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Root growth in higher plants is sensitive to excess ammonium (NH4+). Our study shows that contact of NH4+ with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH4+ nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH4+ inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH4+ fluxes along the primary root, measured using the Scanning Ion-selective Electrode Technique, revealed a significant stimulation of NH4+ efflux at the elongation zone following treatment with elevated NH4+, coincident with the inhibition of root elongation. Stimulation of NH4+ efflux and inhibition of cell expansion were significantly more pronounced in the NH4+-hypersensitive mutant vtc1-1, deficient in the enzyme GDP-mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH4+ fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH4+ toxicity response in Arabidopsis. [ABSTRACT FROM AUTHOR]

Demand for low-input nitrogen sustainable rice is increasing to meet the need for environmentally friendly agriculture and thus development of rice with high nitrogen use efficiency (NUE) is a major objective. Hence, understanding how rice responds to growth under low-nitrogen conditions is essential to devise new ways of manipulating genes to improve rice NUE. In this study, using two rice varieties with different seedling-stage NUE obtained from previous field experiments, we investigated the physiological and molecular responses of young rice to low-nitrogen conditions. Our results suggest that glutamine synthetase (GS) and NADH-dependent glutamate synthase (NADH-GOGAT) play important roles in N assimilation of seedling rice roots under low-nitrogen conditions; the regulatory mechanisms of GS and NADH-GOGAT in seedling rice roots do not occur at the transcription level, and may be posttranscriptional; OsAMT1;1 play important roles in rice N acquisition by partially regulating N uptake under low-nitrogen conditions; and OsAMT1;1 and OsNRT2;1 also play important roles in rice N acquisition by partially regulating root growth and development under low-nitrogen conditions. The challenge for future studies is to characterize the functional roles of GS, NADH-GOGAT, OsAMT1;1, and OsNRT2;1 in young rice NUE using RNAi and mutant techniques. [ABSTRACT FROM AUTHOR]

Abstract  Vegetable cultivation during winter season is economically profitable, but the impact of the intensive production on soil and water quality remains to be studied. The objectives of this study were to investigate the seasonal dynamics of soil nutrients, acidification and salt accumulation in vegetable fields in South-Eastern China. Various vegetables were grown either under open-field conditions or under two different alternating open-field and greenhouse conditions with three replications. Soil samples were collected periodically and analyzed for pH, plant available nitrogen (N), phosphorous (P), potassium (K), electrical conductivity (EC), and urease activity. Water samples from wells located in or near the plots were collected and analyzed for nitrate. Soil nitrate, available phosphate and salt concentrations declined in summer under open-field conditions and significantly increased from December to May under greenhouse conditions. Exchangeable K also decreased in summer season, but did not increase in the spring. Under alternating open-field and greenhouse conditions, nutrient accumulation, soil salinity and acidification were significantly higher for soil used for vegetable cultivation for 2 years (2-y-plot) than that for only half year (0.5-y-plot). The accumulation of nitrate significantly correlated with soil EC and soil acidification. Thirty-two percent of groundwater samples from the 2-y-plot showed a nitrate concentration higher than 50 mg NO3 l−1. Conversely, no groundwater sample of 0.5-y-plot showed such high nitrate concentration. It can be concluded that the nitrate accumulation in soil used for vegetable cultivation under alternating open-field and greenhouse conditions not only causes soil salinization and soil acidification but also presents a high pollution potential for groundwater. [ABSTRACT FROM AUTHOR]

Soil salinity is a major abiotic stress in reducing plant growth and agricultural productivity. In higher plants, 14-3-3 proteins are phosphoserine-binding proteins that regulate the activity of a wide array of targets via protein–protein interaction and play a key role in abiotic stress. We report here that CaMV 35S promoter driven overexpression of the TFT7 gene, one of tomato 14-3-3 proteins, improves plant salt tolerance in Arabidopsis thaliana. In response to salt stress, transgenic plants overexpressing TFT7 improved germination rate, dry mass, total chlorophyll concentration and root length. In contrast, the degree of hydrogen peroxide and malondialdehyde accumulation were more in wild-type plants than in transgenic plants. Therefore, the mechanisms of salt tolerance in transgenic plants can be explained by reduction of salt-induced oxidative stress damage. In addition, under normal growth condition or salt stress, the higher total ascorbate peroxidase (APX) activity in transgenic plants than in the wild-type plants indicates that TFT7 upregulates the activity of APX which plays the indispensable role in salt-induced oxidative stress. These results suggest that in higher plants, TFT7 plays an important role in salt tolerance and may be a candidate gene for developing salt-tolerant crop plants. [ABSTRACT FROM AUTHOR]

A hydroponics experiment was conducted to investigate the effects of iron plaque on root surfaces with respect to selenite uptake and translocation within the seedlings of two cultivars of rice ( Oryza sativa L. cv Xiushui48 and Bing9652). Different amounts of iron plaque were formed by adding 0, 10, 30, 50, 70 mg Fe l−1 in the nutrient solution. After 24 h of growth, the amount of iron plaque was positively correlated with the Fe2+ addition to the nutrient solution. These concentrations of Fe, inducing plaque, had no significant effect on the shoot and root growth of rice plants in 50 µg Se l−1 nutrient solution. The amount of Se accumulated in iron plaque was positively correlated to the amount of iron plaque. Increasing iron plaque decreased the selenium concentration in shoots and in roots. At the same time, the translocation of Se from roots to shoots was reduced with increasing amounts of iron plaque. At both the shorter and longer exposure times, the ratio of root- to-shoot selenium was higher than in the controls. More Se stayed in the roots at the longer exposure time than at the shorter time. The concentration of selenium in the xylem sap was sharply decreased with increasing amount of iron plaque on the rice roots. The DCB (dithionite-citrate-bicarbonate)-extracted Se was up to 89.9–91.1% of the total Se when roots with iron plaque (Fe 70) were incubated in 50 µg Se l−1 solution for 30 min. This DCB-extracted Se, however, accounted for only 21.9–28.7% of total Se when roots with iron plaque were incubated in the same solution for 3 days. Se adsorbed in iron plaque can be desorbed by low-molecular-weight organic acids, similar to the desorption of Se from ferrihydrite. These results suggest that iron plaque might act as a ‘buffer’ for Se in the rhizosphere. [ABSTRACT FROM AUTHOR]

• Background and Aims Mineral nutrient deficiencies and salinity constitute major limitations for crop plant growth on agricultural soils. 14-3-3 proteins are phosphoserine-binding proteins that regulate the activities of a wide array of targets via direct protein–protein interactions and may play an important role in responses to mineral nutrients deficiencies and salt stress. In the present study, the expression profiling of the 14-3-3 gene family in response to salt stress and potassium and iron deficiencies in young tomato (Solanum lycopersicum) roots was investigated in order to analyse the 14-3-3 roles of the proteins in these abiotic stresses.• Methods Sequence identities and phylogenetic tree creation were performed using DNAMAN version 4.0 (Lynnon Biosoft Company). Real-time RT–PCR was used to examine the expression of each 14-3-3 gene in response to salt stress and potassium and iron deficiencies in young tomato roots.• Key Results The phylogenetic tree shows that the 14-3-3 gene family falls into two major groups in tomato plants. By using real-time RT–PCR, it was found that (a) under normal growth conditions, there were significant differences in the mRNA levels of 14-3-3 gene family members in young tomato roots and (b) 14-3-3 proteins exhibited diverse patterns of gene expression in response to salt stress and potassium and iron deficiencies in tomato roots.• Conclusions The results suggest that (a) 14-3-3 proteins may be involved in the salt stress and potassium and iron deficiency signalling pathways in young tomato roots, (b) the expression pattern of 14-3-3 gene family members in tomato roots is not strictly related to the position of the corresponding proteins within a phylogenetic tree, (c) gene-specific expression patterns indicate that isoform-specificity may exist in the 14-3-3 gene family of tomato roots, and (d) 14-3-3 proteins (TFT7) might mediate cross-talk between the salt stress and potassium and iron-deficiency signalling pathways in tomato roots. [ABSTRACT FROM AUTHOR]