The precursor for ethylene synthesis is methionine. This amino acid is converted to S-adenosyl methionine (SAM) from ATP and methionine. The reaction is catalyzed by SAM synthetase (Ravanel et al. 1998). Arabidopsis has two genes encoding this enzyme (Peleman et al. 1989). Analysis for subcel-lular localization signals using Target P (Emanuelsson et al. 2000) did not reveal potential subcellular targeting, suggesting cytosolic localization of the enzyme.
The next step is the conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC). This step is catalyzed by ACC synthase (ACS). This is a cytosolic enzyme that requires PLP (pyridoxal-5'-phosphate) as a cofac-tor (Adams and Yang 1979; Yang and Hoffman 1984). ACS isozymes function as homodimers (Capitani et al. 1999; Yamagami et al. 2003). In Arabidopsis,
ACS is encoded by a gene family containing 12 members (ACS1 and ACS2 of Van Der Straeten et al. 1992 are named ACS2 and ACS4 respectively by Ya-magami et al. 2003). Ten of the 12 family members encode ACS isozymes; of these ACS1 and ACS3 are not biologically active, and ACS10 and ACS12 function as aminotransferases. It has been proven that ACS is encoded by a multigene family in other plants too (for a review, see Vandenbussche et al. 2006). The catalytic activity of ACS results in not only ACC but also 5'-methylthioadenosine (MTA). MTA is recycled to methionine in the Yang cycle (Miyazaki and Yang 1987). ACS is the main enzyme that controls the synthesis of ethylene and is, in turn, controlled by multiple signals. We will discuss these signal interactions in the next paragraphs.
ACO (ACC oxidase) catalyzes the conversion of ACC to ethylene. During this reaction ACC is oxidized and forms ethylene, CO2 and HCN (Yang and Hoffman 1984). In Arabidopsis ACO is part of a multigene family, as is ACS (Gomez-Lim et al. 1993). It was proposed that under particular conditions, such as upon wounding or during ripening and senescence, ACO also plays a role in regulating ethylene levels in plants (Kende 1993).
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