Effects of Potato Spindle Tuber Viroid Infection on Phytohormone and Antioxidant Responses in Symptomless Solanum laxum Plants

Abstract

To better understand the role of phytohormones and antioxidative responses in plant-viroid interaction, changes in endogenous phytohormone contents and activities of key antioxidant enzymes during latent infection of Solanum laxum by potato spindle tuber viroid (PSTVd) were examined. The results showed that endogenous jasmonic acid (JA) and castasterone (CS) content significantly increased, whereas abscisic acid content significantly decreased in symptomless leaves of systemically infected plants compared to that of mock-inoculated control plants. Regarding endogenous salicylic acid and indole-3-acetic acid content, there were no differences between infected and control plants. PSTVd infection also caused excessive production of hydrogen peroxide and enhanced the activity of ascorbate peroxidase (APX) and superoxide dismutase but not the activity of peroxidase and catalase (CAT). When expressed on a protein-weight base, only APX activity increased while CAT activity decreased almost by half in systemic leaves of infected plants. Furthermore, there was a fall in total glutathione content but also a rise in the content of oxidized forms of both ascorbate and glutathione, indicating that the regeneration of glutathione is insufficient. Results of this study strongly suggest the involvement of JA, CS and enzymatic and non-enzymatic antioxidants in the physiological response of S. laxum during latent infection by PSTVd.