RNA interference-mediated change in protein body morphology and seed opacity through loss of different zein proteins.
Plant Physiol. 153:337-47. Abstract
Opaque or nonvitreous phenotypes relate to the seed architecture of maize (Zea mays) and are linked to loci that control the accumulation and proper deposition of storage proteins, called zeins, into specialized organelles in the endosperm, called protein bodies. However, in the absence of null mutants of each type of zein (i.e. alpha, beta, gamma, and delta), the molecular contribution of these proteins to seed architecture remains unclear. Here, a double null mutant for the delta-zeins, the 22-kD alpha-zein, the beta-zein, and the gamma-zein RNA interference (RNAi; designated as z1CRNAi, betaRNAi, and gammaRNAi, respectively) and their combinations have been examined. While the delta-zein double null mutant had negligible effects on protein body formation, the betaRNAi and gammaRNAi alone only cause slight changes. Substantial loss of the 22-kD alpha-zeins by z1CRNAi resulted in protein body budding structures, indicating that a sufficient amount of the 22-kD zeins is necessary for maintenance of a normal protein body shape. Among different mutant combinations, only the combined betaRNAi and gammaRNAi resulted in drastic morphological changes, while other combinations did not. Overexpression of alpha-kafirins, the homologues of the maize 22-kD alpha-zeins in sorghum (Sorghum bicolor), in the beta/gammaRNAi mutant failed to offset the morphological alterations, indicating that beta- and gamma-zeins have redundant and unique functions in the stabilization of protein bodies. Indeed, opacity of the beta/gammaRNAi mutant was caused by incomplete embedding of the starch granules rather than by reducing the vitreous zone.
RNA Interference Can Rebalance the Nitrogen Sink of Maize Seeds without Losing Hard Endosperm.
PLoS One. 7:e32850. Abstract
BACKGROUND: One of the goals of plant breeding is to create crops to provide better nutrition for humans and livestock. Insufficient intake of protein is one of the most severe factors affecting the growth and development of children in developing countries. More than a century ago, in 1896, Hopkins initiated the well-known Illinois long-term selection for maize seed protein concentration, yielding four protein strains. By continuously accumulating QTLs, Illinois High Protein (IHP) reached a protein level 2.5-fold higher than normal maize, with the most increased fraction being the zein protein, which was shown to contain no lysine soon after the long-term selection program initiated. Therefore, IHP is of little value for feeding humans and monogastric animals. Although high-lysine lines of non-vitreous mutants were based on reduced zeins, the kernel soft texture precluded their practical use. Kernel hardness in opaque 2 (o2) could be restored in quality protein maize (QPM) with quantitative trait loci called o2 modifiers (Mo2s), but those did not increase total protein levels. METHODS: The most predominant zeins are the 22- and 19-kDa alpha-zeins. To achieve a combination of desired traits, we used RNA interference (RNAi) against both alpha-zeins in IHP and evaluated the silencing effect by SDS-PAGE. Total protein, amino acid composition and kernel texture were analyzed. CONCLUSIONS: The alpha-zeins were dramatically reduced, but the high total seed protein level remained unchanged by complementary increase of non-zein proteins. Moreover, the residual zein levels still allowed for a vitreous hard seed. Such dramatic rebalancing of the nitrogen sink could have a major impact in world food supply.
The Rice Annotation Project Database (RAP-DB): 2008 update.
Nucleic Acids Res. 36:D1028-33. Abstract
The Rice Annotation Project Database (RAP-DB) was created to provide the genome sequence assembly of the International Rice Genome Sequencing Project (IRGSP), manually curated annotation of the sequence, and other genomics information that could be useful for comprehensive understanding of the rice biology. Since the last publication of the RAP-DB, the IRGSP genome has been revised and reassembled. In addition, a large number of rice-expressed sequence tags have been released, and functional genomics resources have been produced worldwide. Thus, we have thoroughly updated our genome annotation by manual curation of all the functional descriptions of rice genes. The latest version of the RAP-DB contains a variety of annotation data as follows: clone positions, structures and functions of 31 439 genes validated by cDNAs, RNA genes detected by massively parallel signature sequencing (MPSS) technology and sequence similarity, flanking sequences of mutant lines, transposable elements, etc. Other annotation data such as Gnomon can be displayed along with those of RAP for comparison. We have also developed a new keyword search system to allow the user to access useful information. The RAP-DB is available at: http://rapdb.dna.affrc.go.jp/ and http://rapdb.lab.nig.ac.jp/.
Retrotranspositions in orthologous regions of closely related grass species.
BMC evolutionary biology. 6:62. Abstract
BACKGROUND: Retrotransposons are commonly occurring eukaryotic transposable elements (TEs). Among these, long terminal repeat (LTR) retrotransposons are the most abundant TEs and can comprise 50-90% of the genome in higher plants. By comparing the orthologous chromosomal regions of closely related species, the effects of TEs on the evolution of plant genomes can be studied in detail. RESULTS: Here, we compared the composition and organization of TEs within five orthologous chromosomal regions among three grass species: maize, sorghum, and rice. We identified a total of 132 full or fragmented LTR retrotransposons in these regions. As a percentage of the total cumulative sequence in each species, LTR retrotransposons occupy 45.1% of the maize, 21.1% of the rice, and 3.7% of the sorghum regions. The most common elements in the maize retrotransposon-rich regions are the copia-like retrotransposons with 39% and the gypsy-like retrotransposons with 37%. Using the contiguous sequence of the orthologous regions, we detected 108 retrotransposons with intact target duplication sites and both LTR termini. Here, we show that 74% of these elements inserted into their host genome less than 1 million years ago and that many retroelements expanded in size by the insertion of other sequences. These inserts were predominantly other retroelements, however, several of them were also fragmented genes. Unforeseen was the finding of intact genes embedded within LTR retrotransposons. CONCLUSION: Although the abundance of retroelements between maize and rice is consistent with their different genome sizes of 2,364 and 389 Mb respectively, the content of retrotransposons in sorghum (790 Mb) is surprisingly low. In all three species, retrotransposition is a very recent activity relative to their speciation. While it was known that genes re-insert into non-orthologous positions of plant genomes, they appear to re-insert also within retrotransposons, potentially providing an important role for retrotransposons in the evolution of gene function.
Rescue of a dominant mutant with RNA interference.
Genetics. 186:1493-6. Abstract
Maize Mucronate1 is a dominant floury mutant based on a misfolded 16-kDa gamma-zein protein. To prove its function, we applied RNA interference (RNAi) as a dominant suppressor of the mutant seed phenotype. A gamma-zein RNAi transgene was able to rescue the mutation and restore normal seed phenotype. RNA interference prevents gene expression. In most cases, this is used to study gene function by creating a new phenotype. Here, we use it for the opposite purpose. We use it to reverse the creation of a mutant phenotype by restoring the normal phenotype. In the case of the maize Mucronate1 (Mc1) phenotype, interaction of a misfolded protein with other proteins is believed to be the basis for the Mc1 phenotype. If no misfolded protein is present, we can reverse the mutant to the normal phenotype. One can envision using this approach to study complex traits and in gene therapy.
Reconstruction of monocotelydoneous proto-chromosomes reveals faster evolution in plants than in animals.
Proc Natl Acad Sci U S A. 106:14908-13. Abstract
Paleogenomics seeks to reconstruct ancestral genomes from the genes of today's species. The characterization of paleo-duplications represented by 11,737 orthologs and 4,382 paralogs identified in five species belonging to three of the agronomically most important subfamilies of grasses, that is, Ehrhartoideae (rice) Panicoideae (sorghum, maize), and Pooideae (wheat, barley), permitted us to propose a model for an ancestral genome with a minimal size of 33.6 Mb structured in five proto-chromosomes containing at least 9,138 predicted proto-genes. It appears that only four major evolutionary shuffling events (alpha, beta, gamma, and delta) explain the divergence of these five cereal genomes during their evolution from a common paleo-ancestor. Comparative analysis of ancestral gene function with rice as a reference indicated that five categories of genes were preferentially modified during evolution. Furthermore, alignments between the five grass proto-chromosomes and the recently identified seven eudicot proto-chromosomes indicated that additional very active episodes of genome rearrangements and gene mobility occurred during angiosperm evolution. If one compares the pace of primate evolution of 90 million years (233 species) to 60 million years of the Poaceae (10,000 species), change in chromosome structure through speciation has accelerated significantly in plants.
Rapid divergence of prolamin gene promoters of maize after gene amplification and dispersal.
Genetics. 192(2):507-19. Abstract
Seeds have evolved to accommodate complicated processes like senescence, dormancy, and germination. Central to these is the storage of carbohydrates and proteins derived from sugars and amino acids synthesized during photosynthesis. In the grasses, the bulk of amino acids is stored in the prolamin superfamily that specifically accumulates in seed endosperm during senescence. Their promoters contain a conserved cis-element, called prolamin-box (P-box), recognized by the trans-activator P-box binding factor (PBF). Because of the lack of null mutants in all grass species, its physiological role in storage-protein gene expression has been elusive. In contrast, a null mutant of another endosperm-specific trans-activator Opaque2 (O2) has been shown to be required for the transcriptional activation of subsets of this superfamily by binding to the O2 box. Here, we used RNAi to knockdown Pbf expression and found that only 27-kDa γ- and 22-kDa α-zein gene expression were affected, whereas the level of other zeins remained unchanged. Still, transgenic seeds had an opaque seed phenotype. Combination of PbfRNAi and o2 resulted in further reduction of α-zein expression. We also tested the interaction of promoters and constitutively expressed PBF and O2. Whereas transgenic promoters could be activated, endogenous promoters appeared to be not accessible to transcriptional activation, presumably due to differential chromatin states. Although analysis of the methylation of binding sites of PBF and O2 correlated with the expression of endogenous 22-kDa α-zein promoters, a different mechanism seems to apply to the 27-kDa γ-zein promoter, which does not undergo methylation changes.