UC Riverside

Oxygen deficient plants suffer an energy limitation that can suppress growth and development and lead to premature cell death. Underlying adaptation, plants have evolved integrated genetic and cellular programs to respond to low oxygen stress (hypoxia/anoxia). Mechanisms of diverse adaptation share common themes, such as enhanced oxygen-independent energy supply, regulated growth and energy consumption, and prioritization of energy use. Tailored protein synthesis conserves energy by reduction in synthesis of non-stress-relevant proteins (90% of cellular mRNA) involved in normal growth and development, and funnels resources to translation of hypoxia-induced mRNAs encoding proteins that drive carbohydrate mobility and substrate-level phosphorylation. It was hypothesized that the mRNAs that are translationally repressed during hypoxia associate with RNA binding proteins, which protect them or allow their degradation, and facilitate their rapid return of the sequestered mRNAs to translational complexes within minutes of reoxygenation. Stress granules (SGs) and processing bodies (PBs) are two structures of up to 4 µM in diameter, characterized in non-plant eukaryotic cells as sites of mRNA sequestration and degradation during stress. Conservation of genes encoding orthologs of these proteins in plants suggests a corresponding system for management of cytosolic non-translated mRNA . We initiated a survey study of predicted RNA binding proteins that are orthologous to non-plant SG and PB proteins encoded by Arabidopsis thaliana. To determine their roles in normal development, T-DNA insertion mutant alleles were identified and surveyed for abnormal growth and development. In addition, several were fused to fluorescent proteins to determine their subcellular localization and make observations of the effect of overexpression on normal growth. From this survey, plant OLIGOURIDYLATE BINDING PROTEIN (UBP)1C was further characterized for its role during hypoxia stress. UBP1A and UBP1C-GPF fusion proteins relocalized from diffuse to granular cytoplasmic localization upon hypoxia treatment, which was rapidly reversed by reoxygenation, concomitant with restoration of protein synthesis. These granules required completion of translation for their assembly. Mutation of these genes led to altered survival of oxygen deprivation. Global quantitative profiling of the mRNAs associated with UBP1C revealed that transcripts with U-rich 3'-untranslated regions are highly UBP1C-associated during non-stress conditions. Upon treatment with hypoxia most cellular mRNAs increased association with UBP1C with the exception of the mRNAs that were highly induced by hypoxia and associated with polyribosomes under stress and those already highly associated. Upon reoxygenation this association was rapidly reversed. In conclusion, UBP1C association with mRNA during hypoxia is linked to the dynamic sequestration and turnover of transcripts observed during hypoxia in the model plant Arabidopsis thaliana.