Plant Physiol, February 2000, Vol. 122, pp. 543-552
Induction of an Extracellular Cyclic Nucleotide Phosphodiesterase
as an Accessory Ribonucleolytic Activity during Phosphate Starvation of
Cultured Tomato Cells
Steffen
Abel,*
Thorsten
Nürnberger,
Volker
Ahnert,
Gerd-Joachim
Krauss, and
Konrad
Glund
Department of Vegetable Crops, University of California, Davis,
California 95616 (S.A.); Institute of Plant Biochemistry, D-06120
Halle, Germany (T.N.); Department of Biochemistry and
Biotechnology, Martin-Luther-University Halle-Wittenberg, D-06120
Halle, Germany (V.A., G.-J.K.); Probiodrug Gesellschaft für
Arzneimittelforschung mbH, D-06120 Halle, Germany (K.G.).
During growth under conditions of
phosphate limitation, suspension-cultured cells of tomato
(Lycopersicon esculentum Mill.) secrete
phosphodiesterase activity in a similar fashion to phosphate starvation-inducible ribonuclease (RNase LE), a cyclizing
endoribonuclease that generates 2':3'-cyclic nucleoside monophosphates
(NMP) as its major monomeric products (T. Nürnberger, S. Abel, W. Jost, K. Glund [1990] Plant Physiol 92: 970-976). Tomato
extracellular phosphodiesterase was purified to homogeneity from the
spent culture medium of phosphate-starved cells and was characterized
as a cyclic nucleotide phosphodiesterase. The purified enzyme has a
molecular mass of 70 kD, a pH optimum of 6.2, and an isoelectric point
of 8.1. The phosphodiesterase preparation is free of any detectable deoxyribonuclease, ribonuclease, and nucleotidase activity. Tomato extracellular phosphodiesterase is insensitive to EDTA and hydrolyzes with no apparent base specificity 2':3'-cyclic NMP to 3'-NMP and the
3':5'-cyclic isomers to a mixture of 3'-NMP and 5'-NMP. Specific activities of the enzyme are 2-fold higher for 2':3'-cyclic NMP than
for 3':5'-cyclic isomers. Analysis of monomeric products of sequential
RNA hydrolysis with purified RNase LE, purified extracellular
phosphodiesterase, and cleared 
Pi culture medium as a source of
3'-nucleotidase activity indicates that cyclic nucleotide
phosphodiesterase functions as an accessory ribonucleolytic activity
that effectively hydrolyzes primary products of RNase LE to substrates
for phosphate-starvation-inducible phosphomonoesterases. Biosynthetical
labeling of cyclic nucleotide phopshodiesterase upon phosphate
starvation suggests de novo synthesis and secretion of a set of
nucleolytic enzymes for scavenging phosphate from extracellular RNA substrates.
*
Corresponding author; e-mail sabel{at}ucdavis.edu; fax
530-752-9659.
© 2000 American Society of Plant Physiologists
