Review Elicitors and priming agents initiate plant defense responses Paul W. Pare´ 1, *, Mohamed A. Farag 2 , Venkat Krishnamachari 1 , Huiming Zhang 1 , Choong-Min Ryu 3 & Joseph W. Kloepper 4 1 Chemistry & Biochemistry Department, Texas Tech University, Lubbock, TX 79409, USA; 2 Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, OK 73402, USA; 3 Laboratory of Microbial Genomics, Korean Research Institute of Bioscience and Biotechnology, Yusong, Taejon, 306-600, S. Korea; 4 Department of Entomology & Plant Pathology, Auburn University, Auburn, AL 36849, USA; *Author for correspondence (e-mail: Paul.Pare@ttu.edu; fax: +1-806-742-1289) Received 15 October 2004; accepted in revised form 19 January 2005 Key words: chemical elicitors, plant defense responses, plant volatile emissions, priming agents, volicitin Abstract Biotic elicitors produced by plant pathogens or herbivore pests rapidly activate a range of plant chemical defenses when translocated to plant tissue. The fatty acid conjugate volicitin has proven to be a robust elicitor model for studying herbivore-induced plant defense responses. Here we review the role of insect- derived volicitin (N-[17-hydroxylinolenoyl]-L-glutamine) as an authentic elicitor of defense responses, specifically as an activator of signal volatiles that attract natural enemies of herbivore pests. Comparisons are drawn between volicitin as an elicitor of plant defenses and two other classes of signaling molecules, C 6 green-leaf volatiles and C 4 bacterial volatiles that appear to prime plant defenses thereby enhancing the capacity to mobilize cellular defense responses when a plant is faced with herbivore or pathogen attack. Abbreviations: ADH – alcohol dehydrogenase; BAW – beet armyworm (Spodoptera exigua); FACs – fatty acid conjugates; HPL – hydroperoxide lyase; HPLC – high performance liquid chromatography; IF – isom- erization factor; ISR – induced systemic resistance; JA – jasmonic acid; LOX – lipoxygenase; MeJA – met- hyl jasmonate; PAL – phenylalanine ammonia lyase; PGPR – plant growth promoting rhizobacteria; SAR – systemic acquired resistance; VOCs – volatile organic compounds Herbivore elicitors trigger plant defenses Herbivores and plant pathogens alike can trigger responses in their host that cannot be mimicked by mechanical damage alone. Such plant defense responses have been ascribed to a wide array of chemical elicitors that activate specific down stream signal transduction pathways (Table 1). For herbivorous insects, two major classes of elic- itors have been isolated from the oral secretions that alter wound responses in plants. The lytic enzyme group includes b-glucosidases (Mattiacci et al. 1995), glucose oxidases (Felton and Eichen- seer 1999), and alkaline phosphatases (Funk 2001), while the fatty acid–amino acid conjugates (FAC) comprise a family of compounds consisting of 18- carbon polyunsaturated fatty acids coupled to L- glutamine or L-glutamic acid. Three structurally similar amides of linolenic acid, N-[17-hydroxyli- nolenoyl]-L-glutamine (volicitin), N-linolenoyl-L- glutamine, and N-linolenoyl-L-glutamic acid are thought to be responsible for a majority of elicitor activity associated with the oral secretions of the Lepidopteran larvae analyzed so far (Alborn et al. 1997; Pohnert et al. 1999; Halitschke et al. 2001). For example, FACs from Manduca sexta larvae Photosynthesis Research (2005) 85: 149–159 Ó Springer 2005