Nitrate Signaling Research Articles

No Coincident Nitrate Enhancement Events in Polar Ice Cores Following the Largest Known Solar Storms

AbstractKnowledge on the occurrence rate of extreme solar storms is strongly limited by the relatively recent advent of satellite monitoring of the Sun. To extend our perspective of solar storms prior to the satellite era and because atmospheric ionization induced by solar energetic particles (SEPs) can lead to the production of odd nitrogen, nitrate spikes in ice cores have been tentatively used to document both the occurrence and intensity of past SEP events. However, the reliability of the use of nitrate in ice records as a proxy for SEP events is strongly debated. This is partly due to equivocal detection of nitrate spikes in single ice cores and possible alternative sources, such as biomass burning plumes. Here we present new continuous high‐resolution measurements of nitrate and of the biomass burning species ammonium and black carbon, from several Antarctic and Greenland ice cores. We investigate periods covering the two largest known SEP events of 775 and 994 Common Era as well as the Carrington event and the hard SEP event of February 1956. We report no coincident nitrate spikes associated with any of these benchmark events. We also demonstrate the low reproducibility of the nitrate signal in multiple ice cores and confirm the significant relationship between biomass burning plumes and nitrate spikes in individual ice cores. In the light of these new data, there is no line of evidence that supports the hypothesis that ice cores preserve or document detectable amounts of nitrate produced by SEPs, even for the most extreme events known to date.

Overexpression of the Maize ZmNLP6 and ZmNLP8 Can Complement the Arabidopsis Nitrate Regulatory Mutant nlp7 by Restoring Nitrate Signaling and Assimilation

Nitrate is a key nutrient that affects maize growth and yield, and much has yet to be learned about nitrate regulatory genes and mechanisms in maize. Here, we identified nine ZmNLP genes in maize and analyzed the functions of two ZmNLP members in nitrate signaling. qPCR results revealed a broad pattern of expression for ZmNLP genes in different stages and organs with the highest levels of transcript expression of ZmNLP6 and ZmNLP8. When ZmNLP6 and ZmNLP8 were overexpressed in the Arabidopsis nitrate regulatory gene mutant nlp7-4, nitrate assimilation and induction of nitrate-responsive genes in the transgenic plants were recovered to WT levels, indicating that ZmNLP6 and ZmNLP8 can replace the essential roles of the master nitrate regulatory gene AtNLP7 in nitrate signaling and metabolism. ZmNLP6 and ZmNLP8 are localized in the nucleus and can bind candidate nitrate-responsive cis-elements in vitro. The biomass and yield of transgenic Arabidopsis lines overexpressing ZmNLP6 and ZmNLP8 showed significant increase compared with WT and nlp7-4 mutant line in low nitrate conditions. Thus, ZmNLP6 and ZmNLP8 regulate nitrate signaling in transgenic Arabidopsis plants and may be potential candidates for improving nitrogen use efficiency of maize.

Dancing with Hormones: A Current Perspective of Nitrate Signaling and Regulation in Arabidopsis.

In nature and agriculture, nitrate availability is a main environmental cue for plant growth, development and stress responses. Nitrate signaling and regulation are hence at the center of communications between plant intrinsic programs and the environment. It is also well known that endogenous phytohormones play numerous critical roles in integrating extrinsic cues and intrinsic responses, regulating and refining almost all aspects of plant growth, development and stress responses. Therefore, interaction between nitrate and phytohormones, such as auxins, cytokinins, abscisic acid, gibberellins, and ethylene, is prevalent. The growing evidence indicates that biosynthesis, de-conjugation, transport, and signaling of hormones are partly controlled by nitrate signaling. Recent advances with nitrate signaling and transcriptional regulation in Arabidopsis give rise to new paradigms. Given the comprehensive nitrate transport, sensing, signaling and regulations at the level of the cell and organism, nitrate itself is a local and long-distance signal molecule, conveying N status at the whole-plant level. A direct molecular link between nitrate signaling and cell cycle progression was revealed with TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR1-20 (TCP20) – NIN-LIKE PROTEIN 6/7 (NLP6/7) regulatory nexus. NLPs are key regulators of nitrogen responses in plants. TCPs function as the main regulators of plant morphology and architecture, with the emerging role as integrators of plant developmental responses to the environment. By analogy with auxin being proposed as a plant morphogen, nitrate may be an environmental morphogen. The morphogen-gradient-dependent and cell-autonomous mechanisms of nitrate signaling and regulation are an integral part of cell growth and cell identification. This is especially true in root meristem growth that is regulated by intertwined nitrate, phytohormones, and glucose-TOR signaling pathways. Furthermore, the nitrate transcriptional hierarchy is emerging. Nitrate regulators in primary nitrate signaling can individually and combinatorially control downstream transcriptional networks and hormonal pathways for signal propagation and amplification. Under the new paradigms, nitrate-induced hormone metabolism and signaling deserve fresh examination. The close interplay and convergent regulation of nitrate and hormonal signaling at morphological, physiological, and molecular levels have significant effects on important agronomic traits, especially nutrient-dependent adaptive root system growth and architecture.

Nitrate induction of root hair density is mediated by TGA1/TGA4 and CPC transcription factors in Arabidopsis thaliana.

Root hairs are specialized cells that are important for nutrient uptake. It is well established that nutrients such as phosphate have a great influence on root hair development in many plant species. Here we investigated the role of nitrate on root hair development at a physiological and molecular level. We showed that nitrate increases root hair density in Arabidopsis thaliana. We found that two different root hair defective mutants have significantly less nitrate than wild-type plants, suggesting that in A.thaliana root hairs have an important role in the capacity to acquire nitrate. Nitrate reductase-null mutants exhibited nitrate-dependent root hair phenotypes comparable with wild-type plants, indicating that nitrate is the signal that leads to increased formation of root hairs. We examined the role of two key regulators of root hair cell fate, CPC and WER, in response to nitrate treatments. Phenotypic analyses of these mutants showed that CPC is essential for nitrate-induced responses of root hair development. Moreover, we showed that NRT1.1 and TGA1/TGA4 are required for pathways that induce root hair development by suppression of longitudinal elongation of trichoblast cells in response to nitrate treatments. Our results prompted a model where nitrate signaling via TGA1/TGA4 directly regulates the CPC root hair cell fate specification gene to increase formation of root hairs in A.thaliana.

The Arabidopsis CPSF30-L gene plays an essential role in nitrate signaling and regulates the nitrate transceptor gene NRT1.1.

Plants have evolved sophisticated mechanisms to adapt to fluctuating environmental nitrogen availability. However, more underlying genes regulating the response to nitrate have yet to be characterized. We report here the identification of a nitrate regulatory mutant whose mutation mapped to the Cleavage and Polyadenylation Specificity Factor 30 gene (CPSF30-L). In the mutant, induction of nitrate-responsive genes was inhibited independent of the ammonium conditions and was restored by expression of the wild-type 65kDa encoded by CPSF30-L. Molecular and genetic evidence suggests that CPSF30-L works upstream of NRT1.1 and independently of NLP7 in response to nitrate. Analysis of the 3'-UTR of NRT1.1 showed that the pattern of polyadenylation sites was altered in the cpsf30 mutant. Transcriptome analysis revealed that four nitrogen-related clusters were enriched in the differentially expressed genes of the cpsf30 mutant. Nitrate uptake was decreased in the mutant along with reduced expression of the nitrate transporter/sensor gene NRT1.1, while nitrate reduction and amino acid content were enhanced in roots along with increased expression of several nitrate assimilatory genes. These findings indicate that the 65kDa protein encoded by CPSF30-L mediates nitrate signaling in part by regulating NRT1.1 expression, thus adding an important component to the nitrate signaling network.

Up-regulation of CmNRTs and CmANR1 genes expression contribute to root configuration changes for efficient capturing NO3− in the roots of chrysanthemum

Nitrogen is very important to the yield and quality of chrysanthemum. However, excessive application not only negatively affects the yield and quality of chrysanthemum, but also can not be timely absorbed and utilized for chrysanthemum, so causes a lot of waste of fertilizer resources and serious pollution of groundwater environment. Nitrate nitrogen is the main form of nitrogen uptake by the roots of chrysanthemum. The uptake of NO3− by plants is mostly achieved by NO3− transporters (NRTs) and lateral root formation. However, the morphological structure and molecular mechanism of the response to nitrate signaling in the roots of chrysanthemum are poorly understood. In this study, we investigated that the response to NO3− signaling from the viewpoint of roots morphology and anatomical structure, roots NO3− contents, endogenous hormones contents and expression of related genes in the roots of chrysanthemum. A hydroponic experiment was performed using the rooted cuttings of chrysanthemum, with KNO3 treatments (control for Hogland nutrient solution without N element) for seven times at 0, 1, 2, 3, 4, 5, 6 days, respectively. The results showed that the roots morphological indexes increased significantly in the KNO3 treatments compared to those of the controls at 6days after treatments, which were associated with the high expression of CmNRT1.1, CmNRT2.1, CmNAR2.1 and CmANR1, which related to NO3− transport genes and lateral root development gene, the contents of NO3−, indole-3-acetic acid (IAA) and cytokinin (CTK) in the roots and the improved root microscopic structure in KNO3 treatment. The promoting effect of KNO3 shows an obvious time-effect. The results from correlation analysis indicated that the responses of chrysanthemum root to NO3− signaling could be related with the induction of NO3− signaling, the gene expression of NO3− transporter genes and lateral root development gene due to regulating the IAA, CTK levels and adjusting the root architecture of the root system.

Low genetic diversity and functional constraint of miRNA genes participating pollen-pistil interaction in rice.

In this study, we sequenced and analyzed the expression and evolution of rice miRNA genes participating pollen-pistil interaction that is crucial to rice yield. Pollen-pistil interaction is an essential reproductive process for all flowering plants. While microRNAs (miRNAs) are important noncoding small RNAs that regulate mRNA levels in eukaryotic cells, there is little knowledge about which miRNAs involved in the early stages of pollen-pistil interaction in rice and how they evolve under this conserved process. In this study, we sequenced the small RNAs in rice from unpollinated pistil (R0), pistil from 5min and 15min after pollination, respectively, to identify known and novel miRNAs that are involved in this process. By comparing the corresponding mRNA-seq dataset, we identified a group of miRNAs with strong negative expression pattern with their target genes. Further investigation of all miRNA loci (MIRNAs) across 1083 public rice accessions revealed significantly reduced genetic diversity in MIRNAs with strong negative expression of their targets when comparing to those with little or no impact on targets during pollen-pistil interaction. Annotation of targets suggested that those MIRNAs with strong impact on targets were pronounced in cell wall related processes such as xylan metabolism. Additionally, plant conserved miRNAs, such as those with functions in gibberellic acid, auxin and nitrate signaling, were also with strong negative expression of their targets. Overall, our analyses identified key miRNAs participating pollen-pistil interaction and their evolutionary patterns in rice, which can facilitate the understanding of molecular mechanisms associated with seed setting.

Records of sunspots and aurora candidates in the Chinese official histories of the Yuán and Míng dynasties during 1261–1644

Abstract Records of observations of sunspots and auroras in pre-telescopic historical documents provide useful information about past solar activity both in long-term trends and short-term space weather events. In this study, we present the results of a comprehensive survey of the records of sunspots and aurora candidates in the Yuánshĭ and Míngshĭ, Chinese Official Histories spanning 1261–1368 and 1368–1644, based on continuous observations with well-formatted reportds conducted by contemporary professional astronomers. We then provide a brief comparison of these data with Total Solar Irradiance (TSI) as an indicator of the solar activity during the corresponding periods to show significant active phases between the 1350s–80s and 1610s–30s. We then compared the former with contemporary Russian reports concerning naked-eye sunspots and the latter with contemporary sunspot drawings based on Western telescopic observations. Especially some of the latter are consistent with nitrate signals preserved in ice cores. These results show us some insights on and beyond minima and maxima of solar activity during the 13th–17th centuries.

Emerging Players in the Nitrate Signaling Pathway

Nitrogen (N)-based fertilizers are routinely used to increase agricultural productivity for both food and non-food uses of crops. Unfortunately, excess N fertilizers escape to the environment, leading to detrimental effects on the ecosystem and human health. Understanding how plants sense and respond to different N nutrients or metabolites to regulate metabolism, physiology, growth, and development is essential for sustained yields while reducing agriculture's environmental and economic costs. Several recent publications report important components of the nitrate signaling pathway, and the role of Ca2+ as a second messenger downstream of nitrate perception in roots and shoots is now clear.

Genome-wide analysis of NIN-like protein (NLP) family in maize (Zea mays L.) by using bioinformatic methods

The NIN-like proteins (NLP) belong to RWP-RK transcription factor family and possess the similarity characteristics of NIN (Nodules INception). The NLPs regulate the expression of genes which are involved in nitrate signaling in plants. In this work, we have performed a genome-wide analysis of the NLP gene family in maize (Zea mays L.) through the bioinformatic methods. We identified a total of nine NLP encoding genes in whole genome of maize. The genomic sequences of these genes were from 2855 to 8092 nucleotides in length and contained three or four introns. Their predicted protein sizes ranged in size from 786 to 945 amino acids. The theoretical isoelectric point values of most deduced protein were less than 7. The maize NLP proteins possessed conserved regions of plant NLP at N-terminal as well as at C-terminal including the RWP-RK and PB1 domains. Based on the phylogenic analysis, we detected three current whole-genome gene duplication events which occurred in maize genome apter speciation point. All of NLP genes expressed in tissues at different development stages, from germinating seed to maturation seed were examined. The ZmNLP5, ZmNLP6 and ZmNLP7 were weakly expressed in comparison to others genes in most examined tissues.

Nitrate transporters: an overview in legumes.

The nitrate transporters, belonging to NPF and NRT2 families, play critical roles in nitrate signaling, root growth and nodule development in legumes. Nitrate plays an essential role during plant development as nutrient and also as signal molecule, in both cases working via the activity of nitrate transporters. To date, few studies on NRT2 or NPF nitrate transporters in legumes have been reported, and most of those concern Lotus japonicus and Medicago truncatula. A molecular characterization led to the identification of 4 putative LjNRT2 and 37 putative LjNPF gene sequences in L. japonicus. In M. truncatula, the NRT2 family is composed of 3 putative members. Using the new genome annotation of M. truncatula (Mt4.0), we identified, for this review, 97 putative MtNPF sequences, including 32 new sequences relative to previous studies. Functional characterization has been published for only two MtNPF genes, encoding nitrate transporters of M. truncatula. Both transporters have a role in root system development via abscisic acid signaling: MtNPF6.8 acts as a nitrate sensor during the cell elongation of the primary root, while MtNPF1.7 contributes to the cellular organization of the root tip and nodule formation. An in silico expression study of MtNPF genes confirmed that NPF genes are expressed in nodules, as previously shown for L. japonicus, suggesting a role for the corresponding proteins in nitrate transport, or signal perception in nodules. This review summarizes our knowledge of legume nitrate transporters and discusses new roles for these proteins based on recent discoveries.

Nitrate signalling: Calcium bridges the nitrate gap.

The nitrate signalling pathway now has a backbone. CPK calcium-dependent kinases are the long-awaited molecular link between major players in this pathway, the membrane-located nitrate transceptor NRT1.1 and the NLP transcription factors.

QTL analysis of the developmental response to L-glutamate in Arabidopsis roots and its genotype-by-environment interactions.

Primary root growth in Arabidopsis and a number of other species has previously been shown to be remarkably sensitive to the presence of external glutamate, with glutamate signalling eliciting major changes in root architecture. Using two recombinant inbred lines from reciprocal crosses between Arabidopsis accessions C24 and Col-0, we have identified one large-effect quantitative trait locus (QTL), GluS1, and two minor QTLs, GluS2 and GluS3, which together accounted for 41% of the phenotypic variance in glutamate sensitivity. The presence of the GluS1 locus on chromosome 3 was confirmed using a set of C24/Col-0 isogenic lines. GluS1 was mapped to an interval between genes At3g44830-At3g46880. When QTL mapping was repeated under a range of environmental conditions, including temperature, shading and nitrate supply, a strong genotype-by-environment interaction in the controls for the glutamate response was identified. Major differences in the loci controlling this trait were found under different environmental conditions. Here we present evidence for the existence of loci on chromosomes 1 and 5 epistatically controlling the response of the GluS1 locus to variations in ambient temperature, between 20°C and 26°C. In addition, a locus on the long arm of chromosome 1 was found to play a major role in controlling the ability of external nitrate signals to antagonize the glutamate effect. We conclude that there are multiple loci controlling natural variation in glutamate sensitivity in Arabidopsis roots and that epistatic interactions play an important role in modulating glutamate sensitivity in response to changes in environmental conditions.

Nitrate: A Crucial Signal during Lateral Roots Development.

Root plasticity is an important trait for plants to forage nutrient and adapt to survival in a complicated environment. Lateral roots (LRs) are generally more sensitive than primary roots in response to changing environmental conditions. As the main source of nitrogen for most higher plants, nitrate acting as a signal has received great attention in the regulation of LR development. In general, there are dual effects including stimulatory and inhibitory of low nitrate on LR development; while high nitrate supply has an inhibitory effect on LR development; nitrate heterogeneity also has a stimulatory effect on LR development in - rich zone. Here, we focus on recent progresses in the role of a nitrate signal in the regulation of the LRs development.

Nitrate signaling and early responses in Arabidopsis roots.

Nitrogen (N) is an essential macronutrient that impacts many aspects of plant physiology, growth, and development. Besides its nutritional role, N nutrient and metabolites act as signaling molecules that regulate the expression of a wide range of genes and biological processes. In this review, we describe recent advances in the understanding of components of the nitrate signaling pathway. Recent evidence posits that in one nitrate signaling pathway, nitrate sensed by NRT1.1 activates a phospholipase C activity that is necessary for increased cytosolic calcium levels. The nitrate-elicited calcium increase presumably activates calcium sensors, kinases, or phosphatases, resulting in changes in expression of primary nitrate response genes. Consistent with this model, nitrate treatments elicit proteome-wide changes in phosphorylation patterns in a wide range of proteins, including transporters, metabolic enzymes, kinases, phosphatases, and other regulatory proteins. Identifying and characterizing the function of the different players involved in this and other nitrate signaling pathways and their functional relationships is the next step to understand N responses in plants.

Interacting TCP and NLP transcription factors control plant responses to nitrate availability.

Plants have evolved adaptive strategies that involve transcriptional networks to cope with and survive environmental challenges. Key transcriptional regulators that mediate responses to environmental fluctuations in nitrate have been identified; however, little is known about how these regulators interact to orchestrate nitrogen (N) responses and cell-cycle regulation. Here we report that teosinte branched1/cycloidea/proliferating cell factor1-20 (TCP20) and NIN-like protein (NLP) transcription factors NLP6 and NLP7, which act as activators of nitrate assimilatory genes, bind to adjacent sites in the upstream promoter region of the nitrate reductase gene, NIA1, and physically interact under continuous nitrate and N-starvation conditions. Regions of these proteins necessary for these interactions were found to include the type I/II Phox and Bem1p (PB1) domains of NLP6&7, a protein-interaction module conserved in animals for nutrient signaling, and the histidine- and glutamine-rich domain of TCP20, which is conserved across plant species. Under N starvation, TCP20-NLP6&7 heterodimers accumulate in the nucleus, and this coincides with TCP20 and NLP6&7-dependent up-regulation of nitrate assimilation and signaling genes and down-regulation of the G2/M cell-cycle marker gene, CYCB1;1 TCP20 and NLP6&7 also support root meristem growth under N starvation. These findings provide insights into how plants coordinate responses to nitrate availability, linking nitrate assimilation and signaling with cell-cycle progression.

Nitrate Supply-Dependent Shifts in Communities of Root-Associated Bacteria in Arabidopsis.

Root-associated bacterial communities are necessary for healthy plant growth. Nitrate is a signal molecule as well as a major nitrogen source for plant growth. In this study, nitrate-dependent alterations in root-associated bacterial communities and the relationship between nitrate signaling and root-associated bacteria in Arabidopsis were examined. The bacterial community was analyzed by a ribosomal RNA intergenic spacer analysis (RISA) and 16S rRNA amplicon sequencing. The Arabidopsis root-associated bacterial community shifted depending on the nitrate amount and timing of nitrate application. The relative abundance of operational taxonomic units of 25.8% was significantly changed by the amount of nitrate supplied. Moreover, at the family level, the relative abundance of several major root-associated bacteria including Burkholderiaceae, Paenibacillaceae, Bradyrhizobiaceae, and Rhizobiaceae markedly fluctuated with the application of nitrate. These results suggest that the application of nitrate strongly affects root-associated bacterial ecosystems in Arabidopsis. Bulk soil bacterial communities were also affected by the application of nitrate; however, these changes were markedly different from those in root-associated bacteria. These results also suggest that nitrate-dependent alterations in root-associated bacterial communities are mainly affected by plant-derived factors in Arabidopsis. T-DNA insertion plant lines of the genes for two transcription factors involved in nitrate signaling in Arabidopsis roots, NLP7 and TCP20, showed similar nitrate-dependent shifts in root-associated bacterial communities from the wild-type, whereas minor differences were observed in root-associated bacteria. Thus, these results indicate that NLP7 and TCP20 are not major regulators of nitrate-dependent bacterial communities in Arabidopsis roots.

Direct DOC and nitrate determination in water using dual pathlength and second derivative UV spectrophotometry

UV spectrophotometry is largely used for water and wastewater quality monitoring. The measurement/estimation of specific and aggregate parameters such as nitrate and dissolved organic carbon (DOC) is possible with UV spectra exploitation, from 2 to multi wavelengths calibration. However, if nitrate determination from UV absorbance is known, major optical interferences linked to the presence of suspended solids, colloids or dissolved organic matter limit the relevance of UV measurement for DOC assessment. A new method based on UV spectrophotometric measurement of raw samples (without filtration) coupling a dual pathlength for spectra acquisition and the second derivative exploitation of the signal is proposed in this work. The determination of nitrate concentration is carried out from the second derivative of the absorbance at 226 nm corresponding at the inflexion point of nitrate signal decrease. A short optical pathlength can be used considering the strong absorption of nitrate ion around 210 nm. For DOC concentration determination the second derivative absorbance at 295 nm is proposed after nitrate correction. Organic matter absorbing slightly in the 270–330 nm window, a long optical pathlength must be selected in order to increase the sensitivity. The method was tested on several hundred of samples from small rivers of two agricultural watersheds located in Brittany, France, taken during dry and wet periods. The comparison between the proposed method and the standardised procedures for nitrate and DOC measurement gave a good adjustment for both parameters for ranges of 2–100 mg/L NO3 and 1–30 mg/L DOC.

NIN-like protein 8 is a master regulator of nitrate-promoted seed germination in Arabidopsis.

Seeds respond to multiple different environmental stimuli that regulate germination. Nitrate stimulates germination in many plants but how it does so remains unclear. Here we show that the Arabidopsis NIN-like protein 8 (NLP8) is essential for nitrate-promoted seed germination. Seed germination in nlp8 loss-of-function mutants does not respond to nitrate. NLP8 functions even in a nitrate reductase-deficient mutant background, and the requirement for NLP8 is conserved among Arabidopsis accessions. NLP8 reduces abscisic acid levels in a nitrate-dependent manner and directly binds to the promoter of CYP707A2, encoding an abscisic acid catabolic enzyme. Genetic analysis shows that NLP8-mediated promotion of seed germination by nitrate requires CYP707A2. Finally, we show that NLP8 localizes to nuclei and unlike NLP7, does not appear to be activated by nitrate-dependent nuclear retention of NLP7, suggesting that seeds have a unique mechanism for nitrate signalling.

Calcium-Mediated Abiotic Stress Signaling in Roots.

Roots are subjected to a range of abiotic stresses as they forage for water and nutrients. Cytosolic free calcium is a common second messenger in the signaling of abiotic stress. In addition, roots take up calcium both as a nutrient and to stimulate exocytosis in growth. For calcium to fulfill its multiple roles must require strict spatio-temporal regulation of its uptake and efflux across the plasma membrane, its buffering in the cytosol and its sequestration or release from internal stores. This prompts the question of how specificity of signaling output can be achieved against the background of calcium’s other uses. Threats to agriculture such as salinity, water availability and hypoxia are signaled through calcium. Nutrient deficiency is also emerging as a stress that is signaled through cytosolic free calcium, with progress in potassium, nitrate and boron deficiency signaling now being made. Heavy metals have the capacity to trigger or modulate root calcium signaling depending on their dose and their capacity to catalyze production of hydroxyl radicals. Mechanical stress and cold stress can both trigger an increase in root cytosolic free calcium, with the possibility of membrane deformation playing a part in initiating the calcium signal. This review addresses progress in identifying the calcium transporting proteins (particularly channels such as annexins and cyclic nucleotide-gated channels) that effect stress-induced calcium increases in roots and explores links to reactive oxygen species, lipid signaling, and the unfolded protein response.