Parental Genome Imbalance Causes Post-Zygotic Seed Lethality and Deregulates Imprinting in Rice.

BACKGROUND: Reproductive isolation between rice of different ploidy levels is manifested as endosperm and embryo abortion in seeds produced by interploidy crosses. Genomic imprinting is considered to be the underlying mechanism establishing the post-zygotic hybridization barrier. We characterized disrupted seed development in reciprocal crosses between a diploid Japonica rice and a tetraploid Indica rice. RESULTS: Triploid seeds from these crosses had aborted development and could not germinate in soil but could be rescued in culture medium with significantly more seeds developing to seedlings in the 4n × 2n (♀-♂) cross with excess maternal genomes than in the 2n × 4n cross with excess paternal genome. Consistent with previous findings, precocious endosperm cellularization and bigger embryos were observed in the seeds from the maternal excess cross, whereas absence of cellularization and arrested globular embryos were found in the seeds from the paternal excess cross, supporting the idea that endosperm cellularization is an important transition for embryo development. Moreover, we found that starch granules were persistently deposited in the pericarp parenchyma cells of the paternal excess cross, while pericarp starch gradually decreased and relocated to the developing endosperm in balanced and maternal excess crosses in which cellularization and starch deposition occur in endosperm, suggesting that parental genome balance influences pericarp starch relocation via cellularization and starch deposition. Loss of imprinting, or altered expression of imprinted genes and epigenetic regulators, OsFIE2 and OsMET1b were observed, implying the potential role of imprinting and epigenetic mechanisms in regulating the differential parental genome dosage effects on endosperm development. CONCLUSIONS: Our results support the hypothesis that the maternal genome dosage promotes endosperm cellularization and the paternal genome dosage delays or inhibits cellularization via contributing different sets of imprinted genes.

Phycourobilin in trichromatic phycocyanin from oceanic cyanobacteria is formed post-translationally by a phycoerythrobilin lyase-isomerase.

Most cyanobacteria harvest light with large antenna complexes called phycobilisomes. The diversity of their constituting phycobiliproteins contributes to optimize the photosynthetic capacity of these microorganisms. Phycobiliprotein biosynthesis, which involves several post-translational modifications including covalent attachment of the linear tetrapyrrole chromophores (phycobilins) to apoproteins, begins to be well understood. However, the biosynthetic pathway to the blue-green-absorbing phycourobilin (lambda(max) approximately 495 nm) remained unknown, although it is the major phycobilin of cyanobacteria living in oceanic areas where blue light penetrates deeply into the water column. We describe a unique trichromatic phycocyanin, R-PC V, extracted from phycobilisomes of Synechococcus sp. strain WH8102. It is evolutionarily remarkable as the only chromoprotein known so far that absorbs the whole wavelength range between 450 and 650 nm. R-PC V carries a phycourobilin chromophore on its alpha-subunit, and this can be considered an extreme case of adaptation to blue-green light. We also discovered the enzyme, RpcG, responsible for its biosynthesis. This monomeric enzyme catalyzes binding of the green-absorbing phycoerythrobilin at cysteine 84 with concomitant isomerization to phycourobilin. This reaction is analogous to formation of the orange-absorbing phycoviolobilin from the red-absorbing phycocyanobilin that is catalyzed by the lyase-isomerase PecE/F in some freshwater cyanobacteria. The fusion protein, RpcG, and the heterodimeric PecE/F are mutually interchangeable in a heterologous expression system in Escherichia coli. The novel R-PC V likely optimizes rod-core energy transfer in phycobilisomes and thereby adaptation of a major phytoplankton group to the blue-green light prevailing in oceanic waters.

Genetic analysis and gene fine mapping of aroma in rice (Oryza sativa L. Cyperales, Poaceae)

We investigated inheritance and carried out gene fine mapping of aroma in crosses between the aromatic elite hybrid rice Oryza sativa indica variety Chuanxiang-29B (Ch-29B) and the non-aromatic rice O. sativa indica variety R2 and O. sativa japonica Lemont (Le). The F1 grains and leaves were non-aromatic while the F2 non-aroma to aroma segregation pattern was 3:1. The F3 segregation ratio was consistent with the expected 1:2:1 for a single recessive aroma gene in Ch-29B. Linkage analysis between simple sequence repeat (SSR) markers and the aroma locus for the aromatic F2 plants mapped the Ch-29B aroma gene to a chromosome 8 region flanked by SSR markers RM23120 at 0.52 cM and RM3459 at 1.23 cM, a replicate F2 population confirming these results. Three bacterial artificial chromosome (BAC) clones cover chromosome 8 markers RM23120 and RM3459. Our molecular mapping data from the two populations indicated that the aroma locus occurs in a 142.85 kb interval on BAC clones AP005301 or AP005537, implying that it might be the same gene reported by Bradbury et al (2005a; Plant Biotec J. 3:363-370). The flanking markers Aro7, RM23120 and RM3459 identified by us could greatly accelerate the efficiency and precision of aromatic rice breeding programs.

Genome-wide high-frequency non-Mendelian loss of heterozygosity in rice.

Classic Mendelian genetics declares that hybrids inherit genomic information from both male and female parents, and that alleles should be heterozygous in F1 plants. A few exceptions to this principle have been reported, but most of them are restricted to either a limited set of specific genes or specific types of alleles. Here, we show that a rice triploid and diploid hybridization resulted in stable diploid progenies, both in genotypes and phenotypes, through gene homozygosity. Furthermore, their gene homozygosity can be inherited through 8 generations, and they can convert DNA sequences of other rice varieties into their own. Molecular-marker examination confirmed that this type of genome-wide gene conversion occurred at a very high frequency. Possible mechanisms, including RNA-templated repair of double-strand DNA, are discussed.

[Cytological and embryological observation on the cross of autotriploid x diploid in rice].

In this report, the process of hybridization between different ploidy levels was observed in rice. Autotriploid (2n = 36) 149-B, generated naturally from the twin-seedling rice population SAR-2, was used as the female parent crossing with a normal diploid variety Shuhui 363. The processes of fertilization and embryological development were investigated. The pollen could germinate in triploid tissue of pistil normally and arrive at micropyle. But the percentage of double fertilization was only 8.89%, and the percentage of seed setting was 0.566%. The hybridization obstacles mainly occurred in the process of fertilization. Barriers of double fertilization, prolongation and abnormal development of proembryo, and even development stopping or differentiation failure of embryo and/or endosperm could result in abnormal development and abortion of zygote.

[Identification and analysis of F2 stable population derived from the cross of triploid x diploid in rice].

Polyploid strain 149-B, that was generated naturally from a rice twin-seedling population SAR-2, has been determined as triploid (2n = 36). It was then used as the female parent crossing with a normal diploid variety SH R363. From its F2 generation we obtained a genetic-stable population. To prove the uniformity of such a population, SSR markers were used to survey the F2 individual plants. The results showed that F2 individuals carried only one parental molecular marker at each polymorphic locus, and their genotypes were identical with F1 progeny. Based on the above experiments, we consider that this F2 population is definitely an early-generation stable population. Meanwhile, we discussed the possible mechanism of the special phenomenon as well.