Proteínas associadas à defesa em plantas de soja desafiadas com o fungo Cercospora kikuchii

Abstract

Soybean is a crop that has been growing in importance in Brazil due to its high nutritional value and various applications. However, its production is significantly impaired due to phytopathogen attack. The fungus Cercospora kikuchii occurs in all soybean producing Brazilian regions and leads to around 20% losses in yield after its attack and establishment in adult plants. To cope with phytopathogens, plants have evolved both constitutive and induced defense mechanisms. Among the mechanisms induced are the oxidative burst, when there is generation of reactive oxygen species (ROS) above the physiological level, hypersensitive response (HR), which leads, eventually, to the programmed cell death (PCD), the Systemic Acquired Resistance (SAR), and the induction of defense proteins, or PR-Proteins (Pathogenesis Related proteins). The present study aimed to evaluate the biochemical responses associated with constitutive and induced proteins of soybean plants, at different stages of their development cycle, when challenged by C. kikuchii. Comparative analyzes were conducted on protein extracts from leaves from both control (uninfected) and plants challenged with the fungus. Examining the constitutive proteins in young plants, superoxide dismutase, chitinase, β-1,3-glucanase and proteases exhibited the highest activities. In the adult phase, catalase, ascorbate peroxidase, phenol peroxidase, phenylalanine ammonia-lyases and protease inhibitors predominated. In relation to the plants challenged with C. kikuchii, in the young phase, there was a rapid response of β-1,3-glucanase activity at the early stage of infection, as well as a possible accumulation of hydrogen peroxide and modulation of the amount of proteases. In the adult phase, there was a decrease in β-1,3-glucanase activity, but increases in the inhibitory activity of proteases, peroxidases and phenylalanine ammonia-lyases. These results suggest that the soybean defense mechanism used in the young phase against C. kikuchii is related to the increases of chitinase and β-1,3-glucanase activity and SAR induction. In adult plants, it is possible that the decline of glucanase activity is related to callose deposition, while the concomitant increase of phenol peroxidases and phenylalanine ammonia-liases is related to lignification. In short, this work allowed verifying the occurrence of different biochemical responses of soybean, at different developmental stages, when it was challenged by C. kikuchii. This fact might be associated to the establishment of the incompatible relationship of soybean with the fungus in the young phase and compatibility in the adult phase, when the pathogen can establish and disseminate.