Guangjie Li

Ammonium stress in Arabidopsis: signaling, genetic loci, and physiological targets.

Ammonium (NH4(+)) toxicity is a significant ecological and agricultural issue, and an important phenomenon in cell biology. As a result of increasing soil nitrogen input and atmospheric deposition, plants have to deal with unprecedented NH4(+) stress from sources below and above ground. In this review, we describe recent advances in elucidating the signaling pathways and identifying the main physiological targets and genetic loci involved in the effects of NH4(+) stress in the roots and shoots of Arabidopsis thaliana. We outline new experimental approaches that are being used to study NH4(+) toxicity in Arabidopsis and propose an integrated view of behavior and signaling in response to NH4(+) stress in the Arabidopsis system.

Expression of a heterologous SnRK1 in tomato increases carbon assimilation, nitrogen uptake and modifies fruit development

SnRK1 (sucrose non-fermenting-1-related protein kinase 1) plays an important role in plant carbon metabolism and development. To understand the mechanism of carbon and nitrogen metabolism regulated by MhSnRK1 from pingyitiancha (Malus hupehensis Rehd. var. pinyiensis Jiang), two transgenic lines (T2-7 and T2-9) over expressing this gene in tomato were studied. SnRK1 activity in the leaves of 2 transgenic lines was increased by 15-16% compared with that in the wild-type. The leaf photosynthetic rate in transgenic tomatoes was higher than the wild-type. The activity of sucrose synthase breakdown and ADP-glucose pyrophosphorylase was also increased, by approximately 25-36% and 44-48%, respectively, whereas sucrose synthase synthesis and sucrose phosphate synthase activities were unchanged. The content of starch in the leaves and red-ripening fruits was higher than that of the wild-type. The transgenic fruit ripened ∼10 days earlier than the wild-type. The nitrate reductase activity (mgplant⁻¹ h⁻¹) shows no significant difference between the transgenic plant and the wild-type, but the N-uptake efficiency and root/shoot ratio in the T2-9 line were 15% and 35% higher than that in the wild-type, respectively. These results suggest that over expressing MhSnRK1 can increase both the carbon and nitrogen assimilation rate of the plant as well as regulate the development of fruit.

Ethylene is critical to the maintenance of primary root growth and Fe homeostasis under Fe stress in Arabidopsis

Highlight We propose that ethylene is involved in the regulation of excess Fe tolerance by maintenance of tissue Fe and K homeostasis in Arabidopsis.

Ammonium-induced shoot ethylene production is associated with the inhibition of lateral root formation in Arabidopsis

Foliar NH4(+) exposure is linked to inhibition of lateral root (LR) formation. Here, the role of shoot ethylene in NH4(+)-induced inhibition of LR formation in Arabidopsis was investigated using wild-type and mutant lines that show either blocked ethylene signalling (etr1) or enhanced ethylene synthesis (eto1, xbat32). NH4(+) exposure of wild-type Arabidopsis led to pronounced inhibition of LR production chiefly in the distal root, and triggered ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in the shoot. It is shown that shoot contact with NH4(+) is necessary to stimulate shoot ethylene evolution. The ethylene antagonists Ag(+) and aminoethoxyvinylglycine (AVG) mitigated LR inhibition under NH4(+) treatment. The decrease in LR production was significantly greater for eto1-1 and xbat32 and significantly less for etr1-3. Enhanced shoot ethylene synthesis/signalling blocked recovery of LR production when auxin was applied in the presence of NH4(+) and negatively impacted shoot AUX1 expression. The findings highlight the important role of shoot ethylene evolution in NH4(+)-mediated inhibition of LR formation.

Isolation and characterization of a novel ammonium overly sensitive mutant, amos2, in Arabidopsis thaliana

Ammonium (NH4+) toxicity is a significant agricultural problem globally, compromising crop growth and productivity in many areas. However, the molecular mechanisms of NH4+ toxicity are still poorly understood, in part due to a lack of valuable genetic resources. Here, a novel Arabidopsis mutant, amos2 (ammonium overly sensitive 2), displaying hypersensitivity to NH4+ in both shoots and roots, was isolated. The mutant exhibits the hallmarks of NH4+ toxicity at significantly elevated levels: severely suppressed shoot biomass, increased leaf chlorosis, and inhibition of lateral root formation. Amos2 hypersensitivity is associated with excessive NH4+ accumulation in shoots and a reduction in tissue potassium (K+), calcium (Ca2+), and magnesium (Mg2+). We show that the lesion is specific to the NH4+ ion, is independent of NH4+ metabolism, and can be partially rescued by elevated external K+. The amos2 lesion was mapped to a 16-cM interval on top of chromosome 1, where no similar mutation has been previously mapped. Our study identifies a novel locus controlling cation homeostasis under NH4+ stress and provides a tool for the future identification of critical genes involved in the development of NH4+ toxicity.

Auxin Resistant1 and PIN-FORMED2 Protect Lateral Root Formation in Arabidopsis under Iron Stress

Iron stress modulates auxin and ethylene pathways to regulate lateral root formation in Arabidopsis. A stunted root system is a significant symptom of iron (Fe) toxicity, yet little is known about the effects of excess Fe on lateral root (LR) development. In this work, we show that excess Fe has different effects on LR development in different portions of the Arabidopsis (Arabidopsis thaliana) root system and that inhibitory effects on the LR initiation are only seen in roots newly formed during excess Fe exposure. We show that root tip contact with Fe is both necessary and sufficient for LR inhibition and that the auxin, but not abscisic acid, pathway is engaged centrally in the initial stages of excess Fe exposure. Furthermore, Fe stress significantly reduced PIN-FORMED2 (PIN2)-green fluorescent protein (GFP) expression in root tips, and pin2-1 mutants exhibited significantly fewer LR initiation events under excess Fe than the wild type. Exogenous application of both Fe and glutathione together increased PIN2-GFP expression and the number of LR initiation events compared with Fe treatment alone. The ethylene inhibitor aminoethoxyvinyl-glycine intensified Fe-dependent inhibition of LR formation in the wild type, and this inhibition was significantly reduced in the ethylene overproduction mutant ethylene overproducer1-1. We show that Auxin Resistant1 (AUX1) is a critical component in the mediation of endogenous ethylene effects on LR formation under excess Fe stress. Our findings demonstrate the relationship between excess Fe-dependent PIN2 expression and LR formation and the potential role of AUX1 in ethylene-mediated LR tolerance and suggest that AUX1 and PIN2 protect LR formation in Arabidopsis during the early stages of Fe stress.

The Response of the Root Apex in Plant Adaptation to Iron Heterogeneity in Soil

Iron (Fe) is an essential micronutrient for plant growth and development, and is frequently limiting. By contrast, over-accumulation of Fe in plant tissues leads to toxicity. In soils, the distribution of Fe is highly heterogeneous. To cope with this heterogeneity, plant roots engage an array of adaptive responses to adjust their morphology and physiology. In this article, we review root morphological and physiological changes in response to low- and high-Fe conditions and highlight differences between these responses. We especially focus on the role of the root apex in dealing with the stresses resulting from Fe shortage and excess.

Root developmental adaptation to Fe toxicity: Mechanisms and management

ABSTRACT Iron (Fe) is an essential microelement but is highly toxic when in excess. To cope with Fe excess, plants have evolved complex adaptive responses that include morphological and physiological modifications. The highly dynamic adjustments in overall root system architecture (RSA) determine root plasticity and allow plants to efficiently adapt to environmental constraints. However, the effects of Fe excess on RSA are poorly understood. Recently, we showed that excess Fe treatment in Arabidopsis not only directly impairs primary root (PR) growth but also arrests lateral root (LR) formation by acting at the tip of the growing primary root. Such a change is believed to help RSA adjust and restrict excessive Fe absorption in the part of the rhizosphere subject to acute toxicity while maintaining the absorption of other nutrients in the less stressed components of the root system. We further showed that the suppression of PR growth and LR formation under excess Fe is alleviated by K+ addition, providing useful insight into the effectiveness of nutrient management to improve RSA and alleviate Fe toxicity symptoms in the field.

Spatio-temporal dynamics in global rice gene expression (Oryza sativa L.) in response to high ammonium stress

Ammonium (NH4+) is the predominant nitrogen (N) source in many natural and agricultural ecosystems, including flooded rice fields. While rice is known as an NH4+-tolerant species, it nevertheless suffers NH4+ toxicity at elevated soil concentrations. NH4+ excess rapidly leads to the disturbance of various physiological processes that ultimately inhibit shoot and root growth. However, the global transcriptomic response to NH4+ stress in rice has not been examined. In this study, we mapped the spatio-temporal specificity of gene expression profiles in rice under excess NH4+ and the changes in gene expression in root and shoot at various time points by RNA-Seq (Quantification) using Illumina HiSeqTM 2000. By comparative analysis, 307 and 675 genes were found to be up-regulated after 4h and 12h of NH4+ exposure in the root, respectively. In the shoot, 167 genes were up-regulated at 4h, compared with 320 at 12h. According to KEGG analysis, up-regulated DEGs mainly participate in phenylpropanoid (such as flavonoid) and amino acid (such as proline, cysteine, and methionine) metabolism, which is believed to improve NH4+ stress tolerance through adjustment of energy metabolism in the shoot, while defense and signaling pathways, guiding whole-plant acclimation, play the leading role in the root. We furthermore critically assessed the roles of key phytohormones, and found abscisic acid (ABA) and ethylene (ET) to be the major regulatory molecules responding to excess NH4+ and activating the MAPK (mitogen-activated protein kinase) signal-transduction pathway. Moreover, we found up-regulated hormone-associated genes are involved in regulating flavonoid biosynthesis and are regulated by tissue flavonoid accumulation.

Involvement of auxin in the regulation of ammonium tolerance in rice (Oryza sativa L.)

Background and aimsAmmonium (NH4+) is an important nitrogen source and is widely used as a fertilizer in agricultural systems. However, excess NH4+ inhibits root growth, and, subsequently, vegetative shoot growth and yield. This study examines whether auxin is involved in differential NH4+ tolerance in rice (Oryza sativa L.), and how auxin is regulated under high-NH4+ conditions in rice.MethodsAn NH4+-sensitive (Kasalath, Kas) and an NH4+-insensitive (Koshihikari, Kos) rive cultivar were cultured hydroponically with or without exogenous indole-3-acetic acid (IAA) and auxin biosynthesis inhibitors. Root growth, root area, tissue IAA content, and transcription of genes involved in auxin biosynthesis, conjugation and degradation were determined.ResultspDR5::GUS staining and auxin measurement show that high NH4+ can decrease free IAA content in roots. In addition, quantitative RT-PCR, pharmacology, and genetics analysis suggest that Kos possesses a higher capacity for auxin biosynthesis and a weaker capacity for auxin metabolism compared to Kas under high-NH4+ stress.ConclusionWe conclude that the NH4+-tolerant cultivar possesses a higher capacity to maintain auxin homeostasis under high-NH4+ stress, and that this advantage is incurred by promotion of auxin biosynthesis and a suppression of auxin metabolism.