Partitioning Complete Graphs by Heterochromatic Trees

A heterochromatie tree is an edge-colored tree in which any two edges have different colors. The heterochromatic tree partition number of an r-edge-colored graph G, denoted by tr （G）, is the minimum positive integer p such that whenever the edges of the graph G are colored with r colors, the vertices of G can be covered by at most p vertex-disjoint heterochromatic trees. In this paper we determine the heterochromatic tree partition number of r-edge-colored complete graphs. We also find at most tr（Kn） vertex-disjoint heterochromatic trees to cover all the vertices in polynomial time for a given r-edge-coloring of Kn.

内源小RNAs的生物合成及其在植物发育中的作用

在真核生物中,具有20-30个核苷酸的小RNAs能够在DNA或RNA水平上广泛调控复杂生理进程。介绍植物中3种主要内源小RNAs:microRNAs、trans-acting siRNAs和heterochromatic siRNAs的生物合成及其在植物发育中的作用。

Efficient Targeted Genome Modification in Maize Using CRISPR/Cas9 System

CRISPR （clustered regularly interspaced short palindromic repeats）/Cas9 system, which is a newly developed technology for targeted genome modification, has been successfully used in a number of species. In this study, we applied this technology to carry out targeted genome modification in maize. A marker gene Zmzb7 was chosen for targeting. The sgRNA-Cas9 construct was transformed into maize protoplasts, and indel （insertion and deletion） mutations could be detected. A mutant seedling with an expected albino phenotype was obtained from screening 120 seedlings generated from 10 callus events. Mutation efficiency in maize heterochromatic regions was also investigated. Twelve sites with different expression levels in maize centromeres or pericentromere regions were selected. The sgRNA- Cas9 constructs were transformed into protoplasts followed by sequencmg the transformed protoplast genomic DNA. The results show that the genes in heterochromatic regions could be targeted by the CRISPR/Cas9 system efficiently, no matter whether they are expressed or not. Meanwhile, off-target mutations were not found in the similar sites having no PAM （protospacer adjacent motif） or having more than two mismatches. Together. our results show that the CRISPR/Cas9 system is a robust and efficient tool for genome modification in both euchromatic and heterochromatic regions in maize.

siRNA-mediated DNA methylation and H3K9 dimethylation in plants

Heterochromatic siRNAs regulate transcriptional gene silencing by inducing DNA methylation and histone H3K9 dimethylation.Recent advances have revealed the distinct phases involved in siRNA mediated silencing pathway,although the precise functions of a number of factors remain undesignated,putative mechanisms for the con-nection between DNA and histone methylation have been investigated,and much effort has been invested to understand the biological functions of siRNA-mediated epigenetic modifi cation.In this review,we summarize the mechanism of siRNA-mediated epigenetic modification,which involves the production of siRNA and the recruit-ments of DNA and histone methytransferases to the target sequences assisted by complementary pairing between 24-nt siRNAs and nascent scaffold RNAs,the roles of siRNA-mediated epigenetic modifi cation in maintaining genome stability and regulating gene expression have been dis-cussed,newly identified players of the siRNA mediated silencing pathway have also been introduced.

Lens density measurements by two independent psychophysical techniques

Background:Cataract,a leading cause of vision impairment,is due to the lens becoming excessively optically dense.Change in the lens optical density(LOD)could be a useful indicator of incipient nuclear cataract and would necessitate the development of accurate measurement techniques.Mapcat sf^(TM) is a heterochromatic flicker photometer for measuring macular pigment optical density(MPOD)under photopic conditions.In the process,it also measures LOD that is needed in the calculation of MPOD.LOD is then converted by the instrument to“lens equivalent age”(LEA).However,varying cone photoreceptor ratios among individuals could affect the LEA measurement.Scotopic vision is mediated by rod photoreceptors;therefore,LEA measurement under scotopic conditions potentially provides a reliable standard for assessing other methods.The study was conducted to test the level of agreement between the LEA data obtained under photopic and scotopic conditions for a sample population.We also comment on factors that might contribute to any disagreement.Methods:LEAs were obtained by Mapcat sf for 25 subjects and compared with those obtained under absolute scotopic threshold conditions.Results:The mean scotopic LEA for the subjects was 2.7 years higher than the mean photopic LEA,but this difference was not statistically significant.Measurements by the two methods were reasonably correlated(r 2=0.59,p<0.0001).Significant individual differences in LEA by the two methods were found for six of the 25 subjects.Although our calculations included a standard long-to medium-wavelength-sensitive cone ratio,we found that different ratios could be found that rendered the differences in LEA insignificant for two of these six subjects.Variability in pupil diameter during scotopic measurements was considered another potential source of discrepancy between LEAs by the two methods.Conclusion:The absolute threshold technique,with long adaptation times,is probably impractical for routine lens density measurement,whereas Mapcat sf provided a rapid,straightforward test that may find its application in optometric/ophthalmic practice.