eased aminopeptidase activity in the lumen of resistant insects in the absence of exposure to XentariTM corroborated the hypothesis of permanent activation of response genes. Increase in midgut proliferation has been proposed as a mechanism of response to pathogens in the adult from several insect species. Analysis of S. exigua larvae revealed that midgut proliferation was neither increased in resistant insects nor induced by exposure of susceptible larvae to XentariTM, suggesting that mechanisms other than midgut proliferation are involved in the response to B. thuringiensis by S. exigua larvae. Citation: Hernandez-Martinez P, Navarro-Cerrillo G, Caccia S, de Maagd RA, Moar WJ, et al. Constitutive Activation of the Midgut Response to Bacillus thuringiensis in Bt-Resistant Spodoptera exigua. PLoS ONE 5: e12795. doi:10.1371/journal.pone.0012795 Editor: Paulo Lee 16985061 Ho, Instituto Butantan, Brazil Received June 15, 2010; Accepted August 19, 2010; Published KPT-9274 September 17, 2010 Copyright: 2010 Hernandez-Martinez et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors want to thank COST 862 Action from the EU for facilitating the interaction of different groups. This research 17460038 was supported by the Spanish Ministry of Science and Innovation and European FEDER funds. SC was supported by a Marie Curie fellowship PIEF-GA2008-219993 from the EU. Competing Interests: The authors have declared that no competing interests exist. E-mail: [email protected] Current address: Monsanto Company, St. Louis, Missouri, United States of America Introduction Bacillus thuringiensis-based biopesticides are employed for the control of numerous species from Lepidoptera, Diptera, and Coleoptera. The pathological effect of B. thuringiensis is mediated by the effect of crystal proteins produced during sporulation, and accumulating as crystalline inclusions. Cry proteins are pore forming toxins that, after their solubilization and activation in the insect midgut, bind to specific receptors in the microvilli of midgut epithelial cells. The toxins are highly specific to some insects and are innocuous to humans, other vertebrates and plants. These characteristics have made them suitable for their use as the active agents in insect-resistant crops. The ability of insects to develop resistance to B. thuringiensisbased formulated products and/or Cry proteins has been reported for many insect species, mostly in the order Lepidoptera. To date, resistance to B. thuringiensis-based formulated products has only evolved in populations of Plodia interpunctella, Plutella xylostella and Trichoplusia ni, in stored grain and under field or greenhouse conditions, respectively. However, artificial selection using B. thuringiensis formulations or Cry proteins under laboratory conditions has generated B. thuringiensis-resistant colonies in many more insect species. The most accepted and 
studied resistance mechanism is the reduction of Cry protein binding to the insect midgut. In some cases, this reduced binding is associated with mutations or altered expression of genes encoding cadherin, aminopeptidase N or alkaline phosphatase receptors. These mechanisms that generally
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