Extracellular administration of phosphate-sequestering metabolites induces ribonucleases in cultured tomato cells

Abstract.

In cultivated tomato (Lycopersicon esculentum Mill. cv. Lukullus) cells, two ribonucleases (EC 3.1.27.1), extracellular RNase LE and intracellular RNase LX, are induced in response to orthophosphate (Pi) depletion of the cultivation medium. To study the effect of intracellular Pi-pools on the induction process, phosphate-supplied cells were incubated with D-mannose and other metabolites which are known to sequester Pi intracellularly into organic compounds, thus depleting cytoplasmic Pi-pools. By measuring transcript levels, we show here (i) that the induction occurred in the simultaneous presence of phosphate and D-mannose, 2-deoxy-D-glucose, D-galactose, or glycerol, and (ii) that the induction process is transient as shown by a decline in the signals with time. We conclude that the intracellular Pi-pool temporarily decreased, thus activating a signal-transduction chain leading to the induction of phosphate-starvation-responsive proteins.