Negative Regulation of Phosphate Starvation-Induced Genes

Abstract

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...

Phosphate Acquisition

“…In Arabidopsis thaliana nine highaffinity Pi transporter genes have been identified and characterized (Karthikeyan et al, 2002;Muchhal et al, 1996;Mudge et al, 2002;Okumura et al, 1998;Smith et al, 1997). There is growing evidence suggesting that plants in general have a small family of high-affinity Pi transporters (Muchhal et al, 1996;Mudge et al, 2002;Mukatira et al, 2001;Uta et al, 2002).…”

“…Conserved MYB-factor-binding sequences have been identified in promoter regions of several P-starvation-inducible genes. In addition DNA-protein interaction studies revealed the existence of specific protein-binding domains (cis elements) in P-deficiency-induced genes including a Pi transporter (Mukatira et al, 2001). The DNA protein interaction was observed only under Pi sufficiency conditions, suggesting that some of the Pi-starvation-induced genes may be under negative regulation.…”

Molecular mechanisms of phosphate transport in plants

“…Unlike Pho4 in yeast (Kaffman and O'Shea 1999) and NUC-1 from N. crassa (Peleg et al 1996), the two putative transcriptional activators, Psr1 from Chlamydomonas and PHR1 from Arabidopsis, were shown to be permanently present in the nucleus (Wykoff et al 1999;Rubio et al 2001), thus suggesting a different type of Pi-starvation-dependent gene regulation than in yeast, maybe involving heterodimeric interactions between PHR1 and other MYB/CC proteins (Rubio et al 2001). Further evidence for a different type of regulation in plants was provided by Mukatira et al (2001). The authors could show by DNA mobility-shift assays that specific promoter sequences of two Pi-starvation-responsive genes interact with nuclear protein factors from Pi-sufficient plants, while the DNA binding activity disappeared during Pi starvation.…”

Genetic Determinants of Phosphate Use Efficiency in Crops

“…Specifi c interactions between the ARAthPht1;4 promoter and nuclear protein factors obtained from Pisuffi cient plants suggest that Pi -responsive genes are under a negative regulation (Mukatira et al, 2001 ). SPX domain (SYG1/Pho81/XPHR1) has been strongly related with signaling transduction pathway in Saccharomyces cerevisiae , Neurospora crassa , mouse, and human (Spain et al, 1995 ;Lenburg and O ' Shea, 1996 ;Peleg et al, 1996 ;Battini et al, 1999 ).…”

“…It was proposed that AtSPX3 represses ARA;Pht1;4 and modulates Pi allocation, since spx3 plants show an enhanced expression of this transporter and higher Pi shoot concentrations (Duan et al, 2008 ). In rice, suppression of OsSPX1 resulted in the upregulation of OsPT2 and OsPT8 transporter genes under Pi -suffi cient conditions, leading to alter Pi uptake (Wang et al, 2009 ;Mukatira et al, 2001 ;Miura et al, 2005 ;Chen et al, 2008 ;Duan et al, 2008 ;Wang et al, 2009 ). The existence of negative mechanisms of regulation, however, does not preclude the action of a positive transcriptional control, but suggests a mechanism fi nely coordinated to maintain adequate cellular Pi levels, particularly when this nutrient is limited.…”