Ethylene signalling is involved in regulation of phosphate starvation-induced gene expression and production of acid phosphatases and anthocyanin in Arabidopsis

The AtMINPP Gene, Encoding a Multiple Inositol Polyphosphate Phosphatase, Coordinates a Novel Crosstalk between Phytic Acid Metabolism and Ethylene Signal Transduction in Leaf Senescence

Plant senescence is a highly coordinated process that is intricately regulated by numerous endogenous and environmental signals. The involvement of phytic acid in various cell signaling and plant processes has been recognized, but the specific roles of phytic acid metabolism in Arabidopsis leaf senescence remain unclear. Here, we demonstrate that in Arabidopsis thaliana the multiple inositol phosphate phosphatase (AtMINPP) gene, encoding an enzyme with phytase activity, plays a crucial role in regulating leaf senescence by coordinating the ethylene signal transduction pathway. Through overexpressing AtMINPP (AtMINPP-OE), we observed early leaf senescence and reduced chlorophyll contents. Conversely, a loss-of-function heterozygous mutant (atminpp/+) exhibited the opposite phenotype. Correspondingly, the expression of senescence-associated genes (SAGs) was significantly upregulated in AtMINPP-OE but markedly decreased in atminpp/+. Yeast one-hybrid and chromatin immunoprecipitation assays indicated that the EIN3 transcription factor directly binds to the promoter of AtMINPP. Genetic analysis further revealed that AtMINPP-OE could accelerate the senescence of ein3-1eil1-3 mutants. These findings elucidate the mechanism by which AtMINPP regulates ethylene-induced leaf senescence in Arabidopsis, providing insights into the genetic manipulation of leaf senescence and plant growth.

Soil and Mineral Nutrients in Plant Health: A Prospective Study of Iron and Phosphorus in the Growth and Development of Plants

Plants being sessile are exposed to different environmental challenges and consequent stresses associated with them. With the prerequisite of minerals for growth and development, they coordinate their mobilization from the soil through their roots. Phosphorus (P) and iron (Fe) are macro- and micronutrient; P serves as an important component of biological macromolecules, besides driving major cellular processes, including photosynthesis and respiration, and Fe performs the function as a cofactor for enzymes of vital metabolic pathways. These minerals help in maintaining plant vigor via alterations in the pH, nutrient content, release of exudates at the root surface, changing dynamics of root microbial population, and modulation of the activity of redox enzymes. Despite this, their low solubility and relative immobilization in soil make them inaccessible for utilization by plants. Moreover, plants have evolved distinct mechanisms to cope with these stresses and coregulate the levels of minerals (Fe, P, etc.) toward the maintenance of homeostasis. The present study aims at examining the uptake mechanisms of Fe and P, and their translocation, storage, and role in executing different cellular processes in plants. It also summarizes the toxicological aspects of these minerals in terms of their effects on germination, nutrient uptake, plant-water relationship, and overall yield. Considered as an important and indispensable component of sustainable agriculture, a separate section covers the current knowledge on the cross-talk between Fe and P and integrates complete and balanced information of their effect on plant hormone levels.

WRKY33 negatively regulates anthocyanin biosynthesis and cooperates with PHR1 to mediate acclimation to phosphate starvation

Anthocyanin accumulation is acknowledged as a phenotypic indicator of phosphate (Pi) starvation. However, negative regulators of this process and their molecular mechanisms remain largely unexplored. In this study, we demonstrate that WRKY33 acts as a negative regulator of phosphorus-status-dependent anthocyanin biosynthesis. WRKY33 regulates the expression of the gene encoding dihydroflavonol 4-reductase (DFR), a rate-limiting enzyme in anthocyanin production, both directly and indirectly. WRKY33 binds directly to the DFR promoter to repress its expression and also interferes with the MBW complex through interacting with PAP1 to indirectly influence DFR transcriptional activation. Under -Pi conditions, PHR1 interacts with WRKY33, and the protein level of WRKY33 decreases; the repression of DFR expression by WRKY33 is thus attenuated, leading to anthocyanin accumulation in Arabidopsis. Further genetic and biochemical assays suggest that PHR1 is also involved in regulating factors that affect WRKY33 protein turnover. Taken together, our findings reveal that Pi starvation represses WRKY33, a repressor of anthocyanin biosynthesis, to finely tune anthocyanin biosynthesis. This "double-negative logic" regulation of phosphorus-status-dependent anthocyanin biosynthesis is required for the maintenance of plant metabolic homeostasis during acclimation to Pi starvation.

Review: Nutrient-nutrient interactions governing underground plant adaptation strategies in a heterogeneous environment

Plant growth relies on the mineral nutrients present in the rhizosphere. The distribution of nutrients in soils varies depending on their mobility and capacity to bind with soil particles. Consequently, plants often encounter either low or high levels of nutrients in the rhizosphere. Plant roots are the essential organs that sense changes in soil mineral content, leading to the activation of signaling pathways associated with the adjustment of plant architecture and metabolic responses. During differential availability of minerals in the rhizosphere, plants trigger adaptation strategies such as cellular remobilization of minerals, secretion of organic molecules, and the attenuation or enhancement of root growth to balance nutrient uptake. The interdependency, availability, and uptake of minerals, such as phosphorus (P), iron (Fe), zinc (Zn), potassium (K), nitrogen (N) forms, nitrate (NO3-), and ammonium (NH4+), modulate the root architecture and metabolic functioning of plants. Here, we summarized the interactions of major nutrients (N, P, K, Fe, Zn) in shaping root architecture, physiological responses, genetic components involved, and address the current challenges associated with nutrient-nutrient interactions. Furthermore, we discuss the major gaps and opportunities in the field for developing plants with improved nutrient uptake and use efficiency for sustainable agriculture.

A multi-omic survey of black cottonwood tissues highlights coordinated transcriptomic and metabolomic mechanisms for plant adaptation to phosphorus deficiency

Introduction

Phosphorus (P) deficiency in plants creates a variety of metabolic perturbations that decrease photosynthesis and growth. Phosphorus deficiency is especially challenging for the production of bioenergy feedstock plantation species, such as poplars (Populus spp.), where fertilization may not be practically or economically feasible. While the phenotypic effects of P deficiency are well known, the molecular mechanisms underlying whole-plant and tissue-specific responses to P deficiency, and in particular the responses of commercially valuable hardwoods, are less studied.

Methods

We used a multi-tissue and multi-omics approach using transcriptomic, proteomic, and metabolomic analyses of the leaves and roots of black cottonwood (Populus trichocarpa) seedlings grown under P-deficient (5 µM P) and replete (100 µM P) conditions to assess this knowledge gap and to identify potential gene targets for selection for P efficiency.

Results

In comparison to seedlings grown at 100 µM P, P-deficient seedlings exhibited reduced dry biomass, altered chlorophyll fluorescence, and reduced tissue P concentrations. In line with these observations, growth, C metabolism, and photosynthesis pathways were downregulated in the transcriptome of the P-deficient plants. Additionally, we found evidence of strong lipid remodeling in the leaves. Metabolomic data showed that the roots of P-deficient plants had a greater relative abundance of phosphate ion, which may reflect extensive degradation of P-rich metabolites in plants exposed to long-term P-deficiency. With the notable exception of the KEGG pathway for Starch and Sucrose Metabolism (map00500), the responses of the transcriptome and the metabolome to P deficiency were consistent with one another. No significant changes in the proteome were detected in response to P deficiency.

Discussion and conclusion

Collectively, our multi-omic and multi-tissue approach enabled the identification of important metabolic and regulatory pathways regulated across tissues at the molecular level that will be important avenues to further evaluate for P efficiency. These included stress-mediating systems associated with reactive oxygen species maintenance, lipid remodeling within tissues, and systems involved in P scavenging from the rhizosphere.

Correlations between root phosphorus acquisition and foliar phosphorus allocation reveal how grazing promotes plant phosphorus utilization

Overgrazing and phosphorus (P) deficiency are two major factors limiting the sustainable development of grassland ecosystems. Exploring plant P utilization and acquisition strategies under grazing can provide a solid basis for determining a reasonable grazing intensity. Both foliar P allocation and root P acquisition are crucial mechanisms for plants to adapt to environmental P availability; however, their changing characteristics and correlation under grazing remain unknown. Here, we investigated foliar P fractions, root P-acquisition traits and gene expression, as well as rhizosphere and bulk soil properties of two dominant plant species, Leymus chinensis (a rhizomatous grass) and Stipa grandis (a bunchgrass), in a field grazing intensity gradient site in Inner Mongolia. Grazing induced different degrees of compensatory growth in the two dominant plant species, increased rhizosphere P availability, and alleviated plant P limitation. Under grazing, the foliar metabolite P of L. chinensis increased, whereas the nucleic acid P of S. grandis increased. Increased P fractions in L. chinensis were positively correlated with increased root exudates and rapid inorganic P absorption. For S. grandis, increased foliar P fractions were positively correlated with more fine roots, more root exudates, and up-regulated expression of genes involved in defense and P metabolism. Overall, efficient root P mobilization and uptake traits, as well as increases in leaf metabolic activity-related P fractions, supported plant compensatory growth under grazing, a process that differed between tiller types. The highest plant productivity and leaf metabolic activity-related P concentrations under medium grazing intensity clarify the underlying basis for sustainable livestock production.

Soybean type-B response regulator GmRR1 mediates phosphorus uptake and yield by modifying root architecture

Phosphorus (P) plays a pivotal role in plant growth and development. Low P stress can greatly hamper plant growth. Here, we identified a QTL (named QPH-9-1), which is associated with P efficiency across multiple environments through linkage analysis and genome-wide association study. Furthermore, we successfully cloned the underlying soybean (Glycine max) gene GmRR1 (a soybean type-B Response Regulator 1) that encodes a type-B response regulator protein. Knockout of GmRR1 resulted in a substantial increase in plant height, biomass, P uptake efficiency, and yield-related traits due to the modification of root structure. In contrast, overexpression of GmRR1 in plants resulted in a decrease in these phenotypes. Further analysis revealed that knockout of GmRR1 substantially increased the levels of auxin and ethylene in roots, thereby promoting root hair formation and growth by promoting the formation of root hair primordium and lengthening the root apical meristem. Yeast two-hybrid, bimolecular fluorescence complementation, and dual-luciferase assays demonstrated an interaction between GmRR1 and Histidine-containing Phosphotransmitter protein 1. Expression analysis suggested that these proteins coparticipated in response to low P stress. Analysis of genomic sequences showed that GmRR1 underwent a selection during soybean domestication. Taken together, this study provides further insights into how plants respond to low P stress by modifying root architecture through phytohormone pathways.

The Ethylene Biosynthetic Enzymes, 1-Aminocyclopropane-1-Carboxylate (ACC) Synthase (ACS) and ACC Oxidase (ACO): The Less Explored Players in Abiotic Stress Tolerance

Ethylene is an essential plant hormone, critical in various physiological processes. These processes include seed germination, leaf senescence, fruit ripening, and the plant's response to environmental stressors. Ethylene biosynthesis is tightly regulated by two key enzymes, namely 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO). Initially, the prevailing hypothesis suggested that ACS is the limiting factor in the ethylene biosynthesis pathway. Nevertheless, accumulating evidence from various studies has demonstrated that ACO, under specific circumstances, acts as the rate-limiting enzyme in ethylene production. Under normal developmental processes, ACS and ACO collaborate to maintain balanced ethylene production, ensuring proper plant growth and physiology. However, under abiotic stress conditions, such as drought, salinity, extreme temperatures, or pathogen attack, the regulation of ethylene biosynthesis becomes critical for plants' survival. This review highlights the structural characteristics and examines the transcriptional, post-transcriptional, and post-translational regulation of ACS and ACO and their role under abiotic stress conditions. Reviews on the role of ethylene signaling in abiotic stress adaptation are available. However, a review delineating the role of ACS and ACO in abiotic stress acclimation is unavailable. Exploring how particular ACS and ACO isoforms contribute to a specific plant's response to various abiotic stresses and understanding how they are regulated can guide the development of focused strategies. These strategies aim to enhance a plant's ability to cope with environmental challenges more effectively.

A Specific Protective Mechanism Against Chloroplast Photo-Reactive Oxygen Species in Phosphate-Starved Rice Plants

Phosphorus (Pi) starvation prevents a good match between light energy absorption and photosynthetic carbon metabolism, generating photo-reactive oxygen species (photo-ROS) in chloroplasts. Plants have evolved to withstand photo-oxidative stress, but the key regulatory mechanism underlying it remains unclear. In rice (Oryza sativa), DEEP GREEN PANICLE1 (DGP1) is robustly up-regulated in response to Pi deficiency. DGP1 decreases the DNA-binding capacities of the transcriptional activators GLK1/2 on the photosynthetic genes involved in chlorophyll biosynthesis, light harvesting, and electron transport. This Pi-starvation-induced mechanism dampens both electron transport rates through photosystem I and II (ETRI and ETRII) and thus mitigates the electron-excessive stress in mesophyll cells. Meanwhile, DGP1 hijacks glycolytic enzymes GAPC1/2/3, redirecting glucose metabolism toward the pentose phosphate pathway with superfluous NADPH production. Phenotypically, light irradiation induces O2 - production in Pi-starved WT leaves but is observably accelerated in dgp1 mutant and impaired in GAPCsRNAi and glk1glk2 lines. Interestingly, overexpressed DGP1 in rice caused hyposensitivity to ROS-inducers (catechin and methyl viologen), but the dgp1 mutant shows a similar inhibitory phenotype with the WT seedlings. Overall, the DGP1 gene serves as a specific antagonizer against photo-ROS in Pi-starved rice plants, which coordinates light-absorbing and anti-oxidative systems by orchestrating transcriptional and metabolic regulations, respectively.

Phloem-Mobile MYB44 Negatively Regulates Expression of PHOSPHATE TRANSPORTER 1 in Arabidopsis Roots

Phosphorus (P) is an essential plant macronutrient; however, its availability is often limited in soils. Plants have evolved complex mechanisms for efficient phosphate (Pi) absorption, which are responsive to changes in external and internal Pi concentration, and orchestrated through local and systemic responses. To explore these systemic Pi responses, here we identified AtMYB44 as a phloem-mobile mRNA, an Arabidopsis homolog of Cucumis sativus MYB44, that is responsive to the Pi-starvation stress. qRT-PCR assays revealed that AtMYB44 was up-regulated and expressed in both shoot and root in response to Pi-starvation stress. The atmyb44 mutant displayed higher shoot and root biomass compared to wild-type plants, under Pi-starvation conditions. Interestingly, the expression of PHOSPHATE TRANSPORTER1;2 (PHT1;2) and PHT1;4 was enhanced in atmyb44 in response to a Pi-starvation treatment. A split-root assay showed that AtMYB44 expression was systemically regulated under Pi-starvation conditions, and in atmyb44, systemic controls on PHT1;2 and PHT1;4 expression were moderately disrupted. Heterografting assays confirmed graft transmission of AtMYB44 transcripts, and PHT1;2 and PHT1;4 expression was decreased in heterografted atmyb44 rootstocks. Taken together, our findings support the hypothesis that mobile AtMYB44 mRNA serves as a long-distance Pi response signal, which negatively regulates Pi transport and utilization in Arabidopsis.

Hormonal regulation of anthocyanin biosynthesis for improved stress tolerance in plants

Due to unprecedented climate change, rapid industrialization and increasing use of agrochemicals, abiotic stress, such as drought, low temperature, high salinity and heavy metal pollution, has become an increasingly serious problem in global agriculture. Anthocyanins, an important plant pigment, are synthesized through the phenylpropanoid pathway and have a variety of physiological and ecological functions, providing multifunctional and effective protection for plants under stress. Foliar anthocyanin accumulation often occurs under abiotic stress including high light, cold, drought, salinity, nutrient deficiency and heavy metal stress, causing leaf reddening or purpling in many plant species. Anthocyanins are used as sunscreens and antioxidants to scavenge reactive oxygen species (ROS), as metal(loid) chelators to mitigate heavy metal stress, and as crucial molecules with a role in delaying leaf senescence. In addition to environmental factors, anthocyanin synthesis is affected by various endogenous factors. Plant hormones such as abscisic acid, jasmonic acid, ethylene and gibberellin have been shown to be involved in regulating anthocyanin synthesis either positively or negatively. Particularly when plants are under abiotic stress, several plant hormones can induce foliar anthocyanin synthesis to enhance plant stress resistance. In this review, we revisit the role of plant hormones in anthocyanin biosynthesis and the mechanism of plant hormone-mediated anthocyanin accumulation and abiotic stress tolerance. We conclude that enhancing anthocyanin content with plant hormones could be a prospective management strategy for improving plant stress resistance, but extensive further research is essentially needed to provide future guidance for practical crop production.

Control of histone demethylation by nuclear-localized α-ketoglutarate dehydrogenase

Methylations on nucleosomal histones play fundamental roles in regulating eukaryotic transcription. Jumonji C domain-containing histone demethylases (JMJs) dynamically control the level of histone methylations. However, how JMJ activity is generally regulated is unknown. We found that the tricarboxylic acid cycle-associated enzyme α-ketoglutarate (α-KG) dehydrogenase (KGDH) entered the nucleus, where it interacted with various JMJs to regulate α-KG-dependent histone demethylations by JMJs, and thus controlled genome-wide gene expression in plants. We show that nuclear targeting is regulated by environmental signals and that KGDH is enriched at thousands of loci in Arabidopsis thaliana. Chromatin-bound KGDH catalyzes α-KG decarboxylation and thus may limit its local availability to KGDH-coupled JMJs, inhibiting histone demethylation. Thus, our results uncover a regulatory mechanism for histone demethylations by JMJs.

Ethylene signals modulate the survival of Arabidopsis leaf explants

BACKGROUND

Leaf explants are major materials in plant tissue cultures. Incubation of detached leaves on phytohormone-containing media, which is an important process for producing calli and regenerating plants, change their cell fate. Although hormone signaling pathways related to cell fate transition have been widely studied, other molecular and physiological events occurring in leaf explants during this process remain largely unexplored.

RESULTS

Here, we identified that ethylene signals modulate expression of pathogen resistance genes and anthocyanin accumulation in leaf explants, affecting their survival during culture. Anthocyanins accumulated in leaf explants, but were not observed near the wound site. Ethylene signaling mutant analysis revealed that ethylene signals are active and block anthocyanin accumulation in the wound site. Moreover, expression of defense-related genes increased, particularly near the wound site, implying that ethylene induces defense responses possibly by blocking pathogenesis via wounding. We also found that anthocyanin accumulation in non-wounded regions is required for drought resistance in leaf explants.

CONCLUSIONS

Our study revealed the key roles of ethylene in the regulation of defense gene expression and anthocyanin biosynthesis in leaf explants. Our results suggest a survival strategy of detached leaves, which can be applied to improve the longevity of explants during tissue culture.

The proteome and phosphoproteome uncovers candidate proteins associated with vacuolar phosphate signal multipled by Vacuolar phosphate transporter1 (VPT1) in Arabidopsis

Plant vacuoles serve as the primary intracellular compartments for inorganic phosphate (Pi) storage. Passage of Pi across vacuolar membranes plays a critical role in buffering the cytoplasmic Pi level against fluctuations of external Pi and metabolic activities. To gain new insights into the proteins and processes vacuolar Pi level regulated by Vacuolar phosphate transporter1 (VPT1) in Arabidopsis, we carried out TMT labeling proteome and phosphoproteome profiling of Arabidopsis wild-type (WT) and vpt1 loss-of-function mutant plants. The vpt1 mutant had a marked reduced vacuolar Pi level, and an slight increased cytosol Pi level. The mutant was stunted as reflected in the reduction of the fresh weight compared with WT plants, and bolting earlier under normal growth conditions in soil. Over 5566 proteins and 7965 phosphopeptides were quantified. About 146 and 83 proteins were significantly changed at protein abundance or site-specific phosphorylation levels, but only 6 proteins were shared between them. Functional enrichment analysis revealed that the changes of Pi states in vpt1 is associated with photosynthesis, translation, RNA splicing, and defense response, consistent with similar studies in Arabidopsis. Except for PAP26, EIN2, and KIN10, which were reported to be associated with phosphate starvation signal, we also found many differential proteins involved in abscisic acid (ABA) signaling, such as CARK1, SnRK1, and AREB3, were significantly changed in vpt1. Our study illuminates several new aspects of the phosphate response and identifies important targets for further investigation and potential crop improvement.

Integration of transcriptomic and metabolomic analyses provides insights into response mechanisms to nitrogen and phosphorus deficiencies in soybean

Nitrogen (N) and phosphorus (P) are two essential plant macronutrients that can limit plant growth by different mechanisms. We aimed to shed light on how soybean respond to low nitrogen (LN), low phosphorus (LP) and their combined deficiency (LNP). Generally, these conditions triggered changes in gene expression of the same processes, including cell wall organization, defense response, response to oxidative stress, and photosynthesis, however, response was different in each condition. A typical primary response to LN and LP was detected also in soybean, i.e., the enhanced uptake of N and P, respectively, by upregulation of genes for the corresponding transporters. The regulation of genes involved in cell wall organization showed that in LP roots tended to produce more casparian strip, in LN more secondary wall biosynthesis occurred, and in LNP reduction in expression of genes involved in secondary wall production accompanied by cell wall loosening was observed. Flavonoid biosynthesis also showed distinct pattern of regulation in different conditions: more anthocyanin production in LP, and more isoflavonoid production in LN and LNP, which we confirmed also on the metabolite level. Interestingly, in soybean the nutrient deficiencies reduced defense response by lowering expression of genes involved in defense response, suggesting a role of N and P nutrition in plant disease resistance. In conclusion, we provide detailed information on how LN, LP, and LNP affect different processes in soybean roots on the molecular and physiological levels.

Genome-wide investigation and expression pattern of PHR family genes in cotton under low phosphorus stress

Phosphorus starvation response (PHR) protein is an important transcription factor in phosphorus regulatory network, which plays a vital role in regulating the effective utilization of phosphorus. So far, the PHR genes have not been systematically investigated in cotton. In the present study, we have identified 22, 23, 41 and 42 PHR genes in G. arboreum, G. raimondii, G. hirsutum and G. barbadense, respectively. Phylogenetic analysis showed that cotton PHR genes were classified into five distinct subfamilies. The gene structure, protein motifs and gene expression were further investigated. The PHR genes of G. hirsutum from the same subfamily had similar gene structures, all containing Myb_DNA-binding and Myb_CC_LHEQLE conserved domain. The structures of paralogous genes were considerably conserved in exons number and introns length. The cis-element prediction in their promoters showed that genes were not only regulated by light induction, but also were related to auxin, MeJA, abscisic acid-responsive elements, of which might be regulated by miRNA. The expression analysis showed that the GhPHR genes were differentially expressed in different tissues under various stresses. Furthermore, GhPHR6, GhPHR11, GhPHR18 and GhPHR38 were significantly changed under low phosphorus stress. The results of this study provide a basis for further cloning and functional verification of genes related to regulatory network of low phosphorus tolerance in cotton.

An Agrobacterium-mediated transient expression method contributes to functional analysis of a transcription factor and potential application of gene editing in Chenopodium quinoa

An efficient Agrobacterium-mediated transient expression method was developed, which contributed to the functional characterization of the transcription factor CqPHR1, and demonstrates the potential application of gene editing in quinoa. Chenopodium quinoa is a crop expected to ensure global food security in future due to its high resistance to multiple abiotic stresses and nutritional value. We cloned one of the paralogous genes of the Arabidopsis homolog PHR1 (PHOSPHATE STARVATION RESPONSE 1) in quinoa-inbred lines by reverse genetic approach. Overexpression of CqPHR1 driven by the constitutive CaMV 35S promoter in Arabidopsis phr1 mutant can complement its phenotypes, including the induction of phosphate starvation-induced (PSI) genes and anthocyanin accumulation in leaves. By Agrobacterium-mediated gene transient expression, we found that CqPHR1 localized in the nucleus of quinoa cells, and overexpression of CqPHR1 in quinoa cells promoted PSI genes expression, which further revealed the function of CqPHR1 as a transcription factor. We have also shown that the transient expression system can be used to express Cas9 protein in various quinoa-inbred lines and perform effective gene editing in quinoa tissue. The method developed in this study will be useful for verifying the effectiveness of gene-editing systems in quinoa cells and has potential application in the generation of gene-edited quinoa with heritable traits.

SHORT-ROOT stabilizes PHOSPHATE1 to regulate phosphate allocation in Arabidopsis

The coordinated distribution of inorganic phosphate (Pi) between roots and shoots is an important process that plants use to maintain Pi homeostasis. SHORT-ROOT (SHR) is well characterized for its function in root radial patterning. Here we demonstrate a role of SHR in controlling Pi allocation from root to shoot by regulating PHOSPHATE1 in the root differentiation zone. We recovered a weak mutant allele of SHR in Arabidopsis that accumulates much less Pi in the shoot and shows a constitutive Pi starvation response under Pi-sufficient conditions. In addition, Pi starvation suppresses SHR protein accumulation and releases its inhibition on the HD-ZIP III transcription factor PHB. PHB accumulates and directly binds the promoter of PHOSPHATE2 to upregulate its transcription, resulting in PHOSPHATE1 degradation in the xylem-pole pericycle cells. Our findings reveal a previously unrecognized mechanism of how plants regulate Pi translocation from roots to shoots.

Abscisic acid facilitates phosphate acquisition through the transcription factor ABA INSENSITIVE5 in Arabidopsis

Low phosphate (LP) in soil is a common nutrient stress that severely restricts agricultural production, but the role, if any, of the major stress phytohormone abscisic acid (ABA) in plant phosphate (Pi) starvation responses remains elusive. Here, we report that LP-induced ABA accumulation promotes Pi uptake in an ABA INSENSITIVE5 (ABI5)-dependent manner in Arabidopsis thaliana. LP significantly activated plant ABA biosynthesis, metabolism, and stress responses, suggesting a role of ABA in the plant response to Pi availability. LP-induced ABA accumulation and expression of two major high-affinity phosphate transporter genes PHOSPHATE TRANSPORTER1;1/1;4 (PHT1;1/1;4) were severely impaired in a mutant lacking BETA-GLUCOSIDASE1 (BG1), which converts conjugated ABA to active ABA, and the mutant had shorter roots and less Pi content than wild-type plants under LP conditions. Moreover, a mutant of ABI5, which encodes a central transcription factor in ABA signaling, also exhibited suppressed root elongation and had reduced Pi content under LP conditions. ABI5 facilitated Pi acquisition by activating the expression of PHT1;1 by directly binding to its promoter, while overexpression of PHT1;1 completely rescued its Pi content under LP conditions. Together, our findings illustrate a molecular mechanism by which ABA positively modulates phosphate acquisition through ABI5 in the Arabidopsis response to phosphate deficiency.

Nitrogen deficiency- and sucrose-induced anthocyanin biosynthesis is modulated by HISTONE DEACETYLASE15 in Arabidopsis

Anthocyanin accumulation is a hallmark response to nitrogen (N) deficiency in Arabidopsis. Although the regulation of anthocyanin biosynthesis has been extensively studied, the roles of chromatin modification in this process are largely unknown. In this study we show that anthocyanin accumulation induced by N deficiency is modulated by HISTONE DEACETYLASE15 (HDA15) in Arabidopsis seedlings. The hda15-1 T-DNA insertion mutant accumulated more anthocyanins than the wild-type when the N supply was limited, and this was caused by up-regulation of anthocyanin biosynthetic and regulatory genes in the mutant. The up-regulated genes also had increased levels of histone acetylation in the mutant. The accumulation of anthocyanins induced by sucrose and methyl jasmonate, but not that induced by H2O2 and phosphate starvation, was also greater in the hda15-1 mutant. While sucrose increased histone acetylation in the hda15-1 mutant in genes in a similar manner to that caused by N deficiency, methyl jasmonate only enhanced histone acetylation in the genes involved in anthocyanin biosynthesis. Our results suggest that different stresses act through distinct regulatory modules to activate anthocyanin biosynthesis, and that HDA15-mediated histone modification modulates the expression of anthocyanin biosynthetic and regulatory genes to avoid overaccumulation in response to N deficiency and other stresses.

Plant phosphate nutrition: sensing the stress

Phosphorus (P) is obtained by plants as phosphate (Pi) from the soil and low Pi levels affects plant growth and development. Adaptation to low Pi condition entails sensing internal and external Pi levels and translating those signals to molecular and morphophysiological changes in the plant. In this review, we present findings related to local and systemin Pi sensing with focus the molecular mechanisms behind root system architectural changes and the impact of hormones and epigenetic mechanisms affecting those changes. We also present some of the recent advances in the Pi sensing and signaling mechanisms focusing on inositol pyrophosphate InsP8 and its interaction with SPX domain proteins to regulate the activity of the central regulator of the Pi starvation response, PHR.

Editing of the starch branching enzyme gene SBE2 generates high-amylose storage roots in cassava

The production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme gene SBE2 was firstly achieved. High-amylose cassava (Manihot esculenta Crantz) is desirable for starch industrial applications and production of healthier processed food for human consumption. In this study, we report the production of high-amylose cassava through CRISPR/Cas9-mediated mutagenesis of the starch branching enzyme 2 (SBE2). Mutations in two targeted exons of SBE2 were identified in all regenerated plants; these mutations, which included nucleotide insertions, and short or long deletions in the SBE2 gene, were classified into eight mutant lines. Three mutants, M6, M7 and M8, with long fragment deletions in the second exon of SBE2 showed no accumulation of SBE2 protein. After harvest from the field, significantly higher amylose (up to 56% in apparent amylose content) and resistant starch (up to 35%) was observed in these mutants compared with the wild type, leading to darker blue coloration of starch granules after quick iodine staining and altered starch viscosity with a higher pasting temperature and peak time. Further ¹H-NMR analysis revealed a significant reduction in the degree of starch branching, together with fewer short chains (degree of polymerization [DP] 15-25) and more long chains (DP>25 and especially DP>40) of amylopectin, which indicates that cassava SBE2 catalyzes short chain formation during amylopectin biosynthesis. Transition from A- to B-type crystallinity was also detected in the starches. Our study showed that CRISPR/Cas9-mediated mutagenesis of starch biosynthetic genes in cassava is an effective approach for generating novel varieties with valuable starch properties for food and industrial applications.

A systems-biology approach identifies co-expression modules in response to low phosphate supply in maize lines of different breeding history

Plants cope with low phosphorus availability by adjusting growth and metabolism through transcriptomic and proteomic adaptations. We hypothesize that selected genotypes with distinct phosphorous (P) use efficiency covering the breeding history of European Flint heterotic pool provide a tool to reveal general and genotype-specific molecular responses to P limitation. We reconstructed protein and gene co-expression networks by weighted correlation network analysis and related these to phosphate deficiency-induced traits. In roots, low phosphate supply resulted in a decreasing abundance of proteins in the oxidative pentose phosphate pathway and a negative correlation with root and shoot phosphate content. We observed an increase in abundance and positive correlation with root and shoot phosphate content for proteins in sucrose biosynthesis, lipid metabolism, respiration and RNA processing. Purple acid phosphatases, superoxide dismutase and phenylalanine ammonia lyase were identified as being upregulated under low phosphate in all genotypes. Overall, correlations between protein and mRNA abundance changes were limited, with ribosomal proteins and the ubiquitin protein degradation pathway exclusively responding with protein abundance changes. Carbohydrate, phospho- and sulfo-lipid metabolism showed abundance changes at the protein and mRNA levels. These partially non-overlapping proteomic and transcriptomic adjustments to low phosphate suggest sugar and lipid metabolism as metabolic processes associated with improved P use efficiency specifically in Founder Flint lines. We identified a mitogen-activated protein kinase-kinase as a potential genotype-specific regulator of sucrose metabolism at low phosphate in Founder Flint line EP1. We conclude that, during breedingt of Elite Flint lines, regulation of primary metabolism has changed to result in a distinct low phosphate response in Founder lines.

Research Advances in the Mutual Mechanisms Regulating Response of Plant Roots to Phosphate Deficiency and Aluminum Toxicity

Low phosphate (Pi) availability and high aluminum (Al) toxicity constitute two major plant mineral nutritional stressors that limit plant productivity on acidic soils. Advances toward the identification of genes and signaling networks that are involved in both stresses in model plants such as Arabidopsis thaliana and rice (Oryza sativa), and in other plants as well have revealed that some factors such as organic acids (OAs), cell wall properties, phytohormones, and iron (Fe) homeostasis are interconnected with each other. Moreover, OAs are involved in recruiting of many plant-growth-promoting bacteria that are able to secrete both OAs and phosphatases to increase Pi availability and decrease Al toxicity. In this review paper, we summarize these mutual mechanisms by which plants deal with both Al toxicity and P starvation, with emphasis on OA secretion regulation, plant-growth-promoting bacteria, transcription factors, transporters, hormones, and cell wall-related kinases in the context of root development and root system architecture remodeling that plays a determinant role in improving P use efficiency and Al resistance on acidic soils.

Nutrient-hormone relations: Driving root plasticity in plants

Optimal plant development requires root uptake of 14 essential mineral elements from the soil. Since the bioavailability of these nutrients underlies large variation in space and time, plants must dynamically adjust their root architecture to optimize nutrient access and acquisition. The information on external nutrient availability and whole-plant demand is translated into cellular signals that often involve phytohormones as intermediates to trigger a systemic or locally restricted developmental response. Timing and extent of such local root responses depend on the overall nutritional status of the plant that is transmitted from shoots to roots in the form of phytohormones or other systemic long-distance signals. The integration of these systemic and local signals then determines cell division or elongation rates in primary and lateral roots, the initiation, emergence, or elongation of lateral roots, as well as the formation of root hairs. Here, we review the cascades of nutrient-related sensing and signaling events that involve hormones and highlight nutrient-hormone relations that coordinate root developmental plasticity in plants.

Integrative analysis of the metabolome and transcriptome reveal the phosphate deficiency response pathways of alfalfa

Understanding the mechanisms underlying the responses to inorganic phosphate (Pi) deficiency in alfalfa will help enhance Pi acquisition efficiency and the sustainable use of phosphorous resources. Integrated global metabolomic and transcriptomic analyses of mid-vegetative alfalfa seedlings under 12-day Pi deficiency were conducted. Limited seedling growth were found, including 13.24%, 16.85% and 33.36% decreases in height, root length and photosynthesis, and a 24.10% increase in root-to-shoot ratio on day 12. A total of 322 and 448 differentially abundant metabolites and 1199 and 1061 differentially expressed genes were identified in roots and shoots. Increased (>3.68-fold) inorganic phosphate transporter 1;4 and SPX proteins levels in the roots (>2.15-fold) and shoots (>2.50-fold) were related to Pi absorption and translocation. The levels of phospholipids and Pi-binding carbohydrates and nucleosides were decreased, while those of phosphatases and pyrophosphatases in whole seedlings were induced under reduced Pi. In addition, nitrogen assimilation was affected by inhibiting high-affinity nitrate transporters (NRT2.1 and NRT3.1), and nitrate reductase. Increased delphinidin-3-glucoside might contribute to the gray-green leaves induced by Pi limitation. Stress-induced MYB, WRKY and ERF transcription factors were identified. The responses of alfalfa to Pi deficiency were summarized as local systemic signaling pathways, including root growth, stress-related responses consisting of enzymatic and nonenzymatic systems, and hormone signaling and systemic signaling pathways including Pi recycling and Pi sensing in the whole plant, as well as Pi recovery, and nitrate and metal absorption in the roots. This study provides important information on the molecular mechanism of the response to Pi deficiency in alfalfa.

Multilevel regulation of anthocyanin-promoting R2R3-MYB transcription factors in plants

Anthocyanins are common secondary metabolites in plants that confer red, blue, and purple colorations in plants and are highly desired by consumers for their visual appearance and nutritional quality. In the last two decades, the anthocyanin biosynthetic pathway and transcriptional regulation of anthocyanin biosynthetic genes (ABGs) have been well characterized in many plants. From numerous studies on model plants and horticultural crops, many signaling regulators have been found to control anthocyanin accumulation via regulation of anthocyanin-promoting R2R3-MYB transcription factors (so-called R2R3-MYB activators). The regulatory mechanism of R2R3-MYB activators is mediated by multiple environmental factors (e.g., light, temperature) and internal signals (e.g., sugar, ethylene, and JA) in complicated interactions at multiple levels. Here, we summarize the transcriptional control of R2R3-MYB activators as a result of natural variations in the promoter of their encoding genes, upstream transcription factors and epigenetics, and posttranslational modifications of R2R3-MYB that determine color variations of horticultural plants. In addition, we focus on progress in elucidating the integrated regulatory network of anthocyanin biosynthesis mediated by R2R3-MYB activators in response to multiple signals. We also highlight a few gene cascade modules involved in the regulation of anthocyanin-related R2R3-MYB to provide insights into anthocyanin production in horticultural plants.

Ethylene participates in zinc oxide nanoparticles induced biochemical, molecular and ultrastructural changes in rice seedlings

Nowadays, the applications of engineered nanoparticles (ENPs) have been significantly increased, thereby negatively affecting crop production and ultimately contaminating the food chain worldwide. Zinc oxide nanoparticles (ZnO NPs) induced oxidative stress has been clarified in previous studies. But until now, it has not been investigated that how ethylene mediates or participates in ZnO NPs-induced toxicity and related cellular ultrastructural changes in rice seedlings. Here, we reported that 500 mg/L of ZnO NPs reduced the fresh weight (54.75% and 55.64%) and dry weight (40.33% and 47.83%) in shoot and root respectively as compared to control. Furthermore, ZnO NPs (500 mg/L) reduced chlorophyll content (72% Chla, 70% Chlb), induced the stomatal closure and ultrastructural damages by causing oxidative stress in rice seedlings. These cellular damages were significantly increased by exogenous applications of ethylene biosynthesis precursor (ACC) in the presence of ZnO NPs. In contrary, ZnO NPs induced damages on the above-mentioned attributes were reversed through the exogenous supply of ethylene signaling and biosynthesis antagonists such as silver (Ag) and cobalt (Co) respectively. Interestingly, ZnO NPs accelerate ethylene biosynthesis by up-regulating the transcriptome of ethylene biosynthesis responsive genes. The antioxidant enzymes activities and related gene expressions were further increased in ethylene signaling and biosynthesis associated antagonists (Ag and Co) treated seedlings as compared to sole ZnO NPs treatments. In contrary, the above-reported attributes were further decreased by ACC together with ZnO NPs. In a nutshell, ethylene effectively contributes in ZnO NPs induced toxicity and causing ultrastructural and stomatal damage in rice seedlings. Such findings could have potential implications in producing genetic engineered crops, which will be able to tolerate nanoparticles toxicity in the environment.

Phosphate availability regulates flavonoid accumulation associated with photosynthetic carbon partitioning in Cyclocarya paliurus

Cyclocarya paliurus has traditionally been used as medicine and functional food. This study aims at investigating the flavonoid accumulation in C. paliurus dependent on phosphate (Pi) availability and its potential association with internal carbon partitioning. One-year-old seedlings of C. paliurus were planted in four different Pi levels. Low Pi resulted in low phosphorus content within plants, while the nitrogen content increased. Further analysis revealed that the surplus carbon pool was greater and was allocated to N-metabolism and carbohydrate synthesis under low Pi conditions, as shown by the higher levels of free amino acids, starch, and soluble sugars. Low Pi availability also induced higher enzymatic activities of shikimate dehydrogenase (SDH) and flavonoid 3-hydoxylase (FHT), and higher flavonoid accumulation in leaves. Our results indicated that the surplus carbon induced by low-Pi levels can increase flavonoid synthesis in seedlings of C. paliurus. In addition, growth and biomass accumulation were increased by the elevated Pi levels. As a result, the highest flavonoid yield per plant was obtained under relative low Pi conditions. This study can provide the basis for developing new agricultural practices to maintain high yield while still keeping the quality of medicinal plants and crops.

Role of ethylene in effective establishment of the peanut-bradyrhizobia symbiotic interaction

Ethylene has been implicated in nitrogen fixing symbioses in legumes, where rhizobial invasion occurs via infection threads (IT). In the symbiosis between peanut (Arachis hypogaea L.) and bradyrhizobia, the bacteria penetrate the root cortex intercellularly and IT are not formed. Little attention has been paid to the function of ethylene in the establishment of this symbiosis. The aim of this article is to evaluate whether ethylene plays a role in the development of this symbiotic interaction and the participation of Nod Factors (NF) in the regulation of ethylene signalling. Manipulation of ethylene in peanut was accomplished by application of 1-aminocyclopropane-1-carboxylic acid (ACC), which mimics applied ethylene, or AgNO_3, which blocks ethylene responses. To elucidate the participation of NF in the regulation of ethylene signalling, we inoculated plants with a mutant isogenic rhizobial strain unable to produce NF and evaluated the effect of AgNO₃ on gene expression of NF and ethylene responsive signalling pathways. Data revealed that ethylene perception is required for the formation of nitrogen-fixing nodules, while addition of ACC does not affect peanut symbiotic performance. This phenotypic evidence is in agreement with transcriptomic data from genes involved in symbiotic and ethylene signalling pathways. NF seem to modulate the expression of ethylene signalling genes. Unlike legumes infected through IT formation, ACC addition to peanut does not adversely affect nodulation, but ethylene perception is required for establishment of this symbiosis. Evidence for the contribution of NF to the modulation of ethylene-inducible defence gene expression is provided.

Visible light regulates anthocyanin synthesis via malate dehydrogenases and the ethylene signaling pathway in plum (Prunus salicina L.)

Light regulates anthocyanins synthesis in plants. Upon exposure to visible light, the inhibition of photosynthetic electron transfer significantly lowered the contents of anthocyanins and the expression levels of key genes involved in anthocyanins synthesis in plum fruit peel. Meanwhile, the expression levels of PsmMDH2 (encoding the malate dehydrogenase in mitochondria) and PschMDH (encoding the malate dehydrogenase in chloroplasts) decreased significantly. The contents of anthocyanins and the levels of the key genes involved in anthocyanin synthesis decreased significantly with the treatment of 1-MCP (an inhibitor of ethylene perception) but were enhanced by the exogenous application of ethylene. The ethylene treatment could also recover the anthocyanin synthesis capacity lowered by the photosynthetic electron transfer inhibition. Silencing PsmMDH2 and PschMDH significantly lowered the contents of anthocyanins in plum fruit. At low temperature, visible light irradiation induced anthocyanin accumulation in Arabidopsis leaves. However, the mmdh, chmdh, and etr1-1 mutants had significantly lower anthocyanins content and expressions of the key genes involved in anthocyanins synthesis compared to wild type. Overall, the present study demonstrates that both photosynthesis and respiration were involved in the regulation of anthocyanin synthesis in visible light. The visible light regulates anthocyanin synthesis by controlling the malate metabolism via MDHs and the ethylene signaling pathway.

Root developmental responses to phosphorus nutrition

Phosphorus is an essential macronutrient for plant growth and development. Root system architecture (RSA) affects a plant's ability to obtain phosphate, the major form of phosphorus that plants uptake. In this review, I first consider the relationship between RSA and plant phosphorus-acquisition efficiency, describe how external phosphorus conditions both induce and impose changes in the RSA of major crops and of the model plant Arabidopsis, and discuss whether shoot phosphorus status affects RSA and whether there is a universal root developmental response across all plant species. I then summarize the current understanding of the molecular mechanisms governing root developmental responses to phosphorus deficiency. I also explore the possible reasons for the inconsistent results reported by different research groups and comment on the relevance of some studies performed under laboratory conditions to what occurs in natural environments.

Ethylene and Nitric Oxide Involvement in the Regulation of Fe and P Deficiency Responses in Dicotyledonous Plants

Iron (Fe) and phosphorus (P) are two essential elements for plant growth. Both elements are abundant in soils but with poor availability for plants, which favor their acquisition by developing morphological and physiological responses in their roots. Although the regulation of the genes related to these responses is not totally known, ethylene (ET) and nitric oxide (NO) have been involved in the activation of both Fe-related and P-related genes. The common involvement of ET and NO suggests that they must act in conjunction with other specific signals, more closely related to each deficiency. Among the specific signals involved in the regulation of Fe- or P-related genes have been proposed Fe-peptides (or Fe ion itself) and microRNAs, like miR399 (P), moving through the phloem. These Fe- or P-related phloem signals could interact with ET/NO and confer specificity to the responses to each deficiency, avoiding the induction of the specific responses when ET/NO increase due to other nutrient deficiencies or stresses. Besides the specificity conferred by these signals, ET itself could confer specificity to the responses to Fe- or P-deficiency by acting through different signaling pathways in each case. Given the above considerations, there are preliminary results suggesting that ET could regulate different nutrient responses by acting both in conjunction with other signals and through different signaling pathways. Because of the close relationship among these two elements, a better knowledge of the physiological and molecular basis of their interaction is necessary to improve their nutrition and to avoid the problems associated with their misuse. As examples of this interaction, it is known that Fe chlorosis can be induced, under certain circumstances, by a P over- fertilization. On the other hand, Fe oxides can have a role in the immobilization of P in soils. Qualitative and quantitative assessment of the dynamic of known Fe- and P-related genes expression, selected ad hoc and involved in each of these deficiencies, would allow us to get a profound knowledge of the processes that regulate the responses to both deficiencies. The better knowledge of the regulation by ET of the responses to these deficiencies is necessary to properly understand the interactions between Fe and P. This will allow the obtention of more efficient varieties in the absorption of P and Fe, and the use of more rational management techniques for P and Fe fertilization. This will contribute to minimize the environmental impacts caused by the use of P and Fe fertilizers (Fe chelates) in agriculture and to adjust the costs for farmers, due to the high prices and/or scarcity of Fe and P fertilizers. This review aims to summarize the latest advances in the knowledge about Fe and P deficiency responses, analyzing the similarities and differences among them and considering the interactions among their main regulators, including some hormones (ethylene) and signaling substances (NO and GSNO) as well as other P- and Fe-related signals.

A Network of Phosphate Starvation and Immune-Related Signaling and Metabolic Pathways Controls the Interaction between Arabidopsis thaliana and the Beneficial Fungus Colletotrichum tofieldiae

The beneficial root-colonizing fungus Colletotrichum tofieldiae mediates plant growth promotion (PGP) upon phosphate (Pi) starvation in Arabidopsis thaliana. This activity is dependent on the Trp metabolism of the host, including indole glucosinolate (IG) hydrolysis. Here, we show that C. tofieldiae resolves several Pi starvation-induced molecular processes in the host, one of which is the downregulation of auxin signaling in germ-free plants, which is restored in the presence of the fungus. Using CRISPR/Cas9 genome editing, we generated an Arabidopsis triple mutant lacking three homologous nitrilases (NIT1 to NIT3) that are thought to link IG-hydrolysis products with auxin biosynthesis. Retained C. tofieldiae-induced PGP in nit1/2/3 mutant plants demonstrated that this metabolic connection is dispensable for the beneficial activity of the fungus. This suggests that either there is an alternative metabolic link between IG-hydrolysis products and auxin biosynthesis, or C. tofieldiae restores auxin signaling independently of IG metabolism. We show that C. tofieldiae, similar to pathogenic microorganisms, triggers Arabidopsis immune pathways that rely on IG metabolism as well as salicylic acid and ethylene signaling. Analysis of IG-deficient myb mutants revealed that these metabolites are, indeed, important for control of in planta C. tofieldiae growth: however, enhanced C. tofieldiae biomass does not necessarily negatively correlate with PGP. We show that Pi deficiency enables more efficient colonization of Arabidopsis by C. tofieldiae, possibly due to the MYC2-mediated repression of ethylene signaling and changes in the constitutive IG composition in roots.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Essential Roles of the Linker Sequence Between Tetratricopeptide Repeat Motifs of Ethylene Overproduction 1 in Ethylene Biosynthesis

Ethylene Overproduction 1 (ETO1) is a negative regulator of ethylene biosynthesis. However, the regulation mechanism of ETO1 remains largely unclear. Here, a novel eto1 allele (eto1-16) was isolated with typical triple phenotypes due to an amino acid substitution of G480C in the uncharacterized linker sequence between the TPR1 and TPR2 motifs. Further genetic and biochemical experiments confirmed the eto1-16 mutation site. Sequence analysis revealed that G480 is conserved not only in two paralogs, EOL1 and EOL2, in Arabidopsis, but also in the homologous protein in other species. The glycine mutations (eto1-11, eto1-12, and eto1-16) do not influence the mRNA abundance of ETO1, which is reflected by the mRNA secondary structure similar to that of WT. According to the protein-protein interaction analysis, the abnormal root phenotype of eto1-16 might be caused by the disruption of the interaction with type 2 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs) proteins. Overall, these data suggest that the linker sequence between tetratricopeptide repeat (TPR) motifs and the glycine in TPR motifs or the linker region are essential for ETO1 to bind with downstream mediators, which strengthens our knowledge of ETO1 regulation in balancing ACSs.

SPX4 interacts with both PHR1 and PAP1 to regulate critical steps in phosphorus-status-dependent anthocyanin biosynthesis

Phosphate (Pi) is the plant-accessible form of phosphorus, and its insufficiency limits plant growth. The over-accumulation of anthocyanins in plants is often an indication of Pi starvation. However, whether the two pathways are directly linked and which components are involved in this process await identification. Here, we demonstrate that SPX4, a conserved regulator of the Pi response, transduces the Pi starvation signal to anthocyanin biosynthesis in Arabidopsis. When phr1spx4 plants were grown under low Pi conditions, DFR expression and anthocyanin biosynthesis were induced, which distinguished the plant from the behavior reported in the phr1 mutant. We also provide evidence that SPX4 interacts with PAP1, an MYB transcription factor that controls the anthocyanin biosynthetic pathway, in an inositol polyphosphate-dependent manner. Through a physical interaction, SPX4 prevented PAP1 from binding to its target gene promoter; by contrast, during Pi-deficient conditions, in the absence of inositol polyphosphates, PAP1 was released from SPX to activate anthocyanin biosynthesis. Our results reveal a direct link between Pi deficiency and flavonoid metabolism. This new regulatory module, at least partially independent from PHR1, may contribute to developing a strategy for plants to adapt to Pi starvation.

Positive effects of metallic nanoparticles on plants: Overview of involved mechanisms

Engineered nanoparticles (NPs) are considered as potential agents for agriculture as fertilizers, growth enhancers and pesticides. Therefore, understanding the mechanisms that are responsible for their effects is important. Various studies demonstrated that the application of nontoxic concentrations can promote seed germination, enhance plant growth and increase the yield. Moreover, NPs can be used to protect plants from environmental impacts such as salt or drought stress and diminish accumulation and toxicity of heavy metals. NPs can serve as a source of micronutrients (e.g. ZnO, iron- and manganese-based NPs), thus increasing fitness and helps plants to cope with stress conditions. TiO₂ and iron-based NPs are able to delay senescence and speed-up cell division via changes in phytohormonal levels. The application of some NPs can promote the activity of enzymes such as amylase, nitrate reductase, phosphatase, phytase and carbonic anhydrases, which are involved in metabolism and nutrient acquisition. E.g. ZnO and TiO₂ NPs can stimulate chlorophyll biosynthesis and photosynthetic activity. Iron-based and CeO₂ NPs enhance stomata opening resulting in better gas exchange and CO₂ assimilation rate. NPs can also modulate oxidative stress by the stimulation of the antioxidant enzymes such peroxidases and superoxide dismutase. However, the knowledge about the fate, transformation, and accumulation of NPs in the environment and organisms is needed prior to their use in agriculture to avoid negative environmental impacts. Higher or lower toxicity of various NPs was established for microorganisms, plants or animals. In this overview, we focused on the possible mechanisms of Ag, ZnO, TiO₂, Fe-based, CeO₂, Al₂O₃, and manganese-based NPs responsible for their positive effects on plants.

Influence of Ethylene Signaling in the Crosstalk Between Fe, S, and P Deficiency Responses in Arabidopsis thaliana

To cope with P, S, or Fe deficiency, dicot plants, like Arabidopsis, develop several responses (mainly in their roots) aimed to facilitate the mobilization and uptake of the deficient nutrient. Within these responses are the modification of root morphology, an increased number of transporters, augmented synthesis-release of nutrient solubilizing compounds and the enhancement of some enzymatic activities, like ferric reductase activity (FRA) or phosphatase activity (PA). Once a nutrient has been acquired in enough quantity, these responses should be switched off to minimize energy costs and toxicity. This implies that they are tightly regulated. Although the responses to each deficiency are induced in a rather specific manner, crosstalk between them is frequent and in such a way that P, S, or Fe deficiency can induce responses related to the other two nutrients. The regulation of the responses is not totally known but some hormones and signaling substances have been involved, either as activators [ethylene (ET), auxin, nitric oxide (NO)], or repressors [cytokinins (CKs)]. The plant hormone ET is involved in the regulation of responses to P, S, or Fe deficiency, and this could partly explain the crosstalk between them. In spite of these crosslinks, it can be hypothesized that, to confer the maximum specificity to the responses of each deficiency, ET should act in conjunction with other signals and/or through different transduction pathways. To study this latter possibility, several responses to P, S, or Fe deficiency have been studied in the Arabidopis wild-type cultivar (WT) Columbia and in some of its ethylene signaling mutants (ctr1, ein2-1, ein3eil1) subjected to the three deficiencies. Results show that key elements of the ET transduction pathway, like CTR1, EIN2, and EIN3/EIL1, can play a role in the crosstalk among nutrient deficiency responses.

Interplay between Hormones and Several Abiotic Stress Conditions on Arabidopsis thaliana Primary Root Development

As sessile organisms, plants must adjust their growth to withstand several environmental conditions. The root is a crucial organ for plant survival as it is responsible for water and nutrient acquisition from the soil and has high phenotypic plasticity in response to a lack or excess of them. How plants sense and transduce their external conditions to achieve development, is still a matter of investigation and hormones play fundamental roles. Hormones are small molecules essential for plant growth and their function is modulated in response to stress environmental conditions and internal cues to adjust plant development. This review was motivated by the need to explore how Arabidopsis thaliana primary root differentially sense and transduce external conditions to modify its development and how hormone-mediated pathways contribute to achieve it. To accomplish this, we discuss available data of primary root growth phenotype under several hormone loss or gain of function mutants or exogenous application of compounds that affect hormone concentration in several abiotic stress conditions. This review shows how different hormones could promote or inhibit primary root development in A. thaliana depending on their growth in several environmental conditions. Interestingly, the only hormone that always acts as a promoter of primary root development is gibberellins.

Vitamin E Is Superior to Vitamin C in Delaying Seedling Senescence and Improving Resistance in Arabidopsis Deficient in Macro-Elements

Nitrogen (N), phosphorus (P), and potassium (K) are three essential macro-elements for plant growth and development. Used to improve yield in agricultural production, the excessive use of chemical fertilizers often leads to increased production costs and ecological environmental pollution. Vitamins C and E are antioxidants that play an important role in alleviating abiotic stress. However, there are few studies on alleviating oxidative stress caused by macro-element deficiency. Here, we used Arabidopsis vitamin E synthesis-deficient mutant vte4 and vitamin C synthesis-deficient mutant vtc1 on which exogenous vitamin E and vitamin C, respectively, were applied at the bolting stage. In the deficiency of macro-elements, the Arabidopsis chlorophyll content decreased, malondialdehyde (MDA) content and relative electric conductivity increased, and reactive oxygen species (ROS) accumulated. The mutants vtc1 and vte4 are more severely stressed than the wild-type plants. Adding exogenous vitamin E was found to better alleviate stress than adding vitamin C. Vitamin C barely affected and vitamin E significantly inhibited the synthesis of ethylene (ETH) and jasmonic acid (JA) genes, thereby reducing the accumulation of ETH and JA that alleviated the senescence caused by macro-element deficiency at the later stage of bolting in Arabidopsis. A deficiency of macro-elements also reduced the yield and germination rate of the seeds, which were more apparent in vtc1 and vte4, and adding exogenous vitamin C and vitamin E, respectively, could restore them. This study reported, for the first time, that vitamin E is better than vitamin C in delaying seedling senescence caused by macro-element deficiency in Arabidopsis.

MiR399d and epigenetic modification comodulate anthocyanin accumulation in Malus leaves suffering from phosphorus deficiency

Inorganic phosphorus (Pi) deficiency induces anthocyanin accumulation in the leaves of some plant species; however, the molecular mechanisms underlying this phenomenon have not been well characterized. Here, we showed that microRNA399d (miR399d), high-affinity Pi transporter McPHT1;4, and McMYB10 are strongly induced in Malus leaves suffering from Pi deficiency. By culturing explants of transiently transformed plants in MS medium under conditions of Pi sufficiency and Pi deficiency, miR399d and McPHT1;4 were shown to play essential roles in the response to Pi deficiency and to play positive roles in the regulation of anthocyanin biosynthesis. Silencing of McHDA6 expression and treatment with the inhibitor trichostatin A suggested that the low expression of McHDA6 simultaneously reduced the transcription of McMET1 and decreased the methylation level of the McMYB10 promoter; however, the expression of McMYB10 and anthocyanin content were increased. Bimolecular fluorescence complementation and yeast two-hybrid assays revealed that McHDA6 binds directly to McMET1 through its BAH2 and DNMT1-RFD domains. Based on the results of our study, we propose a mechanism for the molecular regulation of anthocyanin biosynthesis, namely, the miR399d and epigenetic modification comodulation model, to explain the phenomenon in which leaves turn red under conditions of Pi deficiency.

An Update on Nitric Oxide Production and Role Under Phosphorus Scarcity in Plants

Phosphate (P) is characterized by its low availability and restricted mobility in soils, and also by a high redistribution capacity inside plants. In order to maintain P homeostasis in nutrient restricted conditions, plants have developed mechanisms which enable P acquisition from the soil solution, and an efficient reutilization of P already present in plant cells. Nitric oxide (NO) is a bioactive molecule with a plethora of functions in plants. Its endogenous synthesis depends on internal and environmental factors, and is closely tied with nitrogen (N) metabolism. Furthermore, there is evidence demonstrating that N supply affects P homeostasis and that P deficiency impacts on N assimilation. This review will provide an overview on how NO levels in planta are affected by P deficiency, the interrelationship with N metabolism, and a summary of the current understanding about the influence of this reactive N species over the processes triggered by P starvation, which could modify P use efficiency.

Functional identification of apple MdMYB2 gene in phosphate-starvation response

Inorganic phosphate (Pi) starvation severely affects the normal growth and development of plants. Here, a Pi-responsive gene, named MdMYB2 (MDP0000823458), was cloned and functionally identified in apple. Overexpression of MdMYB2 regulated the expression of Pi starvation-induced (PSI) genes and then promoted phosphate assimilation and utilization. The ectopic expression of MdMYB2 in Arabidopsis influenced plant growth and flowering, which was partially rescued by application of exogenous gibberellin (GA). These results indicated that MdMYB2 may be an essential regulator in phosphate utilization and GA-regulated plant growth and development.

Zinc finger protein 5 (ZFP5) associates with ethylene signaling to regulate the phosphate and potassium deficiency-induced root hair development in Arabidopsis

Zinc finger protein transcription factor ZFP5 positively regulates root hair elongation in response to Pi and potassium deficiency by mainly activating the expression of EIN2 in Arabidopsis. Phosphate (Pi) and potassium (K⁺) are major plant nutrients required for plant growth and development, and plants respond to low-nutrient conditions via metabolic and morphology changes. The C2H2 transcription factor ZFP5 is a key regulator of trichome and root hair development in Arabidopsis. However, its role in regulating root hair development under nutrient deprivations remains unknown. Here, we show that Pi and potassium deficiency could not restore the short root hair phenotype of zfp5 mutant and ZFP5 RNAi lines to wild type level. The deprivation of either of these nutrients also induced the expression of ZFP5 and the activity of an ethylene reporter, pEBS:GUS. The significant reduction of root hair length in ein2-1 and ein3-1 as compared to wild-type under Pi and potassium deficiency supports the involvement of ethylene in root hair elongation. Furthermore, the application of 1-aminocyclopropane-1-carboxylic acid (ACC) significantly enhanced the expression level of ZFP5 while the application of 2-aminoethoxyvinyl glycine (AVG) had the opposite effect when either Pi or potassium was deprived. Further experiments reveal that ZFP5 mainly regulates transcription of ETHYLENE INSENSITIVE 2 (EIN2) to control deficiency-mediated root hair development through ethylene signaling. Generally, these results suggest that ZFP5 regulates root hair elongation by interacting with ethylene signaling mainly through regulates the expression of EIN2 in response to Pi and potassium deficiency in Arabidopsis.

Plant PHR Transcription Factors: Put on A Map

The phosphate starvation response (PHR) protein family exhibits the MYB and coiled-coil domains. In plants, within the either 5' untranslated regions (UTRs) or promoter regions of phosphate starvation-induced (PSI) genes are characteristic cis-regulatory elements, namely PHR1 binding sequence (P1BS). The most widely studied PHR protein family members, such as AtPHR1 in Arabidopsis thaliana (L.) and OsPHR2 in Oryza sativa (L.), may activate the gene expression of a broad range of PSI genes by binding to such elements in a phosphate (Pi) dependent manner. In Pi signaling, PHR transcription factors (TFs) can be selectively activated or deactivated by other proteins to execute the final step of signal transduction. Several new proteins have been associated with the AtPHR1/OsPHR2 signaling cascade in the last few years. While the PHR TF transcriptional role has been studied intensively, here we highlight the recent findings of upstream molecular components and other signaling pathways that may interfere with the PHR final mode of action in plants. Detailed information about transcriptional regulation of the AtPHR1 gene itself and its upstream molecular events has been reviewed.

Ethylene and Phloem Signals Are Involved in the Regulation of Responses to Fe and P Deficiencies in Roots of Strategy I Plants

Iron (Fe) and phosphorus (P) are two essential mineral nutrients whose acquisition by plants presents important environmental and economic implications. Both elements are abundant in most soils but scarcely available to plants. To prevent Fe or P deficiency dicot plants initiate morphological and physiological responses in their roots aimed to specifically acquire these elements. The existence of common signals in Fe and P deficiency pathways suggests the signaling factors must act in conjunction with distinct nutrient-specific signals in order to confer tolerance to each deficiency. Previous works have shown the existence of cross talk between responses to Fe and P deficiency, but details of the associated signaling pathways remain unclear. Herein, the impact of foliar application of either P or Fe on P and Fe responses was studied in P- or Fe-deficient plants of Arabidopsis thaliana, including mutants exhibiting altered Fe or P homeostasis. Ferric reductase and acid phosphatase activities in roots were determined as well as the expression of genes related to P and Fe acquisition. The results obtained showed that Fe deficiency induces the expression of P acquisition genes and phosphatase activity, whereas P deficiency induces the expression of Fe acquisition genes and ferric reductase activity, although only transitorily. Importantly, these responses were reversed upon foliar application of either Fe or P on nutrient-starved plants. Taken together, the results reveal interactions between P- and Fe-related phloem signals originating in the shoots that likely interact with hormones in the roots to initiate adaptive mechanisms to tolerate deficiency of each nutrient.

Two-factor ANOVA of SSH and RNA-seq analysis reveal development-associated Pi-starvation genes in oilseed rape

The 5-leaf-stage rape seedlings were more insensitive to Pi starvation than that of the 3-leaf-stage plants, which may be attributed to the higher expression levels of ethylene signaling and sugar-metabolism genes in more mature seedlings. Traditional suppression subtractive hybridization (SSH) and RNA-Seq usually screen out thousands of differentially expressed genes. However, identification of the most important regulators has not been performed to date. Here, we employed two methods, namely, a two-round SSH and two-factor transcriptome analysis derived from the two-factor ANOVA that is commonly used in the statistics, to identify development-associated inorganic phosphate (Pi) starvation-induced genes in Brassica napus. Several of these genes are related to ethylene signaling (such as EIN3, ACO3, ACS8, ERF1A, and ERF2) or sugar metabolism (such as ACC2, GH3, LHCB1.4, XTH4, and SUS2). Although sucrose and ethylene may counteract each other at the biosynthetic level, they may also work synergistically on Pi-starvation-induced gene expression (such as PT1, PT2, RNS1, ACP5, AT4, and IPS1) and root acid phosphatase activation. Furthermore, three new transcription factors that are responsive to Pi starvation were identified: the zinc-finger MYND domain-containing protein 15 (MYND), a Magonashi family protein (MAGO), and a B-box zinc-finger family salt-tolerance protein. This study indicates that the two methods are highly efficient for functional gene screening in non-model organisms.

ZmAPRG, an uncharacterized gene, enhances acid phosphatase activity and Pi concentration in maize leaf during phosphate starvation

An uncharacterized gene, ZmAPRG, isolated by map-based cloning, enhances acid phosphatase activity and phosphate concentration in maize leaf during phosphate starvation. Acid phosphatase (APase) plays important roles in the absorption and utilization of phosphate (Pi) during maize growth. The information on genes regulating the acid phosphatase activity (APA) in maize leaves remains obscured. In a previous study, we delimited the quantitative trait locus, QTL-AP9 for APA to a region of about 546 kb. Here, we demonstrate that the GRMZM2G041022 located in the 546 kb region is a novel acid phosphatase-regulating gene (ZmAPRG). Its overexpression significantly increased the APA and Pi concentration in maize and rice leaves. Subcellular localization of ZmAPRG showed that it was anchored on the plasma and nuclear membrane. The transcriptome analysis of maize ZmAPRG overexpressing lines (ZmAPRG OE) revealed 1287 up-regulated and 392 down-regulated genes. Among these, we found APase, protein phosphatase, and phosphate transporter genes, which are known to be implicated in the metabolism and utilization of Pi. We inferred the ZmAPRG functions as an upstream regulation node, directly or indirectly regulating APases, protein phosphatases, and phosphate transporter genes involved in Pi metabolism and utilization in maize. These findings will pave the way for elucidating the mechanism of APase regulation, absorption and utilization of Pi, and would facilitate maize breeding for efficient use of fertilizers.

The FvPHR1 transcription factor control phosphate homeostasis by transcriptionally regulating miR399a in woodland strawberry

Plants have evolved phosphate (Pi) starvation response to adapt the low-Pi environment. The regulation of adaptive responses to phosphorus deficiency by the PHR1-miR399-PHO2 module has been well studied in Arabidopsis thaliana but not in strawberry. Transcription factor PHR1 as the central regulator in the Pi starvation signaling has been revealed in a few plant species. However, the function of PHR1 homologues in strawberry is still unknown. In this study, a total of 13 MYB-CC genes were identified in the woodland strawberry (Fragaria vesca) genome and the FvPHR1 gene was characterized. FvPHR1 contains MYB domain and coiled-coil (CC) domain and is localized in the nucleus. FvPHR1 also exhibits trans-activation ability. Furthermore, the P content in leaves of FvPHR1-overexpressing woodland strawberries was significantly increased by 1.38-fold to 1.78-fold compared with that in the wild type. FvPHR1 was also demonstrated to directly bind to the FvMIR399a promoter and positively regulate the expression of FvmiR399a in woodland strawberry. These results showed that PHR1-miR399 module is involved in the regulation of phosphate-signaling pathway and phosphate homeostasis in woodland strawberry.

CONSTITUTIVE TRIPLE RESPONSE1 and PIN2 act in a coordinate manner to support the indeterminate root growth and meristem cell proliferating activity in Arabidopsis seedlings

The plant hormone ethylene induces auxin biosynthesis and transport and modulates root growth and branching. However, its function on root stem cells and the identity of interacting factors for the control of meristem activity remains unclear. Genetic analysis for primary root growth in wild-type (WT) Arabidopsis thaliana seedlings and ethylene-related mutants showed that the loss-of-function of CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) inhibits cell division and elongation. This phenotype is associated with an increase in the expression of the auxin transporter PIN2 and a drastic decrease in the expression of key factors for stem cell niche maintenance such as PLETHORA1, SHORT ROOT and SCARECROW. While the root stem cell niche is affected in ctr1 mutants, its maintenance is severely compromised in the ctr1-1eir1-1(pin2) double mutant, in which an evident loss of proliferative capacity of the meristematic cells leads to a fully differentiated root meristem shortly after germination. Root traits affected in ctr1-1 mutants could be restored in ctr1-1ein2-1 double mutants. These results reveal that ethylene perception via CTR1 and EIN2 in the root modulates the proliferative capacity of root stem cells via affecting the expression of genes involved in the two major pathways, AUX-PIN-PLT and SCR-SHR, which are key factors for proper root stem cell niche maintenance.