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Inorganic phosphate (Pi) is an essential nutrient for plants, and the low bioavailability of Pi in soils is often a limitation to growth and development. Consequently, plants have evolved a range of regulatory mechanisms to adapt to phosphorus‐starvation in order to optimise uptake and assimilation of Pi. Recently, significant progress has been made in elucidating these mechanisms. The coordinated expression of a large number of genes is important for many of these adaptations. Several global expression studies using microarray analysis have been conducted in Arabidopsis thaliana. These studies provide a valuable basis for the identification of new regulatory genes and promoter elements to further the understanding of Pi‐dependent gene regulation. With focus on the Arabidopsis transcriptome, we extract common findings that point to new groups of putative regulators, including the NAC, MYB, ethylene response factor/APETALA2, zinc‐finger, WRKY and CCAAT‐binding families. With a number of new discoveries of regulatory elements, a complex regulatory network is emerging. Some regulatory elements, e.g. the transcription factor PHR1 and the microRNA (miRNA) miR399 and associated factors are well documented, yet not fully understood, whereas other suggested components need further characterisation. Here, we evaluate the contribution of the regulatory elements to the P‐responses and present a model comprising factors directly or indirectly involved in transcriptional regulation and the role of miRNAs as regulators and long‐distance signals. A striking feature is a series of feedback loops and parallel mechanisms that can modify and attenuate responses. We suggest that these mechanisms are instrumental in providing an accurate response and in keeping P‐homeostasis. [ABSTRACT FROM AUTHOR]

Expression of selected genes in relation to phosphate (Pi) starvation and sugar sensing was studied in leaves of Arabidopsis. Excised leaf segments with different P status were supplied with combinations of Pi and sugars. Sugar-inducible genes, encoding β-amylase (β-AMY) and chalcone synthase (CHS), were also induced by P deficiency, and were more strongly regulated by sugars when leaf segments originated from P-starved plants. Furthermore, transcript levels of the P-starvation-inducible genes ACP5 (encoding an acid phosphatase), RNS1 (encoding a ribonuclease), and IPS1 (unknown function) increased in response to exogenously applied sugars. Supply of Pi to the leaf segments reversed both P-starvation-induced and sugar-induced gene expression. These interactions reveal a close relationship between P and sugar sensing. To differentiate between hexokinase-dependent and hexokinase-independent sugar sensing the effect of the glucose analogue 2-deoxyglucose and gene expression in the hexokinase-1 deficient mutant, gin2-1, were studied. Both β-AMY and CHS were induced by supplying sucrose to excised leaves but not by 2-deoxyglucose, confirming that these genes are regulated by hexokinase-independent sugar sensing. In the gin2-1 mutant both β-AMY and CHS responded clearly to P starvation excluding that hexokinase-1 mediates the response to P. Similarly, the P-responding genes, IPS1 and RNS1 were repressed by addition of Pi also in the gin2-1 mutant. In conclusion, several phosphate starvation-induced genes are also sugar-induced and hexokinase-independent sugar sensing in Arabidopsis is strongly intensified by phosphate starvation. [ABSTRACT FROM AUTHOR]

Expression of phosphate starvation inducible genes was studied during recovery from phosphate starvation of Arabidopsis thaliana. Genes analysed were ACP5 (encoding an acid phosphatase), RNS1 (encoding an RNase), At4 and IPS1 (both of unknown function), Pht1;7 and Pht2;1 (encoding phosphate transporters). Plants grown at limiting phosphate exhibited reduced growth rate and accumulated anthocyanins, soluble sugars, and starch. Re-supply of phosphate resulted in dramatic uptake of phosphate, increased growth rate and decreased levels of anthocyanins, soluble sugars, and starch in leaf tissue. In both shoots and roots re-supplied with Pi, transcript levels decreased rapidly, and first changes were observed within 30 min. These alterations in gene expression occurred before the content of carbohydrates decreased, indicating that transcriptional regulation was due to phosphate sensing, and not a secondary effect of carbon accumulation. The data reveal different response rates for individual genes and demonstrate that roots and shoots can differ with respect to both timing and genes responding. In general, the changes in transcriptional activity in roots preceded the changes in shoots. Furthermore, transcriptional regulation was observed in isolated roots and shoots. This implies that roots do not strictly require a signal from the shoot, and vice versa, in order to respond to phosphate starvation. [ABSTRACT FROM AUTHOR]

Phosphate (Pi) plays a central role as reactant and effector molecule in plant cell metabolism. However, Pi is the least accessible macronutrient in many ecosystems and its low availability often limits plant growth. Plants have evolved an array of molecular and morphological adaptations to cope with Pi limitation, which include dramatic changes in gene expression and root development to facilitate Pi acquisition and recycling. Although physiological responses to Pi starvation have been increasingly studied and understood, the initial molecular events that monitor and transmit information on external and internal Pi status remain to be elucidated in plants. This review summarizes molecular and developmental Pi starvation responses of higher plants and the evidence for coordinated regulation of gene expression, followed by a discussion of the potential involvement of plant hormones in Pi sensing and of molecular genetic approaches to elucidate plant signalling of low Pi availability. Complementary genetic strategies in Arabidopsis thaliana have been developed that are expected to identify components of plant signal transduction pathways involved in Pi sensing. Innovative screening methods utilize reporter gene constructs, conditional growth on organophosphates and the inhibitory properties of the Pi analogue phosphite, which hold the promise for significant advances in our understanding of the complex mechanisms by which plants regulate Pi-starvation responses. [ABSTRACT FROM AUTHOR]