Rapid pollen tube growth places unique demands on energy production and biosynthetic capacity. The aim of this work is to understand how primary metabolism meets the demands of such rapid growth. Aerobically grown pollen produce ethanol in large quantities. The ethanolic fermentation pathway consists of two committed enzymes: pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH). Because adh mutations do not affect male gametophyte function, the obvious question is why pollen synthesize an abundant enzyme if they could do just as well without. Using transposon tagging in Petunia hybrida, we isolated a null mutant in pollen-specific Pdc2. Growth of the mutant pollen tubes through the style is reduced, and the mutant allele shows reduced transmission through the male, when in competition with wild-type pollen. We propose that not ADH but rather PDC is the critical enzyme in a novel, pollen-specific pathway. This pathway serves to bypass pyruvate dehydrogenase enzymes and thereby maintain biosynthetic capacity and energy production under the unique conditions prevailing during pollen-pistil interaction.

SummaryRapid pollen tube growth requires a high rate of sugar metabolism to meet energetic and biosynthetic demands. Previous work on pollen sugar metabolism showed that tobacco pollen carry out ef®cient ethanolic fermentation concomitantly with a high rate of respiration (Bucher et al., 1995). Here we show that the products of fermentation, acetaldehyde and ethanol, are further metabolised in a pathway that bypasses mitochondrial PDH. The enzymes involved in this pathway are pyruvate decarboxylase, aldehyde dehydrogenase and acetyl-CoA synthetase. Radiolabelling experiments show that during tobacco pollen tube growth label of 14 C-ethanol is incorporated into CO 2 as well as into lipids and other higher molecular weight compounds. A role for the glyoxylate cycle appears unlikely since activity of malate synthase, a key enzyme of the glyoxylate cycle, could not be detected.

Methionine (Met) S-methyltransferase (MMT) catalyzes the synthesis of S-methyl-Met (SMM) from Met and S-adenosyl-Met(Ado-Met). SMM can be reconverted to Met by donating a methyl group to homocysteine (homo-Cys), and concurrent operation of this reaction and that mediated by MMT sets up the SMM cycle. SMM has been hypothesized to be essential as a methyl donor or as a transport form of sulfur, and the SMM cycle has been hypothesized to guard against depletion of the free Met pool by excess Ado-Met synthesis or to regulate Ado-Met level and hence the Ado-Met to S-adenosylhomo-Cys ratio (the methylation ratio). To test these hypotheses, we isolated insertional mmt mutants of Arabidopsis and maize (Zea mays). Both mutants lacked the capacity to produce SMM and thus had no SMM cycle. They nevertheless grew and reproduced normally, and the seeds of the Arabidopsis mutant had normal sulfur contents. These findings rule out an indispensable role for SMM as a methyl donor or in sulfur transport. The Arabidopsis mutant had significantly higher Ado-Met and lower S-adenosylhomo-Cys levels than the wild type and consequently had a higher methylation ratio (13.8 versus 9.5). Free Met and thiol pools were unaltered in this mutant, although there were moderate decreases (of 30%-60%) in free serine, threonine, proline, and other amino acids. These data indicate that the SMM cycle contributes to regulation of Ado-Met levels rather than preventing depletion of free Met.S-Methyl-Met (SMM) synthesis is a unique feature of plant sulfur and one-carbon metabolism (Pokorny et al., 1970; Mudd and Datko, 1990; Ranocha et al., 2001). SMM is formed by the S-adenosyl-Met (AdoMet)-dependent methylation of Met, catalyzed by Met S-methyltransferase (MMT; Bourgis et al., 1999). SMM can be reconverted to Met by transferring a methyl group to homo-Cys in a reaction mediated by homo-Cys S-methyltransferase (HMT; Ranocha et al., 2000). The tandem action of MMT and HMT, together with that of Ado-Met synthetase and S-adenoyslhomo-Cys (AdoHcy) hydrolase, sets up a futile cycle (the SMM cycle) in which Met is converted to SMM, and SMM is reconverted to Met (Mudd and Datko, 1990). This cycle in effect short-circuits the activated methyl cycle (Fig. 1), and each of its turns hydrolyzes a molecule of ATP to adenosine, pyrophosphate, and phosphate. The SMM cycle operates throughout the plant, and consumes one-half the Ado-Met produced in Arabidopsis leaves (Ranocha et al., 2001).The functions of SMM and its seemingly wasteful cycle are for the most part unknown. The only established role of SMM is in transporting reduced sulfur in the phloem, for which there is qualitative evidence in a range of plants including Arabidopsis (Bourgis et al., 1999 Article, publication date, and citation information can be found at www.plantphysiol.org/cgi