Heat stress impairs centromere structure and segregation of meiotic chromosomes in Arabidopsis.

Heat stress is a major threat to global crop production, and understanding its impact on plant fertility is crucial for developing climate-resilient crops. Despite the known negative effects of heat stress on plant reproduction, the underlying molecular mechanisms remain poorly understood. Here, we investigated the impact of elevated temperature on centromere structure and chromosome segregation during meiosis in Arabidopsis thaliana. Consistent with previous studies, heat stress leads to a decline in fertility and micronuclei formation in pollen mother cells. Our results reveal that elevated temperature causes a decrease in the amount of centromeric histone and the kinetochore protein BMF1 at meiotic centromeres with increasing temperature. Furthermore, we show that heat stress increases the duration of meiotic divisions and prolongs the activity of the spindle assembly checkpoint during meiosis I, indicating an impaired efficiency of the kinetochore attachments to spindle microtubules. Our analysis of mutants with reduced levels of centromeric histone suggests that weakened centromeres sensitize plants to elevated temperature, resulting in meiotic defects and reduced fertility even at moderate temperatures. These results indicate that the structure and functionality of meiotic centromeres in Arabidopsis are highly sensitive to heat stress, and suggest that centromeres and kinetochores may represent a critical bottleneck in plant adaptation to increasing temperatures.

A CENH3 mutation promotes meiotic exit and restores fertility in SMG7-deficient Arabidopsis.

Meiosis in angiosperm plants is followed by mitotic divisions to form multicellular haploid gametophytes. Termination of meiosis and transition to gametophytic development is, in Arabidopsis, governed by a dedicated mechanism that involves SMG7 and TDM1 proteins. Mutants carrying the smg7-6 allele are semi-fertile due to reduced pollen production. We found that instead of forming tetrads, smg7-6 pollen mother cells undergo multiple rounds of chromosome condensation and spindle assembly at the end of meiosis, resembling aberrant attempts to undergo additional meiotic divisions. A suppressor screen uncovered a mutation in centromeric histone H3 (CENH3) that increased fertility and promoted meiotic exit in smg7-6 plants. The mutation led to inefficient splicing of the CENH3 mRNA and a substantial decrease of CENH3, resulting in smaller centromeres. The reduced level of CENH3 delayed formation of the mitotic spindle but did not have an apparent effect on plant growth and development. We suggest that impaired spindle re-assembly at the end of meiosis limits aberrant divisions in smg7-6 plants and promotes formation of tetrads and viable pollen. Furthermore, the mutant with reduced level of CENH3 was very inefficient haploid inducer indicating that differences in centromere size is not the key determinant of centromere-mediated genome elimination.

The importance of localized auxin production for morphogenesis of reproductive organs and embryos in Arabidopsis.

Plant sexual reproduction involves highly structured and specialized organs: stamens (male) and gynoecia (female, containing ovules). These organs synchronously develop within protective flower buds, until anthesis, via tightly coordinated mechanisms that are essential for effective fertilization and production of viable seeds. The phytohormone auxin is one of the key endogenous signalling molecules controlling initiation and development of these, and other, plant organs. In particular, its uneven distribution, resulting from tightly controlled production, metabolism and directional transport, is an important morphogenic factor. In this review we discuss how developmentally controlled and localized auxin biosynthesis and transport contribute to the coordinated development of plants' reproductive organs, and their fertilized derivatives (embryos) via the regulation of auxin levels and distribution within and around them. Current understanding of the links between de novo local auxin biosynthesis, auxin transport and/or signalling is presented to highlight the importance of the non-cell autonomous action of auxin production on development and morphogenesis of reproductive organs and embryos. An overview of transcription factor families, which spatiotemporally define local auxin production by controlling key auxin biosynthetic enzymes, is also presented.

High penetrance of a pan-canina type rDNA family in intersection Rosa hybrids suggests strong selection of bivalent chromosomes in the section Caninae.

All dogroses (Rosa sect. Caninae) are characterized by the peculiar canina meiosis in which genetic material is unevenly distributed between female and male gametes. The pan-canina rDNA family (termed beta) appears to be conserved in all dogroses analyzed so far. Here, we have studied rDNAs in experimental hybrids obtained from open pollination of F1 plants derived from 2 independent intersectional crosses between the pentaploid dogrose species (2n = 5x = 35) Rosa rubiginosa as female parent (producing 4x egg cells due to the unique asymmetrical canina meiosis) and the tetraploid (2n = 4x = 28) garden rose R. hybrida 'André Brichet' as male parent (producing 2x pollen after normal meiosis). We analyzed the structure of rDNA units by molecular methods [CAPS and extensive sequencing of internal transcribed spacers (ITS)] and determined the number of loci on chromosomes by FISH. FISH showed that R. rubiginosa and 'André Brichet' harbored 5 and 4 highly heteromorphic rDNA loci, respectively. In the second generation of hybrid lines, we observed a reduced number of loci (4 and 5 instead of the expected 6). In R. rubiginosa and 'André Brichet', 2-3 major ITS types were found which is consistent with a weak homogenization pressure maintaining high diversity of ITS types in this genus. In contrast to expectation (the null hypothesis of Mendelian inheritance of ITS families), we observed reduced ITS diversity in some individuals of the second generation which might derive from self-fertilization or from a backcross to R. rubiginosa. In these individuals, the pan-canina beta family appeared to be markedly enriched, while the paternal families were lost or diminished in copies. Although the mechanism of biased meiotic transmission of certain rDNA types is currently unknown, we speculate that the bivalent-forming chromosomes carrying the beta rDNA family exhibit extraordinary pairing efficiency and/or are subjected to strong selection in Caninae polyploids.

Expression of 5 S rRNA genes linked to 35 S rDNA in plants, their epigenetic modification and regulatory element divergence.

BACKGROUND: In plants, the 5 S rRNA genes usually occur as separate tandems (S-type arrangement) or, less commonly, linked to 35 S rDNA units (L-type). The activity of linked genes remains unknown so far. We studied the homogeneity and expression of 5 S genes in several species from family Asteraceae known to contain linked 35 S-5 S units. Additionally, their methylation status was determined using bisulfite sequencing. Fluorescence in situ hybridization was applied to reveal the sub-nuclear positions of rDNA arrays. RESULTS: We found that homogenization of L-type units went to completion in most (4/6) but not all species. Two species contained major L-type and minor S-type units (termed L(s)-type). The linked genes dominate 5 S rDNA expression while the separate tandems do not seem to be expressed. Members of tribe Anthemideae evolved functional variants of the polymerase III promoter in which a residing C-box element differs from the canonical angiosperm motif by as much as 30%. On this basis, a more relaxed consensus sequence of a plant C-box: (5'-RGSWTGGGTG-3') is proposed. The 5 S paralogs display heavy DNA methylation similarly as to their unlinked counterparts. FISH revealed the close association of 35 S-5 S arrays with nucleolar periphery indicating that transcription of 5 S genes may occur in this territory. CONCLUSIONS: We show that the unusual linked arrangement of 5 S genes, occurring in several plant species, is fully compatible with their expression and functionality. This extraordinary 5 S gene dynamics is manifested at different levels, such as variation in intrachromosomal positions, unit structure, epigenetic modification and considerable divergence of regulatory motifs.