Loss of E3 ligase HvST1 function substantially increases distal crossover frequency

This study characterizes the function of a grass-specific E3 ubiquitin ligase, HvST1, in regulating synapsis and crossover (CO) formation during meiosis in barley (Hordeum vulgare). In large-genome cereals, COs are predominantly restricted to distal chromosomal regions, limiting genetic recombination and breeding flexibility. We aimed to identify genetic components regulating CO frequency and distribution. A frameshift mutation in HvST1 was identified in the semi-sterile barley near-isogenic line BW233 through fine mapping of the des12.w locus. The causal role of this mutation was validated via CRISPR/Cas9 gene editing. HvST1 function was investigated using in vitro autoubiquitination and substrate ubiquitination assays, while meiotic progression was assessed using structured illumination microscopy and immunolocalization of key axis and synaptonemal complex (SC) proteins. Loss of HvST1 function led to disrupted SC formation, persistent ASY1 signal, and aberrant ZYP1 polycomplex formation. Despite impaired synapsis, Hvst1 mutants showed a significant increase in distal CO frequency. HvST1 was shown to ubiquitinate ASY1 in vitro, linking its activity to protein turnover at the chromosome axis. HvST1 is critical for normal synapsis and CO regulation during meiosis in barley. Its loss disrupts SC progression but enhances distal CO formation, revealing a previously uncharacterized ubiquitination-based mechanism modulating recombination in grasses.