Two stimuli that have been associated with nutrient remobilization in plants are phosphate (P(i)) starvation and senescence. Little is known about how the nutrient remobilization machinery is induced at the molecular level, but in the case of P(i) starvation, ribonucleases are considered to play important roles in the remobilization process. Here, the control of two closely related ribonuclease genes of Arabidopsis, RNS1 and RNS3 is investigated. The RNS1 gene is sharply induced during starvation for P(i), an effect specific among the major macronutrients, whereas RNS3 transcript levels remain relatively constant. RNS1 and RNS3 produced in yeast co-migrate with Arabidopsis ribonuclease activities that exhibit the same induction properties as the transcripts in both wild-type plants and the pho1 mutant, which is defective in xylem loading of P(i). In contrast to what occurs during P(i) starvation, both RNS1 and RNS3 are modestly induced during senescence, indicating that the two stimuli could trigger different signal transduction pathways. The characterization of RNS1, in particular, provides an important first step towards elucidating the mechanisms by which plants sense and respond to P(i) limitation, a prominent condition in many soil types.