A closer look at centromeres Centromeres are key for anchoring chromosomes to the mitotic spindle, but they have been difficult to sequence because they can contain many repeating DNA elements. These repeats, however, carry regularly spaced, distinctive sequence markers because of sequence heterogeneity between the mostly, but not completely, identical DNA sequence repeats. Such differences aid sequence assembly. Naish et al . used ultra-long-read DNA sequencing to establish a reference assembly that resolves all five centromeres in the small mustard plant Arabidopsis . Their view into the subtly homogenized world of centromeres reveals retrotransposons that interrupt centromere organization and repressive DNA methylation that excludes centromeres from meiotic crossover repair. Thus, Arabidopsis centromeres evolve under the opposing forces of sequence homogenization and retrotransposon disruption. —PJH

Centromeres attach chromosomes to spindle microtubules during cell division and, despite this conserved role, show paradoxically rapid evolution and are typified by complex repeats. We used ultra-long-read sequencing to generate the Col-CEN Arabidopsis thaliana genome assembly that resolves all five centromeres. The centromeres consist of megabase-scale tandemly repeated satellite arrays, which support high CENH3 occupancy and are densely DNA methylated, with satellite variants private to each chromosome. CENH3 preferentially occupies satellites with least divergence and greatest higher-order repetition. The centromeres are invaded by ATHILA retrotransposons, which disrupt genetic and epigenetic organization of the centromeres. Crossover recombination is suppressed within the centromeres, yet low levels of meiotic DSBs occur that are regulated by DNA methylation. We propose that Arabidopsis centromeres are evolving via cycles of satellite homogenization and retrotransposon-driven diversification.

Meiotic recombination is a deeply conserved process within eukaryotes that has a profound effect on patterns of natural genetic variation. During meiosis homologous chromosomes pair and undergo DNA double strand breaks generated by the Spo11 endonuclease. These breaks can be repaired as crossovers that result in reciprocal exchange between chromosomes. The frequency of recombination along chromosomes is highly variable, for example, crossovers are rarely observed in heterochromatin and the centromeric regions. Recent work in plants has shown that crossover hotspots occur in gene promoters and are associated with specific chromatin modifications, including H2A.Z. Meiotic chromosomes are also organized in loop-base arrays connected to an underlying chromosome axis, which likely interacts with chromatin to organize patterns of recombination. Therefore, epigenetic information exerts a major influence on patterns of meiotic recombination along chromosomes, genetic variation within populations and evolution of plant genomes. meiosis, recombination, epigenetic, crossover, chromatin Citation:Yelina N, Diaz P, Lambing C, Henderson IR.

Photosynthesis in eukaryotic cells takes place in specialised plastids. The regulation of plastid development is crucial for multicellular systems such as plants. Two families of transcription factors known as Golden2-like (GLK) and GATA regulate plant chloroplast development, and the miR171-targeted SCARECROW-LIKE (SCL) GRAS transcription factors regulate chlorophyll biosynthesis. The extent to which these proteins carry out conserved roles in non-seed plants such as the liverworts is not known. Here we determine the degree of functional conservation of the GLK, GATA and SCL proteins in controlling chloroplast development in the model liverwort Marchantia polymorpha. Our results indicate that GATA and SCL do not play a detectable role in chloroplast biogenesis but loss of GLK function leads to reduced chloroplast size, underdeveloped thylakoid membranes and lower chlorophyll accumulation. These findings suggest that the functioning of GATA and SCL in chloroplast development either evolved after the divergence of vascular plants from bryophytes, that both roles were secondarily lost in M. polymorpha, or that functional redundancy is masking their roles. In contrast, and consistent with its presence in algae, GLK plays a conserved role in chloroplast biogenesis of liverworts and vascular plants.

BMC Plant Biology 2005, 5(Suppl 1):S1The influence of UV-radiation on the 14 CO 2 assimilation rate of three high mountainous plants (Heracleum lehmannianum Bunge, Prangos pabularia Lind L. and Lathyrus mulkak Lipsky) with different photosynthetic intensity and directivity of photosynthetic carbon metabolism was studied. The investigation was carried out in the Biological Station of The Institute of Plant Physiology and Genetics (Academy of Sciences of Republic of Tajikistan), located on an austral decline (2350 m above sea level) of the Hissar valley (Tajikistan). It is proved, that plants show different responses to dissecting away of UV-rays. Almost tenfold fall in the intensity of 14 CO 2 fixation (during 30 sec) is noted for H. lehmannianum; in P. pabularia there was twofold fall in depression, and in L. mulkak the dissecting away of UV-rays resulted in the minor rising of the photosynthetic rate. Under both film dropping UV-rays and in open area, we have not revealed essential differences in the 14 CO 2 assimilation rate in all three plants. The experiments were carried out in three variants: 1. Control -open place. 2. The plants were covered with polyethylene film. 3. The plants were covered with polyethylene film enriched with 2-oxi-4-alcoxibenzophenol (0.65%), cutting off the UV part of solar spectrum. The study of 14 C incorporation into the products of photosynthesis has manifested essential differences between investigated plants. From the control experiment, using the Heracleum leaves, more than 65% of a label was included into the intermediates of the Calvin cycle (among them about 20% into PGA). The dissecting away of UV-radiation resulted in a depression of 14 C incorporation into PGA and PES. The key products, which, at 30 sec exposure, concentrated the most part of label were sugars, predominantly sucrose. Intermediates of the glycolic pathway concentrated a small part of 14 C (22%). The depressing of CO 2 photosynthetic assimilation in Prangos was accompanied by a sharp slump of 14 C incorporation into intermediates of the Calvin cycle. The label was found out in sugars predominantly and in intermediates of the glycolic pathway. In control plants we detected an increase of 14 C lobe in monosaccharides. The content of label in intermediates of glycolic pathway decreased. In Lathyrus leaves under investigation we revealed intensive label incorporation into intermediates of the Calvin cycle and its decrease in the metabolites of the glycolic pathway. We found minor quantities of 14 C contents in sugars and PEP-products. It is supposed that the UV-rays influence the activity of RUBISCO and other enzymes of the Calvin cycle. The mechanisms of UV-rays influence on carboxylation system and the possibility of their regulatory role in high mountainous plants are under discussion. S2Changes in a pattern of HMW-DNA fragmentation accompany differentiation and ageing of plant cells Nuclear DNA is arranged into loop domains by periodical attachment of chromatin fibers to nuclear matrix at highest level of c...