Broomrape (Orobanche cumana), root parasitic weed, negatively affects sunflower, thereby causing severe yield losses and required controls on O. cumana infestation. Brassinosteroids (BRs) plays key roles in plant growth and provides resilience to weed infection. This designed study evaluated the mechanisms by which BRs ameliorates O. cumana infection in sunflower (Helianthus annuus). The pretreatment of sunflower seeds with various concentrations of BRs (1, 10, and 100 nM) were applied with and without O. cumana infestation. Under O. cumana infection, a significant reduction in plant height (38.61%), fresh/dry biomass, photosynthesis and regulation in plant defense (POX, GST), BRs signaling (BAK1, BSK1 to BSK4) and synthesis (BRI1, BR6OX2) associated genes. Moreover, O. cumana elevated the levels of MDA, OH‒, H2O2 and O2•‒ in leaves/roots by 77%/112%, 62%/104%, 57%/98% and 55%/89.4% and caused ultrastructural damages in leaves/roots, suggesting the severe oxidative or cellular damages in sunflower. In response, plants activate activities of SOD and POD enzymes as well as reduced or oxidized glutathione, but unable to stimulate APX and CAT activities. Addition of BRs, especially at 10 nM notably lowered the production of oxidative stress, activated the enzymatic and non-enzymatic antioxidants. The downregulation in the reported genes by BRs attributed to the increased resilience of sunflower (via susceptible reaction). In nutshell, the pretreatment of BRs enhanced the sunflower resistance to O. cumana infection by improving the planta growth and biomass, photosynthesis, activating the antioxidant defense system, lowering the oxidative stress, cellular damages and modulating the expression of BR synthesis and signaling genes.
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