Effect of Phytoplasma Infection on Primary and Secondary Metabolites andAntioxidative Enzyme Activities of Sweet Orange (Citrus sinenses L.)

Sweet orange (Citrus sinensis L.) is one of the most economically important citrus crops in the world and is the most commonly grown citrus fruit in the world. Nonetheless, its production is constantly threatened by pathogens that cause considerable economic losses and severe social impacts including phytoplasma. Phytoplasma is an emerging threat to sweet orange production which leads to severe yield losses worldwide. Phytoplasmas are phloem-limited pleomorphic bacteria, mainly transmitted through leafhoppers but also by plant propagation materials and seeds. This study aimed to evaluate the biochemical alterations in sweet orange plant as a response to phytoplasma infection. Phytoplasma-infected plants in this study, showed symptoms, such as yellowing of leaves, stunted and rolled foliage, unripened shoots and fruits, stunted roots or plant and “witches' broom”. Nested PCR confirmed the presence of phytoplasma in all the infected plants. Primary metabolites including chlorophyll a, b and total chlorophyll contents were significantly reduced. Meanwhile, carotenoid, proline and soluble protein were increased while soluble sugar, hydrogen peroxide and malondialdehyde were decreased in phytoplasma-infected plants. The secondary metabolites including phenolics, glycine betaine and anthocyanin were increased while ascorbic acid was decreased in the phytoplasma-infected plants. The activities of the antioxidative enzymes: ascorbate peroxidase, superoxide dismutase, catalase, and peroxidase activities were increased in phytoplasma-infected plants. On the basis of comparing our findings with previous reports, it is clear that the responses of host plants to phytoplasma infection are complex and may vary among plants.