Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

Publisher Summary This chapter discusses a search for physiological causes and ecological consequence in reference with the variation in growth rate between higher plants. When grown under optimum conditions, plant species from fertile, productive habitats tend to have inherently higher relative growth rates (RGR) than species from less favorable environments. Under these conditions, fast-growing species produce relatively more leaf area and less root mass, which greatly contributes to their larger carbon gain per unit plant weight. Fast-growing species also have higher respiration rates per unit organ weight, due to demands of a higher RGR and higher rate of nutrient uptake. Fast-growing species have a greater capacity to acquire nutrients, which is likely to be a consequence, rather than the cause, of their higher RGR. This chapter analyses variations in morphological, physiological, chemical, and allocation characteristics underlying variation in RGR, to arrive at an appraisal of its ecological significance. The lower specific leaf area (SLA) of slow-growing species is because of the relatively high concentration of cell-wall material and quantitative secondary compounds, which may protect against detrimental abiotic and biotic factors. This chapter concludes that it is likely that there are trade-offs between growth potential and performance under adverse conditions, however, the current ecophysiological information explaining variation in RGR is too limited to support this contention quantitatively.

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Inherent Variation in Growth Rate Between Higher Plants: A Search for Physiological Causes and Ecological Consequences

This paper discusses whole-plant responses to salinity in order to answer the question of what process limits growth of non-halophytes in saline soils. Leaf growth is more sensitive to salinity than root growth, so we focus on the process or processes that might limit leaf expansion. Effects of short-term exposure (days) are considered separately from long-term exposure (weeks to years). The answer in the short term is probably the water status of the root and we suggest that a message from the root is regulating leaf expansion. The answer to what limits growth in the long term may be the maximum salt concentration tolerated by the fully expanded leaves of the shoot; if the rate of leaf death approaches the rate of new leaf expansion, the photosynthetic area will eventually become too low to support continued growth.

Whole-plant responses to salinity

In this review, recent developments and future prospects of obtaining a better understanding of the regulation of nitrogen use efficiency in the main crop species cultivated in the world are presented. In these crops, an increased knowledge of the regulatory mechanisms controlling plant nitrogen economy is vital for improving nitrogen use efficiency and for reducing excessive input of fertilizers, while maintaining an acceptable yield. Using plants grown under agronomic conditions at low and high nitrogen fertilization regimes, it is now possible to develop whole-plant physiological studies combined with gene, protein, and metabolite profiling to build up a comprehensive picture depicting the different steps of nitrogen uptake, assimilation, and recycling to the final deposition in the seed. A critical overview is provided on how understanding of the physiological and molecular controls of N assimilation under varying environmental conditions in crops has been improved through the use of combined approaches, mainly based on whole-plant physiology, quantitative genetics, and forward and reverse genetics approaches. Current knowledge and prospects for future agronomic development and application for breeding crops adapted to lower fertilizer input are explored, taking into account the world economic and environmental constraints in the next century.

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The challenge of improving nitrogen use efficiency in crop plants: towards a more central role for genetic variability and quantitative genetics within integrated approaches

Most higher plants develop severe toxicity symptoms when grown on ammonium (NH) as the sole nitrogen source. Recently, NH toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying NH toxicity have been extensively sought, the primary events conferring it at the cellular level are not understood. Using a high-precision positron tracing technique, we here present a cell-physiological characterization of NH acquisition in two major cereals, barley (Hordeum vulgare), known to be susceptible to toxicity, and rice (Oryza sativa), known for its exceptional tolerance to even high levels of NH. We show that, at high external NH concentration ([NH]o), barley root cells experience a breakdown in the regulation of NH influx, leading to the accumulation of excessive amounts of NH in the cytosol. Measurements of NH efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for NH, reveal that, at elevated [NH]o, barley cells engage a high-capacity NH-efflux system that supports outward NH fluxes against a sizable gradient. Ammonium efflux is shown to constitute as much as 80% of primary influx, resulting in a never-before-documented futile cycling of nitrogen across the plasma membrane of root cells. This futile cycling carries a high energetic cost (we record a 40% increase in root respiration) that is independent of N metabolism and is accompanied by a decline in growth. In rice, by contrast, a cellular defense strategy has evolved that is characterized by an energetically neutral, near-Nernstian, equilibration of NH at high [NH]o. Thus our study has characterized the primary events in NH nutrition at the cellular level that may constitute the fundamental cause of NH toxicity in plants.

https://www.pnas.org/content/pnas/98/7/4255.full.pdf

Futile transmembrane NH4(+) cycling: a cellular hypothesis to explain ammonium toxicity in plants.

Inorganic nitrogen concentrations in soil solutions vary across several orders of magnitude among different soils and as a result of seasonal changes In order to respond to this heterogeneity, plants have evolved mechanisms to regulate and influx In addition, efflux analysis using (13)N has revealed that there is a co-ordinated regulation of all component fluxes within the root, including biochemical fluxes Physiological studies have demonstrated the presence of two high-affinity transporter systems (HATS) for and one HATS for in roots of higher plants By contrast, in Arabidopsis thaliana there exist seven members of the NRT2 family encoding putative HATS for and five members of the AMT1 family encoding putative HATS for The induction of high-affinity transport and Nrt21 and Nrt22 expression occur in response to the provision of, while down-regulation of these genes appear to be due to the effects of glutamine High-affinity transport and AMT11 expression also appear to be subject to down-regulation by glutamine In addition, there is evidence that accumulated and may act post-transcriptionally on transporter function The present challenge is to resolve the functions of all of these genes In Aspergillus nidulans and Chlamydomonas reinhardtii there are but two high-affinity transporters and these appear to have undergone kinetic differentiation that permits a greater efficiency of absorption over the wide range of concentration normally found in nature Such kinetic differentiation may also have occurred among higher plant transporters The characterization of transporter function in higher plants is currently being inferred from patterns of gene expression in roots and shoots, as well as through studies of heterologous expression systems and knockout mutants

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The regulation of nitrate and ammonium transport systems in plants

(13)NO(3) (-) was used to investigate patterns of NO(3) (-) influx into roots of barley plants (Hordeum vulgare L. cv Klondike) previously grown with (;induced') or without (;uninduced') a source of external NO(3) (-) ([NO(3) (-)](0)). In both induced and uninduced plants, (13)NO(3) (-) influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO(3) (-)](0). In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and V(max) and K(m) values for influx either increased or decreased according to NO(3) (-) pretreatment. By contrast, (13)NO(3) (-) influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system.

/pdf/studies-of-the-uptake-of-nitrate-in-barley-i-kinetics-of-1js3r13brp.pdf

Studies of the uptake of nitrate in barley. I. Kinetics of 13NO3- influx.

Short-term influxes of 13NH4+ were measured in intact roots of 3-week-old rice (Oryza sativa L cv M202) seedlings that were hydroponically grown at 2, 100, or 1000 [mu]M NH4+ Below 1 mM external concentration ([NH4+]0), influx was saturable and due to a high-affinity transport system (HATS) For the HATS, Vmax values were negatively correlated and Km values were positively correlated with NH4+ provision during growth and root [NH4+] Between 1 and 40 mM [NH4+]0, 13NH4+ influx showed a linear response due to a low-affinity transport system (LATS) The 13NH4+ influxes by the HATS, and to a lesser extent the LATS, are energy-dependent processes Selected metabolic inhibitors reduced influx of the HATS by 50 to 80%, but of the LATS by only 31 to 51% Estimated values for Q10 (the ratio of rates at temperatures differing by 10[deg]C) for HATS were greater than 24 at root temperatures from 5 to 10[deg]C and were constant at approximately 15 between 5 and 30[deg]C for the LATS Influx of 13NH4+ by the HATS was insensitive to external pH in the range from 45 to 90, but influx by the LATS declined significantly beyond pH 60 The data presented are discussed in the context of the kinetics, energy dependence, and the regulation of ammonium influx

Ammonium Uptake by Rice Roots (II. Kinetics of 13NH4+ Influx across the Plasmalemma).

Influxes of 13NO3- across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Three kinetically distinct uptake systems for NO3- were identified. In seedlings not previously exposed to external NO3-, a single Michaelis-Menten-type constitutive high-affinity transport system (CHATS) operated in a 2.5 to 500 [mu]M range of external NO3- [NO3-]o. The Vmax of this system was 0.1 [mu]mol g-1 h-1, and the Km was approximately 15 [mu]M. Following exposure to NO3- for 3 d, this CHATS activity was increased approximately 3-fold, with no change of Km. In addition, a NO3--inducible high-affinity system became apparent with a Km of approximately 100[mu]M. The combined Vmax for these discrete saturable components was 0.7 [mu]mol g-1 h-1. In both uninduced and induced plants a linear low-affinity system, additive to CHATS and an NO3--inducible high-affinity system, operated at [NO3-]o [greater than or equal to] 1 mM. The time taken to achieve maximal rates of uptake (full induction) was 2 d from 1.5 mM [NO3-]o and 3 d from 200 [mu]M [NO3-]o.

M. Y. Siddiqi

Papers

Futile transmembrane NH4(+) cycling: a cellular hypothesis to explain ammonium toxicity in plants.

The regulation of nitrate and ammonium transport systems in plants

Studies of the uptake of nitrate in barley. I. Kinetics of 13NO3- influx.

Ammonium Uptake by Rice Roots (II. Kinetics of 13NH4+ Influx across the Plasmalemma).

Kinetics of NO3- Influx in Spruce