Phialocephala fortinii increases aluminum tolerance in Miscanthus sinensis growing in acidic mine soil

Miscanthus sinensis growing in our study mine site contained a high concentration of Al in the adventitious roots. It has a root endophyte, Phialocephala fortinii, in its adventitious roots at a high frequency. The purpose of this study was to elucidate the effects of P. fortinii on Al tolerance mechanisms of M. sinensis and reveal potential underlying mechanisms. In the absence of P. fortinii, M. sinensis produced chlorogenic, citric, and malic acids that could act to alleviate Al toxicity in acidic mine soil. Up on fungal inoculation, the levels of these compounds were reduced, although the growth of seedlings and Mg concentration in the roots were increased. IAA production by the fungus may contribute to enhanced plant growth whereas an increase of Mg uptake could reduce toxicity of reactive oxygen species under Al stress. These actions of P. fortinii could promote growth and survival of M. sinensis in mine sites.     

Differential expression of genes involved in alternative glycolytic pathways,phosphorus scavenging and recycling in response to aluminum and phosphorus interactions in <Emphasis Type="Italic">Citrus</Emphasis> roots

The objective was to determine the possible links between the expression levels of genes involved in alternative glycolytic pathways, phosphorus (P) scavenging and recycling and Citrus tolerance to aluminum (Al) and/or P-deficiency. ‘Xuegan’ (Citrus sinensis) and ‘Sour pummelo’ (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O × 0, 50 and 200 μM KH₂PO₄. C. sinensis displayed more tolerant to Al and P-deficiency than C. grandis. Under Al stress, C. sinensis accumulated more Al in roots and less Al in shoots than C. grandis. P concentration was higher in C. sinensis shoots and roots than in C. grandis ones. C. sinensis roots secreted more malate and citrate than C. grandis ones when exposed to Al. Al-induced-secretion of malate and citrate by excised roots from Al-treated seedlings decreased with increasing P supply. Al-induced-secretion of malate and citrate from roots and Al precipitation by P in roots might be responsible for Al-tolerance of C. sinensis. qRT-PCR analysis showed that Al-activated malate transporter (ALMT1), ATP-dependent phosphofructokinase (ATP-PFK), pyrophosphate-dependent phosphofructokinase (PPi-PFK), tonoplast adenosine-triphosphatase subunit A (V-ATPase A), tonoplast pyrophosphatase (V-PPiase), pyruvate kinase (PK), acid phosphatase (APase), phosphoenolpyruvate carboxylase (PEPC), malic enzyme (ME) and malate dehydrogenase (MDH) genes might contribute to the tolerance of Citrus to Al and/or P-deficiency, but any single gene could not explain the differences between the two species. Citrus tolerance to Al and/or P-deficiency might be caused by the coordinated regulation of gene expression involved in alternative glycolytic pathways, P scavenging and recycling.     

Contribution of constitutive characteristics of lipids and phenolics in roots of tree species in Myrtales to aluminum tolerance

High aluminum (Al) concentration in soil solution is the most important factor restricting plant growth in acidic soils. However, various plant species naturally grow in such soils. Generally, they are highly tolerant to Al, but organic acid exudation, the most common Al tolerance mechanism, cannot explain their tolerance. Lower phospholipid and higher sterol proportions in root plasma membrane enhance Al tolerance. Other cellular components, such as cell walls and phenolics, may also be involved in Al tolerance mechanisms. In this study, the relationships between these cellular components and the Al tolerance mechanisms in Melastoma malabathricum and Melaleuca cajuputi, both highly Al‐tolerant species growing in strongly acidic soils, were investigated. Both species contained lower proportions of phospholipids and higher proportions of sterols in roots, respectively. Concentrations of phenolics in roots of both species were higher than that of rice; their phenolics could form chelates with Al. In these species, phenolic concentrations and composition were the same irrespective of the presence or absence of Al in the medium, suggesting that a higher concentration of phenolics is not a physiological response to Al but a constitutive characteristic. These characteristics of cellular components in roots may be cooperatively involved in their high Al tolerance.     

Oryza sativa Lysine‐Histidine‐type Transporter 1 functions in root uptake and root‐to‐shoot allocation of amino acids in rice

In agricultural soils, amino acids can represent vital nitrogen (N) sources for crop growth and yield. However, the molecular mechanisms underlying amino acid uptake and allocation are poorly understood in crop plants. This study shows that rice (Oryza sativa L.) roots can acquire aspartate at soil concentration, and that japonica subspecies take up this acidic amino acid 1.5‐fold more efficiently than indica subspecies. Genetic association analyses with 68 representative japonica or indica germplasms identified rice Lysine‐Histidine‐type Transporter 1 (OsLHT1) as a candidate gene associated with the aspartate uptake trait. When expressed in yeast, OsLHT1 supported cell growth on a broad spectrum of amino acids, and effectively transported aspartate, asparagine and glutamate. OsLHT1 is localized throughout the rice root, including root hairs, epidermis, cortex and stele, and to the leaf vasculature. Knockout of OsLHT1 in japonica resulted in reduced root uptake of amino acids. Furthermore, in ¹⁵N‐amino acid‐fed mutants versus wild‐type, a higher percentage of ¹⁵N remained in roots instead of being allocated to the shoot. ¹⁵N‐ammonium uptake and subsequently the delivery of root‐synthesized amino acids to Oslht1 shoots were also significantly decreased, which was accompanied by reduced shoot growth. These results together provide evidence that OsLHT1 functions in both root uptake and root to shoot allocation of a broad spectrum of amino acids in rice.     

Biochemical and molecular changes in rice seedlings (Oryza sativa L.) to cope with chromium stress

Chromium (Cr) is very toxic to both humans and plants. This investigation aimed to understand the physiological and molecular responses of rice seedlings to Cr stress. Cr toxicity did not significantly affect morphological features and Cr accumulation in roots and shoots in Pokkali but not in BRRI 51, although there was a reduction in chlorophyll concentration in leaves of both genotypes. These results imply that Pokkali has mechanisms to cope with Cr supplementation. We therefore performed quantitative real‐time PCR on the expression pattern of two chelator genes, OsPCS1 and OsMT1, but there were no significant changes in expression in roots and shoots of Pokkali and BRRI 51 following Cr stress. This suggests that there was no metal sequestration following heavy metal stress in roots of these genotypes. Moreover, no expression of two heavy metal transporter genes, OsHMA3 and OsNRAMP1, was induced after Cr stress in roots and shoots, suggesting that these transporter genes are not induced by Cr stress or might not be involved in Cr uptake in rice. We also performed a targeted study on the effect of Cr on Fe uptake mechanisms. Our studies showed a consistent reduction in Fe uptake, Fe reductase activity and expression of Fe‐related genes (OsFRO1 and OsIRT1) under Cr stress in both roots and leaves of Pokkali. In contrast, these parameters and genes were significantly increased in Cr‐sensitive BRRI 51 under Cr stress. The results confirm that limiting Fe uptake through the down‐regulation of Fe reductase and Fe transporter genes is the main strategy of Cr‐tolerant Pokkali to cope with Cr stress. Finally, increased CAT, POD and GR activity and elevated glutathione and proline synthesis might provide strong antioxidant defence against Cr stress in Pokkali. Taken together, our findings reveal that Cr stress tolerance in rice (Pokkali) is not related to metal sequestration but is associated with reduced Fe transport and increased antioxidant defence.     

First Report of Pseudomonas lurida Causing Bacterial Leaf Spot on Miscanthus sinensis

Miscanthus spp. are large perennial wetland grasses that are receiving considerable attention as bioenergy crops. In late summer 2011, leaf spot symptoms were observed in a field of Miscanthus sinensis in Jeongseon, Gangwon province, Korea. Bacterial strains that belonged to the γ‐Proteobacteria genus Pseudomonas were isolated from the affected leaves. By phylogenetic analysis and phenotypic characterization, the representative strain MDM‐03 was identified as Pseudomonas lurida. Healthy M. sinensis leaves inoculated with MDM‐03 developed leaf spots similar to those observed in field. Bacteria re‐isolated from the leaf lesions were identical to the original strain MDM‐03 based on their cultural characteristics and 16S rDNA sequencing. This is the first report of bacterial leaf spot in Miscanthus sinensis.     

A multidrug and toxic compound extrusion (MATE) transporter modulates auxin levels in root to regulate root development and promotes aluminium tolerance

MATE (multidrug and toxic compound extrusion) transporters play multiple roles in plants including detoxification, secondary metabolite transport, aluminium (Al) tolerance, and disease resistance. Here we identify and characterize the role of the Arabidopsis MATE transporter DETOXIFICATION30. AtDTX30 regulates auxin homeostasis in Arabidopsis roots to modulate root development and Al-tolerance. DTX30 is primarily expressed in roots and localizes to the plasma membrane of root epidermal cells including root hairs. dtx30 mutants exhibit reduced elongation of the primary root, root hairs, and lateral roots. The mutant seedlings accumulate more auxin in their root tips indicating role of DTX30 in maintaining auxin homeostasis in the root. Al induces DTX30 expression and promotes its localization to the distal transition zone. dtx30 seedlings accumulate more Al in their roots but are hyposensitive to Al-mediated rhizotoxicity perhaps due to saturation in root growth inhibition. Increase in expression of ethylene and auxin biosynthesis genes in presence of Al is absent in dtx30. The mutants exude less citrate under Al conditions, which might be due to misregulation of AtSTOP1 and the citrate transporter AtMATE. In conclusion, DTX30 modulates auxin levels in root to regulate root development and in the presence of Al indirectly modulates citrate exudation to promote Al tolerance.     

Aluminium-induced changes in the profiles of both organic acids and phenolic substances underlie Al tolerance in Rumex acetosa L.

The common sorrel, Rumex acetosa L. is well adapted to acid mineral soils with high availability of phytotoxic Al species. The mechanisms of Al resistance in this species are not established. Our goal was to assess the possible implications of organic acids and phenolic substances in Al detoxification in roots and shoots of this plant. R. acetosa plants were exposed in nutrient solution (pH 4.3) to a non-growth reducing Al concentration of 50 μM Al for 5 days. Exclusion of Al from root tips was visualized by haematoxylin staining. Tissue Al and Ca concentrations were analysed by ICP ES. Root and shoot concentrations of organic acids and phenolic substances were analysed by HPLC. A time-dependent (model II type) Al exclusion pattern in root tips was observed. Nonetheless, high Al concentrations accumulated in roots (1170 μg/g) and shoots (275 μg/g). Aluminium supply enhanced root citrate concentrations but decreased shoot organic acid levels. Aluminium induced high levels of anthraquinone in roots and of catechol, catechin and rutin in shoots. Aluminium resistance in R. acetosa implies both exclusion of Al from root tips and tolerance to high Al tissue concentrations. Citrate in roots and phenolics in shoots may bind Al in non-toxic form. Anthraquinones, as strong antioxidants, may play a role in a general defence response to the root stress.     

Localization and compartmentation of Al in the leaves and roots of tea plants

Under acid soil conditions, solubility of aluminum (Al) increases leading to toxicity for plants. Al accumulator species such as tea, however, accumulate high levels of Al in tissues without toxicity symptoms. In this work, Al localization and compartmentation were studied in tea [Camellia sinensis (L.) O. Kuntze] grown hydroponically at 0 or 100 µM Al for eight weeks. Plant dry matter production was significantly higher in the presence of Al and accumulated up to 1.21 and 6.18 mg Al/g DW in the leaves and roots, respectively. About 40-50% of Al was partitioned into cell wall (CW)-bound fraction without any difference among leaves of different age and roots. A significant increase of the soluble phenolics fraction by Al was observed in both leaves and roots. Conventional and confocal laser scanning microscopy images of morin-stained roots indicated a high fluorescence signal in the caps and adjacent mersitematic cells. Towards basal parts, however, Al tended to accumulate mainly in the root hairs, rhizodermal and endodermal cell layers and slightly in the cortex while it was clearly excluded from the central cylinder. A high Al-morin signal was detected from the CW compared with other parts of the cells. Relatively high green fluorescence signal was emitted from the epidermal cell layer, trichomes, vascular bundle region and stomatal cells of particularly young leaves. Our study provides evidences for involvement of both avoidance and tolerance mechanisms for Al in tea plants.     

Glutathione plays an essential role in nitric oxide‐mediated iron‐deficiency signaling and iron‐deficiency tolerance in Arabidopsis

Iron (Fe) deficiency is a common agricultural problem that affects both the productivity and nutritional quality of plants. Thus, identifying the key factors involved in the tolerance of Fe deficiency is important. In the present study, the zir1 mutant, which is glutathione deficient, was found to be more sensitive to Fe deficiency than the wild type, and grew poorly in alkaline soil. Other glutathione‐deficient mutants also showed various degrees of sensitivity to Fe‐limited conditions. Interestingly, we found that the glutathione level was increased under Fe deficiency in the wild type. By contrast, blocking glutathione biosynthesis led to increased physiological sensitivity to Fe deficiency. On the other hand, overexpressing glutathione enhanced the tolerance to Fe deficiency. Under Fe‐limited conditions, glutathione‐deficient mutants, zir1, pad2 and cad2 accumulated lower levels of Fe than the wild type. The key genes involved in Fe uptake, including IRT1, FRO2 and FIT, are expressed at low levels in zir1; however, a split‐root experiment suggested that the systemic signals that govern the expression of Fe uptake‐related genes are still active in zir1. Furthermore, we found that zir1 had a lower accumulation of nitric oxide (NO) and NO reservoir S‐nitrosoglutathione (GSNO). Although NO is a signaling molecule involved in the induction of Fe uptake‐related genes during Fe deficiency, the NO‐mediated induction of Fe‐uptake genes is dependent on glutathione supply in the zir1 mutant. These results provide direct evidence that glutathione plays an essential role in Fe‐deficiency tolerance and NO‐mediated Fe‐deficiency signaling in Arabidopsis.     

The plant beneficial rhizobacterium Achromobacter sp. 5B1 influences root development through auxin signaling and redistribution

Roots provide physical and nutritional support to plant organs that are above ground and play critical roles for adaptation via intricate movements and growth patterns. Through screening the effects of bacterial isolates from roots of halophyte Mesquite (Prosopis sp.) on Arabidopsis thaliana, we identified Achromobacter sp. 5B1 as a probiotic bacterium that influences plant functional traits. Detailed genetic and architectural analyses in Arabidopsis grown in vitro and in soil, cell division measurements, auxin transport and response gene expression and brefeldin A treatments demonstrated that root colonization with Achromobacter sp. 5B1 changes the growth and branching patterns of roots, which were related to auxin perception and redistribution. Expression analysis of auxin transport and signaling revealed a redistribution of auxin within the primary root tip of wild‐type seedlings by Achromobacter sp. 5B1 that is disrupted by brefeldin A and correlates with repression of auxin transporters PIN1 and PIN7 in root provasculature, and PIN2 in the epidermis and cortex of the root tip, whereas expression of PIN3 was enhanced in the columella. In seedlings harboring AUX1, EIR1, AXR1, ARF7ARF19, TIR1AFB2AFB3 single, double or triple loss‐of‐function mutations, or in a dominant (gain‐of‐function) mutant of SLR1, the bacterium caused primary roots to form supercoils that are devoid of lateral roots. The changes in growth and root architecture elicited by the bacterium helped Arabidopsis seedlings to resist salt stress better. Thus, Achromobacter sp. 5B1 fine tunes both root movements and the auxin response, which may be important for plant growth and environmental adaptation.     

Aluminum uptake and aluminum-induced rapid root growth inhibition of rice seedlings

Aluminum (Al) inhibits root growth in acidic soil, but the site of action of Al remains unclear. We investigated whether the rate of Al accumulation correlates to Al-indeced rapid root growth inhibition in rice seedlings (Oryza sativa L. cv. Youngnam). Growth of roots was significantly inhibited by 100 μM AICI₃, as early as 1 h after the treatment. The inhibition of root growth was strongly dependent on Al concentration (l₅₀ = 20 (μM) and Al-exposure time (l₅₀ = 23 min at 25 μM Al) in a solution of 10 mM KCI and 1 mM CaCl₂ buffered by 10 mM Mes/KOH (pH 4.5). Using ICPES, massive uptake of Al by roots was observed even at 15 min treatment of 25 μM Al. The kinetics of Al uptake by the roots closely corresponded to the inhibitory effects of Al on root growth. When the roots of seedlings were exposed to 50 (μM Al for 1 h, then sectioned and stained with hematoxylin, all cell types of the roots showed the presence of Al in the cytoplasm. These results indicate that Al was rapidly taken up into the root cells and thereby reduced root growth.