Phosphate (Pi) and its analog phosphite (Phi) are acquired by plants via Pi transporters. Although the uptake and mobility of Phi and Pi are similar, there is no evidence suggesting that plants can utilize Phi as a sole source of phosphorus. Phi is also known to interfere with many of the Pi starvation responses in plants and yeast (Saccharomyces cerevisiae). In this study, effects of Phi on plant growth and coordinated expression of genes induced by Pi starvation were analyzed. Phi suppressed many of the Pi starvation responses that are commonly observed in plants. Enhanced root growth and root to shoot ratio, a hallmark of Pi stress response, was strongly inhibited by Phi. The negative effects of Phi were not obvious in plants supplemented with Pi. The expression of Pi starvation-induced genes such as LePT1, LePT2, AtPT1, and AtPT2 (high-affinity Pi transporters); LePS2 (a novel acid phosphatase); LePS3 and TPSI1 (novel genes); and PAP1 (purple acid phosphatase) was suppressed by Phi in plants and cell cultures. Expression of luciferase reporter gene driven by the Pi starvation-induced AtPT2 promoter was also suppressed by Phi. These analyses showed that suppression of Pi starvation-induced genes is an early response to addition of Phi. These data also provide evidence that Phi interferes with gene expression at the level of transcription. Synchronized suppression of multiple Pi starvation-induced genes by Phi points to its action on the early molecular events, probably signal transduction, in Pi starvation response.Phosphate (Pi) is one of the major plant nutrients that influences virtually all the biochemical processes and developmental phases of plants. In the absence of Pi, plants exhibit characteristic deficiency symptoms including anthocyanin accumulation, enhanced root growth, and increased root to shoot ratio. The ability of plants to acquire Pi also increases during this period. Molecular dissection of responses to Pi starvation has provided evidence for coordinated expression of genes, including Pi transporters (Raghothama, , 2000. Pi transporters are involved in acquiring Pi against the concentration gradient by an energy-mediated proton cotransport mechanism (Ullrich-Eberius et al., 1981). They are also known to transport ions such as arsenate, vanadate, and phosphite (Phi; Guest and Grant, 1991;Marschner, 1995).Phi (HPO 3 2Ϫ ), also referred to as phosphorous acid or phosphonate, is an isostere of the Pi anion in which one of the oxygens bound to the P atom is replaced by hydrogen. The term Phi is used here to describe the alkali metal salts of phosphorous acid as suggested by Carswell et al. (1996). Phi is used extensively as a fungicide and also sold as a superior source of Pi (Guest and Grant, 1991;Rickard, 2000; McDonald et al., 2001a). Phi is rapidly absorbed and translocated within the plant (Guest and Grant, 1991). The uptake is pH dependent and subject to competition by Pi (Ouimette and Coffey, 1990). Furthermore, mobility of Phi in both xylem and phloem is similar to that of Pi (Ouimette...