The DCD domain-containing protein HaB2 enhances sunflower resistance to Orobanche cumana via the ABA signaling pathway

Broomrape (Orobanche cumana) is a major threat to sunflower (Helianthus annuus) production. Although B2 proteins, a class of DCD (development and cell death) domain-containing proteins, are reported to play a role in responses to a variety of abiotic stresses in plants, their function in biotic stress remains unclear. This study explored the role of HaB2 in sunflower resistance to O. cumana, focusing on its regulation through the ABA signaling pathway. Our results show that O. cumana infection and exogenous ABA treatment strongly induce HaB2 expression in sunflower root. Silencing HaB2 increases parasite attachment, while over-expression of HaB2 reduces the formation of haustoria and xylem bridges. The DCD domain is essential and sufficient for HaB2 resistance function. In addition, we found a physiological interaction between HaB2 and the phosphatase HaFyPP3, whereas HaFyPP3 interacts with the ABA-responsive transcription factor HaABI5. Exogenous application of ABA exhibits a resistance phenotype consistent with that by HaB2 over-expression plant. HaABI5 promotes the expression of HaB2 by binding to the ABRE element on its promoter and therefore enhance sunflower resistance, while HaFyPP3 dampens negatively regulates HaB2-mediated resistance by possibly dephosphorylating both ABI5 and HaB2. These results suggest that HaB2 acts as a positive regulator to enhance sunflower resistance to broomrape through involvement in the ABA signaling pathway, providing new insights into ABA-mediated biotic stress resistance and potential targets for exploitation of superior O. cumana resistant sunflower varieties by molecular breeding.