Vegetable Haploid Induction Technology: From Traditional Methods to Broad-Spectrum Induction via Gene Editing

The generation of homozygous lines represents a critical step in vegetable crop breeding and germplasm improvement. Conventional self-pollination and purification methods are time-consuming and prone to severe inbreeding depression. In contrast, haploid breeding technology enables the rapid acquisition of completely homozygous doubled haploid (DH) lines within one to two generations via chromosome doubling, serving as a revolutionary pathway to substantially shorten breeding cycles and enhance breeding efficiency. In this review, the major evolutionary milestones of plant haploid induction technologies are systematically summarized, tracing their development from spontaneous occurrence to in vitro regulation and modern in vivo precision genetic manipulation. The primary technological pathways for constructing in vivo haploid inducer lines via gynogenesis, androgenesis, distant hybridization, and the recent targeted knockout or modification of key reproductive genes (such as CENH3, MTL/PLA1/NLD and DMP) using CRISPR/Cas9 and other gene-editing tools are emphatically discussed. Furthermore, the advantages, disadvantages, and application scenarios of five major haploid identification technologies are explored: morphological evaluation, stomatal guard cell observation, cytogenetic analysis (chromosome squashing and flow cytometry), molecular markers, and genetic markers (including classical anthocyanin systems, high-oil systems, and novel fluorescent protein reporter systems). In addition, the latest research progress in haploid induction across major vegetable crops belonging to Brassicaceae, Solanaceae, and Cucurbitaceae is detailed, alongside an analysis of current bottlenecks such as strong genotypic dependency and low embryo-to-plantlet conversion rates. Finally, future developmental trends focusing on the deep integration of universal gene-editing induction technologies with high-throughput fluorescent protein sorting systems are outlined. This review aims to provide a systematic theoretical framework and technical reference for establishing large-scale, highly efficient integrated haploid breeding systems in vegetable crops.