SNI1/NSE5 is a cold-sensitive brake on class II crossovers in Arabidopsis.

Temperature reshapes meiotic recombination, but how environmental cues alter crossover pathway choice remains unclear. Here, using interval assays, genome-wide crossover maps, genetic mapping of recombination modifiers and targeted genetics, we show that cold shifts crossovers towards subtelomeric regions and increases class II pathway output. Genetic mapping identified a major temperature-responsive locus on chromosome 4, resolved to a 15.6-kb interval containing SNI1, the Arabidopsis NSE5 orthologue and a component of the SMC5/6 complex. Reduced SNI1 function alters the temperature response of meiotic recombination, promoting a subtelomere-biased crossover landscape. Cytology in zip4 mutants shows that SNI1 limits class II crossover formation at 21 °C and that cold weakens this constraint. SNI1 also promotes longer synaptonemal complexes at 21 °C, although total SC length does not by itself predict crossover output. Our data identify SNI1/NSE5 as a temperature-sensitive brake on class II crossovers.