Phloem iron remodels root development in response to ammonium as the major nitrogen source.

Plants use nitrate and ammonium as major nitrogen (N) sources, each affecting root development through different mechanisms. However, the exact signaling pathways involved in root development are poorly understood. Here, we show that, in Arabidopsis thaliana, either disruption of the cell wall-localized ferroxidase LPR2 or a decrease in iron supplementation efficiently alleviates the growth inhibition of primary roots in response to NH4+ as the N source. Further study revealed that, compared with nitrate, ammonium led to excess iron accumulation in the apoplast of phloem in an LPR2-dependent manner. Such an aberrant iron accumulation subsequently causes massive callose deposition in the phloem from a resulting burst of reactive oxygen species, which impairs the function of the phloem. Therefore, ammonium attenuates primary root development by insufficiently allocating sucrose to the growth zone. Our results link phloem iron to root morphology in response to environmental cues.

Tease out the future: How tea research might enable crop breeding for acid soil tolerance.

Unlike most crops, in which soil acidity severely limits productivity, tea (Camellia sinensis) actually prefers acid soils (pH 4.0-5.5). Specifically, tea is very tolerant of acidity-promoted aluminum (Al) toxicity, a major factor that limits the yield of most other crops, and it even requires Al for optimum growth. Understanding tea Al tolerance and Al-stimulatory mechanisms could therefore be fundamental for the future development of crops adapted to acid soils. Here, we summarize the Al-tolerance mechanisms of tea plants, propose possible mechanistic explanations for the stimulation of tea growth by Al based on recent research, and put forward ideas for future crop breeding for acid soils.

A NAC-type transcription factor confers aluminium resistance by regulating cell wall-associated receptor kinase 1 and cell wall pectin.

Transcriptional regulation is important for plants to respond to toxic effects of aluminium (Al). However, our current knowledge to these events is confined to a few transcription factors. Here, we functionally characterized a rice bean (Vigna umbellata) NAC-type transcription factor, VuNAR1, in terms of Al stress response. We demonstrated that rice bean VuNAR1 is a nuclear-localized transcriptional activator, whose expression was specifically upregulated by Al in roots but not in shoot. VuNAR1 overexpressing Arabidopsis plants exhibit improved Al resistance via Al exclusion. However, VuNAR1-mediated Al exclusion is independent of the function of known Al-resistant genes. Comparative transcriptomic analysis revealed that VuNAR1 specifically regulates the expression of genes associated with protein phosphorylation and cell wall modification in Arabidopsis. Transient expression assay demonstrated the direct transcriptional activation of cell wall-associated receptor kinase 1 (WAK1) by VuNAR1. Moreover, yeast one-hybrid assays and MEME motif searches identified a new VuNAR1-specific binding motif in the promoter of WAK1. Compared with wild-type Arabidopsis plants, VuNAR1 overexpressing plants have higher WAK1 expression and less pectin content. Taken together, our results suggest that VuNAR1 regulates Al resistance by regulating cell wall pectin metabolism via directly binding to the promoter of WAK1 and induce its expression.

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.

Pectin enhances rice (Oryza sativa) root phosphorus remobilization.

Plants growing in phosphorus (P)-deficient conditions can either increase their exploration of the environment (hence increasing P uptake) or can solubilize and reutilize P from established tissue sources. However, it is currently unclear if P stored in root cell wall can be reutilized. The present study shows that culture of the rice cultivars 'Nipponbare' (Nip) and 'Kasalath' (Kas) in P-deficient conditions results in progressive reductions in root soluble inorganic phosphate (Pi). However, Nip consistently maintains a higher level of soluble Pi and lower relative cell wall P content than does Kas, indicating that more cell wall P is released in Nip than in Kas. P-deficient Nip has a greater pectin and hemicellulose 1 (HC1) content than does P-deficient Kas, consistent with the significant positive relationship between pectin and root-soluble Pi levels amongst multiple rice cultivars. These observations suggest that increased soluble Pi might result from increased pectin content during P starvation. In vitro experiments showed that pectin releases Pi from insoluble FePO4. Furthermore, an Arabidopsis thaliana mutant with reduced pectin levels (qua1-2), has less root soluble Pi and is more sensitive to P deficiency than the wild type (WT) Col-0, whereas NaCl-treated WT plants exhibit both an increased root pectin content and an elevated soluble Pi content during P-starvation. These observations indicate that pectin can facilitate the remobilization of P deposited in the cell wall. This is a previously unknown mechanism for the reutilization of P in P-starved plants.

Root-derived auxin contributes to the phosphorus-deficiency-induced cluster-root formation in white lupin (Lupinus albus).

Formation of cluster roots is a typical morphological response to phosphorus (P) deficiency in white lupin (Lupinus albus), but its physiological and molecular mechanisms are still unclear. We investigated the role of auxin in the initiation of cluster roots by distinguishing the sources of auxin, measuring the longitudinal distribution patterns of free indole-3-acetic acid (IAA) along the root and the related gene expressions responsible for polar auxin transport (PAT) in different developmental stages of cluster roots. We found that removal of shoot apex or primary root apex and application of auxin-influx or -efflux transport inhibitors, 3-chloro-4-hydroxyphenylacetic acid, N-1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid, to the stem did not affect the number of cluster roots and the free-IAA concentration in the roots of P-deficient plants, but when these inhibitors were applied directly to the growth media, the cluster-root formation was greatly suppressed, suggesting the fundamental role of root-derived IAA in cluster-root formation. The concentration of free IAA in the roots was higher in P-deficient plants than in P-adequate ones, and the highest in the lateral-root apex and the lowest in the mature cluster roots. Meanwhile the expression patterns of LaAUX1, LaPIN1 and LaPIN3 transcripts related to PAT was consistent with concentrations of free IAA along the lateral root, indicating the contribution of IAA redistribution in the cluster-root development. We proposed that root-derived IAA plays a direct and important role in the P-deficiency-induced formation of cluster roots.

Cell wall polysaccharides are involved in P-deficiency-induced Cd exclusion in Arabidopsis thaliana.

The physiological and molecular mechanisms leading to the competitive interactions between phosphorus (P) and metal elements are a matter of debate. In this study, we found that P deficiency can alleviate cadmium (Cd) toxicity in Arabidopsis thaliana (Col-0). Under P deficiency (-P), less Cd was accumulated in the plants and the root cell walls, indicating the operation of a P-deficiency-induced Cd exclusion mechanism. However, organic acid efflux was similar under -P+Cd and +Cd treatments, suggesting that organic acid efflux is not responsible for the Cd exclusion. Interestingly, P deficiency significantly decreased cell wall polysaccharides (pectin and hemicellulose) contents and pectin methylesterase activity, and decreased the Cd retained by the extracted root cell wall. Therefore, we conclude that the modification of cell wall composition is responsible for the Cd exclusion of the root under P deficiency.

Nitric oxide is the shared signalling molecule in phosphorus- and iron-deficiency-induced formation of cluster roots in white lupin (Lupinus albus).

BACKGROUND AND AIMS: Formation of cluster roots is one of the most specific root adaptations to nutrient deficiency. In white lupin (Lupinus albus), cluster roots can be induced by phosphorus (P) or iron (Fe) deficiency. The aim of the present work was to investigate the potential shared signalling pathway in P- and Fe-deficiency-induced cluster root formation. METHODS: Measurements were made of the internal concentration of nutrients, levels of nitric oxide (NO), citrate exudation and expression of some specific genes under four P × Fe combinations, namely (1) 50 µm P and 10 µm Fe (+P + Fe); (2) 0 P and 10 µm Fe (-P + Fe); (3) 50 µm P and 0 Fe (+P-Fe); and (4) 0 P and 0 Fe (-P-Fe), and these were examined in relation to the formation of cluster roots. KEY RESULTS: The deficiency of P, Fe or both increased the cluster root number and cluster zones. It also enhanced NO accumulation in pericycle cells and rootlet primordia at various stages of cluster root development. The formation of cluster roots and rootlet primordia, together with the expression of LaSCR1 and LaSCR2 which is crucial in cluster root formation, were induced by the exogenous NO donor S-nitrosoglutathione (GSNO) under the +P + Fe condition, but were inhibited by the NO-specific endogenous scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl- 3-oxide (cPTIO) under -P + Fe, +P-Fe and -P-Fe conditions. However, cluster roots induced by an exogenous supply of the NO donor did not secrete citrate, unlike those formed under -P or -Fe conditions. CONCLUSIONS: NO plays an important role in the shared signalling pathway of the P- and Fe-deficiency-induced formation of cluster roots in white lupin.

Genotypic differences in Al resistance and the role of cell-wall pectin in Al exclusion from the root apex in Fagopyrum tataricum.

BACKGROUND AND AIMS: Aluminium (Al) toxicity is one of the factors limiting crop production on acid soils. However, genotypic differences exist among plant species or cultivars in response to Al toxicity. This study aims to investigate genotypic differences among eight cultivars of tatary buckwheat (Fagopyrum tataricum) for Al resistance and explore the possible mechanisms of Al resistance. METHODS: Al resistance was evaluated based on relative root elongation (root elongation with Al/root elongation without Al). Root apex Al content, pectin content and exudation of root organic acids were determined and compared. KEY RESULTS: Genotypic differences among the eight cultivars were correlated with exclusion of Al from the root apex. However, there was a lack of correlation between Al exclusion and Al-induced oxalate secretion. Interestingly, cell-wall pectin content of the root apex was generally lower in Al-resistant cultivars than in Al-sensitive cultivars. Although we were unable to establish a significant correlation between Al exclusion and pectin content among the eight cultivars, a strong correlation could be established among six cultivars, in which the pectin content in the most Al-resistant cultivar 'Chuan' was significantly lower than that in the most Al-sensitive cultivar 'Liuku2'. Furthermore, root apex cell-wall pectin methylesterase activity (PME) was similar in 'Chuan' and 'Liuku2' in the absence of Al, but Al treatment resulted in increased PME activity in 'Liuku2' compared with 'Chuan'. Immunolocalization of pectins also showed that the two cultivars had similar amounts of either low-methyl-ester pectins or high-methyl-ester pectins in the absence of Al, but Al treatment resulted in a more significant increase of low-methyl-ester pectins and decrease of high-methyl-ester pectins in 'Liuku2'. CONCLUSIONS: Cell-wall pectin content may contribute, at least in part, to differential Al resistance among tatary buckwheat cultivars.

Disorganized distribution of homogalacturonan epitopes in cell walls as one possible mechanism for aluminium-induced root growth inhibition in maize.

BACKGROUND AND AIMS: Aluminium (Al) toxicity is one of the most severe limitations to crop production in acid soils. Inhibition of root elongation is the primary symptom of Al toxicity. However, the underlying basis of the process is unclear. Considering the multiple physiological and biochemical functions of pectin in plants, possible involvement of homogalacturonan (HG), one of the pectic polysaccharide domains, was examined in connection with root growth inhibition induced by Al. METHODS: An immunolabelling technique with antibodies specific to HG epitopes (JIM5, unesterified residues flanked by methylesterifed residues; JIM7, methyl-esterified residues flanked by unesterified residues) was used to visualize the distribution of different types of HG in cell walls of root apices of two maize cultivars differing in Al resistance. KEY RESULTS: In the absence of Al, the JIM5 epitope was present around the cell wall with higher fluorescence intensity at cell corners lining the intercellular spaces, and the JIM7 epitope was present throughout the cell wall. However, treatment with 50 microm Al for 3 h produced 10 % root growth inhibition in both cultivars and caused the disappearance of fluorescence in the middle lamella of both epitopes. Prolonged Al treatment (24 h) with 50 % root growth inhibition in 'B73', an Al-sensitive cultivar, resulted in faint and irregular distribution of both epitopes. In 'Nongda3138', an Al-resistant cultivar, the distribution of HG epitopes was also restricted to the lining of intercellular spaces when a 50 % inhibition to root growth was induced by Al (100 microm Al, 9 h). Altered distribution of both epitopes was also observed when of roots were exposed to 50 microm LaCl(3) for 24 h, resulting in 40 % inhibition of root growth. CONCLUSIONS: Changes in HG distribution and root growth inhibition were highly correlated, indicating that Al-induced perturbed distribution of HG epitopes is possibly involved in Al-induced inhibition of root growth in maize.

Protecting cell walls from binding aluminum by organic acids contributes to aluminum resistance.

Aluminum-induced secretion of organic acids from the root apex has been demonstrated to be one major Al resistance mechanism in plants. However, whether the organic acid concentration is high enough to detoxify Al in the growth medium is frequently questioned. The genotypes of Al-resistant wheat, Cassia tora L. and buckwheat secrete malate, citrate and oxalate, respectively. In the present study we found that at a 35% inhibition of root elongation, the Al activities in the solution were 10, 20, and 50 muM with the corresponding malate, citrate, and oxalate exudation at the rates of 15, 20 and 21 nmol/cm(2) per 12 h, respectively, for the above three plant species. When exogenous organic acids were added to ameliorate Al toxicity, twofold and eightfold higher oxalate and malate concentrations were required to produce the equal effect by citrate. After the root apical cell walls were isolated and preincubated in 1 mM malate, oxalate or citrate solution overnight, the total amount of Al adsorbed to the cell walls all decreased significantly to a similar level, implying that these organic acids own an equal ability to protect the cell walls from binding Al. These findings suggest that protection of cell walls from binding Al by organic acids may contribute significantly to Al resistance.

Comparative studies on the effect of a protein-synthesis inhibitor on aluminium-induced secretion of organic acids from Fagopyrum esculentum Moench and Cassia tora L. roots.

Aluminium (Al)-induced secretion of organic acids from plant roots is considered a mechanism of Al resistance, but the processes leading to the secretion of organic acids are still unknown. In the present study, a protein-synthesis inhibitor, cycloheximide (CHM), was used to investigate its effect on Al-induced organic acid secretion in a pattern I (rapid exudation of organic acids under Al stress) plant buckwheat (Fagopyrum esculentum Moench) and a pattern II (exudation of organic acids was delayed by several hours under Al stress) plant Cassia tora L. A dose-response experiment showed that the secretion of oxalate by buckwheat roots was not affected by CHM when added in the range from 0 to 50 microM, with or without exposure to 100 microm Al, but the secretion of citrate was completely inhibited by 30 microM CHM in C. tora. A time-course experiment showed that even prolonged exposure to 20 microM CHM did not affect oxalate secretion in buckwheat, but significantly inhibited citrate secretion in C. tora. However, citrate synthase (CS) activity in C. tora was not affected during 12 h exposure to 100 microM Al when compared with that in control roots, although CHM can inhibit CS activity effectively. These results indicated that CS activity was not related to Al-regulated citrate efflux in C. tora. The total protein was decreased by 14.0% and 32.3% in C. tora and buckwheat root tip, respectively, after 3-h treatment with 20 microM CHM. A 3-h pulse with 20 microM CHM completely inhibited citrate efflux in C. tora during the next 6-h exposure to Al, although a small amount of citrate was exuded after 9-h exposure. However, oxalate efflux in buckwheat was not influenced by a similar treatment. In buckwheat, a 3-h pulse with 100 microM Al maintained oxalate secretion at a high level during the next 9 h, with or without CHM treatment. Conversely, in C. tora a 6-h pulse with 100 microM Al induced significant secretion of citrate which was inhibited by the CHM. Taken together, these findings suggest that both de novo synthesis and activation of an anion channel are needed for Al-induced secretion of citrate in C. tora, but in buckwheat the plasma membrane protein responsible for oxalate secretion pre-exists.

Lead contamination in tea leaves and non-edaphic factors affecting it.

Recent tests have detected high lead (Pb) concentrations in some commercial brands of tea leaves and this finding has raised concerns due to the possible health-related problems associated with Pb poisoning. In present research, we investigated the Pb contamination in tea leaves produced in Zhejiang province in China. Pb concentrations in all tea leaves sampled were below 5 mg/kg, the permissible levels given by Chinese Ministry of Agriculture, indicating that Pb contamination in this province is not excessive. We then investigated the non-edaphic factors that may potentially contribute to Pb accumulation in tea leaves. Pb concentration in tea leaves was found to be positively correlated with the industrialization level of a district (R = 0.83, the significant level at P < 0.05), and greater amounts of Pb were washed from the leaves of plants in districts with more industrial activity. This suggests that Pb accumulation in tea leaves could, in part, be attributed to industrial activity through the precipitation of atmospheric Pb. Furthermore greater amounts of Pb were washed from the leaves of plants growing near road than those growing farther away from road. This trend indicates that automobile activity was another likely contributor to Pb accumulation in tea. Pb content of green tea was also affected by the processing of the leaves in the factory. In particular the twisting and water-removal stages caused increases in Pb content in the tea product. This study suggests that non-edaphic factors also contribute to the Pb accumulation in tea.

Immobilization of aluminum with phosphorus in roots is associated with high aluminum resistance in buckwheat.

Oxalic acid secretion from roots is considered to be an important mechanism for aluminum (Al) resistance in buckwheat (Fygopyrum esculentum Moench). Nonetheless, only a single Al-resistant buckwheat cultivar was used to investigate the significance of oxalic acid in detoxifying Al. In this study, we investigated two buckwheat cultivars, Jiangxi (Al resistant) and Shanxi (Al sensitive), which showed significant variation in their resistance to Al stress. In the presence of 0 to 100 microM Al, the inhibition of root elongation was greater in Shanxi than that in Jiangxi, and the Al content of root apices (0-10 mm) was much lower in Jiangxi. However, the dependence of oxalic acid secretion on external Al concentration and the time course for secretion were similar in both cultivars. Furthermore, the variation in Al-induced oxalic acid efflux along the root was similar, showing a 10-fold greater efflux from the apical 0- to 5-mm region than from the 5- to 10-mm region. These results suggest that both Shanxi and Jiangxi possess an equal capacity for Al-dependent oxalic acid secretion. Another two potential Al resistance mechanisms, i.e. Al-induced alkalinization of rhizosphere pH and root inorganic phosphate release, were also not involved in their differential Al resistance. However, after longer treatments in Al (10 d), the concentrations of phosphorus and Al in the roots of the Al-resistant cultivar Jiangxi were significantly higher than those in Shanxi. Furthermore, more Al was localized in the cell walls of the resistant cultivar. All these results suggest that while Al-dependent oxalic acid secretion might contribute to the overall high resistance to Al stress of buckwheat, this response cannot explain the variation in tolerance between these two cultivars. We present evidence suggesting the greater Al resistance in buckwheat is further related to the immobilization and detoxification of Al by phosphorus in the root tissues.

Lead contamination in tea garden soils and factors affecting its bioavailability.

The consumption of heavy metals is detrimental to human health and most countries restrict the concentration of metals such as lead (Pb) in food and beverages. Recent tests have detected high Pb concentrations in certain commercial brands of tea leaves and this finding has raised concerns for both producers and consumers. To investigate what factors may be contributing to the increase in Pb accumulation in the tea leaves we collected tea leaves and soils from tea producing areas and analyzed them for Pb concentration, pH and organic matter content. The result showed the Pb concentration of 47% investigated tea leaves samples was beyond 2 mg kg(-1), the permissible levels given by China. The total Pb concentration in the surface and subsurface soil layers averaged 36.4 and 32.2 mg kg(-1), respectively which fall below of the 60 mg kg(-1) limit provided for organic tea gardens in China. The pH of the tea garden soils was severely acidic with the lowest pH of 3.37. Soils under older tea gardens tended to have a lower pH and a higher Pb bioavailability which was defined as the amount of lead extracted by CaCl2 solution than those under younger tea gardens. We found that the concentration of bioavailable Pb and the percentage of bioavailable Pb (bioavailable Pb relative to total Pb concentration) were positively correlated with soil H+ activity and soil organic matter content, and the organic matter accumulation contribute more effects on Pb bioavailability in these two factors. We conclude that soil acidification and organic matter accumulation could contribute to increasing Pb bioavailability in soil and that these could increase Pb uptake and accumulation in the tea leaves.