Plants with a winter growth habit flower earlier when exposed for several weeks to cold temperatures, a process called vernalization. We report here the positional cloning of the wheat vernalization gene VRN2, a dominant repressor of flowering that is downregulated by vernalization. Loss of function of VRN2, whether by natural mutations or deletions, resulted in spring lines, which do not require vernalization to flower. Reduction of the RNA level of VRN2 by RNA interference accelerated flowering time of transgenic winter wheat plants more than a month.Common wheat (Triticum aestivum L.) is one of the primary grains consumed by humans and is grown in very different environments. This wide adaptability has been favored by the existence of wheat varieties with different growth habits. Winter wheats require a long exposure to low temperatures to flower (vernalization) and are sown in the fall, whereas spring wheats do not have a vernalization requirement and can be planted in spring or fall. The genes from the vernalization pathway prevent flower development during the winter, providing protection for the temperature-sensitive floral organs against the cold.VRN1 and VRN2 are the central genes in the vernalization pathway in wheat, barley, and other temperate cereals. These two genes have strong epistatic interactions and are likely part of the same regulatory pathway (1, 2). In both diploid wheat (Triticum monococcum L.)

It is generally believed that maize (Zea mays L. ssp. mays) arose as a tetraploid; however, the two progenitor genomes cannot be unequivocally traced within the genome of modern maize. We have taken a new approach to investigate the origin of the maize genome. We isolated and sequenced large genomic fragments from the regions surrounding five duplicated loci from the maize genome and their orthologous loci in sorghum, and then we compared these sequences with the orthologous regions in the rice genome. Within the studied segments, we identified 11 genes that were conserved in location, order, and orientation. We performed phylogenetic and distance analyses and examined the patterns of estimated times of divergence for sorghum and maize gene orthologs and also the time of divergence for maize orthologs. Our results support a tetraploid origin of maize. This analysis also indicates contemporaneous divergence of the ancestral sorghum genome and the two maize progenitor genomes about 11.9 million years ago (Mya). On the basis of a putative conversion event detected for one of the genes, tetraploidization must have occurred before 4.8 Mya, and therefore, preceded the major maize genome expansion by gene amplification and retrotransposition.Maize (Zea mays L. ssp. mays), from the grass tribe Andropogoneae, is an agronomically important crop and also a traditional genetic model. The theory that maize is a tetraploid first arose from the fact that maize has a haploid chromosome number of 10 (2n = 20), whereas many closely related grasses, such as Coix aquatica, Saccharum sp., and Erianthus sp., have only five chromosomes in the haploid nucleus (Celarier 1956;Mehra and Sharma 1975;Mason-Gamer et al. 1998). Rhoades (1951) demonstrated by genetic linkage mapping that nontandem gene duplicates are common in the maize genome. Mapping results of isozymic variants were also consistent with the hypothesis that the maize genome contains large duplicated regions (Goodman et al. 1980;McMillin and Scandalios 1980;Wendel et al. 1986). More recently, molecular mapping studies using RFLP markers have shown that most of the 10 maize chromosomes contain duplicated segments (Helentjaris et al. 1988;Ahn and Tanksley 1993). Comparative genomic studies based on the collinearity of grass genomes (Moore et al. 1995;Gale and Devos 1998) indicated that the maize genome aligns with the diploid rice and sorghum genomes in two chromosome sets, implying wholegenome duplication.Three models can explain the large-scale duplications in the maize genome, that is, segmental duplication (multiple independent duplications within a genome), autotetraploidy (intraspecific genomic duplication), and allotetraploidy (interspecific genome hybridization). Gaut and Doebley (1997) proposed a segmental allotetraploid model and suggested that one of the maize subgenomes is more closely related to sorghum than to the other maize subgenome. However, they deemed their conclusions tentative (Gaut and Doebley 1997;Gaut et al. 2000) in the absence of sorghum sequences. Al...

Sequencing of a contiguous 215-kb interval of Triticum monococcum showed the presence of five genes in the same order as in previously sequenced colinear barley and rice BACs. Gene 2 was in the same orientation in wheat and rice but inverted in barley. Gene density in this region was 1 gene per 43 kb and the ratio of physical to genetic distance was estimated to be 2,700 kb cM -1 . Twenty more-or-less intact retrotransposons were found in the intergenic regions, covering at least 70% of the sequenced region. The insertion times of 11 retrotransposons were less than 5 million years ago and were consistent with their nested structure. Five new families of retroelements and the first full-length elements for two additional retrotransposon families were discovered in this region. Significantly higher values of GC content were observed for Triticeae BACs compared with rice BACs. Relative enrichment or depletion of certain dinucleotides was observed in the comparison of introns, exons and retrotransposons. A higher proportion of transitions in CG and CNG sites that are targets for cytosine methylation was observed in retrotransposons (76%) than in introns (37%). These results showed that the wheat genome is a complex mixture of different sequence elements, but with general patterns of content and interspersion that are similar to those seen in maize and barley.

Maize (Zea mays L. ssp. mays), one of the most important agricultural crops in the world, originated by hybridization of two closely related progenitors. To investigate the fate of its genes after tetraploidization, we analyzed the sequence of five duplicated regions from different chromosomal locations. We also compared corresponding regions from sorghum and rice, two important crops that have largely collinear maps with maize. The split of sorghum and maize progenitors was recently estimated to be 11.9 Mya, whereas rice diverged from the common ancestor of maize and sorghum ∼50 Mya. A data set of roughly 4 Mb yielded 206 predicted genes from the three species, excluding any transposon-related genes, but including eight gene remnants. On average, 14% of the genes within the aligned regions are noncollinear between any two species. However, scoring each maize region separately, the set of noncollinear genes between all four regions jumps to 68%. This is largely because at least 50% of the duplicated genes from the two progenitors of maize have been lost over a very short period of time, possibly as short as 5 million years. Using the nearly completed rice sequence, we found noncollinear genes in other chromosomal positions, frequently in more than one. This demonstrates that many genes in these species have moved to new chromosomal locations in the last 50 million years or less, most as single gene events that did not dramatically alter gene structure.

Colinearity of a large region from barley (Hordeum vulgare) chromosome 5H and rice (Oryza sativa) chromosome 3 has been demonstrated by mapping of several common restriction fragment-length polymorphism clones on both regions. One of these clones, WG644, was hybridized to rice and barley bacterial artificial chromosome (BAC) libraries to select homologous clones. One BAC from each species with the largest overlapping segment was selected by fingerprinting and blot hybridization with three additional restriction fragment-length polymorphism clones. The complete barley BAC 635P2 and a 50-kb segment of the rice BAC 36I5 were completely sequenced. A comparison of the rice and barley DNA sequences revealed the presence of four conserved regions, containing four predicted genes. The four genes are in the same orientation in rice, but the second gene is in inverted orientation in barley. The fourth gene is duplicated in tandem in barley but not in rice. Comparison of the homeologous barley and rice sequences assisted the gene identification process and helped determine individual gene structures. General gene structure (exon number, size, and location) was largely conserved between rice and barley and to a lesser extent with homologous genes in Arabidopsis. Colinearity of these four genes is not conserved in Arabidopsis compared with the two grass species. Extensive similarity was not found between the rice and barley sequences other than within the exons of the structural genes, and short stretches of homology in the promoters and 3Ј untranslated regions. The larger distances between the first three genes in barley compared with rice are explained by the insertion of different transposable retroelements.

No abstract

Microtubule organization is intimately associated with cellulose microfibril deposition, central to plant secondary cell wall development. We have determined that a relatively large suite of eight a-TUBULIN (TUA) and 20 b-TUBULIN (TUB) genes is expressed in the woody perennial Populus. A number of features, including gene number, a:b gene representation, amino acid changes at the C terminus, and transcript abundance in wood-forming tissue, distinguish the Populus tubulin suite from that of Arabidopsis thaliana. Five of the eight Populus TUAs are unusual in that they contain a C-terminal methionine, glutamic acid, or glutamine, instead of the more typical, and potentially regulatory, C-terminal tyrosine. Both C-terminal Y-type (TUA1) and M-type (TUA5) TUAs were highly expressed in wood-forming tissues and pollen, while the Y-type TUA6 and TUA8 were abundant only in pollen. Transcripts of the disproportionately expanded TUB family were present at comparatively low levels, with phylogenetically distinct classes predominating in xylem and pollen. When tension wood induction was used as a model system to examine changes in tubulin gene expression under conditions of augmented cellulose deposition, xylem-abundant TUA and TUB genes were up-regulated. Immunolocalization of TUA and TUB in xylem and phloem fibers of stems further supported the notion of heavy microtubule involvement during cellulose microfibril deposition in secondary walls. The high degree of sequence diversity, differential expansion, and differential regulation of Populus TUA and TUB families may confer flexibility in cell wall formation that is of adaptive significance to the woody perennial growth habit.

No abstract