Floral scent is important to plant reproduction because it attracts pollinators to the sexual organs. Therefore, volatile emission is usually tuned to the foraging activity of the pollinators. In Petunia hybrida, volatile benzenoids determine the floral aroma. Although the pathways for benzenoid biosynthesis have been characterized, the enzymes involved are less well understood. How production and emission are regulated is unknown. By targeted transcriptome analyses, we identified ODORANT1 (ODO1), a member of the R2R3-type MYB family, as a candidate for the regulation of volatile benzenoids in Petunia hybrida cv W115 (Mitchell) flowers. These flowers are only fragrant in the evening and at night. Transcript levels of ODO1 increased before the onset of volatile emission and decreased when volatile emission declined. Downregulation of ODO1 in transgenic P. hybrida Mitchell plants strongly reduced volatile benzenoid levels through decreased synthesis of precursors from the shikimate pathway. The transcript levels of several genes in this pathway were reduced by suppression of ODO1 expression. Moreover, ODO1 could activate the promoter of the 5-enol-pyruvylshikimate-3-phosphate synthase gene. Flower pigmentation, which is furnished from the same shikimate precursors, was not influenced because color and scent biosynthesis occur at different developmental stages. Our studies identify ODO1 as a key regulator of floral scent biosynthesis.

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Summary Phospholipase D (PLD, EC 3.1.4.4.) has been implicated in a variety of plant processes, including signalling. In Arabidopsis thaliana a PLD gene family has been described and individual members classified into α‐, β‐ and γ‐classes. Here we describe a second PLD gene family in tomato (Lycopersicon esculentum) that includes three α‐ and two β‐classes. Different expression patterns in plant organs were observed for each PLD. In testing a variety of stress treatments on tomato cell suspensions, PLDβ1 mRNA was found to rapidly and specifically accumulate in response to the fungal elicitor xylanase. The greatest increase was found 2 h after treatment with 100 µg ml−1 xylanase (ninefold). In vivo PLD activity increased nearly threefold over a 1.5 h period of treatment. When the elicitor was injected into tomato leaves, PLDβ1 mRNA accumulation peaked at 2 h (threefold increase), before decreasing to background levels within 72 h. Mutant, non‐active xylanase was as effective as the active enzyme in eliciting a response, suggesting that xylanase itself, and not the products resulting from its activity, functioned as an elicitor. When chitotetraose was used as elicitor, no PLDβ1 mRNA accumulation was observed, thus it is not a general response to elicitation. Together these data show that PLD genes are differentially regulated, reflecting potential differences in cellular function. The possibility that PLDβ1 is a signalling enzyme is discussed.

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