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 × 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.
This study investigated the potential of azelaic acid (AzA) and hexanoic acid (HxA) to reduce rust development, caused by Phakopsora pachyrhizi, on soybean and the physiological and biochemical changes involved. A 4 × 2 factorial experiment was arranged in a completely randomized design with four replications. The factors studied were as follows: plants sprayed with water (control), acibenzolar‐S‐methyl (ASM; 0.5 g/L) (a well‐recognized resistance inducer), AzA (1 mM) or HxA (20 mM) that were non‐inoculated or inoculated with P. pachyrhizi. Area under rust progress curve significantly decreased by 97%, 95% and 95% for ASM, HxA and AzA treatments, respectively, compared with the control treatment. Infected and AzA and HxA‐sprayed plants showed less impairment on photosynthesis (moderate changes in the values of chlorophyll a fluorescence parameters linked to great concentrations of total chlorophyll a + b and carotenoids) in contrast to infected and water‐sprayed plants. A more robust antioxidative metabolism (great activities of superoxide dismutase, ascorbate peroxidase, catalase, peroxidase and glutathione reductase) on infected and AzA and HxA‐sprayed plants helped to reduce hydrogen peroxide and anion superoxide radical depositions in leaf tissues and their concentrations. Collectively, the physiological and biochemical responses obtained for infected and AzA and HxA‐sprayed plants were comparable with that obtained with ASM. Based on these findings, it is plausible to consider the potential of using AzA and HxA for rust management in soybean toward sustainable agriculture.
Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is a destructive disease affecting soybean production and has been controlled mainly by using different fungicide molecules. The finding of new environmentally-friendly control strategies to minimize fungicide sprays must be prioritized. In this scenario, induced resistance using an array of abiotic or biotic inducers of resistance becomes a very promising alternative. The study investigated the potential of using azelaic acid (AzA), hexanoic acid (HxA) and Mantus ® (copper (20%) and polyphenolic (10%)) to reduce SR symptoms by boosting defense responses. In the first study, the factors studied were plants sprayed with water (control), acibenzolar-S-methyl (ASM), AzA or HxA that were non-inoculated or inoculated with P. pachyrhizi. Both AzA and Hxa significantly inhibited urediniospores germination in vitro. The area under disease progress curve significantly decreased by 95, 93, 92, 82, and 92% for ASM, HxA 5 mM, HxA 20 mM, AzA 0.1 mM, and AzA 1 mM, respectively, compared to control treatment. Hyphae of P. pachyrhizi colonized less abundantly the leaflet tissues of plants sprayed with ASM, HxA (20 mM), and AzA (1 mM) in comparison to water-sprayed plants. Infected and AzA and HxA-sprayed plants showed less impairment on their photos ynthesis and a more robust antioxidative metabolism in contrast to infected and water-sprayed plants. In general, host genes associated with general defense as well as with systemic acquired resistance or induced systemic resistance were strongly up- regulated for infected plants sprayed with AzA, ASM and HxA in comparison to water-sprayed plants. Interestingly, most of the genes were expressed earlier for infected and HxA-sprayed plants than for infected and AzA-sprayed plants, and more expressively in comparison to infected and ASM-sprayed plants. In the second study, the factors studied were plants sprayed with water or Mantus ® (referred to as induced resistance (IR) stimulus thereafter) that were non-inoculated or inoculated with P. pachyrhizi. Urediniospores germination was reduced by 97% by the IR stimulus in vitro. The SR severity and area under disease progress curve decreased by 68 and 35%, respectively, for IR stimulus-sprayed plants compared to water- sprayed plants. Defense-related genes were up-regulated for IR stimulus-sprayed plants compared to water-sprayed plants during the infection process of P. pachyrhizi. Infected and IR stimulus-sprayed plants showed less impairment in their photosynthesis and a more robust antioxidative metabolism in contrast to infected and water-sprayed plants. The results reported here highlight the potential of using AzA and HxA and this IR stimulus for SR management considering its fungistatic effect against urediniospores and by priming soybean resistance more efficiently to cope against P. pachyrhizi infection. Keywords: Glycine max. Disease management. Induced resistance. Plant defense mechanisms. Photosynthesis. Rust. Reactive oxygen species.
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