Submitted to: Annals of Applied Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2012
Publication Date: May 1, 2012
Citation: Wu, W., Ding, Y., Wei, W., Davis, R.E., Lee, I., Hammond, R., Zhao, Y. 2012. Salicylic acid-mediated elicitation of tomato defense against infection by potato purple top phytoplasma. Annals of Applied Biology. 161:36-45. Interpretive Summary: Diseases in many agriculturally important crops are caused by a group of small bacteria called phytoplasmas. These bacteria lack a cell wall, live inside nutrient-transporting vessels of infected plants, and are spread by insects. Control of these bacterial diseases is difficult and has mainly relied on agricultural practices such as the use of healthy planting materials, eradication of infected plants, and use of insecticide treatment against the disease-spreading insects. The present study was aimed at developing more efficient and environmnetally favorable ways to manage such bacterial diseases. We used a naturally-occurring compound termed salicylic acid (SA) to stimulate plants' natural defense system before the plants had a chance to encounter the bacteria. Our results showed that tomato plants that received such SA pretreatment had a much higher resistance to a subsequent phytoplasma attack--more than half of the pretreated plants successfully avoided infection and remained disease-free over the entire study period; the remaining plants became infected, but the numbers of bacterial cells established in the host plants were much fewer and disease symptoms were much milder. Our study also revealed clues to how the SA-pretreated plants were able to protect themselves against phytoplasma challenges. This report will be of interest to growers, extension personnel, and agricultural economists who are concerned with plant disease management and food security. The information is also important to research scientists, students, and university professors who are studying pathogen-host interactions and mechanisms of plant defense.
Technical Abstract: Recent outbreaks and continued spread of phytoplasma infection-associated diseases in potato, tomato, and other vegetable crops in the U.S. accentuates the need for practical strategies to mitigate the impact of the phytoplasmal diseases. The purpose of the current study was to investigate whether exogenous application of salicylic acid (SA) on healthy tomato seedlings would enhance the plants' natural defense against a subsequent potato purple top (PPT) phytoplasma infection. Our results showed that SA pretreatment significantly decreased the rate of PPT phytoplasma infection and impeded disease progression. At 40 days post inoculation (dpi), while 94% of the PPT phytoplasma-inoculated control plants exhibited characteristic PPT symptoms, 53% of the SA-pretreated, PPT phytoplasma-inoculated plants remained symptom-free with no molecular evidence of phytoplasma infection. The remaining 47% plants became infected, but symptoms were much milder and the average phytoplasma titer was more than 300 times lower compared with that of control plants. Real-time qRT-PCR assays revealed that SA-pretreatment caused transcriptional programming of defense related genes. An early surge of expression of a WRKY-type transcriptional factor gene (LeWRKY1) and a mitogen-activated protein kinase gene (LeMPK3) appeared to play important roles in maintaining a sustained higher level expression of downstream PR genes including LePR1 and the induction of resistance to PPT phytoplasma infection. Our findings encourage field trials of SA pretreatment as a possibly practical approach to protect crops from phytoplasmal diseases.