Minisymposium 22: Development

Abs # 36002: Essential roles for PPR proteins in plant embryogenesis revealed by Arabidopsis emb-PPR knock-out mutants

Presenter:	Vernon, Daniel M, vernondm@whitman.edu
Authors	Vernon, Daniel M (A)   Cushing, Daniel A (A)   Gestaut, Daniel R (A)   Houghland, John E (A)   Forsthoefel, Nancy R (A)
Affiliations:	(A): Biology Department, Whitman College

Pentatricopeptide Repeat (PPR) proteins constitute a diverse superfamily characterized by the presence of a tandemly-repeated 35-amino acid motif. These repeat domains are thought to bind RNA or, in some cases, DNA. PPR proteins are widespread among eukaryotes, including fungi, plants, and animals. Most are predicted to localize to mitochodria or chloroplasts, and the few PPR proteins that have been functionally characterized to date take part in organelle RNA processing or post-transcriptional gene regulation. In plants, the PPR superfamily has undergone explosive evolution and is one of the largest plant protein classes: more than 200 PPR genes have been identified in the Arabidopsis genome. Previously, we reported that the Arabidopsis embryo-defective175 locus encodes a PPR protein- the first to be implicated in plant development. Here we describe additional embryo-defective PPR mutants. We searched publicly available emb mutant collections for T-DNA-tagged PPR genes, and identified 11 mutants representing 7 additional loci with prospective roles in embryogenesis. All of the mutant alleles contained inserts within predicted coding regions, 5'-UTRs, or basal promoter regions, and thus likely represent null alleles. We investigated the structures of these genes using available EST & cDNA sequences. Interestingly, 3 of the 8 EMB-PPR genes were unusually close to unrelated adjacent genes, and had been incorrectly annotated as single chimeric transcription units with those genes. [We have termed this type of close association between neighboring loci "gene cuddling".] RT-PCR indicated that the EMB-PPRs are all transcribed in leaves, roots, and flowers, suggesting that their functions are not embryo-specific. As a group the EMB-PPR genes encode diverse proteins containing from 9 to 24 PPR motifs. All are predicted to localize to chloroplasts or mitochondria. Nomarski DIC microscopy of mutant embryos at various developmental stages revealed diverse defects, ranging from consistent arrest at the globular stage in some lines to defects in late-stage embryo morphology, such as enlarged shoot meristems and stunted cotyledons, in others. These emb-PPR mutants provide evidence that multiple members of this large protein family have diverse & essential roles in early plant development. Supported by the Murdock Charitable Trust