A NAC transcription factor represses a module associated with xyloglucan content and regulates aluminum tolerance.

The transcriptional regulation of aluminum (Al) tolerance in plants is largely unknown, although Al toxicity restricts agricultural yields in acidic soils.. Here, we identified a NAM, ATAF1/2, and cup-shaped cotyledon 2 (NAC) transcription factor that participates in Al tolerance in Arabidopsis (Arabidopsis thaliana). Al substantially induced the transcript and protein levels of ANAC070, and loss-of-function anan070 mutants showed remarkably increased Al sensitivity, implying a beneficial role of ANAC070 in plant tolerance to Al toxicity. Further investigation revealed that more Al accumulated in the roots of anac070 mutants, especially in root cell walls, accompanied by a higher hemicellulose and xyloglucan level, implying a possible interaction between ANAC070 and genes that encode proteins responsible for the modification of xyloglucan, including xyloglucan endo-transglycosylases/hydrolase (XTH) or ANAC017. Yeast one hybrid analysis revealed a potential interaction between ANAC070 and ANAC017, but not for other XTHs. Furthermore, dual-luciferase reporter assay, RT-qPCR, and GUS analysis revealed that ANAC070 could directly repress the transcript levels of ANAC017, and knockout of ANAC017 in the anac070 mutant partially restored its Al sensitivity phenotype, indicating that ANAC070 contributes to Al tolerance mechanisms other than suppression of ANAC017 expression. Further analysis revealed that the core transcription factor SENSITIVE TO PROTON RHIZOTOXICITY1 (STOP1) and its target genes, which control Al tolerance in Arabidopsis, may also be involved in ANAC070-regulated Al tolerance. In summary, we identified a transcription factor, ANAC070, that represses the ANAC017-XTH31 module to regulate Al tolerance in Arabidopsis.

Boron supply restores aluminum-blocked auxin transport by the modulation of PIN2 trafficking in the root apical transition zone.

The supply of boron (B) alleviates the toxic effects of aluminum (Al) on root growth; however, the mechanistic basis of this process remains elusive. This study filled this knowledge gap, demonstrating that boron modifies auxin distribution and transport in Al-exposed Arabidopsis roots. In B-deprived roots, treatment with Al induced an increase in auxin content in the root apical meristem zone (MZ) and transition zone (TZ), whereas in the elongation zone (EZ) the auxin content was decreased beyond the level required for adequate growth. These distribution patterns are explained by the fact that basipetal auxin transport from the TZ to the EZ was disrupted by Al-inhibited PIN-FORMED 2 (PIN2) endocytosis. Experiments involving the modulation of protein biosynthesis by cycloheximide (CHX) and transcriptional regulation by cordycepin (COR) demonstrated that the Al-induced increase of PIN2 membrane proteins was dependent upon the inhibition of PIN2 endocytosis, rather than on the transcriptional regulation of the PIN2 gene. Experiments reporting on the profiling of Al3+ and PIN2 proteins revealed that the inhibition of endocytosis of PIN2 proteins was the result of Al-induced limitation of the fluidity of the plasma membrane. The supply of B mediated the turnover of PIN2 endosomes conjugated with indole-3-acetic acid (IAA), and thus restored the Al-induced inhibition of IAA transport through the TZ to the EZ. Overall, the reported results demonstrate that boron supply mediates PIN2 endosome-based auxin transport to alleviate Al toxicity in plant roots.

Auxin acts upstream of nitric oxide to regulate cell wall xyloglucan and its aluminium-binding capacity in Arabidopsis thaliana.

MAIN CONCLUSION: Auxin acts upstream of NO through NOA and XXT5 pathways to regulate the binding capacity of the root cell wall to Al. In our previous study, we identified an unknown mechanism by which 1-naphthaleneacetic acid (NAA) decreased the fixation of aluminum (Al) in the cell wall. Here, we observed that external application of the nitric oxide (NO) donor S-nitrosoglutathion (GSNO) increased the inhibition of Al on root elongation. Further analysis indicated that GSNO could induce Al accumulation in the roots and root cell walls, which is consistent with lower xyloglucan content. In comparison to the Columbia-0 (Col-0) wild type (WT), endogenous NO-reduced mutants noa1 (NOA pathway) and nia1nia2 (NR pathway) were more resistant to Al, with lower root Al content, higher xyloglucan content, and more Al accumulation in the root cell walls. By contrast, the xxt5 mutant with reduced xyloglucan content exhibited an Al-sensitive phenotype. Interestingly, Al treatment increased the endogenous auxin and NO levels, and the auxin levels induced under Al stress further stimulated NO production. Auxin application reduced Al retention in hemicellulose and decreased the xyloglucan content, similar to the effects observed with GSNO. In yucca and aux1-7 mutants, exogenous application of NO resulted in responses similar to those of the WT, whereas exogenous auxin had little effect on the noa1 mutant under Al stress. In addition, as auxin had similar effects on the nia1nia2 mutant and the WT, exogenous auxin and NO had little effect on the xxt5 mutant under Al stress, further confirming that auxin acts upstream of NO through NOA and XXT5 pathways to regulate the binding capacity of the root cell wall to Al.

The MYB transcription factor MYB103 acts upstream of TRICHOME BIREFRINGENCE-LIKE27 in regulating aluminum sensitivity by modulating the O-acetylation level of cell wall xyloglucan in Arabidopsis thaliana.

Modification of the O-acetylation level of xyloglucan (XyG) appears to affect aluminum (Al) sensitivity in Arabidopsis by modulating its binding capacity to Al. However, the transcriptional regulation of this process remains largely unknown. In our previous studies, we found that the expression of TRICHOME BIREFRINGENCE-LIKE27 (TBL27), which is responsible for the O-acetylation of XyG, was downregulated under Al stress. In the present study, we showed that the expression of an R2R3-type transcription factor-encoding gene, MYB103, was also inhibited by Al exposure and exhibited a co-expression pattern with TBL27 in roots and siliques, suggesting a potential link between MYB103 and TBL27. The loss of function of MYB103 resulted in increased Al sensitivity, as indicated by more inhibited root growth and elevated root Al content compared with the wild type. Moreover, we also detected increased Al accumulation in the root cell wall and the hemicellulose fraction, which was attributed to the changes in the O-acetylation level of XyG rather than the XyG content itself. In addition, further analysis revealed that MYB103 positively activated TBL27 expression by directly binding to the TBL27 promoter region, and TBL27 overexpression in the myb103 mutant rescued the Al-sensitive phenotype of the mutant to the wild-type level. Taken together, we conclude that MYB103 acts upstream of TBL27 to positively regulate Al resistance by modulating the O-acetylation of the cell wall XyG.

Involvement of the 4-coumarate:coenzyme A ligase 4CL4 in rice phosphorus acquisition and rhizosphere microbe recruitment via root growth enlargement.

MAIN CONCLUSION: The 4-coumarate:coenzyme A ligase 4CL4 is involved in enhancing rice P acquisition and use in acid soil by enlarging root growth and boosting functional rhizosphere microbe recruitment. Rice (Oryza sativa L.) cannot easily acquire phosphorus (P) from acid soil, where root growth is inhibited and soil P is fixed. The combination of roots and rhizosphere microbiota is critical for plant P acquisition and soil P mobilization, but the associated molecular mechanism in rice is unclear. 4CL4/RAL1 encodes a 4-coumarate:coenzyme A ligase related to lignin biosynthesis in rice, and its dysfunction results in a small rice root system. In this study, soil culture and hydroponic experiments were conducted to examine the role of RAL1 in regulating rice P acquisition, fertilizer P use, and rhizosphere microbes in acid soil. Disruption of RAL1 markedly decreased root growth. Mutant rice plants exhibited decreased shoot growth, shoot P accumulation, and fertilizer P use efficiency when grown in soil-but not under hydroponic conditions, where all P is soluble and available for plants. Mutant ral1 and wild-type rice rhizospheres had distinct bacterial and fungal community structures, and wild-type rice recruited some genotype-specific microbial taxa associated with P solubilization. Our results highlight the function of 4CL4/RAL1 in enhancing rice P acquisition and use in acid soil, namely by enlarging root growth and boosting functional rhizosphere microbe recruitment. These findings can inform breeding strategies to improve P use efficiency through host genetic manipulation of root growth and rhizosphere microbiota.

The NAC transcription factor ANAC017 regulates aluminum tolerance by regulating the cell wall-modifying genes.

Aluminum (Al) toxicity is one of the key factors limiting crop production in acid soils; however, little is known about its transcriptional regulation in plants. In this study, we characterized the role of a NAM, ATAF1/2, and cup-shaped cotyledon 2 (NAC) transcription factors (TFs), ANAC017, in the regulation of Al tolerance in Arabidopsis (Arabidopsis thaliana). ANAC017 was localized in the nucleus and exhibited constitutive expression in the root, stem, leaf, flower, and silique, although its expression and protein accumulation were repressed by Al stress. Loss of function of ANAC017 enhanced Al tolerance when compared with wild-type Col-0 and was accompanied by lower root and root cell wall Al content. Furthermore, both hemicellulose and xyloglucan content decreased in the anac017 mutants, indicating the possible interaction between ANAC017 and xyloglucan endotransglucosylase/hydrolase (XTH). Interestingly, the expression of XTH31, which is responsible for xyloglucan modification, was downregulated in the anac017 mutants regardless of Al supply, supporting the possible interaction between ANAC017 and XTH31. Yeast one-hybrid, dual-luciferase reporter assay, and chromatin immunoprecipitation-quantitative PCR analysis revealed that ANAC017 positively regulated the expression of XTH31 through directly binding to the XTH31 promoter region, and overexpression of XTH31 in the anac017 mutant background rescued its Al-tolerance phenotype. In conclusion, we identified that the tTF ANAC017 acts upstream of XTH31 to regulate Al tolerance in Arabidopsis.

The endo-beta mannase MAN7 contributes to cadmium tolerance by modulating root cell wall binding capacity in Arabidopsis thaliana.

The heavy metal cadmium (Cd) is detrimental to crop growth and threatens human health through the food chain. To cope with Cd toxicity, plants employ multiple strategies to decrease Cd uptake and its root-to-shoot translocation. However, genes that participate in the Cd-induced transcriptional regulatory network, including those encoding transcription factors, remain largely unidentified. In this study, we demonstrate that ENDO-BETA-MANNASE 7 (MAN7) is necessary for the response of Arabidopsis thaliana to toxic Cd levels. We show that MAN7 is responsible for mannase activity and modulates mannose content in the cell wall, which plays a role in Cd compartmentalization in the cell wall under Cd toxicity conditions. Additionally, the repression of root growth by Cd was partially reversed via exogenous application of mannose, suggesting that MAN7-mediated cell wall Cd redistribution depends on the mannose pathway. Notably, we identified a basic leucine zipper (bZIP) transcription factor, bZIP44, that acts upstream of MAN7 in response to Cd toxicity. Transient dual-luciferase assays indicated that bZIP44 directly binds to the MAN7 promoter region and activates its transcription. Loss of bZIP44 function was associated with greater sensitivity to Cd treatment and higher accumulation of the heavy metal in roots and shoots. Moreover, MAN7 overexpression relieved the inhibition of root elongation seen in the bzip44 mutant under Cd toxicity conditions. This study thus reveals a pathway showing that MAN7-associated Cd tolerance in Arabidopsis is controlled by bZIP44 upon Cd exposure.

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.

Node-Localized Transporters of Phosphorus Essential for Seed Development in Rice.

About 60-85% of total phosphorus (P) in cereal crops is finally allocated to seeds, where it is required for seed development, germination and early growth. However, little is known about the molecular mechanisms underlying P allocation to seeds. Here, we found that two members (OsPHO1;1 and OsPHO1;2) of the PHO1 gene family are involved in the distribution of P to seeds in rice. Both OsPHO1;1 and OsPHO1;2 were localized to the plasma membrane and showed influx transport activities for inorganic phosphate. At the reproductive stage, both OsPHO1;1 and OsPHO1;2 showed higher expression in node I, the uppermost node connecting to the panicle. OsPHO1;1 was mainly localized at the phloem region of diffuse vascular bundles (DVBs) of node I, while OsPHO1;2 was expressed in the xylem parenchyma cells of the enlarged vascular bundles (EVBs). In addition, they were also expressed in the ovular vascular trace, the outer layer of the inner integument (OsPHO1;1) and in the nucellar epidermis (OsPHO1;2) of caryopses. Knockout of OsPHO1;2, as well as OsPHO1;1 to a lesser extent, decreased the distribution of P to the seed, resulting in decreased seed size and delayed germination. Taken together, OsPHO1;2 expressed in node I is responsible for the unloading of P from the xylem of EVBs, while OsPHO1;1 is involved in reloading P into the phloem of DVBs for subsequent allocation of P to seeds. Furthermore, OsPHO1;1 and OsPHO1;2 expression in the caryopsis is important for delivering P from the maternal tissues to the filial tissues for seed development.

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.

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

MAIN CONCLUSION: 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.

Preferential Distribution of Boron to Developing Tissues Is Mediated by the Intrinsic Protein OsNIP3.

Boron is especially required for the growth of meristem and reproductive organs, but the molecular mechanisms underlying the preferential distribution of B to these developing tissues are poorly understood. Here, we show evidence that a member of nodulin 26-like intrinsic protein (NIP), OsNIP3;1, is involved in this preferential distribution in rice (Oryza sativa). OsNIP3;1 was highly expressed in the nodes and its expression was up-regulated by B deficiency, but down-regulated by high B. OsNIP3;1 was polarly localized at the xylem parenchyma cells of enlarged vascular bundles of nodes facing toward the xylem vessels. Furthermore, this protein was rapidly degraded within a few hours in response to high B. Knockout of this gene hardly affected the uptake and root-to-shoot translocation of B, but altered B distribution in different organs in the above-ground parts, decreased distribution of B to the new leaves, and increased distribution to the old leaves. These results indicate that OsNIP3;1 located in the nodes is involved in the preferential distribution of B to the developing tissues by unloading B from the xylem in rice and that it is regulated at both transcriptional and protein level in response to external B level.

Two Genes Encoding a Bacterial-Type ATP-Binding Cassette Transporter are Implicated in Aluminum Tolerance in Buckwheat.

Buckwheat (Fagopyrum esculentum Moench) shows high tolerance to aluminum (Al) toxicity, but the molecular mechanisms underlying its high Al tolerance are poorly understood. Here, we functionally characterized two genes (FeSTAR1 and FeSTAR2), which encode a nucleotide-binding domain and a membrane domain, respectively, of a bacterial-type ATP-binding cassette (ABC) transporter. The expression of FeSTAR1 and FeSTAR2 was induced by Al in both roots and leaves with higher expression in the roots. Spatial and tissue-specific expression analysis showed that the Al-induced expression of these two genes was found in both the root tips and basal root regions with higher expression in the root outer cell layers. The expression was neither induced by other metals including Cd and La nor by low pH and phosphorus-deficiency. FeSTAR1 and FeSTAR2 were present in a single copy in the genome, but the Al-induced transcript copy number of FeSTAR1 and FeSTAR2 was much higher than their homologous genes in rice and Arabidopsis. FeSTAR1 and FeSTAR2 form a complex when co-expressed in onion epidermal cells. Introduction of FeSTAR1 and FeSTAR2 into Arabidopsis mutants atstar1 and als3/atstar2, respectively, rescued the sensitivity of the mutants to Al. Taken together, our results indicate that FeSTAR1 and FeSTAR2 are involved in Al tolerance and that their high expression level may contribute to high Al tolerance in buckwheat.

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

MAIN CONCLUSION: 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.

Effective reduction of cadmium accumulation in rice grain by expressing OsHMA3 under the control of the OsHMA2 promoter.

Reducing cadmium (Cd) accumulation in rice grain is an important issue for human health. The aim of this study was to manipulate both expression and tissue localization of OsHMA3, a tonoplast-localized Cd transporter, in the roots by expressing it under the control of the OsHMA2 promoter, which shows high expression in different organs including roots, nodes, and shoots. In two independent transgenic lines, the expression of OsHMA3 was significantly enhanced in all organs compared with non-transgenic rice. Furthermore, OsHMA3 protein was detected in the root pericycle cells and phloem region of both the diffuse vascular bundle and the enlarged vascular bundle of the nodes. At the vegetative stage, the Cd concentration in the shoots and xylem sap of the transgenic rice was significantly decreased, but that of the whole roots and root cell sap was increased. At the reproductive stage, the concentration of Cd, but not other essential metals, in the brown rice of transgenic lines was decreased to less than one-tenth that of the non-transgenic rice. These results indicate that expression of OsHMA3 under the control of the OsHMA2 promoter can effectively reduce Cd accumulation in rice grain through sequestering more Cd into the vacuoles of various tissues.

Effects of cadmium exposure on expression of glutathione synthetase system genes in Acidithiobacillus ferrooxidans.

The glutathione synthetase system (GSS) is an important pathway of glutathione synthesis and plays a key role in heavy metal resistance. In this work, the response of Acidithiobacillus ferrooxidans to extracellular Cd2+ was investigated, and the interplay between Cd2+ resistance and the expression of GSS related-genes was analyzed by reverse-transcription quantitative PCR (RT-PCR). During growth in the presence of 5, 15 and 30 mM Cd2+, the transcript levels of eight GSS pathway genes were affected between 0.81- and 7.12-fold. Increased transcription was also reflected in increased enzyme activities: with those of glutathione reductase (GR) increased by 1.10-, 2.26- and 1.54-fold in the presence of 5, 15 and 30 mM Cd2+, respectively. In contrast, the activities of catalase (CAT) and superoxide dismutase (SOD) were decreased in the presence of Cd2+. At the metabolite level, intracellular methane dicarboxylic aldehyde (MDA) content was increased 1.97-, 3.31- and 1.92-fold in the presence of 5, 15 and 30 mM Cd2+, respectively. These results suggest that Cd2+ directly inhibits the activities of CAT and SOD, breaks the redox balance of the cells, which leads to the activation of the other antioxidant pathway of GSS. Resistance of A. ferrooxidans to Cd2+ may involve modulation of expression levels of glutathione S-transferase (GST), GR, and glutathione synthetase, which may protect against oxidative damage.

An Al-inducible expansin gene, OsEXPA10 is involved in root cell elongation of rice.

Expansins are cell wall loosening proteins, which are encoded by multigene families. However, the physiological role of most expansin genes is still poorly understood. Here, we functionally characterized an Al-inducible expansin gene, OsEXPA10, which is regulated by a C2H2-type zinc-finger transcription factor, ART1 in rice. A detailed expression analysis showed that OsEXPA10 was expressed in both the roots and shoots at a similar level, but only the expression in the roots was rapidly upregulated in response to Al. Furthermore, spatial expression analysis showed that the Al-induced expression was only found in the root tips (0-3 mm), but not in the mature root zones. The expression was neither induced by other metals including Cd and La nor by low pH. Immunostaining showed that OsEXPA10 was localized at all cells of the root tips. Knockout of OsEXPA10 resulted in a significant decrease in the cell elongation of the roots in the absence of Al. In the presence of Al, knockout of OsEXPA10 did not alter the Al sensitivity evaluated by relative root elongation, but the root cell wall of knockout lines accumulated less Al compared to those of the wild-type rice. Collectively, our results indicate that OsEXPA10 expressed in the root tips is required for the root cell elongation, but that the contribution of this gene to high Al tolerance in rice is small.

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

Plants challenged with abiotic stress show enhanced polyamines levels. Here, we show that the polyamine putrescine (Put) plays an important role to alleviate Fe deficiency. The adc2-1 mutant, which is defective in Put biosynthesis, was hypersensitive to Fe deficiency compared with wild type (Col-1 of Arabidopsis [Arabidopsis thaliana]). Exogenous Put decreased the Fe bound to root cell wall, especially to hemicellulose, and increased root and shoot soluble Fe content, thus alleviating the Fe deficiency-induced chlorosis. Intriguingly, exogenous Put induced the accumulation of nitric oxide (NO) under both Fe-sufficient (+Fe) and Fe-deficient (-Fe) conditions, although the ferric-chelate reductase (FCR) activity and the expression of genes related to Fe uptake were induced only under -Fe treatment. The alleviation of Fe deficiency by Put was diminished in the hemicellulose-level decreased mutant-xth31 and in the noa1 and nia1nia2 mutants, in which the endogenous NO levels are reduced, indicating that both NO and hemicellulose are involved in Put-mediated alleviation of Fe deficiency. However, the FCR activity and the expression of genes related to Fe uptake were still up-regulated under -Fe+Put treatment compared with -Fe treatment in xth31, and Put-induced cell wall Fe remobilization was abolished in noa1 and nia1nia2, indicating that Put-regulated cell wall Fe reutilization is dependent on NO. From our results, we conclude that Put is involved in the remobilization of Fe from root cell wall hemicellulose in a process dependent on NO accumulation under Fe-deficient condition in Arabidopsis.

Differential Effects of Nitrogen Forms on Cell Wall Phosphorus Remobilization Are Mediated by Nitric Oxide, Pectin Content, and Phosphate Transporter Expression.

NH4 (+) is a major source of inorganic nitrogen for rice (Oryza sativa), and NH4 (+) is known to stimulate the uptake of phosphorus (P). However, it is unclear whether NH4 (+) can also stimulate P remobilization when rice is grown under P-deficient conditions. In this study, we use the two rice cultivars 'Nipponbare' and 'Kasalath' that differ in their cell wall P reutilization, to demonstrate that NH4 (+) positively regulates the pectin content and activity of pectin methylesterase in root cell walls under -P conditions, thereby remobilizing more P from the cell wall and increasing soluble P in roots and shoots. Interestingly, our results show that more NO (nitric oxide) was produced in the rice root when NH4 (+) was applied as the sole nitrogen source compared with the NO3 (-) The effect of NO on the reutilization of P from the cell walls was further demonstrated through the application of the NO donor SNP (sodium nitroprusside) and c-PTIO (NO scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide). What's more, the P-transporter gene OsPT2 is up-regulated under NH4 (+) supplementation and is therefore involved in the stimulated P remobilization. In conclusion, our data provide novel (to our knowledge) insight into the regulatory mechanism by which NH4 (+) stimulates Pi reutilization in cell walls of rice.

PARVUS affects aluminium sensitivity by modulating the structure of glucuronoxylan in Arabidopsis thaliana.

Glucuronoxylan (GX), an important component of hemicellulose in the cell wall, appears to affect aluminium (Al) sensitivity in plants. To investigate the role of GX in cell-wall-localized xylan, we examined the Arabidopsis thaliana parvus mutant in detail. This mutant lacks α-D-glucuronic acid (GlcA) side chains in GX and has greater resistance to Al stress than wild-type (WT) plants. The parvus mutant accumulated lower levels of Al in its roots and cell walls than WT despite having cell wall pectin content and pectin methylesterase (PME) activity similar to those of WT. Our results suggest that the altered properties of hemicellulose in the mutant contribute to its decreased Al accumulation. Although we observed almost no differences in hemicellulose content between parvus and WT under control conditions, less Al was retained in parvus hemicellulose than in WT. This observation is consistent with the finding that GlcA substitutions in WT GX, but not mutant GX, were increased under Al stress. Taken together, these results suggest that the modulation of GlcA levels in GX affects Al resistance by influencing the Al binding capacity of the root cell wall in Arabidopsis.