Increases in Free Amino Acid Levels in Tomato Plants Accompanying Herbicide-Induced Disease Resistance ALVIN N. STARRATT AND GEORGE LAZAROVITS Agriculture and Agri-Food Canada, Pest Management Research Centre, 1391 Sandford Street, London, Ontario N5V 4T3, Canada Received December 7, 1995; accepted March 7, 1996 Tomato seedlings grown in the presence of low levels of various dinitroaniline herbicides to induce resistance to Fusarium oxysporum Schlecht f. sp. lycopersici had strongly elevated levels of free amino acids. The greatest change occurred with the two most prominent amino acids in tomato seedling tissues, glutamine and asparagine. The dose-related changes in response to herbicide treatment were largest in roots but levels in hypocotyls and cotyledons were also increased significantly. Plants treated similarly with the unrelated herbicide acetochlor, which does not induce resistance, had near normal levels of free amino acids. Tests following removal of the seedlings from contact with the dinitroaniline herbicide trifluralin indicated that disease protection was short-lived and the loss of resistance was accompanied by a decline in the level of the free amino acids. The possible impact of increased levels of free amino acids on resistance to disease is discussed. Although free amino acids are probably not directly related to the observed protection, it is proposed that the levels in the roots of tomato seedlings treated with dinitroaniline herbicides and possibly other inducers can be used as an indicator of the extent of resistance induction. © 1996 Academic Press, Inc. INTRODUCTION A number of chemicals, mostly herbicides and growth regulators, alter the susceptibility of plants to disease such that they become resistant to invasion by parasitic agents (1–4). Of the chemicals tested, members of the dinitroaniline class of herbicides were particularly effective at inducing resistance to bacterial and fungal vas- cular wilt agents in crops such as peppers, to- matoes, eggplants, melons, peanuts, flax, and cotton (3, 5–8). At the levels used, the herbi- cides had no apparent toxicity to the disease- causing agents, leading to the conclusion that protection arose as a result of activation of gen- eral plant defense mechanisms (3, 5–7). The manner in which herbicides activate re- sistance mechanisms remains unknown and only scant information exists regarding resis- tance-associated processes which are induced. In tomato and cotton plants treated with triflu- ralin, resistance was related to the production of fungitoxic compounds upon infection with wilt pathogens (6). Such compounds were not de- tected in uninfected treated plants or in infected untreated plants. In melon seedlings inoculated with Fusarium, the resistance induced by dini- troaniline herbicides was related to suppression of ethylene production (7). In this laboratory, Bolter et al. (9) showed that the level of gluta- thione (GSH) in tomatoes treated with the dini- troanilines was directly related to the degree of disease protection. The unrelated herbicide ace- tochlor did not alter GSH levels or increase re- sistance to disease in tomatoes but was more effective than the dinitroanilines in affecting these responses in melons. Other than inoculation with the pathogen, there is no reliable method for identifying whether a treatment is effective at providing disease protection. Since bioassays require 10 days or more to reveal if a treatment is success- ful, it has been difficult to determine which, if any, of the numerous biochemical and physi- ological processes altered by dinitroaniline her- bicide treatment (10) are related to resistance. Therefore, if this technology is ever to be uti- lized for pest protection, we need to develop more rapid and timely markers than bioassays for identifying treatments that induce effective disease resistance. Although GSH may be one such marker, it is not particularly easy to iden- tify or quantitate. The present study was under- taken to determine if other more readily identi- fiable and measurable biochemical alterations PESTICIDE BIOCHEMISTRY AND PHYSIOLOGY 54, 230–240 (1996) ARTICLE NO. 0027 230 0048-3575/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.