Induction of an extracellular cyclic nucleotide phosphodiesterase as an accessory ribonucleolytic activity during phosphate starvation of cultured tomato cells

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 ribonucleas e (RNase LE), a cyclizing endoribonuclease that generates 2':3'-cyclic nucl eoside monophosphates (NMP) as its major monomeric products (T. Nurnberger, S. Abel, W. Jost, K. Glund [1990] Plant Physiol 92: 970-976). Tomato extra cellular phosphodiesterase was purified to homogeneity from the spent cultu re medium of phosphate-starved cells and was characterized as a cyclic nucl eotide 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 phosphodiestera se preparation is free of any detectable deoxyribonuclease, ribonuclease, a nd nucleotidase activity. Tomato extracellular phosphodiesterase is insensi tive 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'-NM P. 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 sequentia l RNA hydrolysis with purified RNase LE, purified extracellular phosphodies terase, and cleared -Pi culture medium as a source of 3'-nucleotidase activ ity indicates that cyclic nucleotide phosphodiesterase functions as an acce ssory ribonucleolytic activity that effectively hydrolyzes primary products of RNase LE to substrates for phosphate-starvation-inducible phosphomonoes terases. Biosynthetical labeling of cyclic nucleotide phopshodiesterase upo n phosphate starvation suggests de novo synthesis and secretion of a set of nucleolytic enzymes for scavenging phosphate from extracellular RNA substr ates.