Identification and Characterization of a Novel Ethylene Inducing Substance in Tomato

Abstract

A continuously increasing world population, a simultaneously decreasing area of arable land, conflicts and global warming are only a few threats for the global food supply. Deeper knowledge in the plant-microbe interface can help to protect plants against biotic stress, decrease yield loss through adequate safety strategies and is one possible way to feed humanity. Plants possess a complex system to interact with their environment and to survive in their particular ecological niche. Limited resources need to be distributed between growth, developement and defense. Differentiation between friend and enemy, with a reliable information uptake for a adequate immune reaction, is existential to spare energy. Different sets of extracellular and intracellular receptors thereby sense and process abiotic and biotic signals. In this work, 83 bacterial strains were screened for immuno-stimulating activity in the model plant S. pennellii. The release of ethylene, a defense hormone, was used as stress indicator. Activity-based purification led to the isolation of maculosin, a small molecule produced by many organisms across different kingdoms. Further investigations about the quantity of maculosin over the fermentation process and the comparison to the initial screen result revealed a difference in the activity. At least two different compounds were present in the analyzed crude culture broth, which both caused ethylene production in S. pennellii. One bacterial derived and still concealed substance and the identified maculosin, originated from the peptone medium. Maculosin is so far unknown to induce ethylene production and callose deposition in S. pennellii. Additionally, five derivates of maculosin were tested for ethylene releasing activity. Even cyclo(L-Phe-L-Pro) the closest derivate to maculosin, lacking only a hydroxy group, did not show activity. This result underlines the importance of single functional groups in small molecule signaling. A compilation of different plants were also tested for stress response due to the treatment of maculosin and its derivates. Except maculosin in S. pennellii, no other stress induction could be monitored. This property of S .pennellii indicates a specific receptor in this plant. With a forward genetic screen the receptor location of maculosin was narrowed down to a part of chromosome 7. As additional part of this work, first purification steps were performed to isolate the ethylene-inducing compound out of the culture broth of an Ensifer species. Activity-based purification of the supernatant led to two stress causing peptides, one of it with less than 10 kDa in size. Using the example of maculosin, this work contributes to a better understanding in plant microbe interaction, especially in plant-mediated recognition and response to small signal molecules. It further provides a first purification protocol for two potential Ensifer derived elicitors recognized by S. pennellii and S. lycopersicum M82 respectively.