Establishment of systemic acquired resistance (sar) in papaya by external salicylic acid application as a strategy to control dieback disease

Erwinia mallotivora has been identified as the causal pathogen of papaya dieback disease in Malaysia, and has become one of the major threats to the papaya industry. Pathogenesis-related (PR) proteins are considered as plant defense proteins, where they are capable to prevent or reduce the effect of pathogen and insect attacks. Accumulation of PR proteins is associated with the development of systemic acquired resistance (SAR) in plants. The PR gene expressions can be induced by signaling compounds such as abscisic acid (ABA), ethylene, jasmonic acid (JA), salicylic acid (SA), mechanical wounding, osmotic stress and microbial infection. In this study, salicylic acid (SA) was used as a chemical inducer and their effectiveness in inducing SAR in papaya for enhanced disease resistance to papaya dieback was evaluated. Carica papaya was grown in a glasshouse with complete randomized block design (CRBD) arrangement. Salicylic acid (SA) treatment with three concentrations (1 mM, 3 mM, 5 mM) and observation of papaya seedlings physiology were carried out to evaluate the effect of SA on the plant growth. Erwinia mallotivora was inoculated to the papaya seedlings after 1 month of SA applications, and evaluation of disease assessment was done weekly. From statistical analysis (proc ANOVA), no significance difference on the seedlings growth was observed when different concentrations of SA applied. As for the disease assessment, highly significantly differences were recorded between the control and the treatments. Among 10 PR genes analyzed using quantitative real Time PCR, only Osmotin, CPBI_13 and CPBI_17 were down regulated for all SA-treatments; whereas increased of expression were observed for the rest of the genes.