Arabidopsis pdr2 reveals a phosphate-sensitive checkpoint in root development

Plants have evolved complex strategies to maintain phosphate (Pi) homeostasis and to maximize Pi acquisition when the macronutrient is limiting. Adjustment of root system architecture via changes in meristem initiation and activity is integral to the acclimation process. However, the mechanisms that monitor external Pi status and interpret the nutritional signal remain to be elucidated. Here, we present evidence that the Pi deficiency response, pdr2, mutation disrupts local Pi sensing. The sensitivity and amplitude of metabolic Pi-starvation responses, such as Pi-responsive gene expression or accumulation of anthocyanins and starch, are enhanced in pdr2 seedlings. However, the most conspicuous alteration of pdr2 is a conditional short-root phenotype that is specific for Pi deficiency and caused by selective inhibition of root cell division followed by cell death below a threshold concentration of about 0.1 mm external Pi. Measurements of general Pi uptake and of total phosphorus (P) in root tips exclude a defect in high-affinity Pi acquisition. Rescue of root meristem activity in Pi-starved pdr2 by phosphite (Phi), a non-metabolizable Pi analog, and divided-root experiments suggest that pdr2 disrupts sensing of low external Pi availability. Thus, PDR2 is proposed to function at a Pi-sensitive checkpoint in root development, which monitors environmental Pi status, maintains and fine-tunes meristematic activity, and finally adjusts root system architecture to maximize Pi acquisition.     

Root architecture remodeling induced by phosphate starvation

Plants have evolved efficient strategies for utilizing nutrients in the soil in order to survive, grow and reproduce. Inorganic phosphate (Pi) is a major macroelement source for plant growth; however, the availability and distribution of Pi are varying widely across locations. Thus, plants in many areas experience Pi deficiency. To maintain cellular Pi homeostasis, plants have developed a series of adaptive responses to facilitate external Pi acquisition, limit Pi consumption and adjust Pi recycling internally under Pi starvation conditions. This review focuses on the molecular regulators that modulate Pi starvation-induced root architectural changes.Key words: auxin, phosphate deficiency, root system architecture modulation     

Ethylene's role in phosphate starvation signaling: more than just a root growth regulator

Phosphate (Pi) is a common limiter of plant growth due to its low availability in most soils. Plants have evolved elaborate mechanisms for sensing Pi deficiency and for initiating adaptive responses to low Pi conditions. Pi signaling pathways are modulated by both local and long-distance, or systemic, sensing mechanisms. Local sensing of low Pi initiates major root developmental changes aimed at enhancing Pi acquisition, whereas systemic sensing governs pathways that modulate expression of numerous genes encoding factors involved in Pi transport and distribution. The gaseous phytohormone ethylene has been shown to play an integral role in regulating local, root developmental responses to Pi deficiency. Comparatively, a role for ethylene in systemic Pi signaling has been more circumstantial. However, recent studies have revealed that ethylene acts to modulate a number of systemically controlled Pi starvation responses. Herein we highlight the findings from these studies and offer a model for how ethylene biosynthesis and responsiveness are integrated into both local and systemic Pi signaling pathways.     

Strigolactones activate different hormonal pathways for regulation of root development in response to phosphate growth conditions

Background

Strigolactones (SLs) – a group of plant hormones and their derivatives – have been found to play a role in the regulation of root development, in addition to their role in suppression of lateral shoot branching: they alter root architecture and affect root-hair elongation, and SL signalling is necessary for the root response to low phosphate (Pi) conditions. These effects of SLs have been shown to be associated with differential activation of the auxin and ethylene signalling pathways.

Scope

The present review highlights recent findings on the activity of SLs as regulators of root development, in particular in response to low Pi stress, and discusses the different hormonal networks putatively acting with SLs in the root''s Pi response.

Conclusions

SLs are suggested to be key regulators of the adaptive responses to low Pi in the root by modulating the balance between auxin and ethylene signalling. Consequently, they impact different developmental programmes responsible for the changes in root system architecture under differential Pi supply.     

Phosphorus compartmentation in Pinus serotina Michx. (pond pine); observations from in vivo nuclear magnetic resonance spectroscopy

³¹P nuclear magnetic resonance (NMR) spectroscopy was used to estimate the amount of inorganic phosphate (Pi) present in the cytoplasm and vacuole of root tips and subapical root segments of pond pine ( Pinus serotina Michx.). In root tips of seedlings grown with 100 mmol m^–3P (HP) the cytoplasmic Pi content, on a root volume basis, was ≈ 1·5 μ mol cm^–3 and the vacuolar Pi content, on a root volume basis, was ≈ 3·4 μ mol cm^–3. In root tips from Pi starved seedlings the cytoplasmic Pi content, on a root volume basis, was ≈ 0·75 μ mol cm^–3; vacuolar Pi was too low to be reliably estimated. Similar results were obtained with subapical root segments; the Pi concentration in the cytoplasm was maintained at around 2 mol m^–3 while that in the vacuole varied with Pi supply. This work demonstrates for the first time that quantitative measurements of the subcellular compartmentation of Pi can be made in young tissues of a woody species. The results indicate that cytoplasmic Pi levels are maintained across a range of external Pi supplies probably by withdrawing Pi stored in the vacuole.     

HPS4/SABRE regulates plant responses to phosphate starvation through antagonistic interaction with ethylene signalling

The phytohormone ethylene plays important roles in regulating plant responses to phosphate (Pi) starvation. To date, however, no molecular components have been identified that interact with ethylene signalling in regulating such responses. In this work, an Arabidopsis mutant, hps4, was characterized that exhibits enhanced responses to Pi starvation, including increased inhibition of primary root growth, enhanced expression of Pi starvation-induced genes, and overproduction of root-associated acid phosphatases. Molecular cloning indicated that hps4 is a new allele of SABRE, which was previously identified as an important regulator of cell expansion in Arabidopsis. HPS4/SABRE antagonistically interacts with ethylene signalling to regulate plant responses to Pi starvation. Furthermore, it is shown that Pi-starved hps4 mutants accumulate more auxin in their root tips than the wild type, which may explain the increased inhibition of their primary root growth when grown under Pi deficiency.     

Different effects of phospholipase Dζ2 and non‐specific phospholipase C4 on lipid remodeling and root hair growth in Arabidopsis response to phosphate deficiency

Phosphate (Pi) deficiency in soils is a major limiting factor for plant growth. In response to Pi deprivation, one prominent metabolic adaptation in plants is the decrease in membrane phospholipids that consume approximately one‐third cellular Pi. The level of two phospholipid‐hydrolyzing enzymes, phospholipase Dζ2 (PLDζ2) and non‐specific phospholipase C4 (NPC4), is highly induced in Pi‐deprived Arabidopsis. To determine the role of PLDζ2 and NPC4 in plant growth under Pi limitation, Arabidopsis plants deficient in both PLDζ2 and NPC4 (npc4pldζ2) were generated and characterized. Lipid remodeling in leaves and roots was analyzed at three different durations of Pi deficiency. NPC4 affected lipid changes mainly in roots at an early stage of Pi deprivation, whereas PLDζ2 exhibited a more overt effect on lipid remodeling in leaves at a later stage of Pi deprivation. Pi deficiency‐induced galactolipid increase and phospholipid decrease were impeded in pldζ2 and npc4pldζ2 plants. In addition, seedlings of npc4pldζ2 had the same root hair density as pldζ2 but shorter root hair length than pldζ2 in response to Pi deficiency. The loss of NPC4 decreased root hair length but had no effect on root hair density. These data suggest that PLDζ2 and NPC4 mediate the Pi deprivation‐induced lipid remodeling in a tissue‐ and time‐specific manner. PLDζ2 and NPC4 have distinctively different roles in root hair growth and development in response to Pi deprivation; PLDζ2 negatively modulates root hair density and length, whereas NPC4 promotes root hair elongation.     

Differential effects of sucrose and auxin on localized phosphate deficiency-induced modulation of different traits of root system architecture in Arabidopsis

Phosphorus, one of the essential elements for plants, is often a limiting nutrient in soils. Low phosphate (Pi) availability induces sugar-dependent systemic expression of genes and modulates the root system architecture (RSA). Here, we present the differential effects of sucrose (Suc) and auxin on the Pi deficiency responses of the primary and lateral roots of Arabidopsis (Arabidopsis thaliana). Inhibition of primary root growth and loss of meristematic activity were evident in seedlings grown under Pi deficiency with or without Suc. Although auxin supplementation also inhibited primary root growth, loss of meristematic activity was observed specifically under Pi deficiency with or without Suc. The results suggested that Suc and auxin do not influence the mechanism involved in localized Pi sensing that regulates growth of the primary root and therefore delineates it from sugar-dependent systemic Pi starvation responses. However, the interaction between Pi and Suc was evident on the development of the lateral roots and root hairs in the seedlings grown under varying levels of Pi and Suc. Although the Pi+ Suc- condition suppressed lateral root development, induction of few laterals under the Pi- Suc- condition point to increased sensitivity of the roots to auxin during Pi deprivation. This was supported by expression analyses of DR5uidA, root basipetal transport assay of auxin, and RSA of the pgp19 mutant exhibiting reduced auxin transport. A significant increase in the number of lateral roots under the Pi- Suc- condition in the chalcone synthase mutant (tt4-2) indicated a potential role for flavonoids in auxin-mediated Pi deficiency-induced modulation of RSA. The study thus demonstrated differential roles of Suc and auxin in the developmental responses of ontogenetically distinct root traits during Pi deprivation. In addition, lack of cross talk between local and systemic Pi sensing as revealed by the seedlings grown under either the Pi- Suc- condition or in the heterogeneous Pi environment highlighted the coexistence of Suc-independent and Suc-dependent regulatory mechanisms that constitute Pi starvation responses.     

Use of phosphate to avoid uranium toxicity in Arabidopsis thaliana leads to alterations of morphological and physiological responses regulated by phosphate availability

Uranium is an ubiquitous pollutant with known chemical and radiological toxicity, which is naturally present in the plant environment. Due to its high affinity for phosphate, insoluble uranium-phosphate precipitates are formed in soils as well as in contaminated plant cells. To date, consequences of such interactions on uranium toxicity and on phosphate availability and metabolism in plants are unknown. This study aims at evaluating in which extent uranium-phosphate interactions have an effect on physiological and molecular mechanisms involved in plant responses (i) to uranium contamination and (ii) to phosphate availability in Arabidopsis thaliana.Inorganic phosphate (Pi) supply in U-contaminated medium was shown to decrease U bioaccumulation and U toxic effects on plant biomass and root cell viability. Besides, U was shown to disturb plant responses to Pi availability. Indeed, in Pi-sufficient conditions, high U concentrations promoted the induction of phosphate starvation responses in plants. However, the most drastic effects have been observed in Pi-deficient conditions as U affected the following plant responses to Pi-starvation: root architecture modulation, phosphate acquisition and optimization of phosphate allocation. Indeed, despite the low Pi status of these plants, 2 μM U inhibited the primary root growth arrest normally triggered by low Pi. Moreover, Pi uptake and translocation to shoot were reduced. The root concentration of soluble inorganic phosphate decreased in Pi-starved plantlets contaminated with U, despite the enhancement of shoot-to-root remobilization of Pi. The observations of intracellular and apoplastic deposits of U and P in roots using electron microscopy (TEM-EDX) and secondary ion mass spectroscopy (NanoSIMS) provided evidence that Pi flux disturbance is a consequence of the use of Pi to immobilize U within roots.     

SIZ1 regulation of phosphate starvation-induced root architecture remodeling involves the control of auxin accumulation

Phosphate (Pi) limitation causes plants to modulate the architecture of their root systems to facilitate the acquisition of Pi. Previously, we reported that the Arabidopsis (Arabidopsis thaliana) SUMO E3 ligase SIZ1 regulates root architecture remodeling in response to Pi limitation; namely, the siz1 mutations cause the inhibition of primary root (PR) elongation and the promotion of lateral root (LR) formation. Here, we present evidence that SIZ1 is involved in the negative regulation of auxin patterning to modulate root system architecture in response to Pi starvation. The siz1 mutations caused greater PR growth inhibition and LR development of seedlings in response to Pi limitation. Similar root phenotypes occurred if Pi-deficient wild-type seedlings were supplemented with auxin. N-1-Naphthylphthalamic acid, an inhibitor of auxin efflux activity, reduced the Pi starvation-induced LR root formation of siz1 seedlings to a level equivalent to that seen in the wild type. Monitoring of the auxin-responsive reporter DR5::uidA indicated that auxin accumulates in PR tips at early stages of the Pi starvation response. Subsequently, DR5::uidA expression was observed in the LR primordia, which was associated with LR elongation. The time-sequential patterning of DR5::uidA expression occurred earlier in the roots of siz1 as compared with the wild type. In addition, microarray analysis revealed that several other auxin-responsive genes, including genes involved in cell wall loosening and biosynthesis, were up-regulated in siz1 relative to wild-type seedlings in response to Pi starvation. Together, these results suggest that SIZ1 negatively regulates Pi starvation-induced root architecture remodeling through the control of auxin patterning.     

Influence of cytokinins on the expression of phosphate starvation responsive genes in Arabidopsis

The increase in the ratio of root growth to shoot growth that occurs in response to phosphate (Pi) deprivation is paralleled by a decrease in cytokinin levels under the same conditions. However, the role of cytokinin in the rescue system for Pi starvation remains largely unknown. We have isolated a gene from Arabidopsis thaliana (AtIPS1) that is induced by Pi starvation, and studied the effect of cytokinin on its expression in response to Pi deprivation. AtIPS1 belongs to the TPSI1/Mt4 family, the members of which are specifically induced by Pi starvation, and the RNAs of which contain only short, non-conserved open reading frames. Pi deprivation induces AtIPS1 expression in all cells of wild-type plants, whereas in the pho1 mutant grown on Pi-rich soils, AtIPS1 expression in the root was delimited by the endodermis. This supports the view that pho1 is impaired in xylem loading of Pi, and that long-distance signals controlling the Pi starvation responses act via negative control. Exogenous cytokinins repress the expression of AtIPS1 and other Pi starvation-responsive genes in response to Pi deprivation. However, cytokinins did not repress the increase in root-hair number and length induced by Pi starvation, a response dependent on local Pi concentration rather than on whole-plant Pi status. Our results raise the possibility that cytokinins may be involved in the negative modulation of long-distance, systemically controlled Pi starvation responses, which are dependent on whole-plant Pi status.     

Pathways of Phosphorus Absorption and Early Signaling between the Mycorrhizal Fungi and Plants

This review highlights the key role that mycorrhizal fungi play in making phosphorus (Pi) more available to plants, including pathways of phosphorus absorption, phosphate transporters and plant-mycorrhizal fungus symbiosis, especially in conditions where the level of inorganic phosphorus (Pi) in the soil is low. Mycorrhizal fungi colonization involves a series of signaling where the plant root exudates strigolactones, while the mycorrhizal fungi release a mixture of chito-oligosaccharides and liposaccharides, that activate the symbiosis process through gene signaling pathways, and contact between the hyphae and the root. Once the symbiosis is established, the extraradical mycelium acts as an extension of the roots and increases the absorption of nutrients, particularly phosphorus by the phosphate transporters. Pi then moves along the hyphae to the plant root/fungus interface. The transfer of Pi occurs in the apoplectic space; in the case of arbuscular mycorrhizal fungi, Pi is discharged from the arbuscular to the plant’s root symplasm, in the membrane that surrounds the arbuscule. Pi is then absorbed through the plant periarbuscular membrane by plant phosphate transporters. Furthermore, plants can acquire Pi from soil as a direct absorption pathway. As a result of this review, several genes that codify for high-affinity Pi transporters were identified. In plants, the main family is Pht1 although it is possible to find others such as Pht2, Pht3, Pho1 and Pho2. As in plants, mycorrhizal fungi have genes belonging to the Pht1 subfamily. In arbuscular mycorrhizal fungi we found L1PT1, GiPT, MtPT1, MtPT2, MtPT4, HvPT8, ZmPht1, TaPTH1.2, GmosPT and LYCes. HcPT1, HcPT2 and BePT have been characterized in ectomycorrhizal fungi. Each gene has a different way of expressing itself. In this review, we present diagrams of the symbiotic relationship between mycorrhizal fungi and the plant. This knowledge allows us to design solutions to regional problems such as food production in soils with low levels of Pi.