Induction of an Extracellular Ribonuclease in Cultured Tomato Cells upon Phosphate Starvation

Abstract

Suspension-cultured cells of tomato (Lycopersicon esculentum) start to secrete an RNA-degrading enzyme activity during transition from logarithmic to stationary growth phase. Using affinity chromatography on agarose-5-(4-aminophenyl-phosphoryl) uridine 3'(2') monophosphate as a powerful and final enrichment step, the enzyme was purified to homogeneity and characterized as ribonuclease I (RNase I) according to the following data: (a) it has an Mr of 22,000 (sodium dodecyl sulfatepolyacrylamide gel electrophoresis), a pH-optimum of pH 5.5, a pi of 3.9, and its activity was found to be insensitive to EDTA; (b) the enzyme splits single-stranded RNA endonucleolytically by a phosphotransferase reaction yielding 2',3'-cNMPs as primary monomeric products; (c) as studied with diribonucleoside monophosphates as substrates, the enzyme exhibits a pronounced preference for 5' purine residues adjacent to the cleavage site.

Attenuation of Phosphate Starvation Responses by Phosphite in Arabidopsis

“…Biochemical adaptations to Pi starvation include increased synthesis of anthocyanins, presumably to adjust photosynthetic light reactions to the Pi-dependent Calvin cycle, and increased synthesis of enzymes for scavenging intra-and extracellular phosphorus (Trull et al, 1997;Bosse and Kö ck, 1998;Raghothama, 1999). We previously demonstrated that Pi starvation coordinately induces ribonuclease, phosphodiesterase, and acid phosphatase activities in tomato (Lycopersicon esculentum) cell suspension cultures, which sequentially participate in the complete hydrolysis of exogenous RNA to nucleosides and Pi (Nü rnberger et al, 1990;Abel et al, 2000). Here, we found that Phi effectively prevents accumulation of anthocyanins and represses these nucleolytic enzymes in Pi limiting conditions at concentrations that are not growth inhibitory.…”

Phosphate or phosphite addition promotes the proteolytic turnover of phosphate‐starvation inducible tomato purple acid phosphatase isozymes

“…Confirmation that SAP1 and SAP2 are their genuine physiological substrates awaits the purification and kinetic characterization of both proteases. It will also be of interest to assess whether extracellular Pi-inducible proteases degrade additional hydrolytic enzymes that are specifically secreted into the CCF by )Pi tomato cell cultures, namely PSI ribonucleases and phosphodiesterases [2,14,20,21]. Although the functional importance of Pi-inducible secreted proteases in response to Pi-sensing and turnover of extracellular PSI proteins remains to be determined, the current study presents the first evidence for protease upregulation in response to Pi-resupply of )Pi plants.…”

“…SDS-PAGE analyses have revealed that the CCF of +Pi, )Pi, and Pi-resupplied tomato suspension cells contain up to 50 different polypeptides, several of which are unique to )Pi cultures [20,24] (also this study, not shown). This suggests that the pair of Pi-inducible secreted serine protease isoforms of cultured tomato cells (Fig.…”

“…tomato suspension cells Pi starvation of cultured tomato cells elicits the secretion of PSI ribonucleases, phosphodiesterases and APs which collectively facilitate Pi scavenging from extracellular nucleic acids [1,2,8,9,14,20,21]. Conversely, the coordinated repression of a PSI gene (LEPS2) and degradation of the corresponding mRNA transcripts occurred within 12 h of Pi-resupply to )Pi tomato seedlings [22].…”

Physiological and Biochemical Mechanisms of Plant Adaptation to Low-Fertility Acid Soils of the Tropics: The Case of Brachiariagrasses

“…12). Nürnberger et al (1990) and Löffler et al (1992) showed that both extracellular and intracellular RNase activities were induced in tomato-cell culture under P deficiency. Extracellular RNase could help degrade the RNA from senescing cells that have been either damaged or lysed, and also help degrade any RNA that might be present in the rhizosphere.…”