Putrescine Alleviates Iron Deficiency via NO-Dependent Reutilization of Root Cell-Wall Fe in Arabidopsis

A NAC transcription factor NAC50 regulates Fe reutilization in Arabidopsis under Fe-deficient condition

A lack of iron (Fe) inhibits the growth and development of plants, leading to reduced agricultural yields and quality. In the last ten years, numerous studies have focused on the induction of Fe uptake and translocation under Fe deficiency, but the regulatory mechanisms governing Fe reutilization within plants are still not well understood. Here, we demonstrated the involvement of the NAM/ATAF1/2/CUC2 (NAC) transcription factor NAC50 in response to Fe shortage. The content of soluble Fe was greatly reduced in nac50 mutants, leading to increased chlorosis in the newly emerging leaves under the Fe-deficient condition. Subsequent investigation revealed that the cell wall of the nac50 mutants' roots accumulated more Fe, along with an increment in hemicellulose content, indicating that a cell wall-associated Fe reutilization pathway was involved in the NAC50-regulated Fe insufficiency response. Interestingly, the expression of NINE-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3), a key enzyme in the abscisic acid (ABA) biosynthetic pathway, was down-regulated in the Fe-deficient nac50 mutants, resulting in decreased endogenous ABA level and Fe-deficient sensitive phenotype. Since no direct relationship was observed between NAC50 and NCED3, this suggests a potential role of NAC50 in mediating the ABA accumulation. Moreover, exogenous ABA application in the nac50 mutant restored Fe deficiency resistance to the level observed in wild-type plants (WT), indicating that NAC50 induced the cell wall Fe reutilization potentially through the regulation of ABA accumulation.

Metabolism of Malus halliana Roots Provides Insights into Iron Deficiency Tolerance Mechanisms

Iron (Fe) deficiency is one of the most common micronutrient imbalances limiting plant growth globally, especially in arid and saline alkali regions due to the decreased availability of Fe in alkaline soils. Malus halliana grows well in arid regions and is tolerant of Fe deficiency. Here, a physiological and metabolomic approach was used to analyze the short-term molecular response of M. halliana roots to Fe deficiency. On the one hand, physiological data show that the root activity first increased and then decreased with the prolongation of the stress time, but the change trend of root pH was just the opposite. The total Fe content decreased gradually, while the effective Fe decreased at 12 h and increased at 3 d. The activity of iron reductase (FCR) increased with the prolongation of stress. On the other hand, a total of 61, 73, and 45 metabolites were identified by GC-MS in three pairs: R12h (Fe deficiency 12 h) vs. R0h (Fe deficiency 0 h), R3d (Fe deficiency 3 d) vs. R0h, and R3d vs. R12h, respectively. Sucrose, as a source of energy, produces monosaccharides such as glucose by hydrolysis, while glucose accumulates significantly at the first (R12h vs. R0h) and third time points (R3d vs. R0h). Carbohydrates (digalacturonate, L-xylitol, ribitol, D-xylulose, glucose, and glycerol) are degraded into pyruvate through glycolysis and pentose phosphate, which participate in the TCA. Glutathione metabolism and the TCA cycle coordinate with each other, actively respond to Fe deficiency stress, and synthesize secondary metabolites at the same time. This study thoroughly examines the metabolite response to plant iron deficiency, highlighting the crucial roles of sugar metabolism, tricarboxylic acid cycle regulation, and glutathione metabolism in the short-term iron deficiency response of apples. It also lays the groundwork for future research on analyzing iron deficiency tolerance.

Melatonin Increases Root Cell Wall Phosphorus Reutilization via an NO Dependent Pathway in Rice (Oryza sativa)

Melatonin (MT) has been implicated in the plant response to phosphorus (P) stress; however, the precise molecular mechanisms involved remain unclear. This study investigated whether MT controls internal P distribution and root cell wall P remobilization in rice. Rice was treated with varying MT and P levels and analyzed using biochemical and molecular techniques to study phosphorus utilization. The results demonstrated that low P levels lead to a rapid increase in endogenous MT levels in rice roots. Furthermore, the exogenous application of MT significantly improved rice tolerance to P deficiency, as evidenced by the increased biomass and reduced proportion of roots to shoots under P-deficient conditions. MT application also mitigated the decrease in P content regardless in both the roots and shoots. Mechanistically, MT accelerated the reutilization of P, particularly in the root pectin fraction, leading to increased soluble P liberation. In addition, MT enhanced the expression of OsPT8, a gene involved in root-to-shoot P translocation. Furthermore, we observed that MT induced the production of nitric oxide (NO) in P-deficient rice roots and that the mitigating effect of MT on P deficiency was compromised in the presence of the NO inhibitor, c-PTIO, implying that NO is involved in the MT-facilitated mitigation of P deficiency in rice. Overall, our findings highlight the potential of MT as a promising strategy for enhancing rice tolerance to P deficiency and improving P use efficiency in agricultural practices.

Epigenetic modification of a pectin methylesterase gene activates apoplastic iron reutilization in tomato roots

Iron (Fe) distribution and reutilization are crucial for maintaining Fe homeostasis in plants. Here, we demonstrate that the tomato (Solanum lycopersicum) Colorless nonripening (Cnr) epimutant exhibits increased Fe retention in cell wall pectin due to an increase in pectin methylesterase (PME) activity. This ultimately leads to Fe deficiency responses even under Fe-sufficient conditions when compared to the wild type (WT). Whole-genome bisulfite sequencing revealed that modifications to cell wall-related genes, especially CG hypermethylation in the intron region of PECTIN METHYLESTERASE53 (SlPME53), are involved in the Cnr response to Fe deficiency. When this intron hypermethylation of SlPME53 was artificially induced in WT, we found that elevated SlPME53 expression was accompanied by increased PME activity and increased pectin-Fe retention. The manipulation of SlPME53, either through overexpression in WT or knockdown in Cnr, influenced levels of pectin methylesterification and accumulation of apoplast Fe in roots. Moreover, CG hypermethylation mediated by METHYLTRANSFERASE1 (SlMET1) increased SlPME53 transcript abundance, resulting in greater PME activity and higher Fe retention in cell wall pectin. Therefore, we conclude that the Cnr mutation epigenetically modulates SlPME53 expression by SlMET1-mediated CG hypermethylation, and thus the capacity of the apoplastic Fe pool, creating opportunities for genetic improvement of crop mineral nutrition.

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.

A cell-wall-modifying gene-dependent CLE26 peptide signaling confers drought resistance in Arabidopsis

Plants respond to various environmental stimuli in sophisticated ways. Takahashi et al. (2018) revealed that CLAVATA3/EMBRYO SURROUNDING REIGON-related 25 (CLE25) peptide is produced in roots under drought stress and transported to shoots, where it induces abscisic acid biosynthesis, resulting in drought resistance in Arabidopsis. However, the drought-related function of the CLE26 peptide, which has the same amino acid sequence as CLE25 (except for one amino acid substitution), is still unknown. In this study, a phenotypic analysis of Arabidopsis plants under repetitive drought stress treatment indicates that CLE26 is associated with drought stress memory and promotes survival rate at the second dehydration event. Additionally, we find that a loss-of-function mutant of a cell-wall-modifying gene, XYLANASE1 (XYN1), exhibits improved resistance to drought, which is suppressed by the mutation of CLE26. XYN1 is down-regulated in response to drought in wild-type plants. A further analysis shows that the synthetic CLE26 peptide is well transported in both xyn1 and drought-pretreated wild-type plants but not in untreated wild-type plants. These results suggest a novel cell wall function in drought stress memory; short-term dehydration down-regulates XYN1 in xylem cells, leading to probable cell wall modification, which alters CLE26 peptide transport, resulting in drought resistance under subsequent long-term dehydration.

OsUGE2 Regulates Plant Growth through Affecting ROS Homeostasis and Iron Level in Rice

BACKGROUND

The growth and development of rice (Oryza sativa L.) are affected by multiple factors, such as ROS homeostasis and utilization of iron. Here, we demonstrate that OsUGE2, a gene encoding a UDP-glucose 4-epimerase, controls growth and development by regulating reactive oxygen species (ROS) and iron (Fe) level in rice. Knockout of this gene resulted in impaired growth, such as dwarf phenotype, weakened root growth and pale yellow leaves. Biochemical analysis showed that loss of function of OsUGE2 significantly altered the proportion and content of UDP-Glucose (UDP-Glc) and UDP-Galactose (UDP-Gal). Cellular observation indicates that the impaired growth may result from decreased cell length. More importantly, RNA-sequencing analysis showed that knockout of OsUGE2 significantly influenced the expression of genes related to oxidoreductase process and iron ion homeostasis. Consistently, the content of ROS and Fe are significantly decreased in OsUGE2 knockout mutant. Furthermore, knockout mutants of OsUGE2 are insensitive to both Fe deficiency and hydrogen peroxide (H2O2) treatment, which further confirmed that OsUGE2 control rice growth possibly through Fe and H2O2 signal. Collectively, these results reveal a new pathway that OsUGE2 could affect growth and development via influencing ROS homeostasis and Fe level in rice.

NO Is Not the Same as GSNO in the Regulation of Fe Deficiency Responses by Dicot Plants

Iron (Fe) is abundant in soils but with a poor availability for plants, especially in calcareous soils. To favor its acquisition, plants develop morphological and physiological responses, mainly in their roots, known as Fe deficiency responses. In dicot plants, the regulation of these responses is not totally known, but some hormones and signaling molecules, such as auxin, ethylene, glutathione (GSH), nitric oxide (NO) and S-nitrosoglutathione (GSNO), have been involved in their activation. Most of these substances, including auxin, ethylene, GSH and NO, increase their production in Fe-deficient roots while GSNO, derived from GSH and NO, decreases its content. This paradoxical result could be explained with the increased expression and activity in Fe-deficient roots of the GSNO reductase (GSNOR) enzyme, which decomposes GSNO to oxidized glutathione (GSSG) and NH3. The fact that NO content increases while GSNO decreases in Fe-deficient roots suggests that NO and GSNO do not play the same role in the regulation of Fe deficiency responses. This review is an update of the results supporting a role for NO, GSNO and GSNOR in the regulation of Fe deficiency responses. The possible roles of NO and GSNO are discussed by taking into account their mode of action through post-translational modifications, such as S-nitrosylation, and through their interactions with the hormones auxin and ethylene, directly related to the activation of morphological and physiological responses to Fe deficiency in dicot plants.

Indole-3-acetaldoxime delays root iron-deficiency responses and modify auxin homeostasis in Medicago truncatula

Iron (Fe) is an essential plant micronutrient, being a major limiting growth factor in calcareous soils. To increase Fe uptake, plants induce lateral roots growth, the expression of a Fe(III)-chelate reductase (FCR), a Fe(II)-transporter and a H⁺-ATPase and the secretion of flavins. Furthermore, auxin hormone family is involved in the Fe-deficiency responses but the action mechanism remains elusive. In this work, we evaluated the effect of the auxin-precursor indole-3-acetaldoxime (IAOx) on hydroponically grown Medicago truncatula plants under different Fe conditions. Upon 4-days of Fe starvation, the pH of the nutrient solution decreased, while both the FCR activity and the presence of flavins increased. Exogenous IAOx increased lateral roots growth contributing to superroot phenotype, decreased chlorosis, and delayed up to 3-days the pH-decrease, the FCR-activity increase, and the presence of flavins, compared to Fe-deficient plants. Gene expression levels were in concordance with the physiological responses. RESULTS: showed that IAOx was immediately transformed to IAN in roots and shoots to maintain auxin homeostasis. IAOx plays an active role in iron homeostasis delaying symptoms and responses in Fe-deficient plants. We may speculate that IAOx or its derivatives remobilize Fe from root cells to alleviate Fe-deficiency. Overall, these results point out that the IAOx-derived phenotype may have advantages to overcome nutritional stresses.

Comparative Analysis of Bioactive Compounds in Two Globe Artichoke Ecotypes Sanitized and Non-Sanitized from Viral Infections

Globe artichoke ecotypes sanitized from plant pathogen infections are characterized by high vegetative vigor, productivity, and quality of capitula. The recent availability on the market of these plants has renewed the interest of farmers and pharmaceutical industries in the crop. Globe artichoke exhibits interesting nutraceutical properties due to the high content of health-promoting bioactive compounds (BACs), such as polyphenols, that could be extracted from waste biomass. The production of BACs depends on several factors including the plant portion considered, the globe artichoke variety/ecotype, and the physiological status of the plants, linked to biotic and abiotic stresses. We investigated the influence of viral infections on polyphenol accumulation in two Apulian late-flowering ecotypes "Locale di Mola tardivo" and "Troianella", comparing sanitized virus-free material (S) vs. naturally virus-infected (non-sanitized, NS) plants. Transcriptome analysis of the two ecotypes highlighted that differentially expressed genes (DEGs), in the two tested conditions, were mainly involved in primary metabolism and processing of genetic/environmental information. The up-regulation of the genes related to the biosynthesis of secondary metabolites and the analysis of peroxidase activity suggested that their modulation is influenced by the phytosanitary status of the plant and is ecotype-dependent. Conversely, the phytochemical analysis showed a remarkable decrease in polyphenols and lignin accumulation in S artichokes compared to NS plants. This unique study analyzes the potential of growing vigorous, sanitized plants, in order to have high amounts of 'soft and clean' biomass, finalized for BAC extraction for nutraceutical purposes. This, in turn, opens new perspectives for a circular economy of sanitized artichokes, in line with the current phytosanitary standards and sustainable development goals.

Decrease in hemicellulose content and its retention of iron contributes to phosphorus deficiency alleviated iron deficiency in Arabidopsis thaliana

The physiological and molecular mechanisms between phosphorus (P) and iron (Fe) interactions are still elusive although they have been extensively investigated. In this study, we uncovered that limiting P supply could alleviate Fe deficiency in Arabidopsis (Col-0). Under Fe deficiency, P deficiency (-Fe-P) decreased cell wall Fe accumulation in root, but elevated Fe accumulation in the shoot, implying that the reduced Fe retention in the root cell wall may contribute to the P-deficiency-alleviated Fe deficiency in the shoot. On the other hand, increasing P supply could mimic the degree of Fe deficiency in terms of the expressions of genes induced after Fe deficient treatment. The components of the root cell wall showed that there was no distinction in the pectin content and the Fe retention in pectin between -Fe and -Fe-P treatments, while hemicellulose 1 content and Fe retained in it were decreased significantly in -Fe-P treatment as compared with -Fe treatment. The time-course experiment showed that decreasing cell wall retained Fe was mainly from the corresponding decrease in hemicellulose 1 retained Fe. Furthermore, the up-regulation of IRT1 expression in -Fe-P was obviously lower than -Fe. All these suggest that the P deficiency-induced decrease of hemicellulose 1 component leads to reutilization of root cell wall Fe and improvement of Fe nutrition in shoot in Fe deficient Arabidopsis. Our results provide a novel explanation of the interplay between Fe and P in Arabidopsis.

Differential modulation of Target of Rapamycin activity under single and combined iron and sulfur deficiency in tomato plants

Over the past few decades, a close relationship between sulfur (S) and iron (Fe) in terms of functionality and nutrition was demonstrated in the tomato. However, very little is known about the regulatory mechanisms underlying S/Fe interactions. Recently, the potential role of citrate in plant adaptation to Fe deficiency and combined S and Fe deficiency has been described. It is known that an impaired organic acid metabolism may stimulate a retrograde signal, which has been proven to be linked to the Target of Rapamycin (TOR) signaling in yeast and animal cells. Recent reports provided evidence of TOR involvement in S nutrient sensing in plants. This suggestion prompted us to investigate whether TOR may play a role in the cross-talk of signaling pathway occurring during plant adaptation to combined nutrient deficiency of Fe and S. Our results revealed that Fe deficiency elicited an increase of TOR activity associated with enhanced accumulation of citrate. In contrast, S deficiency resulted in decreased TOR activity and citrate accumulation. Interestingly, citrate accumulated in shoots of plants exposed to combined S/Fe deficiency to values between those found in Fe- and S-deficient plants, again correlated with TOR activity level. Our results suggest that citrate might be involved in establishing a link between plant response to combined S/Fe deficiency and the TOR network.

Hydrogen sulfide and nitric oxide regulate the adaptation to iron deficiency through affecting Fe homeostasis and thiol redox modification in Glycine max seedlings

Iron (Fe) is a vital microelement required for the growth and development of plants. Hydrogen sulfide (H₂S) and nitric oxide (NO), as messenger molecules, participated in the regulation of plant physiological processes. Here, we studied the interaction effects of H₂S and NO on the adaptation to Fe deficiency in Glycine max L. Physiological, biochemical and molecular approaches were conducted to analyze the role of H₂S and NO in regulating the adaptation to Fe deficiency in soybean. We found that H₂S and NO had obvious rescuing function on the Fe deficiency-induced the plant growth inhibition, which was significantly correlated with the increase in Fe content in the leaves, stems, and roots of soybean. Meanwhile, H⁺-flux, ferric chelate reductase (FCR) activity, and root apoplast Fe content were significantly affected by H₂S and NO. Under Fe deficiency conditions NO and H₂S regulated the expression of genes related to Fe homeostasis. Moreover, photosynthesis (Pn) and photosystem II (PSII) efficiency were enhanced by H₂S and NO, and thiol redox modification was important for regulating the adaptation of Fe deficiency. The aforementioned affirmative influences caused by H₂S and NO were also totally reversed by cPTIO (a NO scavenger). Our results suggested that H₂S might act upstream of NO in response to Fe deficiency by affecting the Fe homeostasis enzyme activities and gene expression, and by promoting Fe accumulation in plant tissues as well as by enhancing thiol redox modification and photosynthesis in soybean plants.

The oxidative damage of the Lagerstroemia indica chlorosis mutant gl1 involves in ferroptosis

Photooxidation is the major physiological performance of the Lagerstroemia indica chlorosis mutant gl1 under field conditions. The mechanisms of the progressive symptoms of oxidative damage from the lower older leaves to the upper mature leaves are complicated and still unclear. The aim of this work was to investigate the physiological mechanisms of oxidative stress from the perspective of the photosynthetic metabolites. The phytosynthetic metabolites of gl1 mutant changed significantly compared to wild type (WT) L. indica, such as by increasing phenolics, decreasing soluble sugar, protein and ascorbate, and redistributing antioxidant enzyme activities. The co-accumulation of phenolics and guaiacol-POD in gl1 mutant promote the removal of H₂O₂, as well the increase of phenoxyl radicals levels. Furthermore, the ion balance was significantly disturbed and Fe accumulated the most among these fluctuating nutrients in the leaves of gl1 mutant. The accumulated Fe was found neither in the chloroplasts nor in the cell wall of the leaves and became unshielded Fe, which favors the Fenton/Haber-Weiss reaction and stabilizes the phenoxyl radicals in metal complexation. The results suggested that the increase of phenolics and Fe accumulation were obviously involved in oxidative damage of gl1 mutant.

Ammonium improved cell wall and cell membrane P reutilization and external P uptake in a putrescine and ethylene dependent pathway

Ammonium promotes rice P uptake and reutilization better than nitrate, under P starvation conditions; however, the underlying mechanism remains unclear. In this study, ammonium treatment significantly increased putrescine and ethylene content in rice roots under P deficient conditions, by increasing the protein content of ornithine decarboxylase and 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase compared with nitrate treatment. Ammonium treatment increased rice root cell wall P release by increasing pectin content and pectin methyl esterase (PME) activity, increased rice shoot cell membrane P release by decreasing phosphorus-containing lipid components, and maintained internal P homeostasis by increasing OsPT2/6/8 expression compared with nitrate treatment. Ammonium also improved external P uptake by regulating root morphology and increased rice grain yield by increasing the panicle number compared with nitrate treatment. The application of putrescine and ethylene synthesis precursor ACC further improved the above process. Our results demonstrate for the first time that ammonium increases rice P acquisition, reutilization, and homeostasis, and rice grain yield, in a putrescine- and ethylene-dependent manner, better than nitrate, under P starvation conditions.

Phytomelatonin and plant mineral nutrition

Plant mineral nutrition is critical for agricultural productivity and for human nutrition; however, the availability of mineral elements is spatially and temporally heterogeneous in many ecosystems and agricultural landscapes. Nutrient imbalances trigger intricate signalling networks that modulate plant acclimation responses. One signalling agent of particular importance in such networks is phytomelatonin, a pleiotropic molecule with multiple functions. Evidence indicates that deficiencies or excesses of nutrients generally increase phytomelatonin levels in certain tissues, and it is increasingly thought to participate in the regulation of plant mineral nutrition. Alterations in endogenous phytomelatonin levels can protect plants from oxidative stress, influence root architecture, and influence nutrient uptake and efficiency of use through transcriptional and post-transcriptional regulation; such changes optimize mineral nutrient acquisition and ion homeostasis inside plant cells and thereby help to promote growth. This review summarizes current knowledge on the regulation of plant mineral nutrition by melatonin and highlights how endogenous phytomelatonin alters plant responses to specific mineral elements. In addition, we comprehensively discuss how melatonin influences uptake and transport under conditions of nutrient shortage.

The importance of the urea cycle and its relationships to polyamine metabolism during ammonium stress in Medicago truncatula

The ornithine-urea cycle (urea cycle) makes a significant contribution to the metabolic responses of lower photosynthetic eukaryotes to episodes of high nitrogen availability. In this study, we compared the role of the plant urea cycle and its relationships to polyamine metabolism in ammonium-fed and nitrate-fed Medicago truncatula plants. High ammonium resulted in the accumulation of ammonium and pathway intermediates, particularly glutamine, arginine, ornithine, and putrescine. Arginine decarboxylase activity was decreased in roots, suggesting that the ornithine decarboxylase-dependent production of putrescine was important in situations of ammonium stress. The activity of copper amine oxidase, which releases ammonium from putrescine, was significantly decreased in both shoots and roots. In addition, physiological concentrations of ammonium inhibited copper amine oxidase activity in in vitro assays, supporting the conclusion that high ammonium accumulation favors putrescine synthesis. Moreover, early supplementation of plants with putrescine avoided ammonium toxicity. The levels of transcripts encoding urea-cycle-related proteins were increased and transcripts involved in polyamine catabolism were decreased under high ammonium concentrations. We conclude that the urea cycle and associated polyamine metabolism function as important protective mechanisms limiting ammonium toxicity in M. truncatula. These findings demonstrate the relevance of the urea cycle to polyamine metabolism in higher plants.

Genome Editing Targets for Improving Nutrient Use Efficiency and Nutrient Stress Adaptation

In recent years, the development of RNA-guided genome editing (CRISPR-Cas9 technology) has revolutionized plant genome editing. Under nutrient deficiency conditions, different transcription factors and regulatory gene networks work together to maintain nutrient homeostasis. Improvement in the use efficiency of nitrogen (N), phosphorus (P) and potassium (K) is essential to ensure sustainable yield with enhanced quality and tolerance to stresses. This review outlines potential targets suitable for genome editing for understanding and improving nutrient use (NtUE) efficiency and nutrient stress tolerance. The different genome editing strategies for employing crucial negative and positive regulators are also described. Negative regulators of nutrient signalling are the potential targets for genome editing, that may improve nutrient uptake and stress signalling under resource-poor conditions. The promoter engineering by CRISPR/dead (d) Cas9 (dCas9) cytosine and adenine base editing and prime editing is a successful strategy to generate precise changes. CRISPR/dCas9 system also offers the added advantage of exploiting transcriptional activators/repressors for overexpression of genes of interest in a targeted manner. CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) are variants of CRISPR in which a dCas9 dependent transcription activation or interference is achieved. dCas9-SunTag system can be employed to engineer targeted gene activation and DNA methylation in plants. The development of nutrient use efficient plants through CRISPR-Cas technology will enhance the pace of genetic improvement for nutrient stress tolerance of crops and improve the sustainability of agriculture.

Arginine Decarboxylase Gene ADC2 Regulates Fiber Elongation in Cotton

Cotton is an important agro-industrial crop providing raw material for the textile industry. Fiber length is the key factor that directly affects fiber quality. ADC, arginine decarboxylase, is the key rate-limiting enzyme in the polyamine synthesis pathway; whereas, there is no experimental evidence that ADC is involved in fiber development in cotton yet. Our transcriptome analysis of the fiber initiation material of Gossypium arboreum L. showed that the expression profile of GaADC2 was induced significantly. Here, GhADC2, the allele of GaADC2 in tetraploid upland cotton Gossypium hirsutum L., exhibited up-regulated expression pattern during fiber elongation in cotton. Levels of polyamine are correlated with fiber elongation; especially, the amount of putrescine regulated by ADC was increased. Scanning electron microscopy showed that the fiber length was increased with exogenous addition of an ADC substrate or product putrescine; whereas, the fiber density was decreased with exogenous addition of an ADC specific inhibitor. Next, genome-wide transcriptome profiling of fiber elongation with exogenous putrescine addition was performed to determine the molecular basis in Gossypium hirsutum. A total of 3163 differentially expressed genes were detected, which mainly participated in phenylpropanoid biosynthesis, fatty acid elongation, and sesquiterpenoid and triterpenoid biosynthesis pathways. Genes encoding transcription factors MYB109, WRKY1, and TCP14 were enriched. Therefore, these results suggested the ADC2 and putrescine involvement in the development and fiber elongation of G. hirsutum, and provides a basis for cotton fiber development research in future.

Jasmonic acid is involved in root cell wall phosphorus remobilization through the nitric oxide dependent pathway in rice

Jasmonic acid (JA) is involved in phosphorus (P) stress in plants, but its underlying molecular mechanisms are still elusive. In this study, we found root endogenous JA content in rice increased under P deficiency (-P), suggesting that JA might participate in P homeostasis in plants. This hypothesis was further confirmed through the addition of exogenous JA (+JA), as this could increase both the root and shoot soluble P content through regulating root cell wall P reutilization. In addition, -P+JA treatment significantly induced the expression of P transporter gene OsPT2, together with increased xylem P content, implying that JA is also important for P translocation from the root to the shoot in P-deficient rice. Furthermore, the accumulation of the molecular signal nitric oxide (NO) was enhanced under -P+JA treatment when compared with -P treatment alone, while the addition of c-PTIO, a scavenger of NO, could reverse the P-deficient phenotype alleviated by JA. Taken together, our results reveal a JA-NO-cell wall P reutilization pathway under P deficiency in rice.

Putrescine is involved in root cell wall phosphorus remobilization in a nitric oxide dependent manner

Phosphorus (P) deficiency is a key limited factor to affect the crop production in rice (Oryza sativa). Recently, accumulating evidences have shown that root cell wall P reutilization could be released to the cytoplasm to alleviate the P starvation and a set of plant hormone and signal molecules have been identified to be involved in it. However, the role of putrescine (Put) in this process is still unknown. In this study, we found that Put with a concentration of 0.001 mM, 0.01 mM and 0.1 mM increased the root and shoot biomass in Nipponbare (Nip) and Kasalath (Kas) under P deficiency, although only 0.1 mM Put could significantly elevated the root and shoot soluble P concentration in Nip. Exogenous 0.1 mM Put treatment reduced the root cell wall P content through increasing the pectin content and pectin methylesterase (PME) activity, indicating that Put can be involved in the root cell wall P reutilization under P starvation. In addition, Put treatment also stimulated the root-to-shoot translocation of P through upregulating the expression of PHOSPHORUS TRANSPORTER 2 (OsPT2) and OsPT8 that responsible for the long-distance transport. Put under P-deficient condition significantly enhanced the Nitric Oxide (NO) accumulation in root and the application of NO inhibitor carboxy-PTIO (cPTIO) could reverse the Put-alleviated P-deficient phenotype, suggesting this process is mediated by NO. In conclusion, our results demonstrated that Put acts upstream of NO to activate the root cell wall P remobilization in rice.

Modulation of Phosphate Deficiency-Induced Metabolic Changes by Iron Availability in Arabidopsis thaliana

Concurrent suboptimal supply of several nutrients requires the coordination of nutrient-specific transcriptional, phenotypic, and metabolic changes in plants in order to optimize growth and development in most agricultural and natural ecosystems. Phosphate (P_i) and iron (Fe) deficiency induce overlapping but mostly opposing transcriptional and root growth responses in Arabidopsis thaliana. On the metabolite level, P_i deficiency negatively modulates Fe deficiency-induced coumarin accumulation, which is controlled by Fe as well as P_i deficiency response regulators. Here, we report the impact of Fe availability on seedling growth under P_i limiting conditions and on P_i deficiency-induced accumulation of amino acids and organic acids, which play important roles in P_i use efficiency. Fe deficiency in P_i replete conditions hardly changed growth and metabolite profiles in roots and shoots of Arabidopsis thaliana, but partially rescued growth under conditions of P_i starvation and severely modulated P_i deficiency-induced metabolic adjustments. Analysis of T-DNA insertion lines revealed the concerted coordination of metabolic profiles by regulators of Fe (FIT, bHLH104, BRUTUS, PYE) as well as of P_i (SPX1, PHR1, PHL1, bHLH32) starvation responses. The results show the interdependency of P_i and Fe availability and the interplay between P_i and Fe starvation signaling on the generation of plant metabolite profiles.

Inhibition of nitric oxide production under alkaline conditions regulates iron homeostasis in rice

Rice is one of the most susceptible plants to iron (Fe) deficiency under neutral and alkaline conditions. Alkaline stress induces H₂ O₂ production and increases the deposition of Fe on the root surface, which causes leaf chlorosis and Fe deficiency in rice. Gene chip and qRT-PCR analysis indicated that the expression of the nitrate reductase (NR) genes were downregulated by alkaline treatment, which resulted in significantly decreased nitrate activity and nitric oxide (NO) production in the epidermis and stele, where H₂ O₂ accumulated. In contrast, treatment with sodium nitroprusside (SNP), a NO donor, strongly alleviated alkaline-induced Fe deficiency by limiting Fe plaque formation. Increasing the NO signal significantly reduced the accumulation of H₂ O₂ and the lignin barrier but enhanced phenolic acid secretion in the root epidermis and stele under alkaline conditions. The secreted phenolic acid effectively mobilized the apoplast Fe and increased Fe uptake in roots, thereby alleviating the Fe-deficiency response and downregulating the expressions of Fe-uptake genes under alkaline conditions. In conclusion, alkaline stress inhibits NR activity and NO production in the roots of rice, which play vital roles in the mobilization of the apoplast Fe by regulation of H₂ O₂ and phenolic acid concentrations.

Induction of S-nitrosoglutathione reductase protects root growth from ammonium toxicity by regulating potassium homeostasis in Arabidopsis and rice

Ammonium (NH4+) is toxic to root growth in most plants already at moderate levels of supply, but mechanisms of root growth tolerance to NH4+ remain poorly understood. Here, we report that high levels of NH4+ induce nitric oxide (NO) accumulation, while inhibiting potassium (K+) acquisition via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4), leading to the arrest of primary root growth. High levels of NH4+ also stimulated the accumulation of GSNOR (S-nitrosoglutathione reductase) in roots. GSNOR overexpression improved root tolerance to NH4+. Loss of GSNOR further induced NO accumulation, increased SNO1/SOS4 activity, and reduced K+ levels in root tissue, enhancing root growth sensitivity to NH4+. Moreover, the GSNOR-like gene, OsGSNOR, is also required for NH4+ tolerance in rice. Immunoblotting showed that the NH4+-induced GSNOR protein accumulation was abolished in the VTC1- (vitamin C1) defective mutant vtc1-1, which is hypersensititive to NH4+ toxicity. GSNOR overexpression enhanced vtc1-1 root tolerance to NH4+. Our findings suggest that induction of GSNOR increases NH4+ tolerance in Arabidopsis roots by counteracting NO-mediated suppression of tissue K+, which depends on VTC1 function.

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.

Melatonin Enhances the Tolerance and Recovery Mechanisms in Brassica juncea (L.) Czern. Under Saline Conditions

Melatonin has been recently known to stimulate plant growth and induce protective responses against different abiotic stresses. However, the mechanisms behind exogenous melatonin pretreatment and restoration of plant vigor from salinity stress remain poorly understood. The present study aimed to understand the effects of exogenous melatonin pretreatment on salinity-damaged green mustard (Brassica juncea L. Czern.) seedlings in terms of oxidative stress regulation and endogenous phytohormone production. Screening of several melatonin concentrations (0, 0.1, 1, 5, and 10 μM) on mustard growth showed that the 1 μM concentration revealed an ameliorative increase of plant height, leaf length, and leaf width. The second study aimed at determining how melatonin application can recover salinity-damaged plants and studying its effects on physiological and biochemical parameters. Under controlled environmental conditions, mustard seedlings were irrigated with distilled water or 150 mM of NaCl for 7 days. This was followed by 1 μM of melatonin application to determine its recovery impact on the damaged plants. Furthermore, several physiological and biochemical parameters were examined in stressed and unstressed seedlings with or without melatonin application. Our results showed that plant height, leaf length/width, and stem diameter were enhanced in 38-day-old salinity-stressed plants under melatonin treatment. Melatonin application obviously attenuated salinity-induced reduction in gas exchange parameters, relative water content, and amino acid and protein levels, as well as antioxidant enzymes, such as superoxide dismutase and catalase. H₂O₂ accumulation in salinity-damaged plants was reduced by melatonin treatment. A decline in abscisic acid content and an increase in salicylic acid content were observed in salinity-damaged seedlings supplemented with melatonin. Additionally, chlorophyll content decreased during the recovery period in salinity-damaged plants by melatonin treatment. This study highlighted, for the first time, the recovery impact of melatonin on salinity-damaged green mustard seedlings. It demonstrated that exogenous melatonin supplementation significantly improved the physiologic and biochemical parameters in salinity-damaged green mustard seedlings.

Molecular functions of nitric oxide and its potential applications in horticultural crops

Nitric oxide (NO) regulates plant growth, enhances nutrient uptake, and activates disease and stress tolerance mechanisms in most plants, making NO a potential tool for use in improving the yield and quality of horticultural crop species. Although the use of NO in horticulture is still in its infancy, research on NO in model plant species has provided an abundance of valuable information on horticultural crop species. Emerging evidence implies that the bioactivity of NO can occur through many potential mechanisms but occurs mainly through S-nitrosation, the covalent and reversible attachment of NO to cysteine thiol. In this context, NO signaling specifically affects crop development, immunity, and environmental interactions. Moreover, NO can act as a fumigant against a wide range of postharvest diseases and pests. However, for effective use of NO in horticulture, both understanding and exploring the biological significance and potential mechanisms of NO in horticultural crop species are critical. This review provides a picture of our current understanding of how NO is synthesized and transduced in plants, and particular attention is given to the significance of NO in breaking seed dormancy, balancing root growth and development, enhancing nutrient acquisition, mediating stress responses, and guaranteeing food safety for horticultural production.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.

AtHAP5A regulates iron translocation in iron-deficient Arabidopsis thaliana

Iron (Fe) deficient plants employ multiple strategies to increase root uptake and root-to-shoot translocation of Fe. The identification of genes that are responsible for these processes, and a comprehensive understanding of the regulatory effects of transcriptional networks on their expression, including transcription factors (TFs), is underway in Arabidopsis thaliana. Here, we show that a Histone- or heme-associated proteins (HAP) transcription factor (TF), HAP5A, is necessary for the response to Fe deficiency in Arabidopsis. Its expression was induced under Fe deficiency, and the lack of HAP5A significantly decreased Fe translocation from the root to the shoot, resulting in substantial chlorosis of the newly expanded leaves, compared with the wild-type (WT, Col-0). Further analysis found that the expression of a gene encoding nicotianamine (NA) synthase (NAS1) was dramatically decreased in the hap5a mutant, regardless of the Fe status. Yeast-one-hybrid and ChIP analyses suggested that HAP5A directly binds to the promoter region of NAS1. Moreover, overexpression of NAS1 could rescue the chlorosis phenotype of hap5a in Fe deficient conditions. In summary, a novel pathway was elucidated, showing that NAS1-dependent translocation of Fe from the root to the shoot is controlled by HAP5A in Fe-deficient Arabidopsis thaliana.

Direct and Bicarbonate-Induced Iron Deficiency Differently Affect Iron Translocation in Kiwifruit Roots

Bicarbonate-induced iron (Fe) deficiency (+Bic) is frequently observed in kiwifruit orchards, but more research attention has been paid to direct Fe deficiency (-Fe) in plants, including kiwifruit. Here we compared the differences of kiwifruit plants between -Fe and +Bic in: (1) the traits of ⁵⁷Fe uptake and translocation within plants, (2) Fe forms in roots, and (3) some acidic ions and metabolites in roots. The concentration of ⁵⁷Fe derived from nutrient solution (⁵⁷Fedfs) in roots was less reduced in +Bic than -Fe treatment, despite similar decrease in shoots of both treatments. +Bic treatment increased ⁵⁷Fedfs distribution in fine roots but decreased it in new leaves and stem, thereby displaying the inhibition of ⁵⁷Fedfs translocation from roots to shoots and from fine roots to xylem of coarse roots. Moreover, +Bic imposition induced the accumulation of water-soluble Fe and apoplastic Fe in roots. However, the opposite was observed in -Fe-treated plants. Additionally, the cell wall Fe and hemicellulose Fe in roots were less reduced by +Bic than -Fe treatment. +Bic treatment also triggered the reduction in H⁺ extrusion and the accumulation of NH₄⁺, succinic acid, and some amino acids in roots. These results suggest that, contrary to -Fe, +Bic treatment inhibits Fe translocation to shoots by accumulating water-soluble and apoplastic Fe and slowing down the release of hemicellulose Fe in the cell wall in kiwifruit roots, which may be related to the decreased H⁺ extrusion and the imbalance between C and N metabolisms.

Micronutrient homeostasis and chloroplast iron protein expression is largely maintained in a chloroplast copper transporter mutant

PAAI is a P-Type ATPase that functions to import copper (Cu) into the chloroplast. Arabidopsis thaliana (L.) Heynh. paa1 mutants have lowered plastocyanin levels, resulting in a decreased photosynthetic electron transport rate. In nature, iron (Fe) and Cu homeostasis are often linked and it can be envisioned that paa1 acclimates its photosynthetic machinery by adjusting expression of its chloroplast Fe-proteome, but outside of Cu homeostasis paa1 has not been studied. Here, we characterise paa1 ultrastructure and accumulation of electron transport chain proteins in a paa1 allelic series. Furthermore, using hydroponic growth conditions, we characterised metal homeostasis in paa1 with an emphasis on the effects of Fe deficiency. Surprisingly, the paa1 mutation does not affect chloroplast ultrastructure or the accumulation of other photosynthetic electron transport chain proteins, despite the strong decrease in electron transport rate. The regulation of Fe-related photosynthetic electron transport proteins in response to Fe status was maintained in paa1, suggesting that regulation of the chloroplast Fe proteins ignores operational signals from photosynthetic output. The characterisation of paa1 has revealed new insight into the regulation of expression of the photosynthetic electron transport chain proteins and chloroplast metal homeostasis and can help to develop new strategies for the detection of shoot Fe deficiency.

Hydrogen sulphide alleviates iron deficiency by promoting iron availability and plant hormone levels in Glycine max seedlings

BACKGROUND

Hydrogen sulphide (H2S) is involved in regulating physiological processes in plants. We investigated how H2S ameliorates iron (Fe) deficiency in soybean (Glycine max L.) seedlings. Multidisciplinary approaches including physiological, biochemical and molecular, and transcriptome methods were used to investigate the H2S role in regulating Fe availability in soybean seedlings.

RESULTS

Our results showed that H2S completely prevented leaf interveinal chlorosis and caused an increase in soybean seedling biomass under Fe deficiency conditions. Moreover, H2S decreased the amount of root-bound apoplastic Fe and increased the Fe content in leaves and roots by regulating the ferric-chelate reductase (FCR) activities and Fe homeostasis- and sulphur metabolism-related gene expression levels, thereby promoting photosynthesis in soybean seedlings. In addition, H2S changed the plant hormone concentrations by modulating plant hormone-related gene expression abundances in soybean seedlings grown in Fe-deficient solution. Furthermore, organic acid biosynthesis and related genes expression also played a vital role in modulating the H2S-mediated alleviation of Fe deficiency in soybean seedlings.

CONCLUSION

Our results indicated that Fe deficiency was alleviated by H2S through enhancement of Fe acquisition and assimilation, thereby regulating plant hormones and organic acid synthesis in plants.

Impacts of plant growth promoters and plant growth regulators on rainfed agriculture

Demand for agricultural crop continues to escalate in response to increasing population and damage of prime cropland for cultivation. Research interest is diverted to utilize soils with marginal plant production. Moisture stress has negative impact on crop growth and productivity. The plant growth promoting rhizobacteria (PGPR) and plant growth regulators (PGR) are vital for plant developmental process under moisture stress. The current study was carried out to investigate the effect of PGPR and PGRs (Salicylic acid and Putrescine) on the physiological activities of chickpea grown in sandy soil. The bacterial isolates were characterized based on biochemical characters including Gram-staining, P-solubilisation, antibacterial and antifungal activities and catalases and oxidases activities and were also screened for the production of indole-3-acetic acid (IAA), hydrogen cyanide (HCN) and ammonia (NH₃). The bacterial strains were identified as Bacillus subtilis, Bacillus thuringiensis and Bacillus megaterium based on the results of 16S-rRNA gene sequencing. Chickpea seeds of two varieties (Punjab Noor-2009 and 93127) differing in sensitivity to drought were soaked for 3 h before sowing in fresh grown cultures of isolates. Both the PGRs were applied (150 mg/L), as foliar spray on 20 days old seedlings of chickpea. Moisture stress significantly reduced the physiological parameters but the inoculation of PGPR and PGR treatment effectively ameliorated the adverse effects of moisture stress. The result showed that chickpea plants treated with PGPR and PGR significantly enhanced the chlorophyll, protein and sugar contents. Shoot and root fresh (81%) and dry weights (77%) were also enhanced significantly in the treated plants. Leaf proline content, lipid peroxidation and antioxidant enzymes (CAT, APOX, POD and SOD) were increased in reaction to drought stress but decreased due to PGPR. The plant height (61%), grain weight (41%), number of nodules (78%) and pod (88%), plant yield (76%), pod weight (53%) and total biomass (54%) were higher in PGPR and PGR treated chickpea plants grown in sandy soil. It is concluded from the present study that the integrative use of PGPR and PGRs is a promising method and eco-friendly strategy for increasing drought tolerance in crop plants.

Gamma-aminobutyric acid enhances tolerance to iron deficiency by stimulating auxin signaling in cucumber (Cucumis sativusL.)

Iron deficiency severely affects crop yield and quality. Gamma-aminobutyric acid (GABA) plays a vital role in plant responses to multifarious stresses. However, the role of GABA in Fe deficiency responses and the potential mechanisms remain largely unknown in cucumber. Here, we found that Fe deficiency raised the GABA levels in leaves and roots of cucumber. To probe the role of GABA in Fe deficiency, the seedlings were subjected to five levels of GABA concentrations (0, 5, 10, 20 and 40 mmol L^-1) for 7 days under Fe deficiency. The results demonstrated that 20 mM GABA in alleviating the Fe deficiency-induced stress was the most effective. GABA pretreatment reduced the Fe deficiency-induced chlorosis and inhibition of photosynthesis and growth, and significantly enhanced the contents of iron in shoots and roots. Exogenous GABA significantly decreased the pH of nutrient solution and increased ferric-chelate reductase (FCR) activity induced by Fe deficiency and the transcript levels of Fe uptake-related genes HA1, FRO2 and IRT1 in roots. GABA also increased the content of auxin (IAA) and expression of auxin biosynthesis (YUC4), response (IAA1), and transport (PIN1) genes under Fe deficiency. Furthermore, exogenous the auxin transport inhibitor 1-naphthylphthalamic acid (NPA) application abolished the GABA-induced changes in Fe deficiency. In summary, we found that GABA improves tolerance to iron deficiency via an auxin-dependent mechanism in cucumber.

The PAP/SAL1 retrograde signaling pathway is involved in iron homeostasis

There is a link between PAP/SAL retrograde pathway, ethylene signaling and Fe metabolism in Arabidopsis. Nuclear gene expression is regulated by a diversity of retrograde signals that travel from organelles to the nucleus in a lineal or classical model. One such signal molecule is 3'-phosphoadenisine-5'-phosphate (PAP) and it's in vivo levels are regulated by SAL1/FRY1, a phosphatase enzyme located in chloroplast and mitochondria. This metabolite inhibits the action of a group of exorribonucleases which participate in post-transcriptional gene expression regulation. Transcriptome analysis of Arabidopsis thaliana mutant plants in PAP-SAL1 pathway revealed that the ferritin genes AtFER1, AtFER3, and AtFER4 are up-regulated. In this work we studied Fe metabolism in three different mutants of the PAP/SAL1 retrograde pathway. Mutant plants showed increased Fe accumulation in roots, shoots and seeds when grown in Fe-sufficient condition, and a constitutive activation of the Strategy I Fe uptake genes. As a consequence, they grew more vigorously than wild type plants in Fe-deficient medium. However, when mutant plants grown in Fe-deficient conditions were sprayed with Fe in their leaves, they were unable to deactivate root Fe uptake. Ethylene synthesis inhibition revert the constitutive Fe uptake phenotype. We propose that there is a link between PAP/SAL pathway, ethylene signaling and Fe metabolism.

Putrescine alleviates aluminum toxicity in rice (Oryza sativa) by reducing cell wall Al contents in an ethylene-dependent manner

Aluminum (Al³⁺ ) toxicity in acidic soils limits crop productivity worldwide. In this study, we found that putrescine (PUT) significantly alleviates Al toxicity in rice roots. The addition of 0.1 mM PUT promoted root elongation and reduced the Al content in the root apices of Nipponbare (Nip) and Kasalath (Kas) rice under Al toxicity conditions. Exogenous treatment with PUT reduced the cell wall Al content by reducing polysaccharide (pectin and hemicellulose) levels and pectin methylesterase (PME) activity in roots and decreased the translocation of Al from the external environment to the cytoplasm by downregulating the expression of OsNRAT1, which responsible to encode an Al transporter protein Nrat1 (Nramp aluminum transporter 1). The addition of PUT under Al toxicity conditions significantly inhibited ethylene emissions and suppressed the expression of genes involved in ethylene biosynthesis. Treatment with the ethylene precursor 1-aminocylopropane-1-carboxylic acid (ACC) significantly improved ethylene emission, inhibited root elongation, increased the Al accumulation in root tips and the root cell wall, and increased cell wall pectin and hemicellulose contents in both rice cultivars under Al toxicity conditions. The ethylene biosynthesis antagonist aminoethoxyvinylglycine (AVG, inhibitor of the ACC synthase) had the opposite effect and reduced PME activity. Together, our results show that PUT decreases the cell wall Al contents by suppressing ethylene emissions and decreases the symplastic Al levels by downregulating OsNRAT1 in rice.

Ammonium regulates Fe deficiency responses by enhancing nitric oxide signaling in Arabidopsis thaliana

The accumulation of NH₄⁺ in response to Fe deficiency plays a role not only in the remobilization of Fe from the root cell wall, but also in the transportation of Fe from root to shoot. Ammonium (NH₄⁺) plays an important role in phosphorus-deficiency responses in rice, but its role in responses to Fe deficiency remains unknown. Here, we demonstrate that the accumulation of NH₄⁺ plays a pivotal role when Arabidopsis thaliana plants are subject to Fe deficiency. The Arabidopsis amt1-3 mutant, which is defective in endogenous NH₄⁺ sensing, exhibited increased sensitivity to Fe deficiency compared to WT (wild type; Col-0). In addition, exogenous application of NH₄⁺ significantly alleviated Fe deficiency symptoms in plants. NH₄⁺ triggers the production of nitric oxide (NO), which then induces ferric-chelate reductase (FCR) activity and accelerates the release of Fe from the cell wall, especially hemicellulose, thereby increasing the availability of soluble Fe in roots. NH₄⁺ also increases soluble Fe levels in shoots by upregulating genes involved in Fe translocation, such as FRD3 (FERRIC REDUCTASE DEFECTIVE3) and NAS1 (NICOTIANAMINE SYNTHASE1), hence, alleviating leaf chlorosis. Overall, NH₄⁺ plays an important role in the reutilization of Fe from the cell wall and the redistribution of Fe from root to shoot in Fe-deficient Arabidopsis, a process dependent on NO accumulation.

NH4+-N alleviates iron deficiency in rice seedlings under calcareous conditions

Drip-irrigated rice (Oryza sativa L.) in calcareous soil exhibits signs of iron (Fe) deficiency. This study aimed to explore whether NH4+ alleviates Fe deficiency in rice seedlings grown under calcareous conditions. Two rice varieties (cv. 'T43' Fe deficiency-tolerant variety and cv. 'T04' Fe deficiency-sensitive variety) were used to carry out two independent experiments with exposure to different nitrogen (N) forms (nitrate (NO3-) or NH4+) under calcareous conditions. In experiment 1, plants were precultured in a nutrient solution with excess Fe (40 µM Fe(II)-EDTA) for 14 d and then supplied NO3--N (AN) or NH4--N (NN) without Fe for 3, 6, or 12 d. In experiment 2, plants were fed AN or NN with 10 µM Fe(II)-EDTA for 18 d. Compared to plants exposed to AN, leaves of plants exposed to NN showed severe chlorosis and significantly decreased chlorophyll content during Fe starvation. The xylem sap pH and cell wall Fe fraction in both shoots and roots of rice fed NN were significantly higher than those fed AN. However, the Fe concentration in xylem sap, soluble and organelle Fe fractions in both shoots and roots, and the shoot/root Fe content ratio in rice exposed to AN were significantly higher than those in plants exposed to NN. AN reduced the root aerenchyma fraction and root porosity compared to NN, which induced greater water uptake and hydraulic conductance by roots, hence the stronger xylem sap flow rate with AN. The results indicated that NH4+-N alleviated Fe deficiency in rice under calcareous conditions by promoting Fe re-allocation in rice tissues and Fe transportation from roots to shoots.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

A Mass Spectrometry-Based Study Shows that Volatiles Emitted by Arthrobacter agilis UMCV2 Increase the Content of Brassinosteroids in Medicago truncatula in Response to Iron Deficiency Stress

Iron is an essential plant micronutrient. It is a component of numerous proteins and participates in cell redox reactions; iron deficiency results in a reduction in nutritional quality and crop yields. Volatiles from the rhizobacterium Arthrobacter agilis UMCV2 induce iron acquisition mechanisms in plants. However, it is not known whether microbial volatiles modulate other metabolic plant stress responses to reduce the negative effect of iron deficiency. Mass spectrometry has great potential to analyze metabolite alterations in plants exposed to biotic and abiotic factors. Direct liquid introduction-electrospray-mass spectrometry was used to study the metabolite profile in Medicago truncatula due to iron deficiency, and in response to microbial volatiles. The putatively identified compounds belonged to different classes, including pigments, terpenes, flavonoids, and brassinosteroids, which have been associated with defense responses against abiotic stress. Notably, the levels of these compounds increased in the presence of the rhizobacterium. In particular, the analysis of brassinolide by gas chromatography in tandem with mass spectrometry showed that the phytohormone increased ten times in plants grown under iron-deficient growth conditions and exposed to microbial volatiles. In this mass spectrometry-based study, we provide new evidence on the role of A. agilis UMCV2 in the modulation of certain compounds involved in stress tolerance in M. truncatula.

How Plants Handle Trivalent (+3) Elements

Plant development and fitness largely depend on the adequate availability of mineral elements in the soil. Most essential nutrients are available and can be membrane transported either as mono or divalent cations or as mono- or divalent anions. Trivalent cations are highly toxic to membranes, and plants have evolved different mechanisms to handle +3 elements in a safe way. The essential functional role of a few metal ions, with the possibility to gain a trivalent state, mainly resides in the ion's redox activity; examples are iron (Fe) and manganese. Among the required nutrients, the only element with +3 as a unique oxidation state is the non-metal, boron. However, plants also can take up non-essential trivalent elements that occur in biologically relevant concentrations in soils. Examples are, among others, aluminum (Al), chromium (Cr), arsenic (As), and antimony (Sb). Plants have evolved different mechanisms to take up and tolerate these potentially toxic elements. This review considers recent studies describing the transporters, and specific and unspecific channels in different cell compartments and tissues, thereby providing a global vision of trivalent element homeostasis in plants.

Overexpression of S-nitrosoglutathione reductase alleviated iron-deficiency stress by regulating iron distribution and redox homeostasis

Iron (Fe) is an essential micronutrient element for plant growth. The S-nitrosoglutathione reductase (GSNOR) gene's functions under Fe-deficiency conditions are not well understood. Here, GSNOR expression was induced by Fe deficiency in tomato (Solanum lycopersicum L.) leaves and roots, while its overexpression alleviated chlorosis under Fe-deficiency conditions. GSNOR overexpression positively regulated the Fe distribution from root to shoot, which might result from the transcriptional regulation of genes involved in Fe metabolism. Additionally, the overexpression of GSNOR maintained redox homeostasis and protected chloroplasts from Fe-deficiency-related damage, resulting in a greater photosynthetic capacity. As a nitric oxide regulator, GSNOR's overexpression decreased the excessive accumulation of nitric oxide and S-nitrosothiols during the Fe deficiency, and maintained the homeostases of reactive oxygen species and reactive nitrogen species. Moreover, GSNOR overexpression, probably at the level of genes and proteins, along with protein S-nitrosylation, promoted Fe uptake and regulated the shoot/root Fe ratio under Fe-deficiency conditions.

Bacillus subtilis STU6 Ameliorates Iron Deficiency in Tomato by Enhancement of Polyamine-Mediated Iron Remobilization

Iron (Fe) deficiency often triggers arginine overproduction in plants. However, it remains elusive whether Fe deficiency-induced increases of arginine levels are involved in beneficial rhizobacteria recruitment and that the mechanism underlying rhizobacteria induced plant Fe deficiency tolerance. Here, Bacillus subtilis STU6 increased soluble Fe content in tomato, thereby alleviating Fe deficiency-induced chlorosis. In a split-root system, STU6 significantly induced arginine exudation by Fe-deficient roots, and increased arginine levels promoted spermidine (Spd) production by STU6 and bacterial colonization. Deletion of the STU6 speB gene inhibited Spd synthesis and abrogated STU6-induced increments of soluble Fe content in the Fe-deficient plants. Increased host Spd levels by STU6 greatly stimulated the NO accumulation in the Fe-deficient roots. Furthermore, disruption of NO signaling markedly repressed STU6-mediated cell wall Fe remobilization. Collectively, our data provide important evidence that chemical dialogues between tomato and STU6 contribute to enhancement of microbe-mediated plant adaptation to Fe deficiency.

Small-Molecules Selectively Modulate Iron-Deficiency Signaling Networks in Arabidopsis

Plant growth requires optimal levels of iron (Fe). Fe is used for energy production, numerous enzymatic processes, and is indispensable for cellular metabolism. Recent studies have established the mechanism involved in Fe uptake and transport. However, our knowledge of Fe sensing and signaling is limited. Dissecting Fe signaling may be useful for crop improvement by Fe fortification. Here, we report two small-molecules, R3 and R6 [where R denotes repressor of IRON-REGULATED TRANSPORTER 1 (IRT1)], identified through a chemical screening, whose use blocked activation of the Fe-deficiency response in Arabidopsis thaliana. Physiological analysis of plants treated with R3 and R6 showed that these small molecules drastically attenuated the plant response to Fe starvation. Small-molecule treatment caused severe chlorosis and strongly reduced chlorophyll levels in plants. Fe content in shoots was decreased considerably by small-molecule treatments especially in Fe deficiency. Small-molecule treatments attenuated the Fe-deficiency-induced expression of the Fe uptake gene IRT1. Analysis of FER-LIKE IRON-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) and subgroup Ib basic helix-loop-helix (bHLH) gene (bHLH38/39/100/101) expression showed that R3 affects the FIT-network, whereas R6 affects both the FIT and Ib bHLH networks. An assessment of the effects of the structural analogs of R3 and R6 on the induction of Fe-dependent chlorosis revealed the functional motif of the investigated chemicals. Our findings suggest that small-molecules selectively modulate the distinct signaling routes that operate in response to Fe-deficiency. R3 and R6 likely interrupt the activity of key upstream signaling regulators whose activities are required for the activation of the Fe-starvation transcriptional cascade in Arabidopsis roots.

The arginine decarboxylase gene ADC1, associated to the putrescine pathway, plays an important role in potato cold-acclimated freezing tolerance as revealed by transcriptome and metabolome analyses

Low temperature severely influences potato production as the cultivated potato (Solanum tuberosum) is frost sensitive, however the mechanism underlying the freezing tolerance of the potato is largely unknown. In the present research, we studied the transcriptome and metabolome of the freezing-tolerant wild species Solanum acaule (Aca) and freezing-sensitive cultivated S. tuberosum (Tub) to identify the main pathways and important factors related to freezing tolerance. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation indicated that polyamine and amino acid metabolic pathways were specifically upregulated in Aca under cold treatment. The transcriptome changes detected in Aca were accompanied by the specific accumulation of putrescine, saccharides, amino acids and other metabolites. The combination of transcriptome and metabolome analyses revealed that putrescine exhibited an accumulative pattern in accordance with the expression of the arginine decarboxylase gene ADC1. The primary role of putrescine was further confirmed by analyzing all three polyamines (putrescine, spermidine, and spermine) and the genes encoding the corresponding enzymes in two sets of potato genotypes with distinct freezing tolerance, implying that only putrescine and ADC1 were uniquely enhanced by cold in the freezing-tolerant genotypes. The function of putrescine was further analyzed by its exogenous application and the overexpression of SaADC1 in S. tuberosum cv. E3, indicating its important role(s) in cold-acclimated freezing tolerance, which was accompanied with the activation of C-repeat binding factor genes (CBFs). The present research has identified that the ADC1-associated putrescine pathway plays an important role in cold-acclimated freezing tolerance of potato, probably by enhancing the expression of CBF genes.

Ammonium mitigates Cd toxicity in rice (Oryza sativa) via putrescine-dependent alterations of cell wall composition

In plants, different forms of nitrogen (NO₃^- or NH₄⁺) affect nutrient uptake and environmental stress responses. In the present study, we tested whether NO₃^- and NH₄⁺ affect the ability of rice (Oryza sativa) to tolerate the toxic heavy metal cadmium (Cd). Compared with NO₃^-, NH₄⁺ treatment significantly increased chlorophyll contents and reduced Cd²⁺ levels in rice cultivars Nipponbare (japonica) and Kasalath (indica) grown in 0.2 mM Cd²⁺. NH₄⁺ significantly reduced the pectin and hemicellulose contents and inhibited the pectin methylesterase (PME) activity in rice roots, thereby reducing the negative charges in the cell wall and decreasing the accumulation of Cd²⁺ in roots. In addition, NH₄⁺ reduced the absorption and root-to-shoot translocation of Cd²⁺ by decreasing the expression of OsHMA2 and OsNramp5 in the root. Levels of the signaling molecule putrescine were significantly higher in the roots of both rice cultivars provided with NH₄⁺ compared with NO₃^-. The addition of putrescine reduced Cd²⁺ contents in both rice cultivars and increased the chlorophyll content in shoots by reducing root cell wall pectin and hemicellulose contents, inhibiting PME activity and suppressing the expression of OsHMA2 and OsNramp5 in the root. Taken together, these results indicate that NH₄⁺ treatment alleviated Cd toxicity, enabling rice to withstand the noxious effects of Cd by modifying the cell wall Cd-binding capacity due to alterations of pectin and hemicellulose contents and Cd transport, processes induced by increasing putrescine levels. Our findings suggest methods to decrease Cd accumulation in rice by applying NH₄⁺ fertilizers.

Nitrate inhibits the remobilization of cell wall phosphorus under phosphorus-starvation conditions in rice (Oryza sativa)

NO₃^- not only inhibited the reutilization of cell wall P via decreasing root cell wall pectin content and PME activity, but also hampered the P translocation from root to shoot. The rice cultivars 'Kasalath' (Kas) and 'Nipponbare' (Nip) were used to demonstrate that the nitrogen source NO₃^- inhibits internal phosphorus (P) reutilization in rice under P-absence conditions. Analysis using Kas showed that the expression of - P-induced marker genes OsIPS1/2 and OsSPX1/2/3/5 are significantly higher under 1 mM NO ₃^- - P (1N - P) treatment than 0 mM NO ₃^- - P (0N - P) treatment. The absence of NO₃^- from the nutrient solution significantly increased cell wall P release by increasing pectin synthesis and increasing the activity of pectin methylesterase (PME), and also significantly improved the translocation of soluble P from the root to the shoot by increasing xylem sap P content under P-absence conditions. The rice seedlings grown in 0 mM NO₃^- accumulated significantly higher nitric oxide (NO) in the roots than those grown in 1 mM NO₃^-. Exogenously applying the NO donor sodium nitroprusside (SNP) revealed that NO is a major contributor to differential cell wall P remobilization in rice by mediating pectin synthesis and demethylation under different NO₃^- concentrations (0 and 1 mM) under P-deprived conditions.

Tomato plants ectopically expressing Arabidopsis GRF9 show enhanced resistance to phosphate deficiency and improved fruit production in the field

Agronomic performance of transgenic tomato overexpressing functional genes has rarely been investigated in the field. In an attempt to improve low-phosphate (P) stress tolerance of tomato (Solanum lycopersicum) plants and promote tomato fruit production in the field, an expression vector containing cDNA to an Arabidopsis 14-3-3 protein, General Regulatory Factor 9 (GRF9), driven by a cauliflower mosaic virus 35S promoter, was transferred into tomato plants. Transgenic expression of GRF9 was ascertained by quantitative real-time PCR analysis. The degree of low-P tolerance in transgenic plants was found to be significantly greater than that in wild-type plants, and reflected in improved root development and enhanced P content under hydroponic conditions. For transgenic tomato, roots had higher P uptake, as evidenced by tissue P content and relative expression of the genes LePT1 and LePT2 in both normal and low-P hydroponic solutions. GRF9 overexpressors had greatly enhanced proton extrusion from roots and heightened activity of the plasma-membrane H⁺-ATPase (PM H⁺-ATPase) in roots under low-P hydroponic conditions. Thus, in addition to enhanced root development, higher expression of genes coding for phosphate transporters and improved capacity for acidification in the rhizosphere emerged as key mechanisms underpinning improved P acquisition in transgenic tomato plants in soil. Subsequent field trials measuring tomato fruit production at two P levels, indicated that GRF9 can indeed improve total tomato production and may play a role in early fruit maturity. Our results suggest that the heterologous Arabidopsis GRF9 gene can confer resistance to P deficiency in transgenic tomato plants and promote fruit production.

Melatonin Mitigates Salt Stress in Wheat Seedlings by Modulating Polyamine Metabolism

Melatonin, a small molecular weight indoleamine molecule, is involved in various biological processes and responses to environmental cues in plants. However, its function in abiotic stress response and the underlying mechanisms is less clear. In this study, we investigated the effect of melatonin on wheat seedlings growth under salt stress condition. Exogenous melatonin pretreatment partially mitigated the salt-induced inhibition of whole-plant growth as judged from shoot dry weight, IAA content, leaf photosynthesis rate, maximum photochemistry efficiency of photosystem II, and chlorophyll. The mitigation was also observed in reduced accumulation of H₂O₂ in melatonin-pretreated wheat seedlings exposed to salt stress. Exogenous melatonin increased endogenous melatonin content by evaluating the levels of TaSNAT transcript, which encodes a key regulatory enzyme in the melatonin biosynthetic pathway. Furthermore, melatonin increased polyamine contents by accelerating the metabolic flow from the precursor amino acids arginine and methionine to polyamines; melatonin also decreased the degradation of salt-induced polyamines. Taken together, these results provide the evidence that melatonin mitigates salt stress mainly through its regulation on polyamine metabolism of wheat seedlings.

Diurnal Changes in Transcript and Metabolite Levels during the Iron Deficiency Response of Rice

BACKGROUND

Rice (Oryza sativa L.) is highly susceptible to iron (Fe) deficiency due to low secretion levels of the mugineic acid (MA) family phytosiderophore (PS) 2'-deoxymugineic acid (DMA) into the rhizosphere. The low levels of DMA secreted by rice have proved challenging to measure and, therefore, the pattern of DMA secretion under Fe deficiency has been less extensively studied relative to other graminaceous monocot species that secrete high levels of PS, such as barley (Hordeum vulgare L.).

RESULTS

Gene expression and metabolite analyses were used to characterise diurnal changes occurring during the Fe deficiency response of rice. Iron deficiency inducible genes involved in root DMA biosynthesis and secretion followed a diurnal pattern with peak induction occurring 3-5 h after the onset of light; a result consistent with that of other Strategy II plant species such as barley and wheat. Furthermore, triple quadrupole mass spectrometry identified 3-5 h after the onset of light as peak time of DMA secretion from Fe-deficient rice roots. Metabolite profiling identified accumulation of amines associated with metal chelation, metal translocation and plant oxidative stress responses occurring with peak induction 10-12 h after the onset of light.

CONCLUSION

The results of this study confirmed that rice shares a similar peak time of Fe deficiency associated induction of DMA secretion compared to other Strategy II plant species but has less prominent daily fluctuations of DMA secretion. It also revealed metabolic changes associated with the remediation of Fe deficiency and mitigation of damage from resulting stress in rice roots. This study complements previous studies on the genetic changes in response to Fe deficiency in rice and constitutes an important advance towards our understanding of the molecular mechanisms underlying the rice Fe deficiency response.