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Plant Physiol, February 2001, Vol. 125, pp. 856-865
Maintaining Methylation Activities during Salt Stress. The
Involvement of Adenosine Kinase1
Elizabeth A.
Weretilnyk,2*
Kristin J.
Alexander,
Martina
Drebenstedt,
Jamie D.
Snider,
Peter S.
Summers, and
Barbara
A.
Moffatt2
Department of Biology, McMaster University, Hamilton, Ontario,
Canada L8S 4K1 (E.A.W., K.J.A., M.D., P.S.S.); and Biology
Department, University of Waterloo, Waterloo, Ontario, Canada N2L
3G1 (J.D.S., B.A.M.)
Synthesis of the compatible osmolyte Gly betaine is increased in
salt-stressed spinach (Spinacia oleracea). Gly betaine
arises by oxidation of choline from phosphocholine. Phosphocholine is synthesized in the cytosol by three successive
S-adenosyl-Met-dependent N-methylations
of phosphoethanolamine. With each transmethylation, a molecule of
S-adenosylhomo-Cys (SAH) is produced, a potent inhibitor of S-adenosyl-Met-dependent methyltransferases. We
examined two enzymes involved in SAH metabolism: SAH hydrolase (SAHH)
catabolizes SAH to adenosine plus homo-Cys and adenosine kinase (ADK)
converts adenosine to adenosine monophosphate. In vitro SAHH and ADK
activities increased incrementally in extracts from leaves of spinach
plants subjected to successively higher levels of salt stress and these changes reflected increased levels of SAHH and ADK protein and transcripts. Another Gly betaine accumulator, sugar beet (Beta vulgaris), also showed salt-responsive increases in SAHH and
ADK activities and protein whereas tobacco (Nicotiana
tabacum) and canola (Brassica napus), which do
not accumulate Gly betaine, did not show comparable changes in these
enzymes. In spinach, subcellular localization positions SAHH and ADK in
the cytosol with the phospho-base N-methyltransferase
activities. Because SAHH activity is inhibited by its products, we
propose that ADK is not a stress-responsive enzyme per se, but plays a
pivotal role in sustaining transmethylation reactions in general by
serving as a coarse metabolic control to reduce the cellular
concentration of free adenosine. In support of this model, we grew
Arabidopsis under a short-day photoperiod that promotes secondary cell
wall development and found both ADK activity and transcript levels to
increase severalfold.
1
This work was supported by research grants from
the Natural Sciences and Engineering Research Council of Canada (to
E.A.W. and B.A.M.).
2
These authors contributed equally to this work.
*
Corresponding author; e-mail weretil{at}mcmaster.ca; fax
905-522-6066.
© 2001 American Society of Plant Physiologists
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