Adenosine Kinase Deficiency Is Associated with Developmental Abnormalities and Reduced Transmethylation

Abstract

Adenosine (Ado) kinase (ADK; ATP:Ado 5Ј phosphotransferase, EC 2.7.1.20) catalyzes the salvage synthesis of adenine monophosphate from Ado and ATP. In Arabidopsis, ADK is encoded by two cDNAs that share 89% nucleotide identity and are constitutively, yet differentially, expressed in leaves, stems, roots, and flowers. To investigate the role of ADK in plant metabolism, lines deficient in this enzyme activity have been created by sense and antisense expression of the ADK1 cDNA. The levels of ADK activity in these lines range from 7% to 70% of the activity found in wild-type Arabidopsis. Transgenic plants with 50% or more of the wild-type activity have a normal morphology. In contrast, plants with less than 10% ADK activity are small with rounded, wavy leaves and a compact, bushy appearance. Because of the lack of elongation of the primary shoot, the siliques extend in a cluster from the rosette. Fertility is decreased because the stamen filaments do not elongate normally; hypocotyl and root elongation are reduced also. The hydrolysis of S-adenosyl-l-homo-cysteine (SAH) produced from S-adenosyl-l-methionine (SAM)-dependent methylation reactions is a key source of Ado in plants. The lack of Ado salvage in the ADK-deficient lines leads to an increase in the SAH level and results in the inhibition of SAMdependent transmethylation. There is a direct correlation between ADK activity and the level of methylesterified pectin in seed mucilage, as monitored by staining with ruthenium red, immunofluorescence labeling, or direct assay. These results indicate that Ado must be steadily removed by ADK to prevent feedback inhibition of SAH hydrolase and maintain SAM utilization and recycling.

Adenosine Kinase Is Inactivated by Geminivirus AL2 and L2 Proteins

“…This also may be true in plant cells, in which ADK activity has been observed to increase ‫-5.1ف(‬ to 3-fold) in response to methyl demand (Weretilnyk et al, 2001). The fact that transgenic, ADK-deficient Arabidopsis plants have reduced transmethylation activity supports this view (Moffatt et al, 2002). Thus, the increase in ADK activity we observed in infected N. benthamiana plants also might reflect an increased demand for transmethylation activity in response to virus challenge ( Figure 5).…”

“…We cannot explain why only one of the transgenic events caused a difference in ADK activity, despite high levels of transgene expression in the case of the sense and antisense lines. But in several respects, our experience has been similar to that of Moffatt et al (2002), who previously constructed transgenic Arabidopsis ADK lines. These investigators found that all lines examined (eight containing sense and four containing antisense ADK constructs) had reduced ADK activity, suggesting varying degrees of silencing, and that reversion to the wild type was frequent (10 to 15% of plants in each generation).…”

“…However, considering the role of methylation in reinforcing silencing pathways, we speculate that by inhibiting ADK, AL2 and L2 might indirectly suppress silencing by interfering with methylation. In support of this idea, transmethylation deficiency has been cited as a possible explanation for the frequent reversion of ADK-silenced plants (see below) (Moffatt et al, 2002).…”

“…Thus, ADK plays a critical role in the adenine and adenosine salvage pathways and is important for the synthesis of nucleic acids and nucleotide cofactors. By recycling adenosine, ADK also plays a major role in sustaining the methyl cycle and S -adenosylmethionine-dependent methyltransferase activity in both yeast and plants (Lecoq et al, 2001;Weretilnyk et al, 2001;Moffatt et al, 2002). ADK also may regulate the interconversion of cytokinin bases and ribosides (von Schwartzenberg et al, 1998).…”

ArabidopsisSAMT1 Defines a Plastid Transporter Regulating Plastid Biogenesis and Plant Development

“…Purified plastids were extracted with 20% trichloroacetic acid, and SAM recovered in the soluble fraction was converted to fluorescent isoindoles subsequently analyzed by HPLC (Capdevila and Wagner, 1998;Moffatt et al, 2002).…”