Regulation of phosphate starvation responses in higher plants

Background

Phosphorus (P) is often a limiting mineral nutrient for plant growth. Many soils worldwide are deficient in soluble inorganic phosphate (P_i), the form of P most readily absorbed and utilized by plants. A network of elaborate developmental and biochemical adaptations has evolved in plants to enhance P_i acquisition and avoid starvation.

Scope

Controlling the deployment of adaptations used by plants to avoid P_i starvation requires a sophisticated sensing and regulatory system that can integrate external and internal information regarding P_i availability. In this review, the current knowledge of the regulatory mechanisms that control P_i starvation responses and the local and long-distance signals that may trigger P_i starvation responses are discussed. Uncharacterized mutants that have P_i-related phenotypes and their potential to give us additional insights into regulatory pathways and P_i starvation-induced signalling are also highlighted and assessed.

Conclusions

An impressive list of factors that regulate P_i starvation responses is now available, as is a good deal of knowledge regarding the local and long-distance signals that allow a plant to sense and respond to P_i availability. However, we are only beginning to understand how these factors and signals are integrated with one another in a regulatory web able to control the range of responses demonstrated by plants grown in low P_i environments. Much more knowledge is needed in this agronomically important area before real gains can be made in improving P_i acquisition in crop plants.     

Regulation of S-Like Ribonuclease Levels in Arabidopsis. Antisense Inhibition of RNS1 or RNS2 Elevates Anthocyanin Accumulation

The S-like ribonucleases (RNases) RNS1 and RNS2 of Arabidopsis are members of the widespread T₂ ribonuclease family, whose members also include the S-RNases, involved in gametophytic self-incompatibility in plants. Both RNS1 and RNS2 mRNAs have been shown previously to be induced by inorganic phosphate (Pi) starvation. In our study we examined this regulation at the protein level and determined the effects of diminishing RNS1 and RNS2 expression using antisense techniques. The Pi-starvation control of RNS1 and RNS2 was confirmed using antibodies specific for each protein. These specific antibodies also demonstrated that RNS1 is secreted, whereas RNS2 is intracellular. By introducing antisense constructs, mRNA accumulation was inhibited by up to 90% for RNS1 and up to 65% for RNS2. These plants contained abnormally high levels of anthocyanins, the production of which is often associated with several forms of stress, including Pi starvation. This effect demonstrates that diminishing the amounts of either RNS1 or RNS2 leads to effects that cannot be compensated for by the actions of other RNases, even though Arabidopsis contains a large number of different RNase activities. These results, together with the differential localization of the proteins, imply that RNS1 and RNS2 have distinct functions in the plant.     

Regulation of bacterial phosphate taxis and polyphosphate accumulation in response to phosphate starvation stress

Phosphorus (P) is an essential constituent in all types of living organisms. Bacteria, which use inorganic phosphate (P_i), as the preferred P source, have evolved complex systems to survive during P_i starvation conditions. Recently, we found thatPseudomonas aeruginosa, a monoflagellated, obligately aerobic bacterium, is attracted to P_i. The evidence that the chemotactic response to P_i (P_i taxis) was observed only with cells grown in P_i-limiting medium suggests that P_i taxis plays an important role in scavenging P_i residues under conditions of P_i starvation. Many bacteria also exhibit rapid and extensive accumulation of polyphosphate (polyP), when P_i is added to cells previously subjected to P_i starvation stress. Since polyP can serve as a P source during P_i starvation conditions, it is likely that polyP accumulation is a protective mechanism for survival during P_i starvation. In the present review, we summarize our current knowledge on regulation of bacterial P_i taxis and polyP accumulation in response to P_i starvation stress.     

Kinetics of phosphate absorption by mycorrhizal and non-mycorrhizal Scots pine seedlings

Kinetics of net phosphate (P_i) uptake was measured on intact ectomycorrhizal and non‐mycorrhizal Pinus sylvestris seedlings using a semihydroponic cultivation method. The depletion of P_i in a nutrient solution was assessed over a 160–0.2 μM P_i gradient. Growth of the pine seedlings was P limited and measurements were performed 7 and 9 weeks after inoculation. Three ectomycorrhizal fungi were studied: Paxillus involutus, Suillus bovinus and Thelephoraterrestris. P_i uptake was extremely fast in plants colonised by P. involutus. The P_i concentration dropped below 0.2 μM within 4–5 h. In plants colonised with S. bovinus this occurred in 5–6 h and in plants associated with T. terrestris 8 h were needed to run through the whole concentration range. Non‐mycorrhizal plants of similar size and nutrient status decreased P_i to a concentration between 1 and 2 μM in 18 h. Data were curve fitted to a two‐phase Michaelis‐Menten equation. The apparent kinetic constants, K_m and V_max, for the high affinity P_i uptake system of the pine roots could be estimated accurately. V_max of this system was up to 7 times higher in pines associated with P. involutus than in non‐mycorrhizal seedlings. The intact extraradical mycelium greatly increased the absorption surface area of the roots (V_max). Non‐mycorrhizal plants had a K_m between 7.8 and 16.4 μM P_i. Plants mycorrhizal with P. involutus had K_m values between 2.4 and 7.2, plants colonised with S. bovinus had a K_m between 5.1 and 12.3, and seedlings associated with T. terrestris had a K_m from 4.6 to 10.1 μM P_i. All 3 ectomycorrhizal fungi had a strong impact on the P_i absorption capacity of the pine seedlings. The results also demonstrated that there is substantial heterogeneity in kinetic parameters among the different mycorrhizal root systems.     

Characterization of a monomeric heat-labile classical alkaline phosphatase from <Emphasis Type="Italic">Anabaena</Emphasis> sp. PCC7120

Alkaline phosphatases (APs), known inducible enzymes of the Pho regulon and poorly characterized in cyanobacteria, hydrolyze phosphomonoesters to produce inorganic phosphate (P_i) during P_i starvation. In this study, two predicted alkaline phosphatase genes in the genome of Anabaena sp. PCC 7120, all2843 and alr5291, were apparently induced during P_i starvation. Sequence analysis showed that alr5291 encodes a protein that is an atypical alkaline phosphatase like other cyanobacteria PhoAs, but the protein encoded by all2843 is very similar to the classical PhoAs, such as Escherichia coli alkaline phosphatase (EAP). To date, there have been no reports about classical phoA in cyanobacterial genomes. The alkaline phosphatase AP^A, coded by all2843, is characterized as a metalloenzyme containing Mg²⁺ and Zn²⁺ with molar ratio of 1: 2. Site-directed mutagenesis analysis indicated that, though the active center of AP^A is highly conserved in comparison with EAP, differences do exist between AP^A and EAP in metal ion coordination. Besides, biochemical analysis revealed that AP^A is a monomeric protein and inactivated rapidly at 50°C. These results suggest that AP^A is the first monomeric heat-labile classical PhoA found in cyanobacteria.     

Response of Alkaline Phosphatases in the Cyanobacterium Anabaena sp. FACHB 709 to Inorganic Phosphate Starvation

Alkaline phosphatases (APases) play a crucial role in phosphorus (P) metabolism and regulation, but their physiological functions largely remain unclear in cyanobacteria. Here, we identified four putative APase genes, designated as phoA-709, phoD1-709, phoD2-709, and phoS-709, in the cyanobacterium Anabaena sp. FACHB 709, and investigated their response to inorganic phosphate (P_i) starvation. With the exception of phoD2-709, three other APase genes were expressed at a constant and relative low level in P_i-replete medium, whereas the expression of all four APase genes was elevated in response to P_i starvation but phoA-709 significantly. However, disruption of phoA-709 did not affect the total APase activity but caused the expressional up-regulation of phoD1-709 and phoS-709 under P_i-sufficient and P_i-limiting conditions. These suggest that, the four APases of Anabaena sp. FACHB 709 are involved in P metabolism and regulation, and PhoA-709 is the main, yet dispensable, APase.     

Short-term effects of boron, germanium and high light intensity on membrane permeability in boron deficient leaves of sunflower

With the ultimate purpose of clarifying the mechanism for aluminium (Al) toxicity and for Al tolerance, we tried to isolate cDNAs whose expression is induced by Al treatment and phosphate (P_i) starvation. We performed P_i starvation and Al treatment (two-step treatment) on suspension-cultured cells of Nicotiana tabacum L. cv. Samsun and then constructed a cDNA library using poly(A)⁺-RNA derived from the treated cells. Four independent cDNA clones (pAL 111, 139, 141 and 142) were isolated from the library by differential screening. Northern blot hybridization analysis indicated that the expression of these clones was induced by P_i starvation. Furthermore, we found that pAL 111 and pAL 142 are also induced by Al treatment. The complete cDNA sequencing of these 4 clones was determined. The results indicated that pAL111 is identical to the parA gene of N. tabacum, which is described as an auxin-regulated gene and that pAL142 is highly homologous to the parB gene of N. tabacum whose product has glutathione S-transferase (GST, EC 2.5.1.18) activity. Furthermore, we found a cysteine-rich domain in the amino acid sequence of pAL139. No DNA and deduced amino acid sequences homologous to the pAL141 were found.     

The type II secretion system (Xcp) of Pseudomonas putida is active and involved in the secretion of phosphatases

The genome of the Gram‐negative bacterium Pseudomonas putida harbours a complete set of xcp genes for a type II protein secretion system (T2SS). This study shows that expression of these genes is induced under inorganic phosphate (P_i) limitation and that the system enables the utilization of various organic phosphate sources. A phosphatase of the PhoX family, previously designated UxpB, was identified, which was produced under low P_i conditions and transported across the cell envelope in an Xcp‐dependent manner demonstrating that the xcp genes encode an active T2SS. The signal sequence of UxpB contains a twin‐arginine translocation (Tat) motif as well as a lipobox, and both processing by leader peptidase II and Tat dependency were experimentally confirmed. Two different tat gene clusters were detected in the P. putida genome, of which one, named tat‐1, is located adjacent to the uxpB and xcp genes. Both Tat systems appeared to be capable of transporting the UxpB protein. However, expression of the tat‐1 genes was strongly induced by low P_i levels, indicating a function of this system in survival during P_i starvation.     

Transgenic rice with low sucrose-phosphate synthase activities retain more soluble protein and chlorophyll during flag leaf senescence

We investigated whether changes in sucrose-phosphate synthase (EC 2.4.1.14, SPS) activity could alter N remobilization during leaf senescence. Transgenic rice (Oryza sativa L. cv. Nipponbare) with low SPS activities and wild-type rice plants were grown with basal N (1.0 mM NH₄NO₃) until the late vegetative stage. Subsequently, half of the plants were transferred to a low N (0.1 mM NH₄NO₃) condition to accelerate leaf senescence, and the others were continuously grown with basal N. With low N supply, the amounts of chlorophyll and soluble protein in flag leaf blades decreased after anthesis in both the low SPS plants and wild-type plants, although the decrease was less in the low SPS plants. Panicle weights were significantly lower in the low SPS plant than in the wild-type plant. These results suggest that the remobilization of N from flag leaves was diminished by suppressing the development of reproductive sinks in the low SPS plant.