Azelaic and hexanoic acids-inducing resistance in soybean against Phakopsora pachyrhizi infection

Rust in soybean, caused by Phakopsora pachyrhizi, has been controlled using different fungicide molecules. This study pinpoints the use of inducers of resistance as a promising alternative by investigating the hypothesis that reduction in rust symptoms on soybean sprayed with azelaic acid (AzA) and hexanoic acid (HxA) is linked to a fungistatic effect against urediniospore germination of P. pachyrhizi and/or through the potentiation of host defence reactions. A 4 x 2 factorial experiment was designed consisting of plants sprayed with water (control), acibenzolar-S-methyl (ASM), AzA or HxA and either non-inoculated or inoculated with P. pachyrhizi. Both AzA and HxA significantly inhibited urediniospore germination in vitro. The area under disease progress curve significantly decreased for ASM, HxA (5 and 20 mM) and AzA (0.1 and 1 mM), and fewer hyphae of P. pachyrhizi colonized tissues of plants sprayed with ASM, HxA (20 mM) and AzA (1 mM), compared to control. Host defence genes were strongly up-regulated for infected plants sprayed with AzA, ASM and HxA compared to water. Most of these genes were expressed earlier for infected and HxA-sprayed plants than for infected and AzA-sprayed plants and at greater expression levels than infected and ASM-sprayed plants. The physiological, biochemical and molecular responses obtained with AzA- and HxA-sprayed plants were comparable to ASM, a well-known inducer of resistance. This study highlights the potential of using AzA and HxA for rust management considering their fungistatic effect against urediniospores and the capacity to provide soybean plants with a more efficient defence against P. pachyrhizi infection.