Submitted to: Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/2011
Publication Date: 4/26/2012
Citation: Hershkovitz, V., Droby, S., Wisniewski, M.E., Liu, J., Ben-Dayan, C., Raphael, G., Pasmanik-Chor, M., Aly, R., Tworkoski, T. 2012. Global changes in expression of grapefruit peel tissue in response to the yeast biocontrol agent, Metschnikowia fructicola. Molecular Plant Pathology. 13(4):338-349. Interpretive Summary: Developing alternative approaches to disease control is a critical objective of NP-303, Plant Diseases, due to consumer demands to lower exposure to chemicals, and to reduce the impact of agriculture on the environment. In the past decade, USDA-ARS has identified several species of yeasts that can be used as biocontrol agents against postharvest diseases of fruit. In previous studies, we have demonstrated that application of the yeast to the fruit can stimulate defense mechanisms in the fruit that make it more resistant to disease. In the present study in a collaborative project with ARO, Israel, a global analysis of gene expression in citrus in response to the application of the biocontrol yeast, Metschnikowia fructicola, was conducted. The study utilized a newly available citrus microarray that allowed us to study the expression of over 20,000 genes at the same time and represented the first study of its kind. The objective of the study was to develop a more complete understanding of how fruit responds to the application of a biocontrol agent. Results revealed numerous citrus genes related to host defense that were induced specifically by the application of the yeast. Numerous genes involved in the signaling pathway (how organisms perceive a foreign stimulus and transmit a message to the nucleus) of host defense responses were identified. A large accumulation of hydrogen peroxide inside the yeast cells was also observed. The presence of the hydrogen peroxide is believed to be the main signal (oxidative burst) that triggers defense responses in the fruit tissue. Further studies on apples are in progress to determine if the triggering of host defenses is a common response. The information from these studies will be utilized to select the optimum time of application of biocontrol agents and to improve the viability of the use of postharvest biocontrol agents as an alternative disease prevention strategy.
Technical Abstract: To gain a better understanding of the molecular changes taking place in citrus fruit tissue following the application of the yeast biocontrol agent, Metschnikowia fructicola, microarray analysis was performed on grapefruit surface wounds using an Affymetrix Citrus GeneChip. Using a cut off of p<0.05 and 1.5 fold change difference as biologically significant, the data indicated that 1007 putative unigenes showed significant expression changes following wounding and yeast application compared to wounded controls. Microarray results of select genes were validated by reverse transcription-quantitative real-time PCR (RT-qPCR). The data indicated that yeast application induced expression of Rbo, MAPK and MAPKK, G-proteins, CHI, PAL, CHS, 4CL. In contrast, three genes POD, SOD and CAT were down regulated in grapefruit peel tissue treated with the yeast cells. Moreover, suppression was correlated with significantly higher levels of hydrogen peroxide, superoxide anion and hydroxyl radical production in yeast-treated surface wounds. Interestingly, high amounts of hydrogen peroxide were detected inside yeast cells recovered from wounded fruit tissue, indicating the ability of the yeast to actively ROS when it is in contact with plant tissue. This study provides the first global picture of gene expression changes in grapefruit in response to the yeast antagonist M. fructicola.