Phosphorus deficiency is one of the major abiotic stresses affecting plant growth. Plants respond to the persistent deficiency of phosphate (Pi) by coordinating the expression of genes involved in alleviation of the stress. The high-affinity Pi transporters are among the major molecular determinants that are activated during Pi stress. In this study, using three reporter genes (green fluorescent protein, luciferase, and β-glucuronidase) regulated by two Pi transporter promoters, we have carried out an extensive analysis of transcriptional and spatial regulation of gene expression. Activation of the genes was rapid, repressible, and specific in response to changes in Pi availability. The phytohormones auxin and cytokinin suppressed the expression of the reporter gene driven by the AtPT1promoter, and that of the native gene, suggesting that hormones may be involved in regulation of some component(s) of Pi starvation response pathway. These studies also provide molecular evidence for a potential role of high-affinity Pi transporters in mobilizing Pi into reproductive organs. The results suggest that members of the Pi transporter family may have similar but nonredundant functions in plants.

Phosphate (Pi) is one of the least available plant nutrients in soils. It is associated with dynamic changes in carbon fluxes and several crucial processes that regulate plant growth and development. Pi levels regulate the expression of large number of genes including those involved in photosynthesis and carbon metabolism. Herein we show that sugar is required for Pi starvation responses including changes in root architecture and expression of phosphate starvation induced (PSI) genes in Arabidopsis. Active photosynthesis or the supplementation of sugar in the medium was essential for the expression of PSI genes under Pi limiting conditions. Expression of these genes was not only induced by sucrose but also detected, albeit at reduced levels, with other metabolizable sugars. Non-metabolizable sugar analogs did not induce the expression of PSI genes. Although sugar input appears to be down-stream of initial Pi sensing, it is absolutely required for the completion of the PSI signaling pathway. Altered expression of PSI genes in the hexokinase signaling mutant gin2 indicates that hexokinase-dependent signaling is involved in this process. The study provides evidence for requirement of sugars in PSI signaling and evokes a role for hexokinase in some components of Pi response mechanism.

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...

Phosphate (Pi) deficiency is a major nutritional problem faced by plants in many agro-ecosystems. This deficiency results in altered gene expression leading to physiological and morphological changes in plants. Altered gene expression is presumed to be due to interaction of regulatory sequences (cis-elements) present in the promoters with DNA binding factors (trans-factors). In this study, we analyzed the expression and DNA-protein interaction of promoter regions of Pi starvationinduced genes AtPT2 and TPSI1. AtPT2 encodes the high-affinity Pi transporter in Arabidopsis, whereas TPSI1 codes for a novel gene induced in the Pi-starved tomato (Lycopersicon esculentum). Expression of AtPT2 was induced rapidly under Pi deficiency and increased with decreasing concentrations of Pi. Abiotic stresses except Pi starvation had no affect on the expression of TPSI1. DNA mobility-shift assays indicated that specific sequences of AtPT2 and TPSI1 promoter interact with nuclear protein factors. Two regions of AtPT2 and TPSI1 promoters specifically bound nuclear protein factors from Pi-sufficient plants. Interestingly, the DNA binding activity disappeared during Pi starvation, leading to the hypothesis that Pi starvation-induced genes may be under negative regulation.Crop productivity is often limited by phosphate (Pi) availability (Barber, 1980). This limitation initiates a series of physiological and genetic changes leading to increased survival of plants under these conditions (Raghothama, 2000). Altered morphology, physiology, and biochemical pathways allow plants to cope with the nutrient deficiency (Plaxton and Carswell, 1999). It is becoming clear that a coordinated expression of genes during Pi starvation is the underlying factor in all these responses. Many genes including Pi transporters, phosphatases, RNases, ␤-glucosidase, and others of unknown function are induced during Pi starvation . Interestingly, the deficiency of Pi sets these molecular events in motion. One can envision that the Pi starvation-mediated signaling pathway results in specific interactions of trans-factors with conserved cis-elements in Pi deficiency-induced genes. This type of interaction is the basis for activation of suites of genes involved in Pi starvation rescue mechanism in yeast and bacteria.In yeast, both positive and negative regulatory elements control the expression of Pi starvationinduced genes (Oshima, 1997). A key positive regulator, Pho4, controls the expression of multiple genes including phosphatases and Pi transporters. Pho4 has an amphipathic helix-loop-helix-type DNA binding domain that interacts with specific promoter sequences of PHO genes (Okamura et al., 2000). Under Pi sufficiency a complex of two negative regulators, Pho80 (cyclin) and Pho85 (cyclin-dependent protein kinase), render Pho4 inactive by hyper-phosphorylation, thus preventing its nuclear localization (O'Neill et al., 1996). During Pi deficiency the Pho81 another member of PHO regulon, inhibits the function of the Pho80/Pho85 complex, thus allowing Pho4 to int...