Biosynthesis of Ascorbic Acid as a Glucose-Induced Photoprotective Process in the Extremophilic Red Alga Galdieria partita.

The extremophilic red alga Galdieria partita is a facultative heterotroph that occupies mostly low-light microhabitats. However, the exceptional detection of abundant populations of G. partita in sunlight-exposed soil raises the possibility that exogenous organic carbon sources protect cells from photo-oxidative damage. The present study aimed to identify the photoprotective process activated by exogenous glucose under photo-oxidative stress. We demonstrated that exogenous glucose mitigated the photo-oxidative damage of cells exposed to 300 µmol photons m-2 s-1 photosynthetic active radiation. Photosynthesis carbon assimilation scarcely contributed to the cell growth in the presence of glucose, but the photosynthetic apparatus was nevertheless maintained and protected by glucose in a concentration-dependent manner. Supplementation of glucose increased expression of the L-gulonolactone oxidase gene essential for ascorbic acid biosynthesis, whereas no enhanced expression of the genes involved in carotenoid or tocopherol biosynthesis was observed. Under the photo-oxidative stress condition, the ascorbic acid content was strongly enhanced by exogenous glucose. We propose that the biosynthesis of ascorbic acid is one of the major photoprotective processes induced by exogenous glucose. The elucidation of how ascorbic acid is involved in scavenging reactive oxygen species provides key insights into the photoprotective mechanism in red algae.

Protein subcellular relocalization of duplicated genes in Arabidopsis.

Gene duplications during eukaroytic evolution, by successive rounds of polyploidy and by smaller scale duplications, have provided an enormous reservoir of new genes for the evolution of new functions. Preservation of many duplicated genes can be ascribed to changes in sequences, expression patterns, and functions. Protein subcellular relocalization (protein targeting to a new location within the cell) is another way that duplicated genes can diverge. We studied subcellular relocalization of gene pairs duplicated during the evolution of the Brassicaceae including gene pairs from the alpha whole genome duplication that occurred at the base of the family. We analyzed experimental localization data from green fluorescent protein experiments for 128 duplicate pairs in Arabidopsis thaliana, revealing 19 pairs with subcellular relocalization. Many more of the duplicate pairs with relocalization than with the same localization showed an accelerated rate of amino acid sequence evolution in one duplicate, and one gene showed evidence for positive selection. We studied six duplicate gene pairs in more detail. We used gene family analysis with several pairs to infer which gene shows relocalization. We identified potential sequence mutations through comparative analysis that likely result in relocalization of two duplicated gene products. We show that four cases of relocalization have new expression patterns, compared with orthologs in outgroup species, including two with novel expression in pollen. This study provides insights into subcellular relocalization of evolutionarily recent gene duplicates and features of genes whose products have been relocalized.

Organ and cell type-specific complementary expression patterns and regulatory neofunctionalization between duplicated genes in Arabidopsis thaliana.

Duplicated genes can contribute to the evolution of new functions and they are common in eukaryotic genomes. After duplication, genes can show divergence in their sequence and/or expression patterns. Qualitative complementary expression, or reciprocal expression, is when only one copy is expressed in some organ or tissue types and only the other copy is expressed in others, indicative of regulatory subfunctionalization or neofunctionalization. From analyses of two microarray data sets with 83 different organ types, developmental stages, and cell types in Arabidopsis thaliana, we determined that 30% of whole-genome duplicate pairs and 38% of tandem duplicate pairs show reciprocal expression patterns. We reconstructed the ancestral state of expression patterns to infer that considerably more cases of reciprocal expression resulted from gain of a new expression pattern (regulatory neofunctionalization) than from partitioning of ancestral expression patterns (regulatory subfunctionalization). Pollen was an especially common organ type for expression gain, resulting in contrasting expression of some duplicates in pollen. Many of the gene pairs with reciprocal expression showed asymmetric sequence rate evolution, consistent with neofunctionalization, and the more rapidly evolving copy often showed a more restricted expression pattern. A gene with reciprocal expression in pollen, involved in brassinosteroid signal transduction, has evolved more rapidly than its paralog, and it shows evidence for a new function in pollen. This study indicates the evolutionary importance of reciprocal expression patterns between gene duplicates, showing that they are common, often associated with regulatory neofunctionalization, and may be a factor allowing for retention and divergence of duplicated genes.

Dramatic change in function and expression pattern of a gene duplicated by polyploidy created a paternal effect gene in the Brassicaceae.

New gene formation by polyploidy has been an ongoing process during the evolution of various eukaryotes that has contributed greatly to the large number of genes in their genomes. After duplication, some genes that are retained can acquire new functions or expression patterns, or subdivide their functions or expression patterns between duplicates. Here, we show that SHORT SUSPENSOR (SSP) and Brassinosteroid Kinase 1 (BSK1) are paralogs duplicated by a polyploidy event that occurred in the Brassicaceae family about 23 Ma. SSP is involved in paternal control of zygote elongation in Arabidopsis thaliana by transcription in the sperm cells of pollen and then translation in the zygote, whereas BSK1 is involved in brassinosteroid signal transduction. Comparative analysis of expression in 63 different organs and developmental stages revealed that BSK1 and SSP have opposite expression patterns in pollen compared with all other parts of the plant. We determined that BSK1 retains the ancestral expression pattern and function. Thus, SSP has diverged in function after duplication from a component of the brassinosteroid signaling pathway to a paternal regulator of the timing of zygote elongation. The ancestral function of SSP was lost by deletions in the kinase domain. Our sequence rate analysis revealed that SSP but not BSK1 has experienced a greatly accelerated rate of amino acid sequence changes and relaxation of purifying selection. In addition, SSP has been duplicated to create a new gene (SSP-like1) with a completely different expression pattern, a shorter coding sequence that has lost a critical functional domain, and a greatly accelerated rate of amino acid sequence evolution along with evidence for positive selection, together indicative of neofunctionalization. This study illustrates two dramatic examples of neofunctionalization following gene duplication by complete changes in expression pattern and function. In addition, our findings indicate that paternal control of zygote elongation by SSP is an evolutionarily recent innovation in the Brassicaceae family.