Pectin Methylesterase Regulates Methanol and Ethanol Accumulation in Ripening Tomato (Lycopersicon esculentum) Fruit

Abstract

We provide genetic evidence that the production of methanol in tomato fruit is regulated by pectin methylesterase (PME, EC 3.1.1.11), an enzyme that catalyzes demethoxylation of pectins. The role of PME in methanol production in tomato fruit was examined by relating the tissue methanol content to the PME enzymatic activity in wild-type Rutgers and isogenic PME antisense fruits with lowered PME activity. In the wild-type, fruit development and ripening were accompanied by an increase in the abundance of PME protein and activity and a corresponding ripening-related increase in methanol content. In the PME antisense pericarp, the level of methanol was greatly reduced in unripe fruit, and diminished methanol content persisted throughout the ripening process. The close correlation between PME activity and levels of methanol in fruit tissues from wildtype and a PME antisense mutant indicates that PME is the primary biosynthetic pathway for methanol production in tomato fruit. Interestingly, ethanol levels that were low and unchanged during ripening of wild-type tomatoes increased progressively with the ripening of PME antisense fruit. In vitro studies indicate that methanol is a competitive inhibitor of the tomato alcohol dehydrogenase (ADH, EC 1.1.1.1) activity suggesting that ADH-catalyzed production of ethanol may be arrested by methanol accumulation in the wild-type but not in the PME mutant where methanol levels remain low.Methanol is emitted by actively growing plant tissues (1) and ripening fruit (2). A major source of methanol may be pectin methyl esters (3) that are de-esterified to methanol and pectic substances by a PME 1 -catalyzed reaction (4). Although methanol production is correlated with PME activity in germinating seeds or other plant tissues (1), the role of PME in methanol accumulation in plants has not been firmly established (4,5).A developmentally regulated increase in PME gene expression occurs in developing tomato fruit (6) and may be used as a test system to relate PME activity to methanol metabolism. We have created transgenic tomato fruits with severely impaired expression of PME by introducing a fruit-specific PME antisense gene under the control of cauliflower mosaic virus 35S promoter (7). We compared methanol production in the wildtype and the PME antisense tomato fruits to examine whether the arrest of PME gene expression results in diminished production of methanol. Our results show that methanol accumulation in ripening tomato pericarp of either the wild-type or the PME antisense is related to PME activity, suggesting that the PME activity is the primary source of methanol production in tomato fruits. A surprising finding is that the tissue methanol and ethanol content in ripening tomato fruit were inversely related.

Methanol exchange between grassland and the atmosphere

“…Concerning the metabolism related methanol release, Frenkel et al (1998) found that methanol within the leaf is mostly produced as a consequence of the demethylation of the pectin matrix, a necessary step in the extension of the cell walls during plant growth. On the basis of the pectin content, Galbally and Kirstine (2002) distinguished between two major cell wall types with a high or low potential for methanol release.…”

Isolation and study of a ubiquitously expressed tomato pectin methylesterase regulatory region

“…In higher plants, it is known to be a cell wall associated protein and several of the PME cDNA available in the literature, are known to have toward the N-terminal sequence, a characteristic signal peptide which is thought to help in targeting the protein to the plant cell wall (Gaffe et al 1997 and references therein). PME catalyzes the deesterification of galactosyluronate methylesters of pectins, releasing protons and methanol into the media (Frenkel et al 1998). Despite the biochemical mode of action of PME is well known, it have been difficult to demonstrate any role for PME in the physiology of plants.…”

Isolation and Expression analysis of OsPME1, encoding for a putative Pectin Methyl Esterase from Oryza sativa (subsp. indica)

“…Several studies have shown a strong correlation between PME activity or PME gene expression and physiological processes such as hypocotyls elongation (Bordenave and Goldberg, 1994;Derbyshire et al, 2007)), fruit ripening and maturation (Tieman and Handa, 1994;Prasanna et al, 2007), cellular adhesion (Roberts et al, 2000), seed germination (Ren and Kermode, 2000), cambial cell differentiation (Micheli et al, 2000) and cell growth in developing wood cells (Siedlecka et al, 2008). Other studies have shown that PME acts as methanol source (Frenkel et al, 1998) and also as host-cell receptor for Tobacco Mosaic Virus (TMV) (Dorokhov et al, 1999;Chen et al, 2000). Recent studies demonstrated the role of PMEs in disease resistance and abiotic stress tolerance .…”