籼,粳稻及其杂种粗线期的核型分析

研究了典型籼,粳稻及其杂种粗线期的核型,结果表明：（１）籼稻品种南特号具有两对随体染色体,分别为第十和第十二染色体,粳稻品种巴利拉具１对随体染色体,为第十染色体,杂种南特号／巴利拉的随体染色体与南特号相同;（２）在粗线期,两品种均可见单核仁和双核仁的细胞存在,说明细胞贩核仁数目与随体数目并不对应;（３）两品种的对应染色体之间,在染色体相对长度,臂比及染色粒分布上均无明显差异。     

MRE11 is required for homologous synapsis and DSB processing in rice meiosis

Mre11, a conserved protein found in organisms ranging from yeast to multicellular organisms, is required for normal meiotic recombination. Mre11 interacts with Rad50 and Nbs1/Xrs2 to form a complex (MRN/X) that participates in double-strand break (DSB) ends processing. In this study, we silenced the MRE11 gene in rice and detailed its function using molecular and cytological methods. The OsMRE11-deficient plants exhibited normal vegetative growth but could not set seed. Cytological analysis indicated that in the OsMRE11-deficient plants, homologous pairing was totally inhibited, and the chromosomes were completely entangled as a formation of multivalents at metaphase I, leading to the consequence of serious chromosome fragmentation during anaphase I. Immunofluorescence studies further demonstrated that OsMRE11 is required for homologous synapsis and DSB processing but is dispensable for meiotic DSB formation. We found that OsMRE11 protein was located on meiotic chromosomes from interphase to late pachytene. This protein showed normal localization in zep1, Oscom1 and Osmer3, as well as in OsSPO11-1 ^RNAi plants, but not in pair2 and pair3 mutants. Taken together, our results provide evidence that OsMRE11 performs a function essential for maintaining the normal HR process and inhibiting non-homologous recombination during meiosis.     

水稻减数分裂染色体分析方法

减数分裂粗线期染色体研究技术的发展, 很大程度上克服了水稻(Oryza sativa)细胞遗传研究中较小染色体所带来的研究困难。减数分裂染色体的制备与观察已经成为水稻细胞遗传学研究中的常规方法。该文详细描述了水稻中常用的减数分裂染色体制备、荧光原位杂交和免疫荧光染色的实验方法。     

Functional rice centromeres are marked by a satellite repeat and a centromere-specific retrotransposon

The centromere of eukaryotic chromosomes is essential for the faithful segregation and inheritance of genetic information. In the majority of eukaryotic species, centromeres are associated with highly repetitive DNA, and as a consequence, the boundary for a functional centromere is difficult to define. In this study, we demonstrate that the centers of rice centromeres are occupied by a 155-bp satellite repeat, CentO, and a centromere-specific retrotransposon, CRR. The CentO satellite is located within the chromosomal regions to which the spindle fibers attach. CentO is quantitatively variable among the 12 rice centromeres, ranging from 65 kb to 2 Mb, and is interrupted irregularly by CRR elements. The break points of 14 rice centromere misdivision events were mapped to the middle of the CentO arrays, suggesting that the CentO satellite is located within the functional domain of rice centromeres. Our results demonstrate that the CentO satellite may be a key DNA element for rice centromere function.     

DEP and AFO Regulate Reproductive Habit in Rice

Sexual reproduction is essential for the life cycle of most angiosperms. However, pseudovivipary is an important reproductive strategy in some grasses. In this mode of reproduction, asexual propagules are produced in place of sexual reproductive structures. However, the molecular mechanism of pseudovivipary still remains a mystery. In this work, we found three naturally occurring mutants in rice, namely, phoenix (pho), degenerative palea (dep), and abnormal floral organs (afo). Genetic analysis of them indicated that the stable pseudovivipary mutant pho was a double mutant containing both a Mendelian mutation in DEP and a non-Mendelian mutation in AFO. Further map-based cloning and microarray analysis revealed that dep mutant was caused by a genetic alteration in OsMADS15 while afo was caused by an epigenetic mutation in OsMADS1. Thus, OsMADS1 and OsMADS15 are both required to ensure sexual reproduction in rice and mutations of them lead to the switch of reproductive habit from sexual to asexual in rice. For the first time, our results reveal two regulators for sexual and asexual reproduction modes in flowering plants. In addition, our findings also make it possible to manipulate the reproductive strategy of plants, at least in rice.     

Activation of gibberellin 2-oxidase 6 decreases active gibberellin levels and creates a dominant semi-dwarf phenotype in rice （Oryza sativa L.）

Gibberellin (GA) 2-oxidase plays a key role in the GA catabolic pathway through 2β-hydroxylation.In the present study,we isolated a CaMV 35S-enhancer activation tagged mutant,H032.This mutant exhibited a dominant dwarf and GA-deficient phenotype,with a final stature that was less than half of its wild-type counterpart.The endogenous bioactive GAs are markedly decreased in the H032 mutant,and application of bioactive GAs (GA3 or GA4) can reverse the dwarf phenotype.The integrated T-DNA was detected 12.8 kb upstream of the OsGA2ox6 in the H032 genome by TAIL-PCR.An increased level of OsGA2ox6 mRNA was detected at a high level in the H032 mutant,which might be due to the enhancer role of the CaMV 35S promoter.RNAi and ectopic expression analysis of OsGA2ox6 indicated that the dwarf trait and the decreased levels of bioactive GAs in the H032 mutant were a result of the up-regulation of the OsGA2ox6 gene.BLASTP analysis revealed that OsGA2ox6 belongs to the class III of GA 2-oxidases,which is a novel type of GA2ox that uses C20-GAs (GA12 and/or GA53) as the substrates.Interestingly,we found that a GA biosynthesis inhibitor,paclobutrazol,positively regulated the OsGA2ox6 gene.Unlike the over-expression of OsGA2ox1,which led to a high rate of seed abortion,the H032 mutant retained normal flowering and seed production.These results indicate that OsGA2ox6 mainly affects plant stature,and the dominant dwarf trait of the H032 mutant can be used as an efficient dwarf resource in rice breeding.