OsPHR2 is involved in phosphate-starvation signaling and excessive phosphate accumulation in shoots of plants

Previous research has demonstrated that AtPHR1 plays a central role in phosphate (Pi)-starvation signaling in Arabidopsis thaliana. In this work, two OsPHR genes from rice (Oryza sativa) were isolated and designated as OsPHR1 and OsPHR2 based on amino acid sequence homology to AtPHR1. Their functions in Pi signaling in rice were investigated using transgenic plants. Our results showed that both OsPHR1 and OsPHR2 are involved in Pi-starvation signaling pathway by regulation of the expression of Pi-starvation-induced genes, whereas only OsPHR2 overexpression results in the excessive accumulation of Pi in shoots under Pi-sufficient conditions. Under Pi-sufficient conditions, overexpression of OsPHR2 mimics Pi-starvation stress in rice with enhanced root elongation and proliferated root hair growth, suggesting the involvement of OsPHR2 in Pi-dependent root architecture alteration by both systematic and local pathways. In OsPHR2-overexpression plants, some Pi transporters were up-regulated under Pi-sufficient conditions, which correlates with the strongly increased content of Pi. The mechanism behind the OsPHR2 regulated Pi accumulation will provide useful approaches to develop smart plants with high Pi efficiency.     

Nitrate transporter NPF7.3/NRT1.5 plays an essential role in regulating phosphate deficiency responses in Arabidopsis

AtNPF7.3/AtNRT1.5, which is a nitrate transporter that drives root-to-shoot transport of NO₃^?, is also involved in modulating the response to K⁺ deprivation in Arabidopsis by affecting root development and K⁺ transport. However, whether NPF7.3/NRT1.5 functions in regulating plant responses to deficiencies of other nutrients remains unknown. In this study, we found that the expression of AtNPF7.3/AtNRT1.5 was predominant in the roots and was substantially induced by phosphate (Pi) starvation. The atnrt1.5 mutants displayed conspicuously longer primary roots along with a significantly reduced lateral root density under Pi-deficient conditions than did the wild-type plants, and these morphological differences in the roots were eliminated to a certain extent by the ethylene synthesis antagonist Co²⁺. Further analyses revealed that the expression of important Pi starvation-induced genes, which are directly involved in Pi transport, mobilization and distribution, were significantly higher in the atnrt1.5 mutants than that in the wild-type plants under Pi-starvation conditions; therefore, the atnrt1.5 mutants retained higher tissue Pi concentrations. Taken together, our results suggest that NPF7.3/NRT1.5 is an important component in the regulation of phosphate deficiency responses in Arabidopsis.     

Phosphate starvation induced OsPHR4 mediates Pi-signaling and homeostasis in rice

Key message

OsPHR4 mediates the regulation of Pi-starvation signaling and Pi-homeostasis in a PHR1-subfamily dependent manner in rice.

Abstract

Phosphate (Pi) starvation response is a sophisticated process for plant in the natural environment. In this process, PHOSPHATE STARVATION RESPONSE 1 (PHR1) subfamily genes play a central role in regulating Pi-starvation signaling and Pi-homeostasis. Besides the three PHR1 orthologs in Oryza sativa L. (Os) [(Os) PHR1, (Os) PHR2, and (Os) PHR3], which were reported to regulated Pi-starvation signaling and Pi-homeostasis redundantly, a close related PHR1 ortholog [designated as (Os) PHR4] is presented in rice genome with unknown function. In this study, we found that OsPHR4 is a Pi-starvation induced gene and mainly expresses in vascular tissues through all growth and development periods. The expression of OsPHR4 is positively regulated by OsPHR1, OsPHR2 and OsPHR3. The nuclear located OsPHR4 can respectively interact with other three PHR1 subfamily members to regulate downstream Pi-starvation induced genes. Consistent with the positive role of PHR4 in regulating Pi-starvation signaling, the OsPHR4 overexpressors display higher Pi accumulation in the shoot and elevated expression of Pi-starvation induced genes under Pi-sufficient condition. Besides, moderate growth retardation and repression of the Pi-starvation signaling in the OsPHR4 RNA interfering (RNAi) transgenic lines can be observed under Pi-deficient condition. Together, we propose that OsPHR4 mediates the regulation of Pi-starvation signaling and Pi-homeostasis in a PHR1-subfamily dependent manner in rice.

     

The expression of GintPT, the phosphate transporter of Rhizophagus irregularis, depends on the symbiotic status and phosphate availability

The development of mutualistic interactions with arbuscular mycorrhizal (AM) fungi is one of the most important adaptation of terrestrial plants to face mineral nutrition requirements. As an essential plant nutrient, phosphorus uptake is acknowledged as a major benefit of the AM symbiosis, but the molecular mechanisms of its transport as inorganic phosphate (Pi) from the soil to root cells via AM fungi remain poorly known. Here we monitored the expression profile of the high-affinity phosphate transporter (PT) gene (GintPT) of Rhizophagus irregularis (DAOM 197198) in fungal structures (spores, extraradical mycelium and arbuscules), under different Pi availability, and in respect to plant connection. GintPT resulted constitutively expressed along the major steps of the fungal life cycle and the connection with the host plant was crucial to warrant GintPT high expression levels in the extraradical mycelium. The influence of Pi availability on gene expression of the fungal GintPT and the Medicago truncatula symbiosis-specific Pi transporter (MtPT4) was examined by qRT-PCR assay on microdissected arbusculated cells. The expression profiles of both genes revealed that these transporters are sensitive to changing Pi conditions: we observed that MtPT4 mRNA abundance is higher at 320 than at 32 μM suggesting that the flow towards the plant requires high concentrations. Taken on the whole, the findings highlight novel traits for the functioning of the GintPT gene and offer a molecular scenario to the models describing nutrient transfers as a cooperation between the mycorrhizal partners.     

The Down-Regulation of Mt4-Like Genes by Phosphate Fertilization Occurs Systemically and Involves Phosphate Translocation to the Shoots

Mt4 is a cDNA representing a phosphate-starvation-inducible gene from Medicago truncatula that is down-regulated in roots in response to inorganic phosphate (Pi) fertilization and colonization by arbuscular mycorrhizal fungi. Split-root experiments revealed that the expression of the Mt4 gene in M. truncatula roots is down-regulated systemically by both Pi fertilization and colonization by arbuscular mycorrhizal fungi. A comparison of Pi levels in these tissues suggested that this systemic down-regulation is not caused by Pi accumulation. Using a 30-bp region of the Mt4 gene as a probe, Pi-starvation-inducible Mt4-like genes were detected in Arabidopsis and soybean (Glycine max L.), but not in corn (Zea mays L.). Analysis of the expression of the Mt4-like Arabidopsis gene, At4, in wild-type Arabidopsis and pho1, a mutant unable to load Pi into the xylem, suggests that Pi must first be translocated to the shoot for down-regulation to occur. The data from the pho1 and split-root studies are consistent with the presence of a translocatable shoot factor responsible for mediating the systemic down-regulation of Mt4-like genes in roots.     

Ethylene Alleviates the Suppressive Effect of Phosphate on Arbuscular Mycorrhiza Formation

Phosphorus is one of the most important macronutrients required for plant growth. Plants have evolved many strategies for inorganic phosphorus (Pi) acquisition, including the symbiotic pathways, involving the formation of mycorrhiza. With regard to arbuscular mycorrhiza (AM), high Pi availability has long been known to negatively affect this association, although the underlying mechanism is unknown. In the present study, the interactive role played by ethylene and Pi in AM regulation was investigated in the tomato-Rhizophagus irregularis symbiosis. AM fungal colonization was analysed in epi, rin and NRO ethylene-responsive mutants under different Pi availability conditions, with a focus on the late stages of the interaction. Although Pi inhibited mycorrhizal parameters in the ethylene-insensitive rin mutant and wild-type cultivars, it did not alter the mycorrhization of the epi tomato mutant, which exhibits a constitutive ethylene-induced response. As with the colonization parameters, root ethylene content and the expression of AM-related and ethylene receptor 6 genes were inhibited by Pi in wild-type cultivars and rin mutants, but were unaffected or slightly activated in epi plants. The application of ethephon offsets the negative impact on the mycorrhizal development caused by the application of Pi. This compensation effect is dose dependent and was ineffective in the NRO mutant, which is more insensitive to the action of ethylene. Our results provide evidence that ethylene signalling negatively affects the suppressive effect of Pi on AM formation and suggests an overlap between this suppressive effect and the regulatory mechanism of Pi-starvation response pathway in plants mediated by ethylene.     

Low phosphate signaling induces changes in cell cycle gene expression by increasing auxin sensitivity in the Arabidopsis root system

Lateral root development is an important morphogenetic process in plants, which allows the modulation root architecture and substantially determines the plant''s efficiency for water and nutrient uptake. Postembryonic root development is under the control of both endogenous developmental programs and environmental stimuli. Nutrient availability plays a major role among environmental signals that modulate root development. Phosphate (Pi) limitation is a constraint for plant growth in many natural and agricultural ecosystems. Plants posses Pi-sensing mechanisms that enable them to respond and adapt to conditions of limited Pi supply, including increased formation and growth of lateral roots. Root developmental modifications are mainly mediated by the plant hormone auxin. Recently we showed that the alteration of root system architecture under Pi-starvation may be mediated by modifications in auxin sensitivity in root cells via a mechanism involving the TIR1 auxin receptor. In this addendum, we provide additional novel evidence indicating that the low Pi pathway involves changes in cell cycle gene expression. It was found that Pi deprivation increases the expression of CDKA, E2Fa, Dp-E2F and CyCD3. In particular, E2Fa, Dp-E2F and CyCD3 genes were specifically upregulated by auxin in Pi-deprived Arabidopsis seedlings that were treated with the auxin transport inhibitor NPA, indicating that cell cycle modulation by low Pi signaling is independent of auxin transport and dependent on auxin sensitivity in the root.Key words: phosphate signaling, auxin transport, auxin sensitivity, roots     

LjABCB1, an ATP-binding cassette protein specifically induced in uninfected cells of Lotus japonicus nodules

Legume plants develop root nodules through symbiosis with rhizobia, and fix atmospheric nitrogen in this symbiotic organ. Development of root nodules is regulated by many metabolites including phytohormones. Previously, we reported that auxin is strongly involved in the development of the nodule vascular bundle and lenticel formation on the nodules of Lotus japonicus. Here we show that an ATP-binding cassette (ABC) protein, LjABCB1, which is a homologue of Arabidopsis auxin transporter AtABCB4, is specifically expressed during nodulation of L. japonicus. A reporter gene analysis indicated that the expression of LjABCB1 was restricted to uninfected cells adjacent to infected cells in the nodule, while no expression was observed in shoot apical meristems or root tips, in which most auxin transporter genes are expressed. The auxin transport activity of LjABCB1 was confirmed using a heterologous expression system.     

Membrane lipid alteration during phosphate starvation is regulated by phosphate signaling and auxin/cytokinin cross-talk

During phosphate (Pi) starvation in plants, membrane phospholipid content decreases concomitantly with an increase in non-phosphorus glycolipids. Although several studies have indicated the involvement of phytohormones in various physiological changes upon Pi starvation, the regulation of Pi-starvation induced membrane lipid alteration remains unknown. Previously, we reported the response of type B monogalactosyl diacylglycerol synthase genes (atMGD2 and atMGD3) to Pi starvation, and suggested a role for these genes in galactolipid accumulation during Pi starvation. We now report our investigation of the regulatory mechanism for the response of atMGD2/3 and changes in membrane lipid composition to Pi starvation. Exogenous auxin activated atMGD2/3 expression during Pi starvation, whereas their expression was repressed by cytokinin treatment in the root. Moreover, auxin inhibitors and the axr4 aux1 double mutation in auxin signaling impaired the increase of atMGD2/3 expression during Pi starvation, showing that auxin is required for atMGD2/3 activation. The fact that hormonal effects during Pi starvation were also observed with regard to changes in membrane lipid composition demonstrates that both auxin and cytokinin are indeed involved in the dynamic changes in membrane lipids during Pi starvation. Phosphite is not metabolically available in plants; however, when we supplied phosphite to Pi-starved plants, the Pi-starvation response disappeared with respect to both atMGD2/3 expression and changes in membrane lipids. These results indicate that the observed global change in plant membranes during Pi starvation is not caused by Pi-starvation induced damage in plant cells but rather is strictly regulated by Pi signaling and auxin/cytokinin cross-talk.     

<Emphasis Type="Italic">MAX4</Emphasis> gene is involved in the regulation of low inorganic phosphate stress responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

MAX4 gene has been shown to be involved in the regulation of shoot branching in Arabidopsis (Arabidopsis thaliana). However, little is known about the role of MAX4 gene in low inorganic phosphate (Pi) stress response in Arabidopsis. Here we showed that MAX4 gene is involved in the regulation of low Pi stress response in Arabidopsis. MAX4 gene was repressed by low Pi stress, and the max4 mutants showed lower anthocyanin content and longer primary root length. In addition, max4 mutant plants also displayed altered root architecture such as increased root-to-shoot ratio, lower lateral root number and root hair density compared with wild-type plants under low Pi stress. Higher total Pi contents were detected in shoots and roots of max4 plants than those of wild-type plants when subjected to low Pi stress, which was associated, at least in part, with increase in expression of WRKY75 as well as AtPT1 and AtPT2 genes encoding high-affinity Pi transporters. Taken together, all these results suggest that MAX4 gene mediates low Pi stress response, at least in part, by regulating the expression of WRKY75 as well as AtPT1 and AtPT2 genes.