S-Methylmethionine Plays a Major Role in Phloem Sulfur Transport and Is Synthesized by a Novel Type of Methyltransferase

Abstract

All flowering plants produce S-methylmethionine (SMM) from Met and have a separate mechanism to convert SMM back to Met. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by using the aphid stylet collection method together with mass spectral and radiolabeling analyses, we established that l-SMM is a major constituent of the phloem sap moving to wheat ears. The SMM level in the phloem ( approximately 2% of free amino acids) was 1.5-fold that of glutathione, indicating that SMM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, l-SMM was a prominently labeled product in phloem exudates obtained by EDTA treatment of detached leaves from plants of the Poaceae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given l-(35)S-Met. cDNA clones for the enzyme that catalyzes SMM synthesis (S-adenosylMet:Met S-methyltransferase; EC 2.1.1.12) were isolated from Wollastonia biflora, maize, and Arabidopsis. The deduced amino acid sequences revealed the expected methyltransferase domain ( approximately 300 residues at the N terminus), plus an 800-residue C-terminal region sharing significant similarity with aminotransferases and other pyridoxal 5'-phosphate-dependent enzymes. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of SMM synthesis in this group involved a gene fusion event. The resulting bipartite enzyme is unlike any other known methyltransferase.

Phloem-Specific Methionine Recycling Fuels Polyamine Biosynthesis in a Sulfur-Dependent Manner and Promotes Flower and Seed Development

“…AdoMet itself is a major component of Arabidopsis phloem sap (Bourgis et al, 1999). SMM is the phloem-mobile transport form of Met and has a major role for S supply of sink tissues (Bourgis et al, 1999;Tan et al, 2010). These reports are in line with the phloem-specific expression of the Met Cycle genes and indicate that the reduction of AdoMet and SMM in the dep1 and mti1 mutants most likely represents a reduction of reduced S compounds in the phloem.…”

“…In comparison to the Col wild type, the Col-related mutant plants mti1-2, dep1-1, and dep1-2 all showed significant reductions by approximately 25% or 50% of their AdoMet levels when grown in 10 or 20 mM sulfate. Strong differences were also found in the phloem-related metabolite SMM, which serves as storage and transport form of sulfate (Bourgis et al, 1999). In comparison to wild-type plants, SMM levels were reduced by approximately 60% in dep1 mutant plants when grown in 10 or 20 mM sulfate.…”

“…Vascular-specific proteome analyses revealed abundance of SAMS1 and SAMS2 in the phloem sap of Arabidopsis and Brassica napus (Schad et al, 2005;Giavalisco et al, 2006). AdoMet itself is a major component of Arabidopsis phloem sap (Bourgis et al, 1999). SMM is the phloem-mobile transport form of Met and has a major role for S supply of sink tissues (Bourgis et al, 1999;Tan et al, 2010).…”

Phloem-Localizing Sulfate Transporter, Sultr1;3, Mediates Re-Distribution of Sulfur from Source to Sink Organs in Arabidopsis

“…Metabolites generated in the companion cells are transferred to the sieve element through the same pathway. Glutathione and S-methyl-Met are the major sulfur compounds in the phloem sap (Bourgis et al, 1999). Furthermore, glutathione translocated in the phloem is suggested to mediate transmission of the interorgan signal of sulfur status in vascular plants (Lappartient et al, 1999).…”

Seed-Specific Expression of a Feedback-Insensitive Form of CYSTATHIONINE-γ-SYNTHASE in Arabidopsis Stimulates Metabolic and Transcriptomic Responses Associated with Desiccation Stress

“…Recent studies performed in legumes and tobacco (Nicotiana tabacum) seeds show that seed-specific expression of feedback-insensitive mutated forms of Arabidopsis (Arabidopsis thaliana) CGS (AtCGS) leads to significantly higher levels of soluble Met (Hanafy et al, 2013;Matityahu et al, 2013;Song et al, 2013), indicating that Met can be synthesized de novo in these seeds via the Asp family pathway through CGS activity in a similar manner to that in vegetative tissues. However, studies performed on Arabidopsis and wheat (Triticum aestivum) seeds suggest the existence of an alternative pathway of Met synthesis in seeds, in which Met synthesized in vegetative tissues is converted to SMM, which is then transported through the phloem into the developing seeds and reconverted back to soluble Met by the activity of HMT (Bourgis et al, 1999;Lee et al, 2008;Fig. 1).…”