Figure 7. HCR1/PPX1 PP4 control of meiotic crossover recombination in Arabidopsis.

During meiosis, recombination is initiated by DNA double strand breaks (DSB) that can be resected to form single stranded DNA (ssDNA). In the central diagram a resected ssDNA end (blue) from one homolog has invaded the second homolog (red), to form an interhomolog displacement loop (D-loop). A subset of IH D-loops are further processed to form double Holliday junctions (dHJs), which may be resolved into a crossover. The Class I (also known as ZMM) pathway proteins (green) acts at multiple steps within the formation and stabilization of IH D-loops and dHJs and their resolution into interfering crossovers. The activity of the Class I pathway has been shown to be promoted by independent kinase pathways, including CDK, DDK and Mec1/Tel1 (ATM/ATR). We propose that HCR1 acts with PP4R2 and PP4R3 in PP4 phosphatase complexes that antagonize one or more of the pro-recombination kinase pathways on Class I targets and thereby restrict the number of interfering crossovers that form per meiosis. The Class II pathway contributes to ~10% of crossovers in wild type. Our data also indicate a minor role for repression of the Class II pathway by HCR1. On the right is a diagram indicating that during progression of meiotic recombination, the abundance of axis protein ASY1 (green) is depleted, as the synaptonemal complex protein ZYP1 (red) increases.