Evidence for an evolutionarily conserved interaction between cell wall biosynthesis and flowering in maize and sorghum.
BACKGROUND: Factors that affect flowering vary among different plant species, and in the grasses in particular the exact mechanism behind this transition is not fully understood. The brown midrib (bm) mutants of maize (Zea mays L.), which have altered cell wall composition, have different flowering dynamics compared to their wild-type counterparts. This is indicative of a link between cell wall biogenesis and flowering. In order to test whether this relationship also exists in other grasses, the flowering dynamics in sorghum (Sorghum bicolor (L.) Moench) were investigated. Sorghum is evolutionarily closely related to maize, and a set of brown midrib (bmr) mutants similar to the maize bm mutants is available, making sorghum a suitable choice for study in this context. RESULTS: We compared the flowering time (time to half-bloom) of several different bmr sorghum lines and their wild-type counterparts. This revealed that the relationship between cell wall composition and flowering was conserved in sorghum. Specifically, the mutant bmr7 flowered significantly earlier than the corresponding wild-type control, whereas the mutants bmr2, bmr4, bmr6, bmr12, and bmr19 flowered later than their wild-type controls. CONCLUSION: The change in flowering dynamics in several of the brown midrib sorghum lines provides evidence for an evolutionarily conserved mechanism that links cell wall biosynthesis to flowering dynamics. The availability of the sorghum bmr mutants expands the germplasm available to investigate this relationship in further detail. (+info)
Conservation and diversity of gene families explored using the CODEHOP strategy in higher plants.
BACKGROUND: Availability of genomewide information on an increasing but still limited number of plants offers the possibility of identifying orthologues, or related genes, in species with major economical impact and complex genomes. In this paper we exploit the recently described CODEHOP primer design and PCR strategy for targeted isolation of homologues in large gene families. RESULTS: The method was tested with two different objectives. The first was to analyze the evolution of the CYP98 family of cytochrome P450 genes involved in 3-hydroxylation of phenolic compounds and lignification in a broad range of plant species. The second was to isolate an orthologue of the sorghum glucosyl transferase UGT85B1 and to determine the complexity of the UGT85 family in wheat. P450s of the CYP98 family or closely related sequences were found in all vascular plants. No related sequence was found in moss. Neither extensive duplication of the CYP98 genes nor an orthologue of UGT85B1 were found in wheat. The UGT85A subfamily was however found to be highly variable in wheat. CONCLUSIONS: Our data are in agreement with the implication of CYP98s in lignification and the evolution of 3-hydroxylation of lignin precursors with vascular plants. High conservation of the CYP98 family strongly argues in favour of an essential function in plant development. Conversely, high duplication and diversification of the UGT85A gene family in wheat suggests its involvement in adaptative response and provides a valuable pool of genes for biotechnological applications. This work demonstrates the high potential of the CODEHOP strategy for the exploration of large gene families in plants. (+info)
A high-density genetic recombination map of sequence-tagged sites for sorghum, as a framework for comparative structural and evolutionary genomics of tropical grains and grasses.
We report a genetic recombination map for Sorghum of 2512 loci spaced at average 0.4 cM ( approximately 300 kb) intervals based on 2050 RFLP probes, including 865 heterologous probes that foster comparative genomics of Saccharum (sugarcane), Zea (maize), Oryza (rice), Pennisetum (millet, buffelgrass), the Triticeae (wheat, barley, oat, rye), and Arabidopsis. Mapped loci identify 61.5% of the recombination events in this progeny set and reveal strong positive crossover interference acting across intervals of +info)
A complex history of rearrangement in an orthologous region of the maize, sorghum, and rice genomes.
The sequences of large insert clones containing genomic DNA that is orthologous to the maize adh1 region were obtained for sorghum, rice, and the adh1-homoeologous region of maize, a remnant of the tetraploid history of the Zea lineage. By using all four genomes, it was possible to describe the nature, timing, and lineages of most of the genic rearrangements that have differentiated this chromosome segment over the last 60 million years. The rice genome has been the most stable, sharing 11 orthologous genes with sorghum and exhibiting only one tandem duplication of a gene in this region. The lineage that gave rise to sorghum and maize acquired a two-gene insertion (containing the adh locus), whereas sorghum received two additional gene insertions after its divergence from a common ancestor with maize. The two homoeologous regions of maize have been particularly unstable, with complete or partial deletion of three genes from one segment and four genes from the other segment. As a result, the region now contains only one duplicated locus compared with the eight original loci that were present in each diploid progenitor. Deletion of these maize genes did not remove both copies of any locus. This study suggests that grass genomes are generally unstable in local genome organization and gene content, but that some lineages are much more unstable than others. Maize, probably because of its polyploid origin, has exhibited extensive gene loss so that it is now approaching a diploid state. (+info)
Transcriptional regulation of sorghum defense determinants against a phloem-feeding aphid.
When attacked by a phloem-feeding greenbug aphid (Schizaphis graminum), sorghum (Sorghum bicolor) activates jasmonic acid (JA)- and salicylic acid (SA)-regulated genes, as well as genes outside known wounding and SA signaling pathways. A collection of 672 cDNAs was obtained by differential subtraction with cDNAs prepared from sorghum seedlings infested by greenbug aphids and those from uninfested seedlings. Subsequent expression profiling using DNA microarray and northern-blot analyses identified 82 transcript types from this collection responsive to greenbug feeding, methyl jasmonate (MeJA), or SA application. DNA sequencing analyses indicated that these encoded proteins functioning in direct defense, defense signaling, oxidative burst, secondary metabolism, abiotic stress, cell maintenance, and photosynthesis, as well as proteins of unknown function. In response to insect feeding, sorghum increased transcript abundance of numerous defense genes, with some SA-dependent pathogenesis-related genes responding to greenbug more strongly than to SA. In contrast, only weak induction of MeJA-regulated defense genes was observed after greenbug treatment. However, infestation tests confirmed that JA-regulated pathways were effective in plant defense against greenbugs. Activation of certain transcripts exclusively by greenbug infestation was observed, and may represent unique signal transduction events independent of JA- and SA-regulated pathways. Results indicate that plants coordinately regulate defense gene expression when attacked by phloem-feeding aphids, but also suggest that aphids are able to avoid triggering activation of some otherwise potentially effective plant defensive machinery, possibly through their particular mode of feeding. (+info)
Molecular evolution of the phytochrome gene family in sorghum: changing rates of synonymous and replacement evolution.
The photoreceptor phytochromes, encoded by a small gene family, are responsible for controlling the expression of a number of light-responsive genes and photomorphogenic events, including agronomically important phenotypes such as flowering time and shade-avoidance behavior. The understanding and control of flowering time are particularly important goals in sorghum cultivar development for diverse environments, and naturally occurring variation in the phytochrome genes might prove useful in breeding programs. Also of interest is whether variation observed at the phytochrome loci in domesticated sorghum, or in particular races, is a result of human selection. Population genetic studies can reveal evidence of such selection in patterns of polymorphism and divergence. In this study we report a population genetic analysis of the PHY gene family in Sorghum bicolor (L.) Moench in a diverse panel including both cultivated and wild accessions. We show that the level of nucleotide variation in all gene family members is about half the average for this species, consistent with purifying selection acting on these loci. However, the rate of amino acid substitution is accelerated at PHYC compared to the other two loci. In comparisons to a closely related sorghum species, PHYC shows a pattern of intermediate frequency amino acid changes that differ from the patterns observed in comparisons across longer evolutionary distances. There is also a departure from expected patterns of polymorphism and divergence at synonymous sites in PHYC, although the data do not fit a simple model of directional or diversifying selection. Cultivated sorghum has a level of variation similar to that of wild relatives (ssp. verticilliflorum), but many polymorphisms are subspecies-specific, including several amino acid variants. (+info)
Structural determinants of substrate specificity in family 1 beta-glucosidases: novel insights from the crystal structure of sorghum dhurrinase-1, a plant beta-glucosidase with strict specificity, in complex with its natural substrate.
Plant beta-glucosidases play a crucial role in defense against pests. They cleave, with variable specificity, beta-glucosides to release toxic aglycone moieties. The Sorghum bicolor beta-glucosidase isoenzyme Dhr1 has a strict specificity for its natural substrate dhurrin (p-hydroxy-(S)-mandelonitrile-beta-D-glucoside), whereas its close homolog, the maize beta-glucosidase isoenzyme Glu1, which shares 72% sequence identity, hydrolyzes a broad spectrum of substrates in addition to its natural substrate 2-O-beta-D-glucopyranosyl-4-hydroxy-7-methoxy-1,4-benzoxaxin-3-one. Structural data from enzyme.substrate complexes of Dhr1 show that the mode of aglycone binding differs from that previously observed in the homologous maize enzyme. Specifically, the data suggest that Asn(259), Phe(261), and Ser(462), located in the aglycone-binding site of S. bicolor Dhr1, are crucial for aglycone recognition and binding. The tight binding of the aglycone moiety of dhurrin promotes the stabilization of the reaction intermediate in which the glycone moiety is in a deformed (1)S(3) conformation within the glycone-binding site, ready for nucleophilic attack to occur. Compared with the broad specificity maize beta-glucosidase, this different binding mode explains the narrow specificity of sorghum dhurrinase-1. (+info)
Ancient polyploidization predating divergence of the cereals, and its consequences for comparative genomics.
Integration of structural genomic data from a largely assembled rice genome sequence, with phylogenetic analysis of sequence samples for many other taxa, suggests that a polyploidization event occurred approximately 70 million years ago, before the divergence of the major cereals from one another but after the divergence of the Poales from the Liliales and Zingiberales. Ancient polyploidization and subsequent "diploidization" (loss) of many duplicated gene copies has thus shaped the genomes of all Poaceae cereal, forage, and biomass crops. The Poaceae appear to have evolved as separate lineages for approximately 50 million years, or two-thirds of the time since the duplication event. Chromosomes that are predicted to be homoeologs resulting from this ancient duplication event account for a disproportionate share of incongruent loci found by comparison of the rice sequence to a detailed sorghum sequence-tagged site-based genetic map. Differential gene loss during diploidization may have contributed many of these incongruities. Such predicted homoeologs also account for a disproportionate share of duplicated sorghum loci, further supporting the hypothesis that the polyploidization event was common to sorghum and rice. Comparative gene orders along paleo-homoeologous chromosomal segments provide a means to make phylogenetic inferences about chromosome structural rearrangements that differentiate among the grasses. Superimposition of the timing of major duplication events on taxonomic relationships leads to improved understanding of comparative gene orders, enhancing the value of data from botanical models for crop improvement and for further exploration of genomic biodiversity. Additional ancient duplication events probably remain to be discovered in other angiosperm lineages. (+info)