Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2007
Publication Date: 5/18/2007
Citation: Adie, B., Perez-Perez, J.P., Perez-Perez, M.M., Godoy, M., Sanchez-Serrano, J., Schmelz, E.A., Solano, R. 2007. Aba is an essential signal for plant resistance to pathogens affecting ja biosynthesis and the activation of plant defenses in arabidopsis. The Plant Cell. 19:1665-1681. Interpretive Summary: To minimize damage from pathogens, plant possess multiple lines defense ranging from mechanisms that impede penetration to rapidly induced transcriptional, proteinaceous and biochemical responses initiated once cells have been invaded. Inducible defenses are in part mediated by phytohormones which can reciprocally influence other phytohormone responses and even production. Arabidopsis thaliana and the oomycete Pythium irregulare, responsible for ‘damping off’ disease, were utilized as a model plant-pathogen system to probe these interactions. In collaboration with researchers at the Departamento de Genética Molecular de Plantas (Campus Universidad Autónoma; Madrid, Spain) scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have confirmed the important defensive roles of the phytohormones jasmonic acid, ethylene and salicylic acid and also discovered that abscisic acid plays a significant role in reducing disease. Analysis of gene transcription profiles, defined abscisic acid-related mutants, and phytohormone production indicates that, in the context of Pythium irregulare infection, abscisic acid signaling may regulate later changes in jasmonic acid, the key defense regulatory phytohormone. Examination of the physiological and molecular basis of a wide range of plant-biotic interactions is essential to our understanding and ability to improve plant resistance traits.
Technical Abstract: Plant defense responses have been studied through a limited number of models that may have constrained our view of plant-pathogen interactions. Discovery of new defense mechanisms should be favored by broadening the range of pathogens under study. With this aim, Arabidopsis defense response to the ‘damping off’ oomycete pathogen Pythium irregulare has been characterized. Analysis of Arabidopsis defense mutants show that resistance to P. irregulare requires a multi-component defense strategy. Penetration represents a first layer, as indicated by the susceptibility of pen2 mutants, followed by recognition, likely mediated by ERECTA RLKs (receptor-like kinases). Subsequent signaling of inducible defenses is predominantly mediated by jasmonic acid (JA), insensitive coi1 mutants showing, therefore, extreme susceptibility. In contrast to the generally accepted roles of ethylene (ET) and salicylic acid (SA) cooperating or antagonizing, respectively, with JA in the activation of defenses against necrotrophs, both are required to prevent disease progression, although much less so than JA. Meta-analysis of transcriptome profiles confirmed the predominant role of JA in the activation of P. irregulare induced defenses and uncovered abscisic acid (ABA) as an important regulator of defense gene expression. Clustering and analysis of cis-regulatory sequences also uncovered an unexpected overrepresentation of ABA-response elements in the promoters of P. irregulare responsive genes. Subsequent infections of ABA-related mutants and callose-deficient mutants confirmed the importance of this plant hormone to defense, acting through a new mechanism (in part) unrelated to priming of callose. A combination of transcriptomic profiling and analysis of hormone levels in wild-type and ABA-deficient mutants supports a model for ABA preceding JA biosynthesis in the activation of defenses against this oomycete.