Comparative physiological and metabolomic responses of four <Emphasis Type="Italic">Brachypodium distachyon</Emphasis> varieties contrasting in drought stress resistance

Brachypodium distachyon (Brachypodium) is not only a monocot grass species, but also a promising model organism of crop research. In this study, the drought resistance of four Brachypodium varieties was identified including drought stress-tolerant Bd1-1 and Bd21, drought stress-susceptible Bd3-1 and Bd18-1. Physiological assay showed that drought-tolerant varieties (Bd1-1 and Bd21) were more effective in maintenance of leaf water content, activation of catalase and peroxidase activities and accumulation of reduced glutathione, resulting in alleviated cell damage and lower reactive oxygen species level than drought-susceptible varieties (Bd3-1 and Bd18-1) in response to drought stress. In addition, 54 primary metabolites were differentially regulated among Brachypodium varieties and after drought stress treatment, indicating the complexity of Brachypodium response to drought stress. We also identified several commonly regulated metabolites especially some compatible solutes including proline and soluble sugars, which exhibited higher concentrations in the drought-tolerant varieties. Taken together, this study suggested that natural variation of Brachypodium varieties in response to drought stress might be connected with higher leaf water, enhanced accumulation of osmolyte and more effective antioxidant system, as well as the modulation of metabolic profiles under drought stress conditions.     

Identification and molecular characterization of a Brachypodium distachyon GIGANTEA gene: functional conservation in monocot and dicot plants

Developmental phase change and flowering transition are emerging as potential targets for biomass agriculture in recent years. The GIGANTEA (GI) gene is one of the central regulators that direct flowering promotion and phase transition. In this work, we isolated a GI gene orthologue from the small annual grass Brachypodium distachyon inbred line Bd21 (Brachypodium), which is perceived as a potential model monocot for studies on bioenergy grass species. A partial GI gene sequence was identified from a Brachypodium expressed sequence tag library, and a full-size gene (BdGI) was amplified from a Brachypodium cDNA library using specific primer sets designed through analysis of monocot GI gene sequences. The BdGI gene was up-regulated by light and cold. A circadian rhythm set by light–dark transition also regulated the expression of the BdGI gene. The deduced amino acid sequence of the BdGI protein shares higher than 70% of sequence identity with the GI proteins in monocots and Arabidopsis. In addition, the BdGI protein is constitutively targeted to the nucleus and physically interacts with the ZEITLUPE (ZTL) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) proteins, like the Arabidopsis GI protein. Interestingly, heterologous expression of the BdGI gene in a GI-deficient Arabidopsis mutant rescued efficiently the late flowering phenotype. Together, our data indicate that the role of the GI gene in flowering induction is conserved in Arabidopsis and Brachypodium. It is envisioned that the GI genes of bioenergy grasses as well as Brachypodium could be manipulated to improve biomass by engineering developmental timing of phase transitions.     

Structural characterization of Brachypodium genome and its syntenic relationship with rice and wheat

Brachypodium distachyon (Brachypodium) has been recently recognized as an emerging model system for both comparative and functional genomics in grass species. In this study, 55,221 repeat masked Brachypodium BAC end sequences (BES) were used for comparative analysis against the 12 rice pseudomolecules. The analysis revealed that ~26.4% of BES have significant matches with the rice genome and 82.4% of the matches were homologous to known genes. Further analysis of paired-end BES and ~1.0 Mb sequences from nine selected BACs proved to be useful in revealing conserved regions and regions that have undergone considerable genomic changes. Differential gene amplification, insertions/deletions and inversions appeared to be the common evolutionary events that caused variations of microcolinearity at different orthologous genomic regions. It was found that ~17% of genes in the two genomes are not colinear in the orthologous regions. Analysis of BAC sequences also revealed higher gene density (~9 kb/gene) and lower repeat DNA content (~13.1%) in Brachypodium when compared to the orthologous rice regions, consistent with the smaller size of the Brachypodium genome. The 119 annotated Brachypodium genes were BLASTN compared against the wheat EST database and deletion bin mapped wheat ESTs. About 77% of the genes retrieved significant matches in the EST database, while 9.2% matched to the bin mapped ESTs. In some cases, genes in single Brachypodium BACs matched to multiple ESTs that were mapped to the same deletion bins, suggesting that the Brachypodium genome will be useful for ordering wheat ESTs within the deletion bins and developing specific markers at targeted regions in the wheat genome.     

Sequencing and Analysis of Approximately 40 000 Soybean cDNA Clones from a Full-Length-Enriched cDNA Library

A large collection of full-length cDNAs is essential for the correct annotation of genomic sequences and for the functional analysis of genes and their products. We obtained a total of 39 936 soybean cDNA clones (GMFL01 and GMFL02 clone sets) in a full-length-enriched cDNA library which was constructed from soybean plants that were grown under various developmental and environmental conditions. Sequencing from 5′ and 3′ ends of the clones generated 68 661 expressed sequence tags (ESTs). The EST sequences were clustered into 22 674 scaffolds involving 2580 full-length sequences. In addition, we sequenced 4712 full-length cDNAs. After removing overlaps, we obtained 6570 new full-length sequences of soybean cDNAs so far. Our data indicated that 87.7% of the soybean cDNA clones contain complete coding sequences in addition to 5′- and 3′-untranslated regions. All of the obtained data confirmed that our collection of soybean full-length cDNAs covers a wide variety of genes. Comparative analysis between the derived sequences from soybean and Arabidopsis, rice or other legumes data revealed that some specific genes were involved in our collection and a large part of them could be annotated to unknown functions. A large set of soybean full-length cDNA clones reported in this study will serve as a useful resource for gene discovery from soybean and will also aid a precise annotation of the soybean genome.Key words: EST, full-length cDNA, functional annotation, legume, soybean     

Construction of a full-length cDNA library from young spikelets of hexaploid wheat and its characterization by large-scale sequencing of expressed sequence tags

The polyploid nature of wheat is a key characteristic of the plant. Full-length complementary DNAs (cDNAs) provide essential information that can be used to annotate the genes and provide a functional analysis of these genes and their products. We constructed a full-length cDNA library derived from young spikelets of common wheat, and obtained 24056 expressed sequence tags (ESTs) from both ends of the cDNA clones. These ESTs were grouped into 3605 contigs using the phrap method, representing expressed loci from each of the three genomes. Using BLAST, 3605 contigs were grouped into 1902 gene clusters, showing that loci of the three genomes are not always expressed. A homology search of these gene clusters against a wheat EST database (15964 gene clusters) and a rice full-length cDNA database (21447 gene clusters) revealed that a quarter of the wheat full-length cDNAs were novel. A protein database of Arabidopsis was used to examine the functional classification of these gene clusters. The GC-content in the 5 -UTR region of wheat cDNAs was compared to that of rice. Forty-three genes (3.5% of wheat cDNAs homologous to those of rice) possessed distinct GC-content in the 5 -UTR region, suggesting different breeding behaviors of wheat and rice.