Uso de nanopartículas de prata como mitigador de estresse em plantas de arroz infectadas por Bipolaris oryzae

Abstract

Rice (Oryza sativa L.) ranks third among the most cultivated cereals worldwide, playing a crucial role in food security, particularly in underdeveloped countries. However, various biotic and abiotic factors occur simultaneously in the field, causing substantial production losses, such as water stress and brown spot, caused by Bipolaris oryzae (Breda de Haan) Shoemaker. It is imperative to understand the effect of these interactions on rice plants and explore new approaches or mitigating strategies. This study aimed to: i) evaluate the effect of Silver Nanoparticles (AgNPs) in vitro and in vivo on B. oryzae and rice brown spot (BS), ii) understand the changes resulting from water stress in B. oryzae infection in rice plants, as well as the potential of AgNPs in controlling this disease in plants under both combined stresses. To evaluate the effect of AgNPs on the development of B. oryzae, in vitro tests were performed with different concentrations diluted in PDA medium (0, 5, 10, 25, 50, 100, and 200 μL L-1). The concentration with high inhibitory potential was used in the in vivo assay, under greenhouse conditions, to evaluate epidemiological (number and size of lesions, severity, and area under the disease progress curve (AUDPC)), physiological (chlorophyll (Chl) a fluorescence and concentration of photosynthetic pigments), and biochemical effects (activities of antioxidant enzymes (ascorbate peroxidase (APX), catalase (CAT), peroxidase (POX), superoxide dismutase (SOD)) and defense (phenylalanine ammonia-lyase (PAL)) in rice plants infected with B. oryzae. The experimental design used was completely randomized in a 2 x 2 x 2 factorial scheme (Evaluation time, Inoculation, and AgNPs Treatment) with four replications. The treatments used were: distilled water (control treatment) and AgNPs (5 μL L-1). Inoculation with B. oryzae was performed 24 hours after AgNPs application, using a concentration of 1 x 105 conidia mL-1. In the in vitro tests, regression adjustments for the mycelial growth rate index, mycelial diameter, and conidial germination of B. oryzae showed a reduction in these parameters as the doses of AgNPs increased, demonstrating a direct effect of AgNPs on B. oryzae. In the in vivo assay, AgNPs spraying on the plants significantly reduced the intensity of rice BS compared to the control treatment. Leaves of AgNPs-treated plants, due to lower BS severity, showed higher concentrations of Chl a and Chl b, as well as greater photosystem II quantum efficiency. Additionally, the lower cell damage due to the low level of symptoms in AgNPs-treated plants resulted in lower activities of SOD, CAT, POX, and APX. For AgNPs-treated plants, PAL activity was significantly higher compared to the control. In conclusion, the photosynthetic capacity was preserved in AgNPs-treated plants. There was a lower activity of reactive oxygen species scavenging enzymes in plants with AgNPs due to the lower level of cell infection. The increased PAL activity indicates a higher capacity for defense pathway activation in rice plants previously treated with AgNPs. To understand the changes resulting from drought stress in B. oryzae infection in rice plants, as well as the potential of AgNPs in controlling this disease in plants under both combined stresses, an in vivo experiment was conducted under greenhouse conditions, evaluating epidemiological, physiological, and biochemical parameters. The experimental design used was completely randomized in a 2 x 2 x 2 factorial scheme (Evaluation time, Inoculation, and Water Stress) with four replications. Rice plants were subjected to mild drought stress induced by PEG6000. After the stress period, the plants returned to their initial condition for 24 hours of recovery and were sprayed with AgNPs (5 μL L-1), followed by inoculation with B. oryzae using a concentration of 1 x 105 conidia mL-1. Our study demonstrated that plants subjected to drought stress showed reductions in symptoms of rice brown spot. Due to lower severity, the functionality of the photosynthetic apparatus was less affected in these plants, which exhibited higher concentrations of Chl a, Chl b, and carotenoids, as well as greater quantum efficiency of photosystem II in plants under the combination of stresses. The effect of drought stress significantly reduced the water potential of both non-inoculated and pathogen-inoculated plants. The activities of SOD and POX, as well as the proline content, increased in plants under B. oryzae infection, with a higher increase in plants previously subjected to drought stress. The activity of PAL was significantly higher in plants subjected to drought stress and inoculated with B. oryzae. Foliar spraying with AgNPs in plants under combined stresses significantly reduced the number of lesions, lesion size, severity, and AUDPC. Taken together, water stress combined with rice brown spot led to increased levels of enzymatic and non-enzymatic antioxidants, as well as higher PAL activity. Although there was a reduction in disease levels, isolated drought stress resulted in physiological harm to the plants, as indicated by the lower water potential. Foliar treatment with AgNPs reduced the epidemiological parameters of rice brown spot in combination with drought stress, indicating potential use as a stress mitigator.