Differential responses in pear and quince genotypes induced by Fe deficiency and bicarbonate

Transcriptome and biochemical analysis in hexaploid wheat with contrasting tolerance to iron deficiency pinpoints multi-layered molecular process

Iron (Fe) is an essential plant nutrient that is indispensable for many physiological activities. This study is an effort to identify the molecular and biochemical basis of wheat genotypes with contrasting tolerance towards Fe deficiency. Our physiological experiments performed at the early growth stage in cv. Kanchan (KAN) showed Fe deficiency tolerance, whereas cv. PBW343 (PBW) was susceptible. Under Fe deficient condition, KAN showed delayed chlorosis, high SPAD values, and low malondialdehyde content compared to PBW, indicative of Fe deficient condition. Comparative shoot transcriptomics revealed increased expression of photosynthetic pathway genes in PBW, further suggesting its sensitivity to Fe fluctuations. Under Fe deficiency, both the cultivars showed distinct molecular re-arrangements such as high expression of genes involved in Fe uptake (including membrane transporters) and its remobilization. Specifically, in KAN these changes lead to high root phytosiderophores (PS) biosynthesis and its release, resulting in enhanced Fe translocation index. Utilizing the non-transgenic TILLING (Targeting Induced Lesions in Genomes) technology, we identified TaZIFL4.2D as a putative PS efflux transporter. Characterization of the wheat TILLING lines indicated that TaZIFL4.2 functions in PS release and Fe acquisition, thereby imparting tolerance to Fe deficiency. Altogether, this work highlights the mechanistic insight into Fe deficiency tolerance of hexaploid wheat, thus enabling breeders to select suitable genotypes to utilize nutrients for maximum yields.

Bicarbonate rather than high pH in growth medium induced Fe-deficiency chlorosis in dwarfing rootstock quince A (Cydonia oblonga Mill.) but did not impair Fe nutrition of vigorous rootstock Pyrus betulifolia

Introduction

Quince A (Cydonia oblonga Mill.), a typical dwarfing rootstock in pear cultivation, is susceptible to iron (Fe) deficiency in calcareous soils. The aim of this study was to compare the strategies in Fe uptake and utilization in dwarfing rootstock quince A (low Fe efficiency) versus a typical vigorous rootstock Pyrus betulifolia (PB) with high Fe efficiency.

Methods

Quince A and PB were grown in nutrient solution (pH 6.3) for 4 weeks followed by three pH treatments: pH6.3, pH8.3a (adjusted with hydroxide) and pH8.3b (adjusted with bicarbonate). The Fe uptake and utilization indicators of the rootstocks were assessed at the onset of chlorosis symptoms (after 58 days of treatments).

Results and discussion

In contrast to PB, quince A exhibited Fe deficiency chlorosis under bicarbonate (pH8.3b). Bicarbonate stimulated the root proton secretion, inhibited root growth and ferric chelate reductase (FCR) activity in both PB and quince A, whereas high pH without bicarbonate (pH8.3a) stimulated only root proton release. Both species accumulated more Fe in roots under high pH treatments than under pH6.3, resulting in Fe sufficiency in leaves. Both high pH treatments increased the activity of leaf FCR in PB and quince A. However, extractable Fe(II) concentration in leaves was increased by high pH treatments in PB only. This study demonstrated that depressed Fe(III) reduction in leaves caused by bicarbonate rather than high pH explained Fe deficiency in quince A grown in bicarbonate-containing medium.

Evaluation of eleven kiwifruit genotypes for bicarbonate tolerance and characterization of two tolerance-contrasting genotypes

Screening bicarbonate-tolerant genotypes is an environmentally-friendly and long-term effective strategy to cope with bicarbonate-induced chlorosis in fruit crops grown on calcareous soils. We investigated eleven genotypes from four kiwifruit species (Actinidia chinensis, A. macrosperma, A. polygama, and A. valvata) for differences in bicarbonate tolerance. We also characterized the physiological and molecular differences in two contrasting genotypes of this group. In the first experiment, bicarbonate-treated plantlets were irrigated with 3.0 g L^-1 CaCO₃ and 5.04 g L^-1 NaHCO₃ in peat and perlite medium culture. Based on principal component analysis, weight-based membership function method and cluster analysis, the tested genotypes were classified into three groups: (1) tolerant, including YX, Av-1, Acd, Ap, Av-2, and QM; (2) moderately tolerant, including Av-3, Am, Av-4, and HWD; and (3) sensitive, including only QH. In the second experiment, QH (bicarbonate-sensitive) and YX (bicarbonate-tolerant) were grown in sand culture with 4.0 g L^-1 CaCO₃ and 0.84 g L^-1 or 1.26 g L^-1 NaHCO₃. Compared with QH, YX showed a better ability to take up iron (Fe) by roots and to transport Fe from roots to shoots in the bicarbonate treatments, probably due to a better capacity to protect from oxidative damage and to excrete protons, and a differential expression of genes associated with Fe uptake and translocation, including HA8, IRT1, YSL3 and NRAMP3. The results can facilitate identifying potential resources for bicarbonate tolerance and breeding new rootstocks, and contribute to the elucidation of the bicarbonate tolerance mechanisms in the genus Actinidia.

Transcriptome Analysis Revealed the Molecular Response Mechanism of Non-heading Chinese Cabbage to Iron Deficiency Stress

Iron is a trace metal that is found in animals, plants, and the human body. Human iron absorption is hampered by plant iron shortage, which leads to anemia. Leafy vegetables are one of the most direct and efficient sources of iron for humans. Despite the fact that ferrotrophic disorder is common in calcareous soil, however, non-heading Chinese cabbage performs a series of reactions in response to iron deficiency stress that help to preserve iron homeostasis in vivo. In this study, we discovered that iron deficiency stress caused leaf yellowing and impeded plant development in both iron-deficient and control treatments by viewing or measuring phenotypic, chlorophyll content, and Fe²⁺ content in both iron-deficient and control treatments. We found a total of 9213 differentially expressed genes (DEGs) in non-heading Chinese cabbage by comparing root and leaf transcriptome data with iron deficiency and control treatments. For instance, 1927 DEGs co-expressed in root and leaf, including 897 up-regulated and 1030 down-regulated genes, respectively. We selected some key antioxidant genes, hormone signal transduction, iron absorption and transport, chlorophyll metabolism, and transcription factors involved in the regulation of iron deficiency stress utilizing GO enrichment, KEGG enrichment, multiple types of functional annotation, and Weighted Gene Co-expression Network Analysis (WGCNA). This study identifies prospective genes for maintaining iron homeostasis under iron-deficient stress, offering a theoretical foundation for further research into the molecular mechanisms of greater adaptation to iron-deficient stress, and perhaps guiding the development of iron-tolerant varieties.

Plasticity, exudation and microbiome-association of the root system of Pellitory-of-the-wall plants grown in environments impaired in iron availability

The investigation of the adaptive strategies of wild plant species to extreme environments is a challenging issue, which favors the identification of new traits for plant resilience. We investigated different traits which characterize the root-soil interaction of Parietaria judaica, a wild plant species commonly known as "Pellitory-of-the-wall". P. judaica adopts the acidification-reduction strategy (Strategy I) for iron (Fe) acquisition from soil, and it can complete its life cycle in highly calcareous environments without any symptoms of chlorosis. In a field-to-lab approach, the microbiome associated with P. judaica roots was analyzed in spontaneous plants harvested from an urban environment consisting in an extremely calcareous habitat. Also, the phenolics and carboxylates content and root plasticity and exudation were analyzed in P. judaica plants grown under three different controlled conditions mimicking the effect of calcareous environments on Fe availability: results show that P. judaica differentially modulates root plasticity under different Fe availability-impaired conditions, and that it induces, to a high extent, the exudation of caffeoylquinic acid derivatives under calcareous conditions, positively impacting Fe solubility.

Rhizosphere microorganisms enhance in vitro root and plantlet development of Pyrus and Prunus rootstocks

The in vitro application of rhizosphere microorganisms led to a higher rooting percentage in Pyrus Py12 rootstocks and increased plant growth of Pyrus Py170 and Prunus RP-20. The rooting of fruit tree rootstocks is the most challenging step of the in vitro propagation process. The use of rhizosphere microorganisms to promote in vitro rooting and plant growth as an alternative to the addition of chemical hormones to culture media is proposed in the present study. Explants from two Pyrus (Py170 and Py12) rootstocks and the Prunus RP-20 rootstock were inoculated with Pseudomonas oryzihabitans PGP01, Cladosporium ramotenellum PGP02 and Phoma sp. PGP03 following two different methods to determine their effects on in vitro rooting and plantlet growth. The effects of the microorganisms on the growth of fully developed Py170 and RP-20 plantlets were also studied in vitro. All experiments were conducted using vermiculite to simulate a soil system in vitro. When applied to Py12 shoots, which is a hard-to-root plant material, both C. ramotenellum PGP02 and Phoma sp. PGP03 fungi were able to increase the rooting percentage from 56.25% to 100% following auxin indole-3-butyric acid (IBA) treatment. Thus, the presence of these microorganisms clearly improved root development, inducing a higher number of roots and causing shorter roots. Better overall growth and improved stem growth of treated plants was observed when auxin treatment was replaced by co-culture with microorganisms. A root growth-promoting effect was observed on RP-20 plantlets after inoculation with C. ramotenellum PGP02, while P. oryzihabitans PGP01 increased root numbers for both Py170 and RP-20 and increased root growth over stem growth for RP-20. It was also shown that the three microorganisms P. oryzihabitans PGP01, C. ramotenellum PGP02 and Phoma sp. PGP03 were able to naturally produce auxin, including indole-3-acetic acid (IAA), at different levels. Overall, our results demonstrate that the microorganisms P. oryzihabitans PGP01 and C. ramotenellum PGP02 had beneficial effects on in vitro rooting and plantlet growth and could be applied to in vitro tissue culture as a substitute for IBA.

Effects of Fe and Mn deficiencies on the protein profiles of tomato (Solanum lycopersicum) xylem sap as revealed by shotgun analyses

The aim of this work was to study the effects of Fe and Mn deficiencies on the xylem sap proteome of tomato using a shotgun proteomic approach, with the final goal of elucidating plant response mechanisms to these stresses. This approach yielded 643 proteins reliably identified and quantified with 70% of them predicted as secretory. Iron and Mn deficiencies caused statistically significant and biologically relevant abundance changes in 119 and 118 xylem sap proteins, respectively. In both deficiencies, metabolic pathways most affected were protein metabolism, stress/oxidoreductases and cell wall modifications. First, results suggest that Fe deficiency elicited more stress responses than Mn deficiency, based on the changes in oxidative and proteolytic enzymes. Second, both nutrient deficiencies affect the secondary cell wall metabolism, with changes in Fe deficiency occurring via peroxidase activity, and in Mn deficiency involving peroxidase, Cu-oxidase and fasciclin-like arabinogalactan proteins. Third, the primary cell wall metabolism was affected by both nutrient deficiencies, with changes following opposite directions as judged from the abundances of several glycoside-hydrolases with endo-glycolytic activities and pectin esterases. Fourth, signaling pathways via xylem involving CLE and/or lipids as well as changes in phosphorylation and N-glycosylation also play a role in the responses to these stresses. Biological significance In spite of being essential for the delivery of nutrients to the shoots, our knowledge of xylem responses to nutrient deficiencies is very limited. The present work applies a shotgun proteomic approach to unravel the effects of Fe and Mn deficiencies on the xylem sap proteome. Overall, Fe deficiency seems to elicit more stress in the xylem sap proteome than Mn deficiency, based on the changes measured in proteolytic and oxido-reductase proteins, whereas both nutrients exert modifications in the composition of the primary and secondary cell wall. Cell wall modifications could affect the mechanical and permeability properties of the xylem sap vessels, and therefore ultimately affect solute transport and distribution to the leaves. Results also suggest that signaling cascades involving lipid and peptides might play a role in nutrient stress signaling and pinpoint interesting candidates for future studies. Finally, both nutrient deficiencies seem to affect phosphorylation and glycosylation processes, again following an opposite pattern.

A Shotgun Proteomic Approach Reveals That Fe Deficiency Causes Marked Changes in the Protein Profiles of Plasma Membrane and Detergent-Resistant Microdomain Preparations from Beta vulgaris Roots

In the present study we have used label-free shotgun proteomic analysis to examine the effects of Fe deficiency on the protein profiles of highly pure sugar beet root plasma membrane (PM) preparations and detergent-resistant membranes (DRMs), the latter as an approach to study microdomains. Altogether, 545 proteins were detected, with 52 and 68 of them changing significantly with Fe deficiency in PM and DRM, respectively. Functional categorization of these proteins showed that signaling and general and vesicle-related transport accounted for approximately 50% of the differences in both PM and DRM, indicating that from a qualitative point of view changes induced by Fe deficiency are similar in both preparations. Results indicate that Fe deficiency has an impact in phosphorylation processes at the PM level and highlight the involvement of signaling proteins, especially those from the 14-3-3 family. Lipid profiling revealed Fe-deficiency-induced decreases in phosphatidic acid derivatives, which may impair vesicle formation, in agreement with the decreases measured in proteins related to intracellular trafficking and secretion. The modifications induced by Fe deficiency in the relative enrichment of proteins in DRMs revealed the existence of a group of cytoplasmic proteins that appears to be more attached to the PM in conditions of Fe deficiency.

Effects of Fe deficiency on the protein profiles and lignin composition of stem tissues from Medicago truncatula in absence or presence of calcium carbonate

Iron deficiency is a yield-limiting factor with major implications for crop production, especially in soils with high CaCO3. Because stems are essential for the delivery of nutrients to the shoots, the aim of this work was to study the effects of Fe deficiency on the stem proteome of Medicago truncatula. Two-dimensional electrophoresis separation of stem protein extracts resolved 276 consistent spots in the whole experiment. Iron deficiency in absence or presence of CaCO3 caused significant changes in relative abundance in 10 and 31 spots, respectively, and 80% of them were identified by mass spectrometry. Overall results indicate that Fe deficiency by itself has a mild effect on the stem proteome, whereas Fe deficiency in the presence of CaCO3 has a stronger impact and causes changes in a larger number of proteins, including increases in stress and protein metabolism related proteins not observed in the absence of CaCO3. Both treatments resulted in increases in cell wall related proteins, which were more intense in the presence of CaCO3. The increases induced by Fe-deficiency in the lignin per protein ratio and changes in the lignin monomer composition, assessed by pyrolysis-gas chromatography-mass spectrometry and microscopy, respectively, further support the existence of cell wall alterations.

BIOLOGICAL SIGNIFICANCE

In spite of being essential for the delivery of nutrients to the shoots, our knowledge of stem responses to nutrient deficiencies is very limited. The present work applies 2-DE techniques to unravel the response of this understudied tissue to Fe deficiency. Proteomics data, complemented with mineral, lignin and microscopy analyses, indicate that stems respond to Fe deficiency by increasing stress and defense related proteins, probably in response of mineral and osmotic unbalances, and eliciting significant changes in cell wall composition. The changes observed are likely to ultimately affect solute transport and distribution to the leaves.

Flooding impairs Fe uptake and distribution in Citrus due to the strong down-regulation of genes involved in Strategy I responses to Fe deficiency in roots

This work determines the ffects of long-term anoxia conditions--21 days--on Strategy I responses to iron (Fe) deficiency in Citrus and its impact on Fe uptake and distribution. The study was carried out in Citrus aurantium L. seedlings grown under flooding conditions (S) and in both the presence (+Fe) and absence of Fe (-Fe) in nutritive solution. The results revealed a strong down-regulation (more than 65%) of genes HA1 and FRO2 coding for enzymes proton-ATPase and Ferric-Chelate Reductase (FC-R), respectively, in -FeS plants when compared with -Fe ones. H+-extrusion and FC-R activity analyses confirmed the genetic results, indicating that flooding stress markedly repressed acidification and reduction responses to Fe deficiency (3.1- and 2.0-fold, respectively). Waterlogging reduced by half Fe concentration in +FeS roots, which led to 30% up-regulation of Fe transporter IRT1, although this effect was unable to improve Fe absorption. Consequently, flooding inhibited 57Fe uptake in +Fe and -Fe seedlings (29.8 and 66.2%, respectively) and 57Fe distribution to aerial part (30.6 and 72.3%, respectively). This evidences that the synergistic action of both enzymes H+-ATPase and FC-R is the preferential regulator of the Fe acquisition system under flooding conditions and, hence, their inactivation implies a limiting factor of citrus in their Fe-deficiency tolerance in waterlogged soils.

Protein profile of Beta vulgaris leaf apoplastic fluid and changes induced by Fe deficiency and Fe resupply

The fluid collected by direct leaf centrifugation has been used to study the proteome of the sugar beet apoplastic fluid as well as the changes induced by Fe deficiency and Fe resupply to Fe-deficient plants in the protein profile. Plants were grown in Fe-sufficient and Fe-deficient conditions, and Fe resupply was carried out with 45 μM Fe(III)-EDTA for 24 h. Protein extracts of leaf apoplastic fluid were analyzed by two-dimensional isoelectric focusing-SDS-PAGE electrophoresis. Gel image analysis revealed 203 consistent spots, and proteins in 81% of them (164) were identified by nLC-MS/MS using a custom made reference repository of beet protein sequences. When redundant UniProt entries were deleted, a non-redundant leaf apoplastic proteome consisting of 109 proteins was obtained. TargetP and SecretomeP algorithms predicted that 63% of them were secretory proteins. Functional classification of the non-redundant proteins indicated that stress and defense, protein metabolism, cell wall and C metabolism accounted for approximately 75% of the identified proteome. The effects of Fe-deficiency on the leaf apoplast proteome were limited, with only five spots (2.5%) changing in relative abundance, thus suggesting that protein homeostasis in the leaf apoplast fluid is well-maintained upon Fe shortage. The identification of three chitinase isoforms among proteins increasing in relative abundance with Fe-deficiency suggests that one of the few effects of Fe deficiency in the leaf apoplast proteome includes cell wall modifications. Iron resupply to Fe deficient plants changed the relative abundance of 16 spots when compared to either Fe-sufficient or Fe-deficient samples. Proteins identified in these spots can be broadly classified as those responding to Fe-resupply, which included defense and cell wall related proteins, and non-responsive, which are mainly protein metabolism related proteins and whose changes in relative abundance followed the same trend as with Fe-deficiency.

The role of nodules in the tolerance of common bean to iron deficiency

Iron is vital for the establishment and function of symbiotic root nodules of legumes. Although abundant in the environment, Fe is often a limiting nutrient for plant growth due to its low solubility and availability in some soils. We have studied the mechanism of iron uptake in the root nodules of common bean to evaluate the role of nodules in physiological responses to iron deficiency. Based on experiments using full or partial submergence of nodulated roots in the nutrient solution, our results show that the nodules were affected only slightly under iron deficiency, especially when the nodules were submerged in nutrient solution in the tolerant cultivar. In addition, fully submerged root nodules showed enhanced acidification of the nutrient solution and showed higher ferric chelate reductase activity than that of partially submerged roots in plants cultivated under Fe deficiency. The main results obtained in this work suggest that in addition to preferential Fe allocation from the root system to the nodules, this symbiotic organ probably develops some mechanisms to respond to iron deficiency. These mechanisms were implied especially in nodule Fe absorption efficiency and in the ability of this organ to take up Fe directly from the medium.

Changes in the concentration of organic acids in roots and leaves of carob-tree under Fe deficiency

Several fruit trees are able to cope with iron (Fe) deficiency when grown in calcareous soils in the Mediterranean region, although information regarding well adapted slow-growing species is scarce, and the mechanisms activated by these species are not described in the literature. A crucial issue related to tolerance is the need to transport Fe over relatively long distances inside the plant. To evaluate the possible role of organic acids in the movement of Fe in tolerant plants, we studied the concentration of low molecular weight organic acids in several organs of 1-year old carob plants grown for 55 days in nutrient solutions without Fe (0µM Fe) or with 1µM Fe and 10µM Fe. Roots, stems and leaves were harvested, and the biomass, Fe and organic acid contents quantified. Total leaf chlorophyll (Chl) was evaluated in young leaves over the experimental period and the activity of root ferric chelate-reductase (FC-R; EC 1.16.1.17) was determined after 35 days, when deficiency symptoms appeared. Iron chlorosis was observed only at the end of the experiment in plants grown in the absence of Fe, and these plants had a smaller DW of leaves and also significant greater activity of root FC-R. Iron deficiency (Fe0 and Fe1 treatments) induced significant changes in the concentrations of succinic, malic, citric and fumaric acids, which increased in roots, or in basal, middle and apical leaves. There were significant correlations between most organic acids (with the exceptions of 2-oxoglutaric and tartaric acids) and leaf Chl. Analysis of each type of leaf showed that more succinic and malic acids were present in young chlorotic leaves while the reverse was true for quinic acid. These changes in organic acids followed a root-to-foliage pathway that was similar in all leaf types and particularly evident in young chlorotic leaves. We hypothesised that it was associated with Fe transport from roots to aboveground tissues, as there were significant differences in Fe contents between treatments with and without Fe.

Bicarbonate blocks iron translocation from cotyledons inducing iron stress responses in Citrus roots

The effect of bicarbonate ion (HCO3(-)) on the mobilization of iron (Fe) reserves from cotyledons to roots during early growth of citrus seedlings and its influence on the components of the iron acquisition system were studied. Monoembryonic seeds of Citrus limon (L.) were germinated "in vitro" on two iron-deprived media, supplemented or not with 10mM HCO3(-) (-Fe+Bic and -Fe, respectively). After 21d of culture, Fe concentration in seedling organs was measured, as well as gene expression and enzymatic activities. Finally, the effect of Fe resupply on the above responses was tested in the presence and absence of HCO3(-) (+Fe+Bic or +Fe, respectively). -Fe+Bic seedlings exhibited lower Fe concentration in shoots and roots than -Fe ones but higher in cotyledons, associated to a significative inhibition of NRAMP3 expression. HCO3(-) upregulated Strategy I related genes (FRO1, FRO2, HA1 and IRT1) and FC-R and H(+)-ATPase activities in roots of Fe-starved seedlings. PEPC1 expression and PEPCase activity were also increased. When -Fe+Bic pre-treated seedlings were transferred to Fe-containing media for 15d, Fe content in shoots and roots increased, although to a lower extent in the +Fe+Bic medium. Consequently, the above-described root responses became markedly repressed, however, this effect was less pronounced in +Fe+Bic seedlings. In conclusion, it appears that HCO3(-) prevents Fe translocation from cotyledons to shoot and root, therefore reducing their Fe levels. This triggers Fe-stress responses in the root, enhancing the expression of genes related with Fe uptake and the corresponding enzymatic activities.

Signals from chloroplasts and mitochondria for iron homeostasis regulation

Iron (Fe) is an essential element for human nutrition. Given that plants represent a major dietary source of Fe worldwide, it is crucial to understand plant Fe homeostasis fully. A major breakthrough in the understanding of Fe sensing and signaling was the identification of several transcription factor cascades regulating Fe homeostasis. However, the mechanisms of activation of these cascades still remain to be elucidated. In this opinion, we focus on the possible roles of mitochondria and chloroplasts as cellular Fe sensing and signaling sites, offering a new perspective on the integrated regulation of Fe homeostasis and its interplay with cellular metabolism.

Metabolic responses to iron deficiency in roots of Carrizo citrange [Citrus sinensis (L.) Osbeck. x Poncirus trifoliata (L.) Raf]

The effects of iron (Fe) deficiency on the low-molecular-weight organic acid (LMWOA) metabolism have been investigated in Carrizo citrange (CC) [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] roots. Major LMWOAs found in roots, xylem sap and root exudates were citrate and malate and their concentrations increased with Fe deficiency. The activities of several enzymes involved in the LMWOA metabolism were also assessed in roots. In the cytosolic fraction, the activities of malate dehydrogenase (cMDH) and phosphoenolpyruvate carboxylase (PEPC) enzymes were 132 and 100% higher in Fe-deficient conditions, whereas the activity of pyruvate kinase was 31% lower and the activity of malic enzyme (ME) did not change. In the mitochondrial fraction, the activities of fumarase, MDH and citrate synthase enzymes were 158, 117 and 53% higher, respectively, in Fe-deficient extracts when compared with Fe-sufficient controls, whereas no significant differences between treatments were found for aconitase (ACO) activity. The expression of their corresponding genes in roots of Fe-deficient plants was higher than that measured in Fe-sufficient controls, except for ACO and ME. Also, dicarboxylate-tricarboxylate carrier (DTC) expression was significantly increased in Fe-deficient roots. In conclusion, Fe deficiency in CC seedlings causes a reprogramming of the carbon metabolism that involves an increase of anaplerotic fixation of carbon via PEPC and MDH activities in the cytosol and a shift of the Krebs cycle in the mitochondria towards a non-cyclic mode, as previously described in herbaceous species. In this scheme, DTC could play an important role shuttling both malate and reducing equivalents between the cytosol and the mitochondria. As a result of this metabolic switch malate and citrate concentrations in roots, xylem sap and root exudates increase.

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.

Differential degradation of photosystem I subunits under iron deficiency in rice

Rice (Oryza sativa) is one of the staple foods of the world. Iron (Fe) deficiency is a major abiotic stress factor that contributes world-wide to losses in crop yield and decline in nutritional quality. As cofactor for many enzymes and proteins, iron is an essential element. It plays a pivotal role in chlorophyll (Chl) biosynthesis, and iron deficiency may result in decreased Chl production and, thus, reduced photosynthetic capacity. Photosystem I (PSI) is a prime target of iron deficiency because of its high iron content (12 Fe per PS). To understand the protein level changes in the light-harvesting complex (LHC) of PSI (LHCI) under iron deficiency, rice seedlings were grown in Hoagland's nutrient medium with and without Fe. Chlorophyll content and photosynthetic efficiency decreased under iron deficiency. Protein gel blots probed with antibodies against the PSI core and Lhca 1-4 proteins revealed that the core subunits PsaA and PsaB remained stable under iron deficiency, whereas PsaC and PsaD decreased by about 50%, and PsaE was completely degraded. Among the LHCI subunits, Lhca1 and Lhca2 decreased by 40 and 50%, respectively, whereas Lhca3 and Lhca4 were completely degraded. We propose that the dissociation of LHCI subunits may be due to increased levels of reactive oxygen species, which is suggested by the increased activity of superoxide dismutase.

Physiological responses and differential gene expression in Prunus rootstocks under iron deficiency conditions

Two Prunus rootstocks, the Myrobalan plum P 2175 and the interspecific peach-almond hybrid, Felinem, were studied to characterize their biochemical and molecular responses induced under iron-Deficient conditions. Plants of both genotypes were submitted to different treatments using a hydroponic system that permitted removal of Fe from the nutrient solution. Control plants were grown in 90 μM Fe (III)-EDTA, Deficient plants were grown in an iron free solution, and plants submitted to an Inductor treatment were resupplied with 180 μM Fe (III)-EDTA over 1 and 2 days after a period of 4 or 15 days of growth on an iron-free solution. Felinem increased the activity of the iron chelate reductase (FC-R) in the Inductor treatment after 4 days of iron deprivation. In contrast, P 2175 did not show any response after at least 15 days without iron. The induction of the FC-R activity in this genotype was coincident in time with the medium acidification. These results suggest two different mechanisms of iron chlorosis tolerance in both Strategy I genotypes. Felinem would use the iron reduction as the main mechanism to capture the iron from the soil, and in P 2175, the mechanism of response would be slower and start with the acidification of the medium synchronized with the gradual loss of chlorophyll in leaves. To better understand the control of these responses at the molecular level, the differential expression of PFRO2, PIRT1 and PAHA2 genes involved in the reductase activity, the iron transport in roots, and the proton release, respectively, were analyzed. The expression of these genes, estimated by quantitative real-time PCR, was different between genotypes and among treatments. The results were in agreement with the physiological responses observed.

Metabolic response in roots of Prunus rootstocks submitted to iron chlorosis

Iron deficiency induces several responses to iron shortage in plants. Metabolic changes occur to sustain the increased iron uptake capacity of Fe-deficient plants. We evaluated the metabolic changes of three Prunus rootstocks submitted to iron chlorosis and their different responses for tolerance using measurements of metabolites and enzymatic activities. The more tolerant rootstocks Adesoto (Prunus insititia) and GF 677 (Prunus amygdalus×Prunus persica), and the more sensitive Barrier (P. persica×Prunus davidiana) were grown hydroponically in iron-sufficient and -deficient conditions over two weeks. Sugar, organic and amino acid concentrations of root tips were determined after two weeks of iron shortage by proton nuclear magnetic resonance spectroscopy of extracts. Complementary analyses of organic acids were performed by liquid chromatography coupled to mass spectrometry. The major soluble sugars found were glucose and sucrose. The major organic acids were malic and citric acids, and the major amino acid was asparagine. Iron deficiency increased root sucrose, total organic and amino acid concentrations and phosphoenolpyruvate carboxylase activity. After two weeks of iron deficiency, the malic, citric and succinic acid concentrations increased in the three rootstocks, although no significant differences were found among genotypes with different tolerance to iron chlorosis. The tolerant rootstock Adesoto showed higher total organic and amino acid concentrations. In contrast, the susceptible rootstock Barrier showed lower total amino acid concentration and phosphoenolpyruvate carboxylase activity values. These results suggest that the induction of this enzyme activity under iron deficiency, as previously shown in herbaceous plants, indicates the tolerance level of rootstocks to iron chlorosis. The analysis of other metabolic parameters, such as organic and amino acid concentrations, provides complementary information for selection of genotypes tolerant to iron chlorosis.

Oxidative stress responses and root lignification induced by Fe deficiency conditions in pear and quince genotypes

We analysed Pyrus communis cv. Conference and Cydonia oblonga BA29, differently tolerant to lime-induced chlorosis, to identify the key mechanisms involved in their different performance under Fe deficiency induced by the absence of Fe (-Fe) or by the presence of bicarbonate (+FeBic). Under our experimental conditions, a decrease in root elongation was observed in BA29 under bicarbonate supply. Superoxide dismutase (SOD) and peroxidase (POD) activities were analysed and the relative isoforms were detected by native electrophoresis. The data obtained for both genotypes under -Fe and for BA29 +FeBic suggest the occurrence of overproduction of reactive oxygen species (ROS) and, at the same time, of a scarce capacity to detoxify them. The detection of ROS (O(2)(-) and H(2)O(2)) through histochemical localization supports these results and suggests that they could account for the modifications of mechanical properties of the cell wall during stress adaptation. On the other hand, in the cv. Conference +FeBic, an increase in non-specific POD activity was detected, confirming its higher level of protection in particular against H(2)O(2) accumulation. Peroxidases involved in lignification were assayed and histochemical analysis was performed. The results suggest that only in BA29 under bicarbonate supply can the presence of ROS in root apoplast be correlated with lignin deposits in external layers and in endodermis as a consequence of the shift of PODs towards a lignification role. We suggest that in BA29 the decrease in root growth could impair mineral nutrition, generating susceptibility to calcareous soils. In the cv. Conference, the allocation of new biomass to the root system could improve soil exploration and consequently Fe uptake.

A role for ferritin in the antioxidant system in coffee cell cultures

Iron (Fe) is an essential nutrient for plants, but it can generate oxidative stress at high concentrations. In this study, Coffea arabica L. cell suspension cultures were exposed to excess Fe (60 and 240 μM) to investigate changes in the gene expression of ferritin and antioxidant enzymes. Iron content accumulated during cell growth, and Western blot analysis showed an increase of ferritin in cells treated with Fe. The expression of two ferritin genes retrieved from the Brazilian coffee EST database was studied. CaFER1, but not CaFER2, transcripts were induced by Fe exposure. Phylogenetic analysis revealed that CaFER1 is not similar to CaFER2 or to any ferritin that has been characterised in detail. The increase in ferritin gene expression was accompanied by an increase in the activity of antioxidant enzymes. Superoxide dismutase, guaiacol peroxidase, catalase, and glutathione reductase activities increased in cells grown in the presence of excess Fe, especially at 60 μM, while the activity of glutathione S-transferase decreased. These data suggest that Fe induces oxidative stress in coffee cell suspension cultures and that ferritin participates in the antioxidant system to protect cells against oxidative damage. Thus, cellular Fe concentrations must be finely regulated to avoid cellular damage most likely caused by increased oxidative stress induced by Fe. However, transcriptional analyses indicate that ferritin genes are differentially controlled, as only CaFER1 expression was responsive to Fe treatment.