Fangpu Han

Somatic chromosome spreads from maize (Zea mays L.) plants containing B-A translocation chromosomes undergoing the chromosome type breakage-fusion-bridge cycle were examined by FISH. The size and type of extra chromosomes varied among cells of the same individual. A collection of minichromosomes derived from the chromosome type breakage-fusion-bridge cycle was examined for the presence of stable dicentric chromosomes. Six of 23 chromosomes in the collection contained two regions with DNA sequences typical of centromeres. Functional analysis and immunolabeling of CENH3, the centromere-specific histone H3 variant, revealed only one functional centromere per chromosome, despite the duplicate centromere sequences. One plant was found with an inactive B centromere that had been translocated to the short arm of chromosome 9. The translocated centromere region appeared identical to that of a normal B chromosome. The inactivation of the centromeres was stable for at least four generations. By using dicentrics from dispensable chromosomes, centromere inactivation was found to be quite common under these circumstances.

Allopolyploidization has been a driving force in plant evolution. Formation of common wheat (Triticum aestivum L.) represents a classic example of successful speciation via allopolyploidy. Nevertheless, the immediate chromosomal consequences of allopolyploidization in wheat remain largely unexplored. We report here an in-depth investigation on transgenerational chromosomal variation in resynthesized allohexaploid wheats that are identical in genome constitution to common wheat. We deployed sequential FISH, genomic in situ hybridization (GISH), and homeolog-specific pyrosequencing, which enabled unequivocal identification of each of the 21 homologous chromosome pairs in each of >1,000 individual plants from 16 independent lines. We report that whole-chromosome aneuploidy occurred ubiquitously in early generations (from selfed generation S(1) to >S(20)) of wheat allohexaploidy although at highly variable frequencies (20-100%). In contrast, other types of gross structural variations were scant. Aneuploidy included an unexpected hidden type, which had a euploid chromosome number of 2n = 42 but with simultaneous loss and gain of nonhomeologous chromosomes. Of the three constituent subgenomes, B showed the most lability for aneuploidy, followed by A, but the recently added D subgenome was largely stable in most of the studied lines. Chromosome loss and gain were also unequal across the 21 homologous chromosome pairs. Pedigree analysis showed no evidence for progressive karyotype stabilization even with multigenerational selection for euploidy. Profiling of two traits directly related to reproductive fitness showed that although pollen viability was generally reduced by aneuploidy, the adverse effect of aneuploidy on seed-set is dependent on both aneuploidy type and synthetic line.

We describe a comprehensive and general approach for mapping centromeres and present a detailed characterization of two maize centromeres. Centromeres are difficult to map and analyze because they consist primarily of repetitive DNA sequences, which in maize are the tandem satellite repeat CentC and interspersed centromeric retrotransposons of maize (CRM). Centromeres are defined epigenetically by the centromeric histone H3 variant, CENH3. Using novel markers derived from centromere repeats, we have mapped all ten centromeres onto the physical and genetic maps of maize. We were able to completely traverse centromeres 2 and 5, confirm physical maps by fluorescence in situ hybridization (FISH), and delineate their functional regions by chromatin immunoprecipitation (ChIP) with anti-CENH3 antibody followed by pyrosequencing. These two centromeres differ substantially in size, apparent CENH3 density, and arrangement of centromeric repeats; and they are larger than the rice centromeres characterized to date. Furthermore, centromere 5 consists of two distinct CENH3 domains that are separated by several megabases. Succession of centromere repeat classes is evidenced by the fact that elements belonging to the recently active recombinant subgroups of CRM1 colonize the present day centromeres, while elements of the ancestral subgroups are also found in the flanking regions. Using abundant CRM and non-CRM retrotransposons that inserted in and near these two centromeres to create a historical record of centromere location, we show that maize centromeres are fluid genomic regions whose borders are heavily influenced by the interplay of retrotransposons and epigenetic marks. Furthermore, we propose that CRMs may be involved in removal of centromeric DNA (specifically CentC), invasion of centromeres by non-CRM retrotransposons, and local repositioning of the CENH3.

BACKGROUND AND AIMS: Phosphorus deficiency is a major limiting factor for crop yield worldwide. Previous studies revealed that PHR1 and it homologues play a key role in regulating the phosphate starvation response in plants. However, the function of PHR homologues in common wheat (Triticum aestivum) is still not fully understood. The aim of the study was to characterize the function of PHR1 genes in regulating phosphate signalling and plant growth in wheat. METHODS: Wheat transgenic lines over-expressing a wheat PHR1 gene were generated and evaluated under phosphorus-deficient and -sufficient conditions in hydroponic culture, a soil pot trial and two field experiments. KEY RESULTS: Three PHR1 homologous genes Ta-PHR1-A1, B1 and D1 were isolated from wheat, and the function of Ta-PHR1-A1 was analysed. The results showed that Ta-PHR1-A1 transcriptionally activated the expression of Ta-PHT1.2 in yeast cells. Over-expressing Ta-PHR1-A1 in wheat upregulated a subset of phosphate starvation response genes, stimulated lateral branching and improved phosphorus uptake when the plants were grown in soil and in nutrient solution. The data from two field trials demonstrated that over-expressing Ta-PHR1-A1 increased grain yield by increasing grain number per spike. CONCLUSIONS: TaPHR1 is involved in phosphate signalling in wheat, and was valuable in molecular breeding of crops, with improved phosphorus use efficiency and yield performance.