Analysis of Mutator-induced mutations in the iojap gene of maize

The conserved theme of ribosome hibernation: from bacteria to chloroplasts of plants

Cells are highly adaptive systems that respond and adapt to changing environmental conditions such as temperature fluctuations or altered nutrient availability. Such acclimation processes involve reprogramming of the cellular gene expression profile, tuning of protein synthesis, remodeling of metabolic pathways and morphological changes of the cell shape. Nutrient starvation can lead to limited energy supply and consequently, remodeling of protein synthesis is one of the key steps of regulation since the translation of the genetic code into functional polypeptides may consume up to 40% of a cell's energy during proliferation. In eukaryotic cells, downregulation of protein synthesis during stress is mainly mediated by modification of the translation initiation factors. Prokaryotic cells suppress protein synthesis by the active formation of dimeric so-called 'hibernating' 100S ribosome complexes. Such a transition involves a number of proteins which are found in various forms in prokaryotes but also in chloroplasts of plants. Here, we review the current understanding of these hibernation factors and elaborate conserved principles which are shared between species.

The molecular cloning and clarification of a photorespiratory mutant, oscdm1, using enhancer trapping

Enhancer trap systems have been demonstrated to increase the effectiveness of gene identification in rice. In this study, a chlorophyll-deficient mutant, named oscdm1, was screened and characterized in detail from a T-DNA enhancer-tagged population. The oscdm1 plants were different from other chlorophyll-deficient mutants; they produced chlorotic leaves at the third leaf stage, which gradually died with further growth of the plants. However, the oscdm1 plants were able to survive exposure to elevated CO₂ levels, similar to photorespiratory mutants. An analysis of the T-DNA flanking sequence in the oscdm1 plants showed that the T-DNA was inserted into the promoter region of a serine hydroxymethyltransferase (SHMT) gene. OsSHMT1 is a key enzyme that is ubiquitous in nature and structurally conserved across kingdoms. The enzyme is responsible for the interconversion of serine and glycine and is essential for cellular one-carbon metabolism. Full-length OsSHMT1 complemented the oscdm1 phenotype, and the downregulation of OsSHMT1 in wild-type plants by RNA interference (RNAi) produced plants that mimicked the oscdm1 phenotype. GUS assays and quantitative PCR revealed the preferential expression of OsSHMT1 in young leaves. TEM revealed serious damage to the thylakoid membrane in oscdm1 chloroplasts. The oscdm1 plants showed more extensive damage than wild type using an IMAGING-PAM fluorometer, especially under high light intensities. OsSHMT1-GFP localized exclusively to mitochondria. Further analysis revealed that the H₂O₂ content in the oscdm1 plants was twice that in wild type at the fourth leaf stage. This suggests that the thylakoid membrane damage observed in the oscdm1 plants was caused by excessive H₂O₂. Interestingly, OsSHMT1-overexpressing plants exhibited increased photosynthetic efficiency and improved plant productivity. These results lay the foundation for further study of the OsSHMT1 gene and will help illuminate the functional role of OsSHMT1 in photorespiration in rice.

RNA silencing mediated by direct repeats in maize: a potential tool for functional genomics

It has been shown in tobacco and Arabidopsis that transgenes with multiple direct repeats induce RNA silencing at high frequency. In this study, we tried to establish a direct repeat-induced RNA silencing system in maize and evaluate whether it can be developed as a high throughput tool for functional genomics. Our results showed that the construct phC4, which carries four direct repeats of a chloramphenicol acetyl-transferase (CAT) gene, was able to induce silencing of itself with high efficiency in maize. Using a transient expression system, we further demonstrated that construct phC3G with a beta-glucuronidase (GUS) gene located downstream of three direct repeats of CAT gene silenced not only itself in maize calli but also an "endogenous" GUS gene, which was stably expressed in maize calli. Most importantly, when constructs with the maize iojap (ij) gene inserted in either sense or antisense orientation into the downstream of four direct repeats of CAT gene were transformed into maize plants, co-suppression of endogenous and transgenic ij genes was detected in majority of transgenic maize plants. Our co-suppression results suggest that with improvements, this new approach has the potential to become an efficient research tool for high throughput functional genomics.

RNA editing in ribosome-less plastids of iojap maize

In maize chloroplasts, 28 C-to-U editing events have been identified in the transcripts of 14 different genes. The iojap mutant of maize, which lacks chloroplast ribosomes, affords the opportunity to examine whether any chloroplast translation products are required for the editing of any of the 28 sites. Furthermore, the mode of action of the IOJAP protein itself is unknown, so we explored the possibility that homozygous ij1/ ij1 plants are defective in RNA editing. Current knowledge of RNA editing in chloroplasts indicates the existence of site-specific factors responsible for recognizing C targets of editing, but the factors have not yet been identified and their encoding genes are unknown. Our results indicate that all 28 editing sites can be recognized and processed in ribosome-less plastids. Transcripts of rpoB are more abundant and more highly edited in iojap mutants. The editing site in rpl2, which creates the mRNA start codon, is the most severely affected in homozygous ij/ ij plants, but nevertheless exhibits at least 10% editing in all mutant lines. Reduced editing of rpl2 may be an indirect effect of reduced splicing, rather than a defect caused by the iojap mutation. We conclude that neither the IOJAP protein nor chloroplast translation products are required for editing any of the 28 C targets of editing in maize chloroplast RNAs.

NUCLEAR CONTROL OF PLASTID AND MITOCHONDRIAL DEVELOPMENT IN HIGHER PLANTS

The nucleus must coordinate organelle biogenesis and function on a cell and tissue-specific basis throughout plant development. The vast majority of plastid and mitochondrial proteins and components involved in organelle biogenesis are encoded by nuclear genes. Molecular characterization of nuclear mutants has illuminated chloroplast development and function. Fewer mutants exist that affect mitochondria, but molecular and biochemical approaches have contributed to a greater understanding of this organelle. Similarities between organelles and prokaryotic regulatory molecules have been found, supporting the prokaryotic origin of chloroplasts and mitochondria. A striking characteristic for both mitochondria and chloroplast is that most regulation is posttranscriptional.