involved in ovule growth implies complex regulation of this process at the levels of transcription, signal transduction, and metabolism.

SHORT INTEGUMENTS 2 (SIN2) is required for sustaining cell divisions during integument development (BROADHVEST et al. …

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