Natural variation of Arabidopsis response to nitrogen availability

Our understanding of plant growth in response to nitrogen (N) supply is mainly based on studies of mutants and transformants. This study explored the natural variability of Arabidopsis thaliana first to find out its global response to N availability and secondly to characterize the plasticity for growth and N metabolism among 23 genetically distant accessions under normal (N+), limited (N-), and starved (N0) N supplies. Plant growth was estimated by eight morphological traits characterizing shoot and root growth and 10 metabolic parameters that represented N and carbon metabolism. Most of the studied traits showed a large variation linked to genotype and nutrition. Furthermore, Arabidopsis growth was coordinated by master traits such as the shoot to root ratio of nitrate content in N+, root fresh matter and root amino acids in N-, and shoot fresh matter together with root thickness in N0. The 23 accessions could be gathered into four different groups, according to their growth in N+, N-, and N0. Phenotypic profiling characterized four different adaptative responses to N- and N0. Class 1 tolerated N limitation with the smallest decrease in shoot and root biomass compared with N+, while class 2 presented the highest resistance to N starvation by preferential increased root growth, huge starch accumulation, and high shoot nitrate content. In contrast, class 3 plants could tolerate neither N limitation nor N starvation. Small plants of class 4 were different, with shoot biomass barely affected in N- and root biomass unaffected in N0.     

Enhanced root growth in phosphate‐starved Arabidopsis by stimulating de novo phospholipid biosynthesis through the overexpression of LYSOPHOSPHATIDIC ACID ACYLTRANSFERASE 2 (LPAT2)

Upon phosphate starvation, plants retard shoot growth but promote root development presumably to enhance phosphate assimilation from the ground. Membrane lipid remodelling is a metabolic adaptation that replaces membrane phospholipids by non‐phosphorous galactolipids, thereby allowing plants to obtain scarce phosphate yet maintain the membrane structure. However, stoichiometry of this phospholipid‐to‐galactolipid conversion may not account for the massive demand of membrane lipids that enables active growth of roots under phosphate starvation, thereby suggesting the involvement of de novo phospholipid biosynthesis, which is not represented in the current model. We overexpressed an endoplasmic reticulum‐localized lysophosphatidic acid acyltransferase, LPAT2, a key enzyme that catalyses the last step of de novo phospholipid biosynthesis. Two independent LPAT2 overexpression lines showed no visible phenotype under normal conditions but showed increased root length under phosphate starvation, with no effect on phosphate starvation response including marker gene expression, root hair development and anthocyanin accumulation. Accompanying membrane glycerolipid profiling of LPAT2‐overexpressing plants revealed an increased content of major phospholipid classes and distinct responses to phosphate starvation between shoot and root. The findings propose a revised model of membrane lipid remodelling, in which de novo phospholipid biosynthesis mediated by LPAT2 contributes significantly to root development under phosphate starvation.     

OsARF16, a transcription factor,is required for auxin and phosphate starvation response in rice (Oryza sativa L.)

Plant responses to auxin and phosphate (Pi) starvation are closely linked. However, the underlying mechanisms connecting auxin to phosphate starvation (?Pi) responses are largely unclear. Here, we show that OsARF16, an auxin response factor, functions in both auxin and ?Pi responses in rice (Oryza sativa L.). The knockout of OsARF16 led to primary roots (PR), lateral roots (LR) and root hair losing sensitivity to auxin and ?Pi response. OsARF16 expression and OsARF16::GUS staining in PR and LR of rice Nipponbare (NIP) were induced by indole acetic acid and ?Pi treatments. In ?Pi conditions, the shoot biomass of osarf16 was slightly reduced, and neither root growth nor iron content was induced, indicating that the knockout of OsARF16 led to loss of response to Pi deficiency in rice. Six phosphate starvation‐induced genes (PSIs) were less induced by ?Pi in osarf16 and these trends were similar to a knockdown mutant of OsPHR2 or AtPHR1, which was a key regulator under ?Pi. These data first reveal the biological function of OsARF16, provide novel evidence of a linkage between auxin and ?Pi responses and facilitate the development of new strategies for the efficient utilization of Pi in rice.     

Characterization of low phosphorus insensitive mutants reveals a crosstalk between low phosphorus-induced determinate root development and the activation of genes involved in the adaptation of Arabidopsis to phosphorus deficiency

Low phosphorus (P) availability is one of the most limiting factors for plant productivity in many natural and agricultural ecosystems. Plants display a wide range of adaptive responses to cope with low P stress, which generally serve to enhance P availability in the soil and to increase its uptake by roots. In Arabidopsis (Arabidopsis thaliana), primary root growth inhibition and increased lateral root formation have been reported to occur in response to P limitation. To gain knowledge of the genetic mechanisms that regulate root architectural responses to P availability, we designed a screen for identifying Arabidopsis mutants that fail to arrest primary root growth when grown under low P conditions. Eleven low phosphorus insensitive (lpi) mutants that define at least four different complementation groups involved in primary root growth responses to P availability were identified. The lpi mutants do not show the typical determinate developmental program induced by P stress in the primary root. Other root developmental aspects of the low P rescue system, including increased root hair elongation and anthocyanin accumulation, remained unaltered in lpi mutants. In addition to the insensitivity of primary root growth inhibition, when subjected to P deprivation, lpi mutants show a reduced induction in the expression of several genes involved in the P starvation rescue system (PHOSPHATE TRANSPORTER 1 and 2, PURPLE ACID PHOSPHATASE 1, ACID PHOSPHATASE 5, and INDUCED BY PHOSPHATE STARVATION 1). Our results provide genetic support for the role of P as an important signal for postembryonic root development and root meristem maintenance and show a crosstalk in developmental and biochemical responses to P deprivation.     

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.     

Strigolactone Regulates Anthocyanin Accumulation,Acid Phosphatases Production and Plant Growth under Low Phosphate Condition in Arabidopsis

Phosphate is an essential macronutrient in plant growth and development; however, the concentration of inorganic phosphate (Pi) in soil is often suboptimal for crop performance. Accordingly, plants have developed physiological strategies to adapt to low Pi availability. Here, we report that typical Pi starvation responses in Arabidopsis are partially dependent on the strigolactone (SL) signaling pathway. SL treatment induced root hair elongation, anthocyanin accumulation, activation of acid phosphatase, and reduced plant weight, which are characteristic responses to phosphate starvation. Furthermore, the expression profile of SL-response genes correlated with the expression of genes induced by Pi starvation. These results suggest a potential overlap between SL signaling and Pi starvation signaling pathways in plants.     

Characterization of the soybean GmALMT family genes and the function of GmALMT5 in response to phosphate starvation

A potential mechanism to enhance utilization of sparingly soluble forms of phosphorus (P) is the root secretion of malate, which is mainly mediated by the ALMT gene family in plants. In this study, a total of 34 GmALMT genes were identified in the soybean genome. Expression patterns diverged considerably among GmALMTs in response to phosphate (Pi) starvation in leaves, roots and flowers, with expression altered by P availability in 26 of the 34 GmALMTs. One root‐specific GmALMT whose expression was significantly enhanced by Pi‐starvation, GmALMT5, was studied in more detail to determine its possible role in soybean P nutrition. Analysis of GmALMT5 tissue expression patterns, subcellular localization, and malate exudation from transgenic soybean hairy roots overexpressing GmALMT5, demonstrated that GmALMT5 is a plasma membrane protein that mediates malate efflux from roots. Furthermore, both growth and P content of transgenic Arabidopsis overexpressing GmALMT5 were significantly increased when sparingly soluble Ca‐P was used as the external P source. Taken together, these results indicate that members of the soybean GmALMT gene family exhibit diverse responses to Pi starvation. One member of this family, GmALMT5, might contribute to soybean P efficiency by enhancing utilization of sparingly soluble P sources under P limited conditions.     

OsCYCP4s coordinate phosphate starvation signaling with cell cycle progression in rice

Phosphate starvation leads to a strong reduction in shoot growth and yield in crops. The reduced shoot growth is caused by extensive gene expression reprogramming triggered by phosphate deficiency, which is not itself a direct consequence of low levels of shoot phosphorus. However, how phosphate starvation inhibits shoot growth in rice is still unclear. In this study, we determined the role of OsCYCP4s in the regulation of shoot growth in response to phosphate starvation in rice. We demonstrate that the expression levels of OsCYCP4s, except OsCYCP4;3, were induced by phosphate starvation. Overexpression of the phosphate starvation induced OsCYCP4s could compete with the other cyclins for the binding with cyclin‐dependent kinases, therefore suppressing growth by reducing cell proliferation. The phosphate starvation induced growth inhibition in the loss‐of‐function mutants cycp4;1, cycp4;2, and cycp4;4 is partially compromised. Furthermore, the expression of some phosphate starvation inducible genes is negatively modulated by these cyclins, which indicates that these OsCYCP4s may also be involved in phosphate starvation signaling. We conclude that phosphate starvation induced OsCYCP4s might coordinate phosphate starvation signaling and cell cycle progression under phosphate starvation stress.     

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.     

Maize root system growth and development as influenced by phosphorus deficiency

Effects on leaf growth, biomass accumulation and root morphogenesis associated with the establishment of phosphorus (P) deficiency were studied on maize in order to test the hypothesis that the root system response can be accounted for by the effect of P deficiency on the carbon budget of the plant. P deprivation had a large and rapid negative effect on leaf expansion. For 7 d after P deprivation, the total dry matter production per plant was almost fully accounted for by the effect of P starvation on leaf growth and its subsequent effect on photosynthetically active radiation (PAR) interception. No strong effect of P deficiency was observed on the radiation use efficiency during this first period, although it was reduced thereafter. Root growth was slightly enhanced a few days after P starvation, but strongly reduced thereafter. The elongation rate of axile roots was maintained throughout the experiment, whereas emergence of new axile roots and elongation of first-order laterals were drastically reduced. The density of first-order laterals was not severely affected. These morphological responses are very similar to what is observed when root growth is limited by the availability in carbohydrates. The results are therefore compatible with the hypothesis that P deficiency mainly affects the root system morphology through its effect on the carbon budget of the plant with no additional specific effect of P deficiency on root morphogenesis. The drastic and early reduction of shoot growth after P deprivation may explain that more carbohydrates were available for root growth which was observed a few days after P starvation and reported by several authors. Later on, however, because of the reduced leaf area of P-deprived plants, their capacity to intercept light was severely reduced so that root growth was finally reduced.Keywords: Zea mays L., maize, phosphorus, root, root morphogenesis.      

Long‐distance movement of phosphate starvation‐responsive microRNAs in Arabidopsis

• Plant microRNAs are small RNAs that are important for genetic regulation of processes such as plant development or environmental responses. Specific microRNAs accumulate in the phloem during phosphate starvation, and may act as long‐distance signalling molecules.
• We performed quantitative PCR on Arabidopsis hypocotyl micrograft tissues of wild‐type and hen1‐6 mutants to assess the mobility of several phosphate starvation‐responsive microRNA species.
• In addition to the previously confirmed mobile species miR399d, the corresponding microRNA* (miR399d*) was identified for the first time as mobile between shoots and roots. Translocation by phosphate‐responsive microRNAs miR827 and miR2111a between shoots and roots during phosphate starvation was evident, while their respective microRNA*s were not mobile.
• The results suggest that long‐distance mobility of microRNA species is selective and can occur without the corresponding duplex strand. Movement of miR399d* and root‐localised accumulation of miR2111a* opens the potential for persisting microRNA*s to be mobile and functional in novel pathways during phosphate starvation responses.

     

Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis

Plant root systems can respond to nutrient availability and distribution by changing the three-dimensional deployment of their roots: their root system architecture (RSA). We have compared RSA in homogeneous and heterogeneous nitrate and phosphate supply in Arabidopsis. Changes in nitrate and phosphate availability were found to have contrasting effects on primary root length and lateral root density, but similar effects on lateral root length. Relative to shoot dry weight (DW), primary root length decreased with increasing nitrate availability, while it increased with increasing phosphate supply. Lateral root density remained constant across a range of nitrate supplies, but decreased with increasing phosphate supply. In contrast, lateral root elongation was suppressed both by high nitrate and high phosphate supplies. Local supplies of high nitrate or phosphate in a patch also had different effects. Primary root growth was not affected by a high nitrate patch, but growth through a high phosphate patch reduced primary root growth after the root left the patch. A high nitrate patch induced an increase in lateral root density in the patch, whereas lateral root density was unaffected by a high phosphate patch. However, both phosphate- and nitrate-rich patches induced lateral root elongation in the patch and suppressed it outside the patch. This co-ordinated response of lateral roots also occurs in soil-grown plants exposed to a nutrient-rich patch. The auxin-resistant mutants axrl, axr4 and aux1 all showed the wild-type lateral root elongation responses to a nitrate-rich patch, suggesting that auxin is not required for this response.     

Conditional identification of phosphate-starvation-response mutants in Arabidopsis thaliana

Plants have evolved elaborate metabolic and developmental adaptations to low phosphorus availability. Biochemical responses to phosphate limitation include increased production and secretion of phosphate-acquisition proteins such as nucleases, acid phosphatases, and high-affinity phosphate transporters. However, the signal transduction pathways that sense phosphate availability and integrate the phosphate-starvation response in plants are unknown. We have devised a screen for conditional mutants in Arabidopsis thaliana (L.) Heynh. to dissect signaling of phosphate limitation. Our genetic screen is based on the facultative ability of wild-type Arabidopsis plants to metabolize exogenous DNA when inorganic phosphate is limiting. After screening 50,000 M2 seedlings, we isolated 22 confirmed mutant lines that showed severely impaired growth on medium containing DNA as the only source of phosphorus, but which recovered on medium containing soluble inorganic phosphate. Characterization of nine such mutant lines demonstrated an inability to utilize either DNA or RNA. One mutant line, psr1 (phosphate starvation response), had significantly reduced activities of phosphate-starvation-inducible isoforms of ribonuclease and acid phosphatase under phosphate-limiting conditions. The data suggest that a subset of the selected mutations impairs the expression of more than one phosphate-starvation-inducible enzyme required for utilization of exogenous nucleic acids, and may thus affect regulatory components of a Pi starvation response pathway in higher plants. Received: 23 September 1999 / Accepted: 10 November 1999     

Phosphate availability regulates root system architecture in Arabidopsis

Plant root systems are highly plastic in their development and can adapt their architecture in response to the prevailing environmental conditions. One important parameter is the availability of phosphate, which is highly immobile in soil such that the arrangement of roots within the soil will profoundly affect the ability of the plant to acquire this essential nutrient. Consistent with this, the availability of phosphate was found to have a marked effect on the root system architecture of Arabidopsis. Low phosphate availability favored lateral root growth over primary root growth, through increased lateral root density and length, and reduced primary root growth mediated by reduced cell elongation. The ability of the root system to respond to phosphate availability was found to be independent of sucrose supply and auxin signaling. In contrast, shoot phosphate status was found to influence the root system architecture response to phosphate availability.