Abstract
The natural resistance of plants to disease is based not only on preformed mechanisms, but also on induced mechanisms. The defense mechanisms present in resistant plants may also be found in susceptible ones. This study attempted to analyze the metabolic alterations in plants of the potato Solanum tuberosum L. cv. Agata that were inoculated with the incompatible plant-pathogenic bacteria X. axonopodis and R. solanacearum, and the compatible bacterium E. carotovora. Levels of total phenolic compounds, including the flavonoid group, and the activities of polyphenol oxidase (PPO) and peroxidase (POX) were evaluated. Bacteria compatibility was evaluated by means of infiltration of tubers. The defense response was evaluated in the leaves of the potato plants. Leaves were inoculated depending on their number and location on the stem. Multiple-leaf inoculation was carried out on basal, intermediate, and apical leaves, and single inoculations on intermediate leaves. Leaves inoculated with X. axonopodis and with R. solanacearum showed hypersensitive responses within 24 hours post-inoculation, whereas leaves inoculated with E. carotovora showed disease symptoms. Therefore, the R. solanacearum isolate used in the experiments did not exhibit virulence to this potato cultivar. Regardless of the bacterial treatments, the basal leaves showed higher PPO and POX activities and lower levels of total phenolic compounds and flavonoids, compared to the apical leaves. However, basal and intermediate leaves inoculated with R. solanacearum and X. axonopodis showed increases in total phenolic compounds and flavonoid levels. In general, multiple-leaf inoculation showed the highest levels of total phenolics and flavonoids, whereas the single inoculations resulted in the highest increase in PPO activity. The POX activity showed no significant difference between single- and multiple-leaf inoculations. Plants inoculated with E. carotovora showed no significant increase in defense mechanisms such as enzyme activity and phenolic compounds. Therefore, resistance or susceptibility in S. tuberosum cv. Agata might depend on leaf age, type of inoculation performed (single or multiple), and the interaction between plant and pathogen.
Highlights
The potato (Solanum tuberosum L.), the fourth most important commercial crop in the world, can be affected by many biotic and abiotic factors, including pathogens and environmental stresses (Juskiewicz et al, 2005; Rauscher et al, 2006)
Some of the important diseases are the bacterial wilt caused by Ralstonia solanacearum, the bacterial soft rot caused by Erwinia carotovora, and the late blight and the early blight caused by the fungi Phytophthora infestans and Alternaria solani, respectively (Nasareno and Jaccoud Filho, 2003; Oliveira et al, 2003)
Neither a hypersensitive response (HR) signal nor disease symptoms were observed in the tubers inoculated with X. axonopodis
Summary
The potato (Solanum tuberosum L.), the fourth most important commercial crop in the world, can be affected by many biotic and abiotic factors, including pathogens and environmental stresses (Juskiewicz et al, 2005; Rauscher et al, 2006). This is a serious economic problem in countries where potatoes are cultivated over large areas. Agata is the most promising because of the uniformity and good features of its tubers. Some of the important diseases are the bacterial wilt caused by Ralstonia solanacearum, the bacterial soft rot caused by Erwinia carotovora, and the late blight and the early blight caused by the fungi Phytophthora infestans and Alternaria solani, respectively (Nasareno and Jaccoud Filho, 2003; Oliveira et al, 2003)
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