SummaryMulticellular organisms produce small cysteine-rich antimicrobial peptides as an innate defense against pathogens. While defensins, a well-known class of such peptides, are common among eukaryotes, there are other classes restricted to the plant kingdom. These include thionins, lipid transfer proteins and snakins. In earlier work, we identified several divergent classes of small putatively secreted cysteine-rich peptides (CRPs) in legumes [Graham et al. (2004) Plant Physiol. 135, 1179. Here, we built sequence motif models for each of these classes of peptides, and iteratively searched for related sequences within the comprehensive UniProt protein dataset, the Institute for Genomic Research's 33 plant gene indices, and the entire genomes of the model dicot, Arabidopsis thaliana, and the model monocot and crop species, Oryza sativa (rice). Using this search strategy, we identified 13 000 plant genes encoding peptides with common features: (i) an N-terminal signal peptide, (ii) a small divergent charged or polar mature peptide with conserved cysteines, (iii) a similar intron/exon structure, (iv) spatial clustering in the genomes studied, and (v) overrepresentation in expressed sequences from reproductive structures of specific taxa. The identified genes include classes of defensins, thionins, lipid transfer proteins, and snakins, plus other protease inhibitors, pollen allergens, and uncharacterized gene families. We estimate that these classes of genes account for 2-3% of the gene repertoire of each model species. Although 24% of the genes identified were not annotated in the latest Arabidopsis genome releases (TIGR5, TAIR6), we confirmed expression via RT-PCR for 59% of the sequences attempted. These findings highlight limitations in current annotation procedures for small divergent peptide classes.

Carotenoids are thought to be the precursors of terpenoid volatile compounds that contribute to flavor and aroma. One such volatile, b-ionone, is important to fragrance in many flowers, including petunia (Petunia hybrida). However, little is known about the factors regulating its synthesis in vivo. The petunia genome contains a gene encoding a 9,10(9#,10#) carotenoid cleavage dioxygenase, PhCCD1. The PhCCD1 is 94% identical to LeCCD1A, an enzyme responsible for formation of b-ionone in tomato (Lycopersicon esculentum; Simkin AJ, Schwartz SH, Auldridge M, Taylor MG, Plant J [in press]). Reduction of PhCCD1 transcript levels in transgenic plants led to a 58% to 76% decrease in b-ionone synthesis in the corollas of selected petunia lines, indicating a significant role for this enzyme in volatile synthesis. Quantitative reverse transcription-PCR analysis revealed that PhCCD1 is highly expressed in corollas and leaves, where it constitutes approximately 0.04% and 0.02% of total RNA, respectively. PhCCD1 is light-inducible and exhibits a circadian rhythm in both leaves and flowers. b-Ionone emission by flowers occurred principally during daylight hours, paralleling PhCCD1 expression in corollas. The results indicate that PhCCD1 activity and b-ionone emission are likely regulated at the level of transcript.Apocarotenoids are a class of compounds derived from oxidative cleavage of carotenoids that are important contributors to flavor and fragrance of foods (Walhberg and Eklund, 1998). Until recently, the derivation of many of these compounds from carotenoids was largely based on structural considerations and correlations between levels of substrates and products (Buttery et al., 1988). With more than 600 carotenoids identified to date, apocarotenoids constitute one of the largest classes of molecules in nature. Some of these apocarotenoids are essential and valuable constituents of color, flavor, and aroma (Winterhalter and Rouseff, 2002).Recently, a family of enzymes that could potentially generate many apocarotenoids has been described. This family, the carotenoid cleavage dioxygenases (CCDs), has been shown to cleave multiple carotenoids at specific double bonds within the substrate (Schwartz et al., 2001;Giuliano et al., 2003). One of the best-characterized apocarotenoids is the hormone abscisic acid (ABA). ABA is a C 15 compound derived from 11,12 cleavage of the epoxy-carotenoids 9-cisvioloaxanthin and 9-cis-neoxanthin by VP14 to produce xanthoxin Tan et al., 1997). VP14 is the founding member of this unique family of dioxygenases. In Arabidopsis (Arabidopsis thaliana), there are nine members of the CCD family, five of which are believed to be involved in ABA synthesis (Tan et al., 2003). One member of the family, AtCCD1 that is not involved in ABA synthesis, symmetrically cleaves the 9,10(9#,10#) double bonds of multiple carotenoid substrates in vitro. Homologues of this enzyme, which generates a C 14 dialdehyde and two C 13 products, have been identified in Phaseolus vulgaris (Schwartz et al., 2001), crocus (Cr...

The molecular mechanisms responsible for postpollination changes in floral scent emission were investigated in snapdragon cv Maryland True Pink and petunia cv Mitchell flowers using a volatile ester, methylbenzoate, one of the major scent compounds emitted by these flowers, as an example. In both species, a 70 to 75% pollination-induced decrease in methylbenzoate emission begins only after pollen tubes reach the ovary, a process that takes between 35 and 40 h in snapdragon and ‫ف‬ 32 h in petunia. This postpollination decrease in emission is not triggered by pollen deposition on the stigma. Petunia and snapdragon both synthesize methylbenzoate from benzoic acid and S -adenosyl-L -methionine (SAM); however, they use different mechanisms to downregulate its production after pollination. In petunia, expression of the gene responsible for methylbenzoate synthesis is suppressed by ethylene. In snapdragon, the decrease in methylbenzoate emission is the result of a decrease in both S -adenosyl-L -methionine:benzoic acid carboxyl methyltransferase (BAMT) activity and the ratio of SAM to S -adenosyl-L -homocysteine ("methylation index") after pollination, although the BAMT gene also is sensitive to ethylene.