María Begoña González-Moro

Zea mays L. amylacea from the Lluta Valley (Arica-Chile) tolerates salinity stress when high levels of boron are available

Elevated levels of boron occurring naturally in soil or irrigation waters are detrimental to many crops grown in agricultural regions of the world. If such levels of boron are accompanied by conditions of excessive salinity, as occurs in the Lluta valley in Northern Chile, the consequences can be drastic for crops. A variety of sweet corn from this valley (Zea mays L. amylacea) has arisen as a consequence of practiced seed selection, suggesting that it is extremely tolerant to high salt and boron levels. In the present study, seeds ofZea mays L. amylacea were collected in order to study their physiological mechanisms of tolerance to high levels of NaCl and boron. Concentrations of 100 and 430 mM NaCl and 20 and 40 mg kg−1 boron were imposed as treatments. The plants did not exhibit symptoms of toxicity to either NaCl and boron during the 20 days of treatment. Na+ accumulation was substantial in roots, while boron was translocated to leaves. Boron alleviated the negative effect of salinity on tissue K+ and maintained membrane integrity. The higher values of water potential seem to be related to the capacity of this ecotype to maintain a better relative water content in leaves. Despite the fact that boron enhanced slightly the effect of salinity on CO2 assimilation, no effect on photochemical parameters was observed in this ecotype. Osmotic adjustment allows this ecotype to survive in high saline soils; however the presence of boron makes this strategy unnecessary since boron contributed to the maintenance of cell wall elasticity.

Depletion of the heaviest stable N isotope is associated with NH4+/NH3 toxicity in NH4+-fed plants

BackgroundIn plants, nitrate (NO3-) nutrition gives rise to a natural N isotopic signature (δ15N), which correlates with the δ15N of the N source. However, little is known about the relationship between the δ15N of the N source and the 14N/15N fractionation in plants under ammonium (NH4+) nutrition. When NH4+ is the major N source, the two forms, NH4+ and NH3, are present in the nutrient solution. There is a 1.025 thermodynamic isotope effect between NH3 (g) and NH4+ (aq) which drives to a different δ15N. Nine plant species with different NH4+-sensitivities were cultured hydroponically with NO3- or NH4+ as the sole N sources, and plant growth and δ15N were determined. Short-term NH4+/NH3 uptake experiments at pH 6.0 and 9.0 (which favours NH3 form) were carried out in order to support and substantiate our hypothesis. N source fractionation throughout the whole plant was interpreted on the basis of the relative transport of NH4+ and NH3.ResultsSeveral NO3--fed plants were consistently enriched in 15N, whereas plants under NH4+ nutrition were depleted of 15N. It was shown that more sensitive plants to NH4+ toxicity were the most depleted in 15N. In parallel, N-deficient pea and spinach plants fed with 15NH4+ showed an increased level of NH3 uptake at alkaline pH that was related to the 15N depletion of the plant. Tolerant to NH4+ pea plants or sensitive spinach plants showed similar trend on 15N depletion while slight differences in the time kinetics were observed during the initial stages. The use of RbNO3 as control discarded that the differences observed arise from pH detrimental effects.ConclusionsThis article proposes that the negative values of δ15N in NH4+-fed plants are originated from NH3 uptake by plants. Moreover, this depletion of the heavier N isotope is proportional to the NH4+/NH3 toxicity in plants species. Therefore, we hypothesise that the low affinity transport system for NH4+ may have two components: one that transports N in the molecular form and is associated with fractionation and another that transports N in the ionic form and is not associated with fractionation.

Exploring ammonium tolerance in a large panel of Arabidopsis thaliana natural accessions

Summary Ammonium nutrition is toxic to many plants. Arabidopsis displays high intraspecific variability in ammonium tolerance (shoot biomass), and ammonium accumulation seems to be an important player in this variability.

Comparative study of the inhibition of photosynthesis caused by aminooxyacetic acid and phosphinothricin in Zea mays

Summary Maize plants fed with PPT (a glutamine synthetase inhibitor) accumulate ammonia. Approximately 50 % of the ammonia accumulated seems to come from the photorespiratory pathway, while the nonphotorespiratory ammonia is derived from nitrate reductase activity. The ammonia accumulated, however, is not sufficient to increase GDH activity. We also carried out a comparative study of the effect of PPT and AOA on photosynthesis. Both compounds inhibit the photorespiratory pathway, causing glycolate accumulation and diminishing photosynthesis.

Root phosphoenolpyruvate carboxylase and NAD-malic enzymes activity increase the ammonium-assimilating capacity in tomato

Plant ammonium tolerance has been associated with the capacity to accumulate large amounts of ammonium in the root vacuoles, to maintain carbohydrate synthesis and especially with the capacity of maintaining high levels of inorganic nitrogen assimilation in the roots. The tricarboxylic acid cycle (TCA) is considered a cornerstone in nitrogen metabolism, since it provides carbon skeletons for nitrogen assimilation. The hypothesis of this work was that the induction of anaplerotic routes of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (NAD-ME) would enhance tolerance to ammonium nutrition. An experiment was established with tomato plants (Agora Hybrid F1) grown under different ammonium concentrations. Growth parameters, metabolite contents and enzymatic activities related to nitrogen and carbon metabolism were determined. Unlike other tomato cultivars, tomato Agora Hybrid F1 proved to be tolerant to ammonium nutrition. Ammonium was assimilated as a biochemical detoxification mechanism, thus leading to the accumulation of Gln and Asn as free amino acids in both leaves and roots as an innocuous and transitory store of nitrogen, in addition to protein synthesis. When the concentration of ammonium in the nutrient solution was high, the cyclic operation of the TCA cycle seemed to be interrupted and would operate in two interconnected branches to provide α-ketoglutarate for ammonium assimilation: one branch supported by malate accumulation and by the induction of anaplerotic PEPC and NAD-ME in roots and MDH in leaves, and the other branch supported by stored citrate in the precedent dark period.

CO2 enrichment modulates ammonium nutrition in tomato adjusting carbon and nitrogen metabolism to stomatal conductance

Ammonium (NH4(+)) toxicity typically occurs in plants exposed to high environmental NH4(+) concentration. NH4(+) assimilating capacity may act as a biochemical mechanism avoiding its toxic accumulation but requires a fine tuning between nitrogen assimilating enzymes and carbon anaplerotic routes. In this work, we hypothesized that extra C supply, exposing tomato plants cv. Agora Hybrid F1 to elevated atmospheric CO2, could improve photosynthetic process and thus ameliorate NH4(+) assimilation and tolerance. Plants were grown under nitrate (NO3(-)) or NH4(+) as N source (5-15mM), under two atmospheric CO2 levels, 400 and 800ppm. Growth and gas exchange parameters, (15)N isotopic signature, C and N metabolites and enzymatic activities were determined. Plants under 7.5mM N equally grew independently of the N source, while higher ammonium supply resulted toxic for growth. However, specific stomatal closure occurred in 7.5mM NH4(+)-fed plants under elevated CO2 improving water use efficiency (WUE) but compromising plant N status. Elevated CO2 annulled the induction of TCA anaplerotic enzymes observed at non-toxic NH4(+) nutrition under ambient CO2. Finally, CO2 enrichment benefited tomato growth under both nutritions, and although it did not alleviate tomato NH4(+) tolerance it did differentially regulate plant metabolism in N-source and -dose dependent manner.

Elevated CO2 Induces Root Defensive Mechanisms in Tomato Plants When Dealing with Ammonium Toxicity

An adequate carbon supply is fundamental for plants to thrive under ammonium stress. In this work, we studied the mechanisms involved in tomato (Solanum lycopersicum L.) response to ammonium toxicity when grown under ambient or elevated CO2 conditions (400 or 800 p.p.m. CO2). Tomato roots were observed to be the primary organ dealing with ammonium nutrition. We therefore analyzed nitrogen (N) and carbon (C) metabolism in the roots, integrating the physiological response with transcriptomic regulation. Elevated levels of CO2 preferentially stimulated root growth despite the high ammonium content. The induction of anaplerotic enzymes from the tricarboxylic acid (TCA) cycle led to enhanced amino acid synthesis under ammonium nutrition. Furthermore, the root transcriptional response to ammonium toxicity was improved by CO2-enriched conditions, leading to higher expression of stress-related genes, as well as enhanced modulation of genes related to signaling, transcription, transport and hormone metabolism. Tomato roots exposed to ammonium stress also showed a defense-like transcriptional response according to the modulation of genes related to detoxification and secondary metabolism, involving principally terpenoid and phenolic compounds. These results indicate that increasing C supply allowed the co-ordinated regulation of root defense mechanisms when dealing with ammonium toxicity.

15N NATURAL ABUNDANCE EVIDENCES A BETTER USE OF N SOURCES BY LATE NITROGEN APPLICATION IN BREAD WHEAT

This work explores whether the natural abundance of N isotopes technique could be used to understand the movement of N within the plant during vegetative and grain filling phases in wheat crop (Triticum aestivum L.) under different fertilizer management strategies. We focus on the effect of splitting the same N dose through a third late amendment at flag leaf stage (GS37) under humid Mediterranean conditions, where high spring precipitations can guarantee the incorporation of the lately applied N to the soil-plant system in an efficient way. The results are discussed in the context of agronomic parameters as N content, grain yield and quality, and show that further splitting the same N dose improves the wheat quality and induces a better nitrogen use efficiency. The nitrogen isotopic natural abundance technique shows that N remobilization is a discriminating process that leads to an impoverishment in 15N of senescent leaves and grain itself. This technique also reflects the more efficient use of N resources (fertilizer and native soil-N) when plants receive a late N amendment.

Interactive effects of excess boron and salinity on response curves of gas exchange to increase in the intensity of light of Zea mays amylacea from the Lluta Valley (Arica-Chile)

espanolLos altos niveles de B (boro) acompanados por condiciones de excesiva salinidad, como ocurre en el valle de Lluta en el norte de Chile; cuyas consecuencias pueden ser drasticas para los cultivos. En el presente estudio, semillas de Zea mays L. amylacea fueron sembradas con el fin de estudiar las curvas de respuesta de intercambio gaseoso con el aumento la intensidad de la luz en condiciones de altos niveles de NaCl y B. Las concentraciones fueron de 100 mM NaCl (baja salinidad) o 430 mM NaCl (alta salinidad), o un exceso de B suministrado como acido borico para obtener concentraciones 20 y 40 mg kg-1 B que se aplico en la solucion nutritiva durante 20 dias. Nuestros resultados complementan los estudios anteriores del ecotipo amylacea y confirman el alto grado de tolerancia a la salinidad y al exceso de B. Una alta intensidad luminica intensifica los parametros de intercambio gaseoso como tasa fotosintetica, tasa de transpiracion y la conductancia estomatica que aumenta en forma gradual. La concentracion de CO2 intercelular y la eficiencia del uso del agua (EUA) no mostraron diferencias entre los tratamientos, excepto a alta salinidad. Las plantas que crecen en condiciones de alta salinidad, independiente de la presencia de B, mostraron un alto requerimiento cuantico a altas intensidad de luz. EnglishHigh levels of B (boron) are accompanied by conditions of excessive salinity, as occurs in the Lluta Valley in northern Chile; the consequences can be drastic for crops. In the present study, seeds of Zea mays L. amylacea were grown in order to study the response curves of gas exchange to increase in the intensity of light at high levels of NaCl and B. Concentrations of 100 mM NaCl (low salinity) or 430 mM NaCl (high salinity), or an excess of B supplied as boric acid to obtain 20 and 40 mg kg-1 B were applied in the nutrient solution for 20 days. Our results complement other studies with the amylacea ecotype and confirm the high degree of tolerance to salinity and excess boron. Higher light intensified the gas exchange parameters photosynthetic rate, transpiration rate and CO2 stomatal conductance, which gradually increased. Intercellular CO2 concentration and water-use efficiency (WUE) showed no differences between treatments, except for high leaf CO2 at high salinity. The plants grown under high salt, independent of the presence of B, showed a high quantum requirement at higher light intensities.

Combined effects of excess boron and salinity on root histology of Zea mays L. amylacea from the Lluta Valley (Arica, Chile)

espanolLa estructura celular y las alteraciones en la organizacion del tejido de raiz se analizaron en Zea mays L. amylacea como consecuencia de altos niveles de boro (B) y de salinidad. Concentraciones de los tratamientos de salinidad fueron 100 mM NaCl (baja salinidad, L) y 430 mM NaCl (alta salinidad, H). El exceso de B se suministro como acido borico para obtener 20 (334 µM) y 40 (668 µM) B mg kg-1 en la solucion de nutrientes durante 20 dias. Nuestros resultados complementan otros estudios sobre el ecotipo amylacea y confirman el alto grado de tolerancia a la salinidad y el exceso de B mostrada por esta variedad. La aplicacion de B bajo condiciones sin sal y baja salinidad no dio lugar a cambios en la estructura de las celulas de la corteza de la raiz ni el cilindro vascular. Bajo condiciones de alta salinidad celulas de la raiz amylacea mostraron alteraciones leves, como un aumento en el numero de filas de celulas. Estas condiciones de alta salinidad no resultaron en el espesor de la estela. EnglishCell structure and alterations in tissue organization were analyzed for roots of Zea mays L. amylacea as a consequence of high salinity and boron (B) levels. Saline treatment concentrations were 100 mM NaCl (Low salinity, L) and 430 mM NaCl (High salinity, H). An excess of B was supplied as boric acid to obtain 20 (334 µM) and 40 (668 µM) mg B kg-1 in the nutrient solution for 20 days. Our results complement other studies on the amylacea ecotype and confirm the high degree of tolerance to salinity and excess B shown by this variety. The application of B under no salt and low salinity conditions did not result in structure changes in root cortex cells nor the vascular cylinder. Under high salinity conditions amylacea root cells showed slight alterations, such as an increase in the number of rows of cells. These high salinity conditions did not result in thickness of the stele.