ZIP4 in homologous chromosome synapsis and crossover formation in rice meiosis

In budding yeast, the ZMM complex is closely associated with class I crossovers and synaptonemal complex (SC) formation. However, the relationship between the ZMM genes remains unclear in most higher eukaryotes. Here, we identify the rice ZIP4 homolog, a member of the ZMM gene group, and explore its relationship with two other characterized ZMM genes, MER3 and ZEP1. Our results show that in the rice zip4 mutant, the chiasma frequency is greatly reduced, although synapsis proceeds with only mild defects. Immunocytological analyses of wild-type rice reveal that ZIP4 presents as punctuate foci and colocalizes with MER3 in prophase I meiocytes. Additionally, ZIP4 is essential for the loading of MER3 onto chromosomes, but not vice versa. Double-mutant analyses show that zip4 mer3 displays a greater decrease in the mean number of chiasmata than either of the zip4 or mer3 single mutants, suggesting that ZIP4 and MER3 work cooperatively to promote CO formation but their individual contributions are not completely identical in rice. Although zep1 alone gives an increased chiasma number, both zip4 zep1 and mer3 zep1 show a much lower chiasma number than the zip4 or mer3 single mutants. These results imply that the normal functions of ZIP4 and MER3 are required for the regulation of COs by ZEP1.