Summary• Flowers have a high risk of pathogen attack because of their rich nutrient and moisture content, and high frequency of insect visitors. We investigated the role of (E)-b-caryophyllene in floral defense against a microbial pathogen. This sesquiterpene is a common volatile compound emitted from flowers, and is a major volatile released from the stigma of Arabidopsis thaliana flowers.• Arabidopsis thaliana lines lacking a functional (E)-b-caryophyllene synthase or constitutively overexpressing this gene were challenged with Pseudomonas syringae pv. tomato DC3000, which is a bacterial pathogen of brassicaceous plants.• Flowers of plant lines lacking (E)-b-caryophyllene emission showed greater bacterial growth on their stigmas than did wild-type flowers, and their seeds were lighter and misshapen. By contrast, plant lines with ectopic (E)-b-caryophyllene emission from vegetative parts were more resistant than wild-type plants to pathogen infection of leaves, and showed reduced cell damage and higher seed production. Based on in vitro experiments, (E)-b-caryophyllene seems to act by direct inhibition of bacterial growth, rather than by triggering defense signaling pathways.• (E)-b-Caryophyllene thus appears to serve as a defense against pathogens that invade floral tissues and, like other floral volatiles, may play multiple roles in defense and pollinator attraction.

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Terpene volatiles play important roles in plant-organism interactions as attractants of pollinators or as defense compounds against herbivores. Among the most common plant volatiles are homoterpenes, which are often emitted from night-scented flowers and from aerial tissues upon herbivore attack. Homoterpene volatiles released from herbivore-damaged tissue are thought to contribute to indirect plant defense by attracting natural enemies of pests. Moreover, homoterpenes have been demonstrated to induce defensive responses in plant-plant interaction. Although early steps in the biosynthesis of homoterpenes have been elucidated, the identity of the enzyme responsible for the direct formation of these volatiles has remained unknown. Here, we demonstrate that CYP82G1 (At3g25180), a cytochrome P450 monooxygenase of the Arabidopsis CYP82 family, is responsible for the breakdown of the C 20 -precursor (E,E)-geranyllinalool to the insect-induced C 16 -homoterpene (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). Recombinant CYP82G1 shows narrow substrate specificity for (E,E)-geranyllinalool and its C 15 -analog (E)-nerolidol, which is converted to the respective C 11 -homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT). Homology-based modeling and substrate docking support an oxidative bond cleavage of the alcohol substrate via syn-elimination of the polar head, together with an allylic C-5 hydrogen atom. CYP82G1 is constitutively expressed in Arabidopsis stems and inflorescences and shows highly coordinated herbivoreinduced expression with geranyllinalool synthase in leaves depending on the F-box protein COI-1. CYP82G1 represents a unique characterized enzyme in the plant CYP82 family with a function as a DMNT/TMTT homoterpene synthase.floral scent | herbivory | terpene biosynthesis P lants interact with the environment by producing a variety of chemical compounds. In particular, volatile compounds emitted from flowers and vegetative plant tissues serve as attractants for pollinators or exert defensive activities against herbivores, thereby contributing to plant survival and reproductive success. Among the most common plant volatiles are the irregular C 16 -homoterpene (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) and its C 11 -analog (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), both of which are widespread floral odor constituents contributing to the "white-floral image" of night scented flowers (1). Moreover, TMTT and DMNT are released in response to herbivore attack from the foliage of gymnosperms (2) and numerous angiosperms, both monocots and dicots (3-7). Several studies have indicated a role of homoterpene volatiles in the attraction of herbivore predators in indirect plant defense. For example, de Boer et al. (8) demonstrated that TMTT influenced the foraging behavior of predatory mites when emitted in the presence of other induced volatiles from lima bean leaves infested by spider mites (Tetranychus urticae). In addition, treatment of lima bean with the terpenoid pathway inhibitor fosmidomycin, wh...

The effect of light on melatonin biosynthesis was examined in detached rice (Oryza sativa cv. Asahi) leaves during the senescence process. The detached leaves were exposed to senescence treatment either in constant darkness or in constant light, and subjected to HPLC analysis for melatonin and its precursors. Higher melatonin levels were detected in rice leaves under constant light while very low levels were observed in constant darkness. Levels of the melatonin intermediates, tryptamine, serotonin, and N-acetylserotonin significantly decreased in the dark compared to those in the light. Furthermore, relative mRNA levels of melatonin biosynthetic genes and their corresponding proteins decreased accordingly in constant darkness. The most striking difference between constant light and dark was observed in levels of the protein tryptamine 5-hydroxylase. These results suggest that melatonin biosynthesis during senescence is dependent on light signals in rice leaves, contrary to the response found in animals.