Certain plant growth-promoting rhizobacteria (PGPR), in the absence of physical contact with a plant stimulate growth via volatile organic compound (VOC) emissions, through largely unknown mechanisms. To probe how PGPR VOCs trigger growth in plants, RNA transcript levels of Arabidopsis seedlings exposed to Bacillus subtilus (strain GB03) were examined using oligonucleotide microarrays. In screening over 26,000 protein-coded transcripts, a group of approximately 600 differentially expressed genes related to cell wall modifications, primary and secondary metabolism, stress responses, hormone regulation and other expressed proteins were identified. Transcriptional and histochemical data indicate that VOCs from the PGPR strain GB03 trigger growth promotion in Arabidopsis by regulating auxin homeostasis. Specifically, gene expression for auxin synthesis was up regulated in aerial regions of GB03-exposed plants; auxin accumulation decreased in leaves and increased in roots with GB03 exposure as revealed in a transgenic DR5::GUS Arabidopsis line, suggesting activation of basipetal auxin transport. Application of the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) restricted auxin accumulation to sites of synthesis thereby preventing GB03-mediated decreases in shoot auxin levels as well as thwarting GB03-mediated growth promotion. In addition, microarray data revealed coordinated regulation of cell wall loosening enzymes that implicated cell expansion with GB03 exposure, which was confirmed by comparative cytological measurements. The discovery that bacterial VOCs, devoid of auxin or other known plant hormones regulate auxin homeostasis and cell expansion provides a new paradigm as to how rhizobacteria promote plant growth.

Four oxidized flavonoid derivatives generated from reacting quercetin (a pentahydroxylated flavone) with the peroxyl radical generator 2,2'-azobis-isobutyronitrile (AIBN) were isolated by chromatographic methods and identified by NMR and MS analyses. Compounds included 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone (2); 1,3,11a-trihydroxy-9-(3,5,7-trihydroxy-4H-1-benzopyran-4-on-2-yl)-5a-(3,4-dihydroxyphenyl)-5,6,11-hexahydro-5,6,11-trioxanaphthacene-12-one (3); 2-(3,4-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid (4); and methyl 3,4-dihydroxyphenylglyoxylate (5). Product ratios under different hydrogen ion concentrations and external nucleophiles revealed that two of the products, namely the substituted benzofuranone (2) and the depside (4), are generated from a common carbocation intermediate. Indirect evidence for the operation of a cyclic concerted mechanism in the formation of the dimeric product (3) is provided. The identification of these products supports the model that the principal site of scavenging reactive oxygen species (ROS) in quercetin is the o-dihydroxyl substituent in the B-ring, as well as the C-ring olefinic linkage.

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.

Flavonols are potent naturally occurring antioxidants. Chemical oxidation reactions in combination with modern spectroscopic techniques have been employed to identify oxidized flavonoid products. Although many oxidized derivatives have been generated from commercially available starting materials, few studies have developed a sequence of flavonoid substrates with a particular hydroxylation pattern to probe the mechanism of flavonoid oxidation. Here, we use AIBN (2,2'-azobisisobutyronitrile) in combination with a series of hydroxylated flavonols to probe the mechanism of flavonoid dimer formation and the role of singly or doubly oxidized species in generating and promoting oxidized flavonoid products. 3-Methoxyquercetin (3-hydroxyl group blocked) and luteolin (lack of 3-hydroxyl) were reacted with AIBN alone or with a second flavonol to serve as a C-3 hydroxyl donor to examine the mechanism of dimer formation. 3-Hydroxyflavones with increasing hydroxyl substitutions in the B ring were also reacted with AIBN in the presence or absence of an external nucleophile to examine the role of various hydroxyls in the formation of a carbocation intermediate via a doubly oxidized species. The presence of a free C-3 hydroxyl, coupled with a B ring ortho hydroxy unit, appears essential for dimer formation. An increase in the number of hydroxyls in the B ring facilitates products generated from a doubly oxidized species.