engleski

Antioxidative response of SA-deficient and wild type potato plants (Solanum tuberosum L.) to infection with potato spindle tuber viroid

Viroids are single-stranded, circular, noncoding RNA molecules that lack an envelope protein or other protein, and as such they are the smallest known plant pathogens. The most prominent symptoms of Potato spindle tuber viroid (PSTVd) infection are growth reduction and deformation of potato tubers, whereas symptoms on leaves and stems are weak and appear at a late stage of infection, which can lead to delayed diagnosis and severe yield losses. The pathogenicity of noncoding viroid RNAs is mainly due to the combined effects of the RNA-silencing machinery and other components of host plant immunity, including hormone signaling and antioxidative responses. Salicylic acid (SA) is a phytohormone important for the induction of defense responses of dicotyledons against a wide range of pathogens. To investigate the possible biological relevance of endogenous SA in the potato-PSTVd interaction, we used potato plants expressing the NahG transgene, which prevents the accumulation of endogenous SA, and wild-type control plants. First, we quantified viroid RNAs during infection in the leaves of potato plants using one-step real-time PCR. Transgenic SA-deficient plants showed increased susceptibility to PSTVd compared with wild-type plants, as evidenced by both early symptom expression and increased viroid RNA accumulation 4 to 5 weeks after inoculation (wpi). Symptoms on PSTVd-infected NahG plants included small and spindle-shaped young leaves, and chlorosis and decay of old leaves, suggesting that the infection induces oxidative stress in systemically infected leaves of transgenic plants. Second, we determined hydrogen peroxide (H2O2) concentration in freshly collected leaves of PSTVd- and mock-inoculated plants using a spectrophotometric method. Histochemical changes of H2O2 were detected by 3, 3’-diaminobenzidine (DAB) staining method. The results showed increased H2O2 accumulation in systemically infected leaves of PSTVd-infected NahG plants at 6 and 8 wpi compared with the leaves of PSTVd-infected wild-type plants. Third, expression analysis of genes encoding antioxidant enzymes such as peroxidase, catalase and ascorbate peroxidase was examined by real-time PCR. Expression analysis showed increased transcription of a peroxidase in systemic leaves of infected NahG plants, but not wild-type plants, at 6 wpi. We show here that inactivation of the SA signaling pathway increases the virulence of PSTVd in susceptible potato plants. Taken together, these results suggest that SA is an important component of the basal defense of potato plants against PSTVd.

PSTVd, salicylic acid, potato (Solanum tuberosum L.)

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