The number of meiotic crossovers (COs) is tightly regulated within a narrow range, despite a large excess of molecular precursors. The factors that limit COs remain largely unknown. Here, using a genetic screen in Arabidopsis thaliana, we identified the highly conserved FANCM helicase, which is required for genome stability in humans and yeasts, as a major factor limiting meiotic CO formation. The fancm mutant has a threefold-increased CO frequency as compared to the wild type. These extra COs arise not from the pathway that accounts for most of the COs in wild type, but from an alternate, normally minor pathway. Thus, FANCM is a key factor imposing an upper limit on the number of meiotic COs, and its manipulation holds much promise for plant breeding.

The success of the first meiotic division relies (among other factors) on the formation of bivalents between homologous chromosomes, the monopolar orientation of the sister kinetochores at metaphase I and the maintenance of centromeric cohesion until the onset of anaphase II. The meiotic cohesin subunit, Rec8 has been reported to be one of the key players in these processes, but its precise role in kinetochore orientation is still under debate. By contrast, much less is known about the other non-SMC cohesin subunit, Scc3. We report the identification and the characterisation of AtSCC3, the sole Arabidopsis homologue of Scc3. The detection of AtSCC3 in mitotic cells, the embryo lethality of a null allele Atscc3-2, and the mitotic defects of the weak allele Atscc3-1 suggest that AtSCC3 is required for mitosis. AtSCC3 was also detected in meiotic nuclei as early as interphase, and bound to the chromosome axis from early leptotene through to anaphase I. We show here that both AtREC8 and AtSCC3 are necessary not only to maintain centromere cohesion at anaphase I, but also for the monopolar orientation of the kinetochores during the first meiotic division. We also found that AtREC8 is involved in chromosome axis formation in an AtSPO11-1-independent manner. Finally, we provide evidence for a role of AtSPO11-1 in the stability of the cohesin complex.

In yeast, the DMC1 gene is required for interhomolog recombination, which is an essential step for bivalent formation and the correct partition of chromosomes during meiosis I. By using a reverse genetics approach, we were able to identify a T-DNA insertion in AtDMC1 , the Arabidopsis homolog of DMC1 . Homozygotes for the AtDMC1 insertion failed to express AtDMC1 , and their residual fertility was 1.5% that of the wild type. Complete fertility was restored in mutant plants when a wild-type copy of the AtDMC1 gene was reintroduced. Cytogenetical analysis points to a correlation of the sterility phenotype with severely disturbed chromosome behavior during both male and female meiosis. In this study, our data demonstrate that AtDMC1 function is crucial for meiosis in Arabidopsis. However, meiosis can be completed in the Arabidopsis dmc1 mutant, which is not the case for mouse or some yeast mutants.

In yeast, the DMC1 gene is required for interhomolog recombination, which is an essential step for bivalent formation and the correct partition of chromosomes during meiosis I. By using a reverse genetics approach, we were able to identify a T-DNA insertion in AtDMC1, the Arabidopsis homolog of DMC1. Homozygotes for the AtDMC1 insertion failed to express AtDMC1, and their residual fertility was 1.5% that of the wild type. Complete fertility was restored in mutant plants when a wild-type copy of the AtDMC1 gene was reintroduced. Cytogenetical analysis points to a correlation of the sterility phenotype with severely disturbed chromosome behavior during both male and female meiosis. In this study, our data demonstrate that AtDMC1 function is crucial for meiosis in Arabidopsis. However, meiosis can be completed in the Arabidopsis dmc1 mutant, which is not the case for mouse or some yeast mutants.