RESUMEN
The brown seaweed (Sargassum polycystum C. Agardh-Sargassaceae) extract was examined for its bioelicitor properties in the rubber tree seedling (Hevea brasiliensis (Willd. ex A.Juss.) Müll.Arg. - Euphorbiaceae) and its application to reduce the leaf fall disease caused by Phytophthora palmivora (Edwin John) Butler, 1917 (Peronosporaceae). The major purpose of this study was to apply this seaweed extract (SWE) to improve the disease resistance in rubber tree seedling compared to a chemical fungicide (1% metalaxyl). After foliar spraying of SWE solution, two antioxidant enzymes, catalase (CAT) and peroxidase (POD) and systemic acquired resistance (SAR)-triggered enzyme, ß-1,3-glucanase (GLU), were analyzed. Both secondary metabolites, a phytoalexin scopoletin (Scp) and a signaling molecule salicylic acid (SA) were measured by high performance liquid chromatography (HPLC). Both SWE- and metalaxyl-treated plants had a close disease index (DI)-score which were 16.90 ± 1.93 and 15.54 ± 1.25, respectively, while the positive control sprayed with P. palmivora showed DI-score of 29.27 ± 1.89 which was much higher than those treated with SWE or fungicide. CAT, POD, and GLU were increased in rubber tree leaves treated with SWE solution. Furthermore, Scp and SA were significantly increased in SWE-treated leaves. Enhanced systemic acquired resistance induction, 2.09 folds of SA accumulation, was observed in the distal area comparing to the local area of SWE application. In conclusion, the positive effects of SWE elicitation from these studies revealed that SWE could be used as an alternative biocontrol agent for foliar spraying to enhance the defense responses in rubber tree seedling against P. palmivora.
RESUMEN
Induced resistance by elicitors is considered to be an eco-friendly strategy to stimulate plant defense against pathogen attack. In this study, we elucidated the effect of salicylic acid (SA) on induced resistance in rubber tree against Phytophthora palmivora and evaluated the possible defense mechanisms that were involved. For SA pretreatment, rubber tree exhibited a significant reduction in disease severity by 41%. Consistent with the occurrence of induced resistance, the pronounced increase in H2O2 level, catalase (CAT) and peroxidase (POD) activities were observed. For defense reactions, exogenous SA promoted the increases of H2O2, CAT, POD and phenylalanine ammonia lyase (PAL) activities, including lignin, endogenous SA and scopoletin (Scp) contents. However, SA had different effects on the activity of each CAT isoform in the particular rubber tree organs. Besides, three partial cDNAs encoding CAT (HbCAT1, HbCAT2 and HbCAT3) and a partial cDNA encoding PAL (HbPAL) were isolated from rubber tree. Moreover, the expressions of HbCAT1, HbPAL and HbPR1 were induced by SA. Our findings suggested that, upon SA priming, the elevated H2O2, CAT, POD and PAL activities, lignin, endogenous SA and Scp contents, including the up-regulated HbCAT1, HbPAL and HbPR1 expressions could potentiate the resistance in rubber tree against P. palmivora.