The specific features of methionine biosynthesis and metabolism in plants

Abstract

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

Figure 1

Methionine biosynthesis and structure of enzyme inhibitors. Enzymes: 1, cystathionine γ-synthase; 2, cystathionine β-lyase; 3, methionine synthase; 10, serine hydroxymethyltransferase; 11, 5,10-CH₂H₄PteGlu_n reductase.

Figure 2

Relationships among methionine, threonine, AdoMet, polyamine, biotin, and ethylene biosynthetic pathways in higher plants. Enzymes: 1, cystathionine γ-synthase; 2, cystathionine β-lyase; 3, methionine synthase; 4, AdoMet synthetase; 5, AdoMet-dependent methylase; 6, AdoHcy hydrolase; 7, 1-aminocyclopropane-1-carboxylic acid synthase; 8, AdoMet decarboxylase; 9, threonine synthase. Note that AVG inhibits both cystathionine γ-synthase and 1-aminocyclopropane-1-carboxylic acid synthase. For more details about reactions 1, 2, and 3, see Fig. 1.

Figure 3

Impediment of Arabidopsis growth by PAG and its reversal by methionine. Left, control plants; Center, plants sprayed daily (from 16 days after sowing) with a PAG solution (2 mM); Right, plants sprayed daily (from 16 days after sowing) with a PAG (2 mM) plus methionine (2 mM) solution. Plants shown are 23-days-old.

Figure 4

Methionine is essential for Arabidopsis growth. (A) Sensitivity of Arabidopsis to exogeneously added lysine plus threonine (Center) and restoration of growth by exogeneously added methionine (Right). Plants were sprayed every 2 days (from 8 days after sowing) with 10 mM lysine and 10 mM threonine (Center) or with 10 mM lysine, 10 mM threonine, or 2 mM methionine (Right). Control plants, Left. Plants shown are 15-days-old. (B) Growth of wild-type Arabidopsis (Left) and cystathionine γ-synthase antisense plants (Right). Plants shown are 33-days-old.

Figure 5

Three models for the compartmentation of methionine- and AdoMet-synthesizing/recycling enzymes in plant cells. (A) All enzymes up to Hcy synthesis are present in plastids. The last steps of methionine and AdoMet syntheses occur in the cytosol. (B) The plastid is autonomous for methionine synthesis and the last step of AdoMet synthesis occurs in the cytosol. (C) The plastid is autonomous for methionine and AdoMet biosyntheses. Recycling of methionine from AdoMet metabolism occurs in the cytosol. Nomenclature for enzymes as in Figs. 1 and 2.