Sandrine Ruffel

Nitrogen economics of root foraging: Transitive closure of the nitrate–cytokinin relay and distinct systemic signaling for N supply vs. demand

As sessile organisms, root plasticity enables plants to forage for and acquire nutrients in a fluctuating underground environment. Here, we use genetic and genomic approaches in a “split-root” framework—in which physically isolated root systems of the same plant are challenged with different nitrogen (N) environments—to investigate how systemic signaling affects genome-wide reprogramming and root development. The integration of transcriptome and root phenotypes enables us to identify distinct mechanisms underlying “N economy” (i.e., N supply and demand) of plants as a system. Under nitrate-limited conditions, plant roots adopt an “active-foraging strategy”, characterized by lateral root outgrowth and a shared pattern of transcriptome reprogramming, in response to either local or distal nitrate deprivation. By contrast, in nitrate-replete conditions, plant roots adopt a “dormant strategy”, characterized by a repression of lateral root outgrowth and a shared pattern of transcriptome reprogramming, in response to either local or distal nitrate supply. Sentinel genes responding to systemic N signaling identified by genome-wide comparisons of heterogeneous vs. homogeneous split-root N treatments were used to probe systemic N responses in Arabidopsis mutants impaired in nitrate reduction and hormone synthesis and also in decapitated plants. This combined analysis identified genetically distinct systemic signaling underlying plant N economy: (i) N supply, corresponding to a long-distance systemic signaling triggered by nitrate sensing; and (ii) N demand, experimental support for the transitive closure of a previously inferred nitrate–cytokinin shoot–root relay system that reports the nitrate demand of the whole plant, promoting a compensatory root growth in nitrate-rich patches of heterogeneous soil.

A framework integrating plant growth with hormones and nutrients

It is well known that nutrient availability controls plant development. Moreover, plant development is finely tuned by a myriad of hormonal signals. Thus, it is not surprising to see increasing evidence of coordination between nutritional and hormonal signaling. In this opinion article, we discuss how nitrogen signals control the hormonal status of plants and how hormonal signals interplay with nitrogen nutrition. We further expand the discussion to include other nutrient-hormone pairs. We propose that nutrition and growth are linked by a multi-level, feed-forward cycle that regulates plant growth, development and metabolism via dedicated signaling pathways that mediate nutrient and hormonal regulation. We believe this model will provide a useful concept for past and future research in this field.

The recessive potyvirus resistance gene pot-1 is the tomato orthologue of the pepper pvr2-eIF4E gene

The translation initiation factor 4E (eIF4E) has been implicated in naturally occurring resistance to Potato virus Y (PVY) determined by the pvr2 locus in pepper (Capsicum annuum). Here, the molecular basis of the recessive resistance to PVY and Tobacco etch virus (TEV) controlled by the pot-1 locus in tomato (Lycopersicon esculentum; now Solanum lycopersicum) was investigated. On the basis of genetic mapping data that indicated that pot-1 and pvr2 are located in syntenic regions of the tomato and pepper genomes, the possible involvement of eIF4E in pot-1-mediated resistance was assessed. Genetic mapping of members of the eIF4E multigenic family in tomato introgression lines revealed that an eIF4E locus indeed maps in the same genomic region as pot-1. By comparing eIF4E coding sequences between resistant and susceptible Lycopersicon genotypes, a small number of polymorphisms that co-segregate with the pot-1 locus were identified, suggesting that this gene could be involved in resistance to potyviruses. Functional complementation experiments using Potato virus X-mediated transient expression of eIF4E from a susceptible genotype in a resistant pepper genotype confirmed that a small number of amino acid substitutions in the eIF4E protein indeed account for resistance/susceptibility to both the PVY and TEV, and consequently that pot-1 and pvr2 are orthologues. Taken together, these results support the role of this eIF4E gene as a key component of recessive resistance to potyviruses, and validate the comparative genomic approach for the molecular characterization of recessive resistance genes.

Systemic Signaling of the Plant Nitrogen Status Triggers Specific Transcriptome Responses Depending on the Nitrogen Source in Medicago truncatula

Legumes can acquire nitrogen (N) from NO3−, NH4+, and N2 (through symbiosis with Rhizobium bacteria); however, the mechanisms by which uptake and assimilation of these N forms are coordinately regulated to match the N demand of the plant are currently unknown. Here, we find by use of the split-root approach in Medicago truncatula plants that NO3− uptake, NH4+ uptake, and N2 fixation are under general control by systemic signaling of plant N status. Indeed, irrespective of the nature of the N source, N acquisition by one side of the root system is repressed by high N supply to the other side. Transcriptome analysis facilitated the identification of over 3,000 genes that were regulated by systemic signaling of the plant N status. However, detailed scrutiny of the data revealed that the observation of differential gene expression was highly dependent on the N source. Localized N starvation results, in the unstarved roots of the same plant, in a strong compensatory up-regulation of NO3− uptake but not of either NH4+ uptake or N2 fixation. This indicates that the three N acquisition pathways do not always respond similarly to a change in plant N status. When taken together, these data indicate that although systemic signals of N status control root N acquisition, the regulatory gene networks targeted by these signals, as well as the functional response of the N acquisition systems, are predominantly determined by the nature of the N source.

AtNIGT1/HRS1 integrates nitrate and phosphate signals at the Arabidopsis root tip

Nitrogen and phosphorus are among the most widely used fertilizers worldwide. Nitrate (NO3−) and phosphate (PO43−) are also signaling molecules whose respective transduction pathways are being intensively studied. However, plants are continuously challenged with combined nutritional deficiencies, yet very little is known about how these signaling pathways are integrated. Here we report the identification of a highly NO3−-inducible NRT1.1-controlled GARP transcription factor, HRS1, document its genome-wide transcriptional targets, and validate its cis-regulatory-elements. We demonstrate that this transcription factor and a close homolog repress primary root growth in response to P deficiency conditions, but only when NO3− is present. This system defines a molecular logic gate integrating P and N signals. We propose that NO3− and P signaling converge via double transcriptional and post-transcriptional control of the same protein, HRS1

HIGH NITROGEN INSENSITIVE 9 (HNI9)-mediated systemic repression of root NO3− uptake is associated with changes in histone methylation

In plants, root nitrate uptake systems are under systemic feedback repression by the N satiety of the whole organism, thus adjusting the N acquisition capacity to the N demand for growth; however, the underlying molecular mechanisms are largely unknown. We previously isolated the Arabidopsis high nitrogen-insensitive 9-1 (hni9-1) mutant, impaired in the systemic feedback repression of the root nitrate transporter NRT2.1 by high N supply. Here, we show that HNI9 encodes Arabidopsis INTERACT WITH SPT6 (AtIWS1), an evolutionary conserved component of the RNA polymerase II complex. HNI9/AtIWS1 acts in roots to repress NRT2.1 transcription in response to high N supply. At a genomic level, HNI9/AtIWS1 is shown to play a broader role in N signaling by regulating several hundred N-responsive genes in roots. Repression of NRT2.1 transcription by high N supply is associated with an HNI9/AtIWS1-dependent increase in histone H3 lysine 27 trimethylation at the NRT2.1 locus. Our findings highlight the hypothesis that posttranslational chromatin modifications control nutrient acquisition in plants.

A Systems View of Responses to Nutritional Cues in Arabidopsis: Toward a Paradigm Shift for Predictive Network Modeling

One of the major challenges of plant biology is to understand and model plant molecular networks coordinating growth and development, in response to nutritional cues. Further, the predictions and/or the genetic modifications of these responses would have the potential of providing solutions to tweak

TARGET: A transient transformation system for genome-wide transcription factor target discovery

Dear Editor, Determining the fundamental structure of gene regulatory networks (GRN) is a major challenge of systems biology. In particular, inferring GRN structure from comprehensive gene expression and transcription factor (TF)–promoter interaction data sets has become an increasingly sought-after aim in both fundamental and agronomical research in plant biology (Bonneau et al., 2007; Ruffel et al., 2010). A crucial step for the assessment of GRN is the identification of the direct TF-target genes. Transgenic plant lines expressing tagged versions of the TF-of-interest can be used together with transcriptomic and DNA-binding analyses to obtain high-confidence lists of direct targets (e.g. Mönke et al., 2012). However, the generation of such transgenics can be a limiting factor, especially in largescale studies or in non-model species. Transient transformation of protoplasts is therefore often employed for the study of TF–promoter interactions, using co-expression of effector constructs with a TF-of-interest and reporter constructs with a promoter-of-interest. We have developed a rapid technique to study the genome-wide effects of TF activation in protoplasts that uses transient expression of a glucocorticoid receptor (GR)-tagged TF. We demonstrate here that this system can be used to rapidly retrieve information on direct target genes in less than 2 weeks’ time. As a proof-of-principle candidate, we used the well-studied TF, ABSCICIC ACID INSENSITIVE 3 (ABI3; Koornneef et al., 1989; Mönke et al., 2012) and established the de novo identification of the abscisic acid response element (ABRE) and a majority of the previously classified direct targets. We have named this technique TARGET, for Transient Assay Reporting Genome-wide Effects of Transcription factors. Technically, plant protoplasts are transfected with a plasmid (pBeaconRFP_GR) that expresses the TF-of-interest fused to GR, which allows the controlled entry of the chimeric GR–TF into the nucleus by addition of the GR–ligand dexamethasone (DEX; Schena and Yamamoto, 1988). In addition, the vector contains a separate expression cassette with a positive fluorescent selection marker (red fluorescent protein; RFP) which enables fluorescence-activated cell sorting (FACS) of successfully transformed protoplasts (Figure 1A; Bargmann and Birnbaum, 2009). This purification step allows reliable qPCR or transcriptomic analysis of multiple independent transfections, which would otherwise be hampered by the presence of a population of untransformed cells that varies from experiment to experiment. Lastly, the effect of target gene induction by DEX treatment is measured in the presence or absence of the translation inhibitor cycloheximide (CHX), allowing for the distinction of direct and indirect target genes of the TF under study (Supplementary Data). pBeaconRFP_GR–ABI3 was used to transfect protoplasts prepared from the roots of Arabidopsis seedlings, where ABI3, known largely for its role in seed development, has also been shown to be involved in development (Brady et al., 2003). As a first test of the TARGET system, the expression of known direct ABI3 targets PER1 and CRU3 was assayed by qPCR. Compared to control gene expression, both PER1 and CRU3 showed significant induction of transcript levels upon DEX treatment in the ABI3–GR-transfected protoplasts in the presence of CHX (Figure 1B and 1C, and Supplementary Data). PER1 and CRU3 expression in protoplasts transformed with an empty vector control showed no significant induction by DEX treatment (Figure 1B and 1C). Significant induction of CRU3 expression could only be measured when CHX was present, indicating that the effects of CHX may in some cases facilitate ABI3 function. Enhancement of ABA signaling output by protein synthesis inhibitors that could explain this phenomenon has been noted before by independent studies (Reeves et al., 2011). For the transcriptomic analysis, using ATH1 Genome Array chips, a two-way analysis of variance (ANOVA) was performed, followed by a Tukey post hoc test to identify genes whose expression is differentially regulated in response to DEX treatment in the absence or presence of CHX (p < 0.05, fold change >1.5; Supplementary Data). Genes found to be significantly regulated by DEX treatment in the empty vector control were omitted from further analysis. This analysis yielded a total of 668 unique genes whose expression was affected by DEX-induced nuclear localization of ABI3, 227 regulated genes without CHX, and 458 regulated genes with CHX (microarray results were validated by qPCR; Supplementary Data). There was just a 17-gene overlap with and without CHX, reiterating that (as was seen for CRU3 in preliminary qPCR analysis) there are many genes whose response to GR–ABI3 was facilitated by the presence of the protein synthesis inhibitor CHX. The 210 genes regulated only in the absence of CHX were categorized

Signal interactions in the regulation of root nitrate uptake

In most aerobic soils, nitrate (NO3(-)) is the main nitrogen source for plants and is often limiting for plant growth and development. To adapt to a changing environment, plants have developed complex regulatory mechanisms that involve short and long-range signalling pathways in response to both NO3(-) availability in the soil and other physiological processes like growth or nitrogen (N) and carbon (C) metabolisms. Over the past decade, transcriptomic approaches largely contributed to the identification of molecular elements involved in these regulatory mechanisms, especially at the level of root NO3(-)uptake. Most strikingly, the data obtained revealed the high level of interaction between N and both hormone and C signalling pathways, suggesting a strong dependence on growth, development, and C metabolism to adapt root NO3(-) uptake to both external NO3(-) availability and the N status of the plant. However, the signalling mechanisms involved in the cross-talk between N, C, and hormones for the regulation of root NO3(-) uptake remain largely obscure. The aim of this review is to discuss the recent advances concerning the regulatory pathways controlling NO3(-) uptake in response to N signalling, hormones, and C in the model plant Arabidopsis thaliana. Then, to further characterize the level of interaction between these signalling pathways we built on publicly available transcriptome data to determine how hormones and C treatments modify the gene network connecting root NO3(-) transporters and their regulators.

RootScape: A Landmark-Based System for Rapid Screening of Root Architecture in Arabidopsis

A landmark-based system quantifies root architecture using holistic trait capture methods. The architecture of plant roots affects essential functions including nutrient and water uptake, soil anchorage, and symbiotic interactions. Root architecture comprises many features that arise from the growth of the primary and lateral roots. These root features are dictated by the genetic background but are also highly responsive to the environment. Thus, root system architecture (RSA) represents an important and complex trait that is highly variable, affected by genotype × environment interactions, and relevant to survival/performance. Quantification of RSA in Arabidopsis (Arabidopsis thaliana) using plate-based tissue culture is a very common and relatively rapid assay, but quantifying RSA represents an experimental bottleneck when it comes to medium- or high-throughput approaches used in mutant or genotype screens. Here, we present RootScape, a landmark-based allometric method for rapid phenotyping of RSA using Arabidopsis as a case study. Using the software AAMToolbox, we created a 20-point landmark model that captures RSA as one integrated trait and used this model to quantify changes in the RSA of Arabidopsis (Columbia) wild-type plants grown under different hormone treatments. Principal component analysis was used to compare RootScape with conventional methods designed to measure root architecture. This analysis showed that RootScape efficiently captured nearly all the variation in root architecture detected by measuring individual root traits and is 5 to 10 times faster than conventional scoring. We validated RootScape by quantifying the plasticity of RSA in several mutant lines affected in hormone signaling. The RootScape analysis recapitulated previous results that described complex phenotypes in the mutants and identified novel gene × environment interactions.