| Plants defend themselves by numbers of active mechanisms. Basal defense pathways leading to non-specific resistance to different categories of pathogens are mediated by plant hormones salicylates (SA), jasmonates (JA), and ethylene (ET), regulated by NPR1, EIN2 and ERF1, and COI1 genes, and associated with induced expression of PR genes, the Hel, and PDF1.2 genes, respectively. These pathways often cross-talk with pathways that contribute to insect resistance, plant growth and development in response to stimulations by various exogenous signals, including biotic and abiotic elicitors. Elicitors that can activate distinct signaling pathways in plants are desirable to determine what critical components are involved in a pathway and what common components could regulate different pathways. Such common signaling components are the keys to understanding signaling networks in plant defense and relevant processes, and have a great practical potential in crop improvement. Studies involved hi the Ph.D project are aimed to determine signaling pathways and components hi plants responding to harpins and riboflavin; both compounds can trigger plant defense and growth pathways, and activate signaling components apparently converging at several points from upstream to downstream of signal transduction processes.Dissection of Signaling in Effects of Harpins on PlantsHarpins are glycine-rich, protease-sensitive, heat-stable, acidic proteins produced by Gram-negative plant pathogenic bacteria, and are required for induction of the hypersensitive response (HR) or hypersensitive cell death (HCD) in nonhost plants of bacteria. Application of harpins to many plants can enhance plant growth, induce resistance to pathogens and insects. These effects have been observed in plants treated with harpinEa from Erwinia amylovora, harpinpss from Pseudomonas syringae pv, syringae, harpinpsph from P. syringae pv. phaseolicola, and harpinxoo,a harpin recently identified in our lab from a Japanese strain of Xanthomonas oryzae pv. oryzae. How harpins perform these diverse functions have been unclear. In this study, three approaches were used to determine signaling pathways, components, and transcriptional basis of regulation of signaling responses involved in the multiple effects of harpins on plants. The first approach is the assays of plants treated with harpinEa or harpinXoo for signaling events, In the secondHARPIN AND RIBOFLAVIN SIGNALING FOR PLANT DEFENSE AND GROWTHapproach, signaling processes in transgenic Arabidopsis and tobacco plants expressing the harpinxoo gene (hrfA) were tested. In the third approach, transgenic Arabidopsis and tobacco engineered with pathogen-inducible giant gromoters (PPPs) were assayed to determine whether harpinxoo and its functional domains confer different effects on plants by activating the promoters through roles of specific cis-acting elements.1. HarpinEa elicits coordinated hypersensitive cell death and pathogen resistance with requirement for the NDR1 and EDS1 genes: Plants sprayed with individuals of several hatpins develop systemic acquired resistance (SAR) without macroscopic necrosis. HCD sometimes accompanies the development of resistance conferred by resistance genes, some of which require the signaling component NDR1 or EDS1 for function. This study addresses whether HCD, NDR1, and EDS1 are required for induction of SAR by harpinEa. When Arabidopsis and tobacco leaves were sprayed with harpinEa, microscopic hypersensitive response (micro-HR) lesions developed. Systemic expression of PR genes and the development of resistance were accompanied by the micro-HR, except in the ndr1-1 mutant, in which harpin induced micro-HR formation without the development of resistance and expression of PR-1 gene. Cell death and resistance did not occur following treatment with harpinEa in plants that could not accumulate SA. Harpin also failed to induce resistance m Arabidopsis edsl-1 mutants. Therefore, harpin-induced resistance seems to develop concomitantly with cell death and r...
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