Iron deficiency-induced increase of root branching contributes to the enhanced root ferric chelate reductase activity

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.

The activation of iron deficiency responses of grapevine rootstocks is dependent to the availability of the nitrogen forms

BACKGROUND

In viticulture, iron (Fe) chlorosis is a common abiotic stress that impairs plant development and leads to yield and quality losses. Under low availability of the metal, the applied N form (nitrate and ammonium) can play a role in promoting or mitigating Fe deficiency stresses. However, the processes involved are not clear in grapevine. Therefore, the aim of this study was to investigate the response of two grapevine rootstocks to the interaction between N forms and Fe uptake. This process was evaluated in a hydroponic experiment using two ungrafted grapevine rootstocks Fercal (Vitis berlandieri x V. vinifera) tolerant to deficiency induced Fe chlorosis and Couderc 3309 (V. riparia x V. rupestris) susceptible to deficiency induced Fe chlorosis.

RESULTS

The results could differentiate Fe deficiency effects, N-forms effects, and rootstock effects. Interveinal chlorosis of young leaves appeared earlier on 3309 C from the second week of treatment with NO3-/NH4+ (1:0)/-Fe, while Fercal leaves showed less severe symptoms after four weeks of treatment, corresponding to decreased chlorophyll concentrations lowered by 75% in 3309 C and 57% in Fercal. Ferric chelate reductase (FCR) activity was by trend enhanced under Fe deficiency in Fercal with both N combinations, whereas 3309 C showed an increase in FCR activity under Fe deficiency only with NO3-/NH4+ (1:1) treatment. With the transcriptome analysis, Gene Ontology (GO) revealed multiple biological processes and molecular functions that were significantly regulated in grapevine rootstocks under Fe-deficient conditions, with more genes regulated in Fercal responses, especially when both forms of N were supplied. Furthermore, the expression of genes involved in the auxin and abscisic acid metabolic pathways was markedly increased by the equal supply of both forms of N under Fe deficiency conditions. In addition, changes in the expression of genes related to Fe uptake, regulation, and transport reflected the different responses of the two grapevine rootstocks to different N forms.

CONCLUSIONS

Results show a clear contribution of N forms to the response of the two grapevine rootstocks under Fe deficiency, highlighting the importance of providing both N forms (nitrate and ammonium) in an appropriate ratio in order to ease the rootstock responses to Fe deficiency.

Comparative transcriptomic and metabolite profiling reveals genotype-specific responses to Fe starvation in chickpea

Iron deficiency is a major nutritional stress that severely impacts crop productivity worldwide. However, molecular intricacies and subsequent physiological and metabolic changes in response to Fe starvation, especially in leguminous crops like chickpea, remain elusive. In the present study, we investigated physiological, transcriptional, and metabolic reprogramming in two chickpea genotypes (H6013 and L4958) with contrasting seed iron concentrations upon Fe deficiency. Our findings revealed that iron starvation affected growth and physiological parameters of both chickpea genotypes. Comparative transcriptome analysis led to the identification of differentially expressed genes between the genotypes related to strategy I uptake, metal ions transporters, reactive oxygen species-associated genes, transcription factors, and protein kinases that could mitigate Fe deficiency. Our gene correlation network discovered several putative candidate genes like CIPK25, CKX3, WRKY50, NAC29, MYB4, and PAP18, which could facilitate the investigation of the molecular rationale underlying Fe tolerance in chickpea. Furthermore, the metabolite analysis also illustrated the differential accumulation of organic acids, amino acids and other metabolites associated with Fe mobilization in chickpea genotypes. Overall, our study demonstrated the comparative transcriptional dynamics upon Fe starvation. The outcomes of the current endeavor will enable the development of Fe deficiency tolerant chickpea cultivars.

Effects of Cadmium on Root Morpho-Physiology of Durum Wheat

Durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] can accumulate a high level of Cd in grains with a significant variability depending on cultivars. Understanding how this toxic element is distributed in cereal tissues and grains is essential to improve the nutritional quality of cereal-based products. The main objective of this work was to investigate roots of durum wheat plants (cv. Iride) exposed to different Cd concentrations (0.5 and 5.0 μM) to identify the mechanisms involved in Cd management. Results showed that the root morphology was altered by Cd treatment both at macroscopic (increased number of tips and primary root length) and ultrastructural levels (cell membrane system damaged, cell walls thickened and enriched in suberin). On the other side, Cd was localized in vesicles and in cell walls, and the metal colocalized with the phytosiderophore nicotianamine (NA). Overall, data suggest that Cd is chelated by NA and then compartmentalized, through vesicular trafficking, in the root thickened walls reducing Cd translocation to the aerial organs of the plant.

Selenite as a Lipid Inductor in Marine Microalga Dunaliella tertiolecta: Comparison of One-Stage and Two-Stage Cultivation Strategies

Microalgae have emerged as one of the most promising alternative sources of biofuels due to their high lipid accumulation ability. High lipid content is of pivotal importance for biodiesel production. In order to obtain high lipid content, modifications of culture conditions and development of an efficient lipid induction method are called for. In the present study, the possibility of using selenium in a form of sodium selenite as a lipid inductor in marine microalga Dunaliella tertiolecta was investigated during one- and two-stage cultivation modes. The effects of selenite on algal growth, pigment content, oxidative stress, and neutral lipid content were determined during both cultivation modes. The results revealed that the two-stage cultivation on 10.00-40.00 mg L-1 of selenite resulted in up to twofold higher algal cell density compared to the one-stage cultivation. Selenite concentrations from 2.50 to 20.00 mg L-1 increased lipid peroxidation during both cultivation modes, emphasizing the selenite-induced oxidative stress accompanied by the increased lipid accumulation in microalgae cells. During one- and two-stage cultivation on 20.00 mg L-1 of selenite, lipid content increased 2.39- and 5.73-fold at days 9 and 14 of cultivation, respectively. Moreover, the highest obtained neutral lipid content during the two-stage cultivation was 5.40-fold higher than lipid content obtained during the one-stage cultivation. Collectively, these results suggest that the two-stage cultivation strategy, initiated with optimal culture conditions for biomass production and followed by the addition of selenite as a stress inductor, can be successfully deployed to enhance the lipid content in D. tertiolecta.

Identification of Quantitative Trait Loci Associated With Iron Deficiency Tolerance in Maize

Iron (Fe) is a limiting factor in crop growth and nutritional quality because of its low solubility. However, the current understanding of how major crops respond to Fe deficiency and the genetic basis remains limited. In the present study, Fe-efficient inbred line Ye478 and Fe-inefficient inbred line Wu312 and their recombinant inbred line (RIL) population were utilized to reveal the physiological and genetic responses of maize to low Fe stress. Compared with the Fe-sufficient conditions (+Fe: 200 μM), Fe-deficient supply (-Fe: 30 μM) significantly reduced shoot and root dry weights, leaf SPAD of Fe-efficient inbred line Ye478 by 31.4, 31.8, and 46.0%, respectively; decreased Fe-inefficient inbred line Wu312 by 72.0, 45.1, and 84.1%, respectively. Under Fe deficiency, compared with the supply of calcium nitrate (N1), supplying ammonium nitrate (N2) significantly increased the shoot and root dry weights of Wu312 by 37.5 and 51.6%, respectively; and enhanced Ye478 by 23.9 and 45.1%, respectively. Compared with N1, N2 resulted in a 70.0% decrease of the root Fe concentration for Wu312 in the -Fe treatment, N2 treatment reduced the root Fe concentration of Ye478 by 55.8% in the -Fe treatment. These findings indicated that, compared with only supplying nitrate nitrogen, combined supply of ammonium nitrogen and nitrate nitrogen not only contributed to better growth in maize but also significantly reduced Fe concentration in roots. In linkage analysis, ten quantitative trait loci (QTLs) associated with Fe deficiency tolerance were detected, explaining 6.2-12.0% of phenotypic variation. Candidate genes considered to be associated with the mechanisms underlying Fe deficiency tolerance were identified within a single locus or QTL co-localization, including ZmYS3, ZmPYE, ZmEIL3, ZmMYB153, ZmILR3 and ZmNAS4, which may form a sophisticated network to regulate the uptake, transport and redistribution of Fe. Furthermore, ZmYS3 was highly induced by Fe deficiency in the roots; ZmPYE and ZmEIL3, which may be involved in Fe homeostasis in strategy I plants, were significantly upregulated in the shoots and roots under low Fe stress; ZmMYB153 was Fe-deficiency inducible in the shoots. Our findings will provide a comprehensive insight into the physiological and genetic basis of Fe deficiency tolerance.

NRAMP1 promotes iron uptake at the late stage of iron deficiency in poplars

Iron (Fe) is an essential micronutrient for plant survival and proliferation. Plants have evolved complex mechanisms to maintain Fe homeostasis in response to Fe deficiency. In this study, we evaluated the physiological, biochemical and transcriptomic differences between poplars grown under Fe-sufficient and Fe-deficient conditions to elucidate the mechanistic responses of poplars to Fe deficiency. Our results revealed that chlorophyll synthesis and photosynthesis were inhibited under Fe-deficient conditions. The inhibition of these pathways caused chlorosis and reduced shoot growth. Although both photosynthetic systems (PSI and PSII) were inhibited under Fe limitation, PSI was affected more severely and earlier than PSII. Fe deficiency also promoted root growth and increased the accumulation of divalent metal ions in roots. IRT1 and NRAMP1 are both Fe2+ transporters for iron uptake in Arabidopsis. In this study, however, only NRAMP1 was induced to promote Fe2+ uptake in roots at the late stage of Fe deficiency response. It indicated that NRAMP1, rather than the more well-known IRT1, might be a major Fe2+ transporter at the late stage of Fe-deficiency in poplars.

Airborne signals from Trichoderma fungi stimulate iron uptake responses in roots resulting in priming of jasmonic acid-dependent defences in shoots of Arabidopsis thaliana and Solanum lycopersicum

Root colonization by Trichoderma fungi can trigger induced systemic resistance (ISR). In Arabidopsis, Trichoderma-ISR relies on the transcription factor MYB72, which plays a dual role in the onset of ISR and the activation of Fe uptake responses. Volatile compounds (VCs) from rhizobacteria are important elicitors of MYB72 in Arabidopsis roots. Here, we investigated the mode of action of VCs from Trichoderma fungi in the onset of ISR and Fe uptake responses. VCs from Trichoderma asperellum and Trichoderma harzianum were applied in an in vitro split-plate system with Arabidopsis or tomato seedlings. Locally, Trichoderma-VCs triggered MYB72 expression and molecular, physiological and morphological Fe uptake mechanisms in Arabidopsis roots. In leaves, Trichoderma-VCs primed jasmonic acid-dependent defences, leading to an enhanced resistance against Botrytis cinerea. By using Arabidopsis micrografts of VCs-exposed rootstocks and non-exposed scions, we demonstrated that perception of Trichoderma-VCs by the roots leads to a systemic signal that primes shoots for enhanced defences. Trichoderma-VCs also elicited Fe deficiency responses and shoot immunity in tomato, suggesting that this phenomenon is expressed in different plant species. Our results indicate that Trichoderma-VCs trigger locally a readjustment of Fe homeostasis in roots, which links to systemic elicitation of ISR by priming of jasmonic acid-dependent defences.

Iron and Immunity

Iron is an essential nutrient for most life on Earth because it functions as a crucial redox catalyst in many cellular processes. However, when present in excess iron can lead to the formation of harmful hydroxyl radicals. Hence, the cellular iron balance must be tightly controlled. Perturbation of iron homeostasis is a major strategy in host-pathogen interactions. Plants use iron-withholding strategies to reduce pathogen virulence or to locally increase iron levels to activate a toxic oxidative burst. Some plant pathogens counteract such defenses by secreting iron-scavenging siderophores that promote iron uptake and alleviate iron-regulated host immune responses. Mutualistic root microbiota can also influence plant disease via iron. They compete for iron with soil-borne pathogens or induce a systemic resistance that shares early signaling components with the root iron-uptake machinery. This review describes the progress in our understanding of the role of iron homeostasis in both pathogenic and beneficial plant-microbe interactions.

H+ -pyrophosphatase IbVP1 promotes efficient iron use in sweet potato [Ipomoea batatas (L.) Lam.]

Iron (Fe) deficiency is one of the most common micronutrient deficiencies limiting crop production globally, especially in arid regions because of decreased availability of iron in alkaline soils. Sweet potato [Ipomoea batatas (L.) Lam.] grows well in arid regions and is tolerant to Fe deficiency. Here, we report that the transcription of type I H+ -pyrophosphatase (H+ -PPase) gene IbVP1 in sweet potato plants was strongly induced by Fe deficiency and auxin in hydroponics, improving Fe acquisition via increased rhizosphere acidification and auxin regulation. When overexpressed, transgenic plants show higher pyrophosphate hydrolysis and plasma membrane H+ -ATPase activity compared with the wild type, leading to increased rhizosphere acidification. The IbVP1-overexpressing plants showed better growth, including enlarged root systems, under Fe-sufficient or Fe-deficient conditions. Increased ferric precipitation and ferric chelate reductase activity in the roots of transgenic lines indicate improved iron uptake, which is also confirmed by increased Fe content and up-regulation of Fe uptake genes, e.g. FRO2, IRT1 and FIT. Carbohydrate metabolism is significantly affected in the transgenic lines, showing increased sugar and starch content associated with the increased expression of AGPase and SUT1 genes and the decrease in β-amylase gene expression. Improved antioxidant capacities were also detected in the transgenic plants, which showed reduced H2 O2 accumulation associated with up-regulated ROS-scavenging activity. Therefore, H+ -PPase plays a key role in the response to Fe deficiency by sweet potato and effectively improves the Fe acquisition by overexpressing IbVP1 in crops cultivated in micronutrient-deficient soils.

The Interaction between Auxin and Nitric Oxide Regulates Root Growth in Response to Iron Deficiency in Rice

Fe deficiency (-Fe) is a common abiotic stress that affects the root development of plants. Auxin and nitric oxide (NO) are key regulator of root growth under -Fe. However, the interactions between auxin and NO regulate root growth in response to Fe deficiency are complex and unclear. In this study, the indole-3-acetic acid (IAA) and NO levels in roots, and the responses of root growth in rice to different levels of Fe supply were investigated using wild type (WT), ospin1b and osnia2 mutants. -Fe promoted LR formation but inhibited seminal root elongation. IAA levels, [³H] IAA transport, and expression levels of PIN1a-c genes in roots were reduced under -Fe, suggesting that polar auxin transport from shoots to roots was decreased. Application of IAA to -Fe seedlings restored seminal root length, but not LR density, to levels similar to those under normal Fe (+Fe), and the seminal root length was shorter in two ospin1b mutants relative to WT under +Fe, but not under -Fe, confirming that auxin transport participates in -Fe-inhibited seminal root elongation. Moreover, -Fe-induced LR density and -Fe-inhibited seminal root elongation paralleled NO production in roots. Interestingly, similar NO accumulation and responses of LR density and root elongation were observed in osnia2 mutants compared to WT, and the higher expression of NOA gene under -Fe, suggesting that -Fe-induced NO was generated via the NO synthase-like pathway rather than the nitrate reductase pathway. However, IAA could restore the functions of NO in inhibiting seminal root elongation, but did not replace the role of NO-induced LR formation under -Fe. Overall, our findings suggested that NO functions downstream of auxin in regulating LR formation; NO-inhibited seminal root elongation by decreasing meristem activity in root tips under -Fe, with the involvement of auxin.

Involvement of endogenous salicylic acid in iron-deficiency responses in Arabidopsis

Several phytohormones have been demonstrated to be involved in iron (Fe) homeostasis. We took advantage of a salicylic acid (SA) biosynthesis defective mutant phytoalexin deficient 4 (pad4: T-DNA Salk_089936) to explore the possible effects of endogenous SA on the morphological and physiological responses to Fe deprivation. The morphological and physiological analysis was carried out between Col-0 and the pad4 mutant. Under an Fe-deficiency treatment, Col-0 showed more severe leaf chlorosis and root growth inhibition compared with the pad4 mutant. The soluble Fe concentrations were significantly higher in pad4 than in Col-0 under the Fe-deficiency treatment. Fe deficiency significantly induced SA accumulation in Col-0 and the loss-of-function of PAD4 blocked this process. The requirement of endogenous SA accumulation for Fe-deficiency responses was confirmed using a series of SA biosynthetic mutants and transgenic lines. Furthermore, a comparative RNA sequencing analysis of the whole seedling transcriptomes between Col-0 and the pad4 mutant was also performed. Based on the transcriptome data, the expression levels of many auxin- and ethylene-response genes were altered in pad4 compared with Col-0. Fe deficiency increases SA contents which elevates auxin and ethylene signalling, thereby activating Fe translocation via the bHLH38/39-mediated transcriptional regulation of downstream Fe genes.

Long-term iron deficiency: Tracing changes in the proteome of different pea (Pisum sativum L.) cultivars

Iron deficiency (-Fe) is one of the major problems in crop production. Dicots, like pea (Pisum sativum L.), are Strategy I plants, which induce a group of specific enzymes such as Fe(III)-chelate reductase (FRO), Fe responsive transporter (IRT) and H(+)-ATPase (HA) at the root plasma membrane under -Fe. Different species and cultivars have been shown to react diversely to -Fe. Furthermore, different kinds of experimental set-ups for -Fe have to be distinguished: i) short-term vs. long-term, ii) constant vs. acute alteration and iii) buffered vs. unbuffered systems. The presented work compares the effects of constant long-term -Fe in an unbuffered system on roots of four different pea cultivars in a timely manner (12, 19 and 25days). To differentiate the effects of -Fe and plant development, control plants (+Fe) were analyzed in comparison to -Fe plants. Besides physiological measurements, an integrative study was conducted using a comprehensive proteome analysis. Proteins, related to stress adaptation (e.g. HSP), reactive oxygen species related proteins and proteins of the mitochondrial electron transport were identified to be changed in their abundance. Regulations and possible functions of identified proteins are discussed.

SIGNIFICANCE

Pea (Pisum sativum L.) belongs to the legume family (Fabaceae) and is an important crop plant due to high Fe, starch and protein contents. According to FAOSTAT data (September 2015), world production of the garden pea quadrupled from 1970 to 2012. Since the initial studies by Gregor Mendel, the garden pea became the most-characterized legume and has been used in numerous investigations in plant biochemistry and physiology, but is not well represented in the "omics"-related fields. A major limitation in pea production is the Fe availability from soils. Adaption mechanisms to Fe deficiency vary between species, and even cultivars have been shown to react diversely. A label-free proteomic approach, in combination with physiological measurements, was chosen to observe four different pea cultivars for 5 to 25days. Physiological and proteome data showed that cultivar Blauwschokker and Vroege were more susceptible to -Fe than cultivar Kelvedon (highly efficient) and GftR (semi-efficient). Proteomic data hint that the adaptation process to long-term -Fe takes place between days 19 and 25. Results show that adaptation processes of efficient cultivars are able to postpone secondary negative effects of long-term -Fe, possibly by stabilizing the protein metabolic processing and the mitochondrial electron transport components. This maintains the cellular energy proliferation, keeps ROS production low and postpones the mitochondrial cell death signal.

Ethylene Participates in the Regulation of Fe Deficiency Responses in Strategy I Plants and in Rice

Iron (Fe) is very abundant in most soils but its availability for plants is low, especially in calcareous soils. Plants have been divided into Strategy I and Strategy II species to acquire Fe from soils. Strategy I species apply a reduction-based uptake system which includes all higher plants except the Poaceae. Strategy II species apply a chelation-based uptake system which includes the Poaceae. To cope with Fe deficiency both type of species activate several Fe deficiency responses, mainly in their roots. These responses need to be tightly regulated to avoid Fe toxicity and to conserve energy. Their regulation is not totally understood but some hormones and signaling substances have been implicated. Several years ago it was suggested that ethylene could participate in the regulation of Fe deficiency responses in Strategy I species. In Strategy II species, the role of hormones and signaling substances has been less studied. However, in rice, traditionally considered a Strategy II species but that possesses some characteristics of Strategy I species, it has been recently shown that ethylene can also play a role in the regulation of some of its Fe deficiency responses. Here, we will review and discuss the data supporting a role for ethylene in the regulation of Fe deficiency responses in both Strategy I species and rice. In addition, we will review the data about ethylene and Fe responses related to Strategy II species. We will also discuss the results supporting the action of ethylene through different transduction pathways and its interaction with other signals, such as certain Fe-related repressive signals occurring in the phloem sap. Finally, the possible implication of ethylene in the interactions among Fe deficiency responses and the responses to other nutrient deficiencies in the plant will be addressed.

The 14-3-3 protein GENERAL REGULATORY FACTOR11 (GRF11) acts downstream of nitric oxide to regulate iron acquisition in Arabidopsis thaliana

Here we report the function of a general regulatory factor, GENERAL REGULATORY FACTOR11 (GRF11), in terms of the iron (Fe) deficiency response. Physiological and molecular responses of the loss-of-function Arabidopsis thaliana grf11 mutant to Fe supply were investigated. Genes involved in posttranscriptional regulation of FER-LIKE FE DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) were also analyzed. In addition, the molecular link between the signaling molecule nitric oxide (NO) and Fe deficiency responses was further dissected. Our results suggest that GRF11 is necessary for induction of Fe-deficiency-tolerance mechanisms. The FIT protein can bind to the promoter of GRF11, which contains an E-box motif. GRF11 also positively affects FIT transcription but has no influence on the genes involved in posttranscriptional regulation of FIT. Furthermore, NO positively regulates GRF11 induction upon the onset of Fe deficiency. We propose that, upon the onset of Fe deficiency, induction of FIT expression is dependent on GRF11, which acts downstream of NO to mediate Fe deficiency responses.

Adaptive strategies of Parietaria diffusa (M.&K.) to calcareous habitat with limited iron availability

The study of native plants growing in hostile environments is useful to understand how these species respond to stress conditions. Parietaria diffusa (M.&K.) is able to survive in highly calcareous soils and extreme environments, such as house walls, without displaying any chlorotic symptoms. Here, we have investigated the existence of Strategy I complementary/alternative mechanism(s) involved in Fe solubilization and uptake and responsible for Parietaria's extraordinary efficiency. After assessing the specific traits involved in a calcicole-behaviour in the field, we have grown plants in conditions of Fe deficiency, either direct (-Fe) or induced by the presence of bicarbonate (+FeBic). Then, the growth performance, physiological and biochemical responses of the plants were investigated. The study shows that in Parietaria+FeBic, the classical responses of Strategy I plants are activated to a lower extent than in -Fe. In addition, there is a greater production of phenolics and organic acids that are both exuded and accumulated in the roots, which in turn show structures similar to 'proteoid-like roots'. We suggest that in the presence of this constraint, Parietaria undergoes some metabolic rearrangements that involve PEP-consuming reactions and an enhancement of the shikimate pathway.

NO synthase-generated NO acts downstream of auxin in regulating Fe-deficiency-induced root branching that enhances Fe-deficiency tolerance in tomato plants

In response to Fe-deficiency, various dicots increase their root branching which contributes to the enhancement of ferric-chelate reductase activity. Whether this Fe-deficiency-induced response eventually enhances the ability of the plant to tolerate Fe-deficiency or not is still unclear and evidence is also scarce about the signals triggering it. In this study, it was found that the SPAD-chlorophyll meter values of newly developed leaves of four tomato (Solanum lycocarpum) lines, namely line227/1 and Roza and their two reciprocal F(1) hybrid lines, were positively correlated with their root branching under Fe-deficient conditions. It indicates that Fe-deficiency-induced root branching is critical for plant tolerance to Fe-deficiency. In another tomato line, Micro-Tom, the increased root branching in Fe-deficient plants was accompanied by the elevation of endogenous auxin and nitric oxide (NO) levels, and was suppressed either by the auxin transport inhibitors NPA and TIBA or the NO scavenger cPTIO. On the other hand, root branching in Fe-sufficient plants was induced either by the auxin analogues NAA and 2,4-D or the NO donors NONOate or SNP. Further, in Fe-deficient plants, NONOate restored the NPA-terminated root branching, but NAA did not affect the cPTIO-terminated root branching. Fe-deficiency-induced root branching was inhibited by the NO-synthase (NOS) inhibitor L-NAME, but was not affected by the nitrate reductase (NR) inhibitor NH(4)(+), tungstate or glycine. Taking all of these findings together, a novel function and signalling pathway of Fe-deficiency-induced root branching is presented where NOS-generated rather than NR-generated NO acts downstream of auxin in regulating this Fe-deficiency-induced response, which enhances the plant tolerance to Fe-deficiency.

Nitric oxide acts downstream of auxin to trigger root ferric-chelate reductase activity in response to iron deficiency in Arabidopsis

In response to iron (Fe) deficiency, dicots employ a reduction-based mechanism by inducing ferric-chelate reductase (FCR) at the root plasma membrane to enhance Fe uptake. However, the signal pathway leading to FCR induction is still unclear. Here, we found that the Fe-deficiency-induced increase of auxin and nitric oxide (NO) levels in wild-type Arabidopsis (Arabidopsis thaliana) was accompanied by up-regulation of root FCR activity and the expression of the basic helix-loop-helix transcription factor (FIT) and the ferric reduction oxidase 2 (FRO2) genes. This was further stimulated by application of exogenous auxin (α-naphthaleneacetic acid) or NO donor (S-nitrosoglutathione [GSNO]), but suppressed by either polar auxin transport inhibition with 1-naphthylphthalamic acid or NO scavenging with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, tungstate, or N(ω)-nitro-L-arginine methyl ester hydrochloride. On the other hand, the root FCR activity, NO level, and gene expression of FIT and FRO2 were higher in auxin-overproducing mutant yucca under Fe deficiency, which were sharply restrained by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide treatment. The opposite response was observed in a basipetal auxin transport impaired mutant aux1-7, which was slightly rescued by exogenous GSNO application. Furthermore, Fe deficiency or α-naphthaleneacetic acid application failed to induce Fe-deficiency responses in noa1 and nial nia2, two mutants with reduced NO synthesis, but root FCR activities in both mutants could be significantly elevated by GSNO. The inability to induce NO burst and FCR activity was further verified in a double mutant yucca noa1 with elevated auxin production and reduced NO accumulation. Therefore, we presented a novel signaling pathway where NO acts downstream of auxin to activate root FCR activity under Fe deficiency in Arabidopsis.

Plant Fe status affects the composition of siderophore-secreting microbes in the rhizosphere

BACKGROUND AND AIMS

Soil microbes have been demonstrated to play an important role in favouring plant iron (Fe) uptake under Fe-limiting conditions. However, the mechanisms involved are still unclear. This present study reported the effects of plant Fe status on the composition of siderophore-secreting microbes in the rhizosphere, and their potential function in improving plant Fe nutrition.

METHODS

An Fe-efficient plant, red clover (Trifolium pratense 'Kenland') was cultured in a calcareous soil to obtain rhizosphere soils with (Fe-sufficient) or without (Fe-stressed) foliar FeEDTA spraying. The siderophore-producing ability of rhizospheric microbes was measured. The bioavailability of the siderophore-solubilized Fe from iron oxides/hydroxides was tested in hydroponic culture.

KEY RESULTS

In rhizosphere soil, the number of microbes that secreted siderophores quickly was more in the Fe-stressed treatment than in the Fe-sufficient one, while the number of microbes that did not secret siderophores was the opposite. A significantly higher concentration of phenolics was detected in the rhizosphere soil of Fe-stressed plants. Moreover, after the soil was incubated with phenolic root exudates, the composition of the siderophore-secreting microbial community was similar with that of the rhizosphere of Fe-stressed plant. Additionally, the siderophores produced by a rhizospheric microbe isolated from the Fe-stressed treatment can well solubilize iron oxides/hydroxides, and the utilization of the siderophore-solubilized Fe by plant was even more efficient than EDTA-Fe.

CONCLUSIONS

Iron-deficiency stress of red clover would alter the composition of siderophore-secreting microbes in the rhizosphere, which is probably due to the phenolics secretion of the root, and may in turn help to improve the solubility of Fe in soils and plant Fe nutrition via elevated microbial siderophore secretion.