Phloem-Specific Expression of Yang Cycle Genes and Identification of Novel Yang Cycle Enzymes inPlantagoandArabidopsis   

Abstract

The 5-methylthioadenosine (MTA) or Yang cycle is a set of reactions that recycle MTA to Met. In plants, MTA is a byproduct of polyamine, ethylene, and nicotianamine biosynthesis. Vascular transcriptome analyses revealed phloem-specific expression of the Yang cycle gene 5-METHYLTHIORIBOSE KINASE1 (MTK1) in Plantago major and Arabidopsis thaliana. As Arabidopsis has only a single MTK gene, we hypothesized that the expression of other Yang cycle genes might also be vascular specific. Reporter gene studies and quantitative analyses of mRNA levels for all Yang cycle genes confirmed this hypothesis for Arabidopsis and Plantago. This includes the Yang cycle genes 5-METHYLTHIORIBOSE-1-PHOSPHATE ISOMERASE1 and DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1. We show that these two enzymes are sufficient for the conversion of methylthioribose-1-phosphate to 1,2-dihydroxy-3-keto-5-methylthiopentene. In bacteria, fungi, and animals, the same conversion is catalyzed in three to four separate enzymatic steps. Furthermore, comparative analyses of vascular and nonvascular metabolites identified Met, S-adenosyl Met, and MTA preferentially or almost exclusively in the vascular tissue. Our data represent a comprehensive characterization of the Yang cycle in higher plants and demonstrate that the Yang cycle works primarily in the vasculature. Finally, expression analyses of polyamine biosynthetic genes suggest that the Yang cycle in leaves recycles MTA derived primarily from polyamine biosynthesis.

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

“…We had previously shown that MTI1 and DEP1 complemented the growth reduction of yeast mutants defective in the conversion of MTR1-P to MTRu1-P or of MTRu-1-P to DHKMP (Fig. 1A) on medium with MTA as sole S source (Pommerrenig et al, 2011). To analyze the function of both enzymes also in planta, we isolated mti1 and dep1 mutants and used them for growth assays on agar medium containing MTA or on hydroponic culture with different levels of S supply.…”

“…On the basis of an expression analysis of MTI1 and DEP1 in rosette leaves (Pommerrenig et al, 2011), we used reporter lines carrying promoter-gene-GFP and promoter-gene-GUS fusions of MTI1 and DEP1 for a more comprehensive expression analysis. These analyses revealed tissue-specific and overlapping localization patterns of MTI1 and DEP1.…”

“…MTI1 has a surprising and high similarity to eukaryotic translation initiation factor eIF-2B family proteins. Phylogenetic analyses and complementation of a yeast strain with a defect in the corresponding MRI1 gene confirmed the enzymatic conversion of MTR-1P to MTRu-1-P by the MTI1 protein (Pommerrenig et al, 2011). …”

“…However, Met Cycle activities are not restricted to seedlings and fruits, since the levels of both mRNA of Met Cycle genes and Met Cycle-related metabolites were found to accumulate in the vasculature of adult rosette leaves of Arabidopsis and Plantago major (Pommerrenig et al, 2011). The specific expression of Met Cycle genes in the vasculature is in line with the second essential function of the Met Cycle, which is the degradation of MTA, the by-product of ethylene, nicotianamine, and polyamine biosynthesis.…”

“…The number of the remaining enzymatic steps and genes of the Met Cycle in Arabidopsis remained unclear, as the conversion of 5-methyl-thioribulose-1-P (MTRu-1-P) to 1,2-dihydroxy-3-keto-5-methylthiopentene (DHKMP) is catalyzed by two enzymes in certain organisms and by three enzymes in others (Sekowska et al, 2004). Recently, two novel higher plant Met Cycle genes, 5-METHYLTHIORIBOSE-1-PHOSPHATE ISOMERASE1 (MTI1) and DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1 (DEP1), have been identified (Pommerrenig et al, 2011). DEP1 is a trifunctional enzyme that catalyzes the conversion of MTRu-1-P to DHKMP.…”