Snapdragon flowers emit two monoterpene olefins, myrcene and ( E )-␤ -ocimene, derived from geranyl diphosphate, in addition to a major phenylpropanoid floral scent component, methylbenzoate. Emission of these monoterpenes is regulated developmentally and follows diurnal rhythms controlled by a circadian clock. Using a functional genomics approach, we have isolated and characterized three closely related cDNAs from a snapdragon petal-specific library that encode two myrcene synthases ( ama1e20 and ama0c15 ) and an ( E )-␤ -ocimene synthase ( ama0a23 ). Although the two myrcene synthases are almost identical (98%), except for the N-terminal 13 amino acids, and are catalytically active, yielding a single monoterpene product, myrcene, only ama0c15 is expressed at a high level in flowers and contributes to floral myrcene emission. ( E )-␤ -Ocimene synthase is highly similar to snapdragon myrcene synthases (92% amino acid identity) and produces predominantly ( E )-␤ -ocimene (97% of total monoterpene olefin product) with small amounts of ( Z )-␤ -ocimene and myrcene. These newly isolated snapdragon monoterpene synthases, together with Arabidopsis AtTPS14 (At1g61680), define a new subfamily of the terpene synthase (TPS) family designated the Tps-g group. Members of this new Tps-g group lack the RRx 8 W motif, which is a characteristic feature of the Tps-d and Tps-b monoterpene synthases, suggesting that the reaction mechanism of Tps-g monoterpene synthase product formation does not proceed via an RR-dependent isomerization of geranyl diphosphate to 3S -linalyl diphosphate, as shown previously for limonene cyclase. Analyses of tissue-specific, developmental, and rhythmic expression of these monoterpene synthase genes in snapdragon flowers revealed coordinated regulation of phenylpropanoid and isoprenoid scent production.

Orthologous adh regions of the sorghum and maize genomes were sequenced and analyzed. Nine known or candidate genes, including adh1, were found in a 225-kilobase (kb) maize sequence. In a 78-kb space of sorghum, the nine homologues of the maize genes were identified in a colinear order, plus five additional genes. The major fraction of DNA in maize, occupying 166 kb (74%), is represented by 22 long terminal repeat (LTR) retrotransposons. About 6% of the sequence belongs to 33 miniature inverted-repeat transposable elements (MITEs), remnants of DNA transposons, 4 simple sequence repeats, and low-copy-number DNAs of unknown origin. In contrast, no LTR retroelements were detected in the orthologous sorghum region. The unconserved sorghum DNA is composed of 20 putative MITEs, transposon-like elements, 5 simple sequence repeats, and low-copy-number DNAs of unknown origin. No MITEs were discovered in the 166 kb of DNA occupied by the maize LTR retrotransposons. In both species, MITEs were found in the space between genes and inside introns, indicating specific insertion and͞or retention for these elements. Two adjacent sorghum genes, including one gene missing in maize, had colinear homologues on Arabidopsis chromosome IV, suggesting two rearrangements in the sorghum and three in the maize genome in comparison to a four-gene region of Arabidopsis. Hence, multiple small rearrangements may be present even in largely colinear genomic regions. These studies revealed a much higher degree of diversity at a microstructural level than predicted by genetic mapping studies for closely related grass species, as well as for comparisons of monocots and dicots.The grasses belong to a family of monocotyledonous angiosperms that are well differentiated morphologically from the other angiosperm families and have a single (monophyletic) origin. Their genome sizes, however, may vary a great deal between species. Thus, rice has an estimated genome size of 430 megabases, which is Ϸ11ϫ smaller than barley, 6ϫ smaller than maize, and 2ϫ smaller than sorghum. These large differences in genome sizes, coupled with differences in the degree and the nature of their investigations, have obscured some common features of grass genomic design. Recent studies comparing high-density linkage maps with DNA markers revealed extensive synteny of chromosomal segments between related species (1-5). Valuable as it is, full genome recombinational mapping of DNA markers is not an efficient approach for detecting small rearrangements. Because the available high-resolution maps based on completed nucleotide sequence are largely restricted to individual genes and their proximal neighborhoods, we are left with two obvious questions that cannot be answered at a full-genome level of analysis. These questions are, will the colinearity observed at the 2-to 20-centimorgan level, the sensitivity level of standard recombinational mapping, be preserved or will it break down at a local level (5), and what will the pattern of gene distribution be, relative to the no...

Emission of methyl benzoate, one of the most abundant scent compounds of bee-pollinated snapdragon flowers, occurs in a rhythmic manner, with maximum emission during the day, and coincides with the foraging activity of bumblebees. Rhythmic emission of methyl benzoate displays a "free-running" cycle in the absence of environmental cues (in continuous dark or continuous light), indicating the circadian nature of diurnal rhythmicity. Methyl benzoate is produced in upper and lower snapdragon petal lobes by enzymatic methylation of benzoic acid in the reaction catalyzed by S -adenosyl-L -methionine:benzoic acid carboxyl methyltransferase (BAMT). When a detailed time-course analysis of BAMT activity in upper and lower petal lobes during a 48-hr period was performed, high BAMT activity was found at night as well as in continuous darkness, indicating that the BAMT activity is not an oscillation-determining factor. Analysis of the level of benzoic acid during a 24-hr period revealed oscillations in the amount of benzoic acid during the daily light/dark cycle that were retained in continuous darkness. These data clearly show that the total amount of substrate (benzoic acid) in the cell is involved in the regulation of the rhythmic emission of methyl benzoate. Our results also suggest that similar molecular mechanisms are involved in the regulation of methyl benzoate production in diurnally (snapdragon) and nocturnally (tobacco and petunia) emitting plants. INTRODUCTIONFlowers of many plant species attract pollinators by producing different complex mixtures of volatile compounds that give each species a unique and characteristic fragrance. Such volatile compounds emitted from flowers may function as both long-distance and short-distance attractants and play a prominent role in the localization and selection of flowers by insects, especially in moth-pollinated flowers, which are detected and visited at night (Dobson, 1994). Plants need to, and often do, vary the floral scent they emit during the life span of the flower, both in total output and in specific composition. These changes occur in relation to flower age (Tollsten, 1993;Pichersky et al., 1994;Wang et al., 1997; Dudareva et al., 1998Dudareva et al., , 2000, pollination status (Tollsten, 1993; Schiestl et al., 1997), environmental conditions (Jakobsen and Olsen, 1994), and diurnal endogenous rhythms (summarized by Dudareva et al., 1999). In addition to reduced costs for scent production, these intraspecific changes in floral fragrances also can influence the activity of flower visitors. For example, a quantitative and/or qualitative change of floral bouquets after pollination might lead to a lower attractiveness of these flowers and help to direct pollinators to the unpollinated flowers, thereby maximizing the reproductive success of the plant.It is an evolutionary advantage that plants have scent output at maximal levels only when potential pollinators are active. Plants that are pollinated by insects with maximum activity during the day (e.g., bees) show a diurnal...

In snapdragon flowers, the volatile ester methyl benzoate is the most abundant scent compound. It is synthesized by and emitted from only the upper and lower lobes of petals, where pollinators (bumblebees) come in contact with the flower. Emission of methyl benzoate occurs in a rhythmic manner, with maximum emission during the day, which correlates with pollinator activity. A novel S -adenosyl-L -methionine:benzoic acid carboxyl methyl transferase (BAMT), the final enzyme in the biosynthesis of methyl benzoate, and its corresponding cDNA have been isolated and characterized. The complete amino acid sequence of the BAMT protein has only low levels of sequence similarity to other previously characterized proteins, including plant O -methyl transferases. During the life span of the flower, the levels of methyl benzoate emission, BAMT activity, BAMT gene expression, and the amounts of BAMT protein and benzoic acid are developmentally and differentially regulated. Linear regression analysis revealed that production of methyl benzoate is regulated by the amount of benzoic acid and the amount of BAMT protein, which in turn is regulated at the transcriptional level.

In snapdragon flowers, the volatile ester methyl benzoate is the most abundant scent compound. It is synthesized by and emitted from only the upper and lower lobes of petals, where pollinators (bumblebees) come in contact with the flower. Emission of methyl benzoate occurs in a rhythmic manner, with maximum emission during the day, which correlates with pollinator activity. A novel S-adenosyl-l-methionine:benzoic acid carboxyl methyl transferase (BAMT), the final enzyme in the biosynthesis of methyl benzoate, and its corresponding cDNA have been isolated and characterized. The complete amino acid sequence of the BAMT protein has only low levels of sequence similarity to other previously characterized proteins, including plant O-methyl transferases. During the life span of the flower, the levels of methyl benzoate emission, BAMT activity, BAMT gene expression, and the amounts of BAMT protein and benzoic acid are developmentally and differentially regulated. Linear regression analysis revealed that production of methyl benzoate is regulated by the amount of benzoic acid and the amount of BAMT protein, which in turn is regulated at the transcriptional level.