Genetic dissection of MutL complexes in Arabidopsis meiosis

During meiosis, homologous chromosomes exchange genetic material through crossing-over. The main crossover pathway relies on ZMM proteins, including ZIP4 and HEI10, and is typically resolved by the MLH1/MLH3 heterodimer, MutLγ. Our analysis of plant fertility and bivalent formation revealed that the MUS81 endonuclease can partially compensate for the MutLγ loss. Comparing genome-wide crossover maps of the mlh1 mutant with ZMM-deficient mutants and lines with varying HEI10 levels reveals that while crossover interference persists in mlh1 , it is weakened. Additionally, mlh1 show reduced crossover assurance, leading to a higher incidence of aneuploidy in offspring. This is likely due to MUS81 resolving intermediates without the crossover bias seen in MutLγ. Comparing mlh1 mlh3 mus81 and zip4 mus81 mutants suggests that additional crossover pathways emerge in the absence of both MutLγ and MUS81. The loss of MutLγ can also be suppressed by eliminating the FANCM helicase. Elevated expression of MLH1 or MLH3 increases crossover frequency, while their overexpression significantly reduces crossover numbers and plant fertility, highlighting the importance for tight control of MLH1/MLH3 levels. By contrast, PMS1, a component of the MutLα endonuclease, appears not to be involved in crossing-over. Together, these findings demonstrate the unique role of MutLγ in ZMM-dependent crossover regulation.