A new biostimulant derived from soybean by-products enhances plant tolerance to abiotic stress triggered by ozone

Abstract

Background: Tropospheric ozone is an air pollutant that causes negative effects on vegetation, leading to significant losses in crop productivity. It is generated by chemical reactions in the presence of sunlight between primary pollutants resulting from human activity, such as nitrogen oxides and volatile organic compounds. Due to the constantly increasing emission of ozone precursors, together with the influence of a warming climate on ozone levels, crop losses may be aggravated in the future. Therefore, the search for solutions to mitigate these losses becomes a priority. Ozone-induced abiotic stress is mainly due to reactive oxygen species generated by the spontaneous decomposition of ozone once it reaches the apoplast. In this regard, compounds with antioxidant activity offer a viable option to alleviate ozone-induced damage. Using enzymatic technology, we have developed a process that enables the production of an extract with biostimulant properties from okara, an industrial soybean byproduct. The biostimulant, named as OEE (Okara Enzymatic Extract), is water-soluble and is enriched in bioactive compounds present in okara, such as isoflavones. Additionally, it contains a significant fraction of protein hydrolysates contributing to its functional effect. Given its antioxidant capacity, we aimed to investigate whether OEE could alleviate ozone-induced damage in plants. For that, pepper plants (Capsicum annuum) exposed to ozone were treated with a foliar application of OEE.

Results: OEE mitigated ozone-induced damage, as evidenced by the net photosynthetic rate, electron transport rate, effective quantum yield of PSII, and delayed fluorescence. This protection was confirmed by the level of expression of genes associated with photosystem II. The beneficial effect was primarily due to its antioxidant activity, as evidenced by the lipid peroxidation rate measured through malondialdehyde content. Additionally, OEE triggered a mild oxidative response, indicated by increased activities of antioxidant enzymes in leaves (catalase, superoxide dismutase, and guaiacol peroxidase) and the oxidative stress index, providing further protection against ozone-induced stress.

Conclusions: The present results support that OEE protects plants from ozone exposure. Taking into consideration that the promotion of plant resistance against abiotic damage is an important goal of biostimulants, we assume that its use as a new biostimulant could be considered.

Keywords: Biostimulant; Capsicum; Enzymatic extract; Okara; Ozone; ROS.

Fig. 1

Chromatography profile of the soluble protein content of OEE according to its molecular weight using a Superdex Peptide 10/300 GL column

Fig. 2

Physiological parameters. A A_N; B ETR and C PhiPSII in response to O₃ (0 and 100 ppm) under a treatment without and with OEE. Values represent mean ± SD, n = 5. Different letters indicate means that are significantly different from each other (two-way ANOVA, O₃ exposition × OEE treatment; HSD test, P < 0.05). O₃ exposition and OEE treatment in the corner of the panel indicate main or interaction significant effects (*P < 0.05; **P < 0.01; ***P < 0.0005; ****P < 0.0001)

Fig. 3

Delayed fluorescence in leaves of pepper plants in response to ozone (O3) (0 and 100 ppm) under a treatment without and with OEE). A Counts per second (cps) values for each treatment. Values represent mean ± SD, n = 5. Different letters indicate means that are significantly different from each other (two-way ANOVA, O₃ exposition × OEE treatment; HSD test, P < 0.05). O₃ exposition and OEE treatment in the corner of the panel indicate main or interaction significant effects (*P < 0.05; **P < 0.01; ***P < 0.0005; ****P < 0.0001). B photographs taken by the plant imaging system NightShade LB 985. Delayed fluorescence was used as a direct indicator of the chlorophyll content. The color scale reflects the detected counts per second (cps) of delayed fluorescence emission in leaves. Red colour indicates high intensities representing high chlorophyll content, blue colour indicated low intensities of fluorescence, indicating low amounts of chlorophyll

Fig. 4

Fold-change of differentially expressed genes related to Photosystem II. Genes differentially expressed when ozone is applied to the plant are shown in blue, and in yellow when ozone plus the treatment with OEE was applied

Fig. 5

Antioxidant enzyme activities. A CAT B SOD and C GPX in response to O₃ (0 and 100 ppm) under a treatment without and with OEE. Values represent mean ± SD, n = 5. Different letters indicate means that are significantly different from each other (two-way ANOVA, O₃ exposition × OEE treatment; HSD test, P < 0.05). O₃ exposition and OEE treatment in the corner of the panel indicate main or interaction significant effects (*P < 0.05; **P < 0.01; ***P < 0.0005; ****P < 0.0001)

Fig. 6

MDA content (A) and OSI (B) in leaves of pepper plants in response to ozone (O₃) (0 and 100 ppm) under a treatment without and with OEE. Values represent mean ± SD, n = 5. Different letters indicate means that are significantly different from each other (two-way ANOVA, O₃ exposition × OEE treatment; HSD test, P < 0.05). O₃ exposition and OEE treatment in the corner of the panel indicate main or interaction significant effects (*P < 0.05; **P < 0.01; ***P < 0.0005; ****P < 0.0001)