Breeding Sunflower (Helianthus annuus L.) for Resistance to Biotic and Abiotic Stress: A Critical Review of Genetic and Genomic Advances

Abstract

Cultivated sunflower (Helianthus annuus L.) is grown across temperate and semi-arid regions worldwide as a principal source of vegetable oil, yet its productivity remains constrained by a shifting complex of fungal, oomycete and parasitic-weed pathogens together with recurrent drought, heat and salinity. Breeding programmes have historically addressed these constraints separately, drawing on the exceptionally wide gene pool contained in the approximately fifty wild Helianthus species native to North America. This review critically synthesises the genetic, genomic and methodological literature on sunflower breeding for biotic and abiotic stress resistance, integrating evidence on major-gene resistance to downy mildew, Sclerotinia diseases, Verticillium wilt, charcoal rot and the parasitic weed Orobanche cumana with quantitative genetic evidence on drought, salinity and combined-stress tolerance. Particular attention is given to the transition from classical interspecific introgression and marker-assisted selection towards genome-wide association mapping, genomic selection and, more recently, CRISPR/Cas9-based functional validation in hairy-root systems. The synthesis indicates that resistance to biotrophic and hemi-biotrophic constraints such as downy mildew and broomrape has been comparatively tractable through major-gene deployment, whereas resistance to necrotrophic pathogens and to abiotic stress is typically polygenic, environmentally contingent and less durable. Recurrent breakdown of vertical resistance, the polygenic architecture of drought and salinity tolerance, and the continued recalcitrance of sunflower to stable genetic transformation are identified as recurring sources of uncertainty across the reviewed evidence. The review further finds that single-stress breeding paradigms are increasingly incongruent with field conditions in which biotic and abiotic pressures co-occur, and it argues for a more integrated resilience-oriented breeding framework. Priority research needs include multi-environment validation of candidate genomic regions, pyramiding of resistance sources with durable and combined-stress mechanisms, and improved functional genomics infrastructure. The conclusions drawn are calibrated to the strength of the underlying evidence, distinguishing well-replicated major-gene findings from preliminary quantitative trait loci requiring independent confirmation.