Phloem Amino Acid Research Articles

Arsenophonus Interacts with Buchnera to Improve Growth Performance of Aphids under Amino Acid Stress.

Amino acids play a crucial role in the growth and development of insects. Aphids cannot ingest enough amino acids in plant phloem to meet their requirements, and therefore, they are mainly dependent on the obligate symbiont Buchnera aphidicola to synthesize essential amino acids. Besides Buchnera, aphids may harbor another facultative symbiont, Arsenophonus, which alters the requirement of the cotton-melon aphid Aphis gossypii for amino acid. However, it is unclear how Arsenophonus regulates the requirement. Here, we found that Arsenophonus ameliorated growth performance of A. gossypii on an amino acid-deficient diet. A deficiency in lysine (Lys) or methionine (Met) led to changes in the abundance of Arsenophonus. Arsenophonus suppressed the abundance of Buchnera when aphids were fed a normal amino acid diet, but this suppression was eliminated or reversed when aphids were on a Lys- or Met-deficient diet. The relative abundance of Arsenophonus was positively correlated with that of Buchnera, but neither of them was correlated with the body weight of aphids. The relative expression levels of Lys and Met synthase genes of Buchnera were affected by the interaction between Arsenophonus infections and Buchnera abundance, especially in aphids reared on a Lys- or Met-deficient diet. Arsenophonus coexisted with Buchnera in bacteriocytes, which strengthens the interaction. IMPORTANCE The obligate symbiont Buchnera can synthesize amino acids for aphids. In this study, we found that a facultative symbiont, Arsenophonus, can help improve aphids' growth performance under amino acid deficiency stress by changing the relative abundance of Buchnera and the expression levels of amino acid synthase genes. This study highlights the interaction between Arsenophonus and Buchnera to ameliorate aphid growth under amino acid stress.

Nitrogen application at anthesis increases barley grain protein by enhancing phloem amino acid mobilisation

Context In the malting industry, the low protein content of barley grains is a major issue. A useful strategy to overcome this problem is to complement the initial fertilisation with a foliar application of nitrogen (N) near anthesis. Aims This study aimed to advance knowledge of the metabolic adjustments displayed by plants grown under common management practices in response to foliar N fertilisation at anthesis. Methods Field experiments with two different rates of foliar N application near anthesis were performed at three commercial sites under the common practice of each farmer. Key results Grain protein content increased upon foliar fertilisation in all sites without affecting leaf senescence or N remobilisation efficiency. Barley plants also showed a rapid assimilation of the N applied at anthesis, increasing the leaf N organic pools and the global phloem amino acid mobilisation during grain filling. These results could be attributed to the increase in the gene expression of glutamine synthetase 1 and two amino acid transporters (HvAAP6 and HvAAP7). Conclusions Foliar N applied was rapidly assimilated and exported with high efficiency to the grain with no negative impact over the contribution of N from vegetative organs. Furthermore, phloem N contribution was shown to be of major importance for grain protein content. Implications Foliar fertilisation near anthesis is an efficient strategy to correct grain protein content to meet maltsters’ requirements because it did not affect pre-assimilated N remobilisation, and was useful under the different nutrient availabilities explored here (N sufficiency, N deficiency and sulfur deficiency).

Why is phloem sap nitrate kept low?

Since the first description of phloem sap composition nearly 60 years ago, it is generally assumed that phloem sap does not contain nitrate and that there is little or no backflow of nitrate from shoots to roots. While it is true that nitrate can occasionally be absent from phloem sap, there is now substantial evidence that phloem can carry nitrate and furthermore, transporters involved in nitrate redistribution to shoot sink organs and roots have been found. This raises the question of why nitrate may or may not be present in phloem sap, why its concentration is generally kept low, and whether plant shoot-root nutrient cycling also involves nitrate. We propose here that phloem sap nitrate is not only an essential component of plant nutritional signaling but also contributes to physical properties of phloem sap and as such, its concentration must be controlled to ensure proper coordination of plant development and nutrient transport. This article is protected by copyright. All rights reserved.

Regulation of senescence-associated protease genes by sulphur availability according to barley (Hordeum vulgare L.) phenological stage.

Proteases are responsible for protein degradation during leaf senescence, allowing nutrients to be redirected to sink tissues. In a previous work, we reported that sulphur deficiency produced a delay in the leaf senescence of barley (Hordeum vulgare L.) plants, at both vegetative and reproductive stages. In this work, we analyse the effect of sulphur deficiency on the expression of several genes coding for proteases of different catalytic groups, which have been strongly associated with leaf senescence. Four independent experiments were performed in order to impose low sulphur availability conditions: one of steady-state sulphur deficiency during the vegetative stage and three of sulphur starvation during vegetative and reproductive stages. Sulphur deficiency inhibited or reduced the senescence-associated induction of seven of the eight proteases analysed. Their induction, as well as senescence and phloem amino acid remobilization, could be achieved with senescence inducers such as methyl-jasmonate (a hormonal stimulus) and darkness, but with different rates of induction dependent on each gene. Sulphur deficiency also exerted an opposite effect on the expression of two cysteine-protease genes (HvSAG12 and HvLEGU) as well as on one serine-protease gene (HvSUBT) according to leaf age and plant phenological stages. All three genes were induced in green leaves but were repressed in senescent leaves of sulphur-deficient plants at the vegetative stage. At the reproductive stage, both cysteine-proteases were only repressed in senescent leaves, while the serine-protease was induced in green and senescent leaves by sulphur deficiency. Our results highlight the relevance of adequate sulphur nutrition in order to ensure leaf senescence onset and induction of protease genes, which will consequently impact on grain protein composition and quality. In addition, our results provide evidence that leaf age, plant developmental stage and the nature of the stress modulate the sulphur responses.

Fatal attraction of non‐vector impairs fitness of manipulating plant virus

Abstract Host manipulation refers to the expression of a host phenotype that is partly under the genetic control of a parasite. This phenomenon can enhance parasite transmission rates and is responsible for biological marvels such as “Zombie‐ants” and the “fatal attraction” of Toxoplasma‐infected rodents to their feline predators. Such host manipulation has evolved in all major phylogenetic lineages of parasites and is assumed to enhance the fitness of the parasite. However, the capacity to manipulate is not ubiquitous; that is, many clades of parasites comprise manipulating and non‐manipulating species. This pattern leads to the prediction of costs that select against the evolution of manipulation, but this has been difficult to show empirically. In the present study, we used a tripartitate system consisting of chili (Capsicum annuum) plants infected with Pepper golden mosaic virus and colonized by non‐vector whiteflies (Trialeurodes vaporariorum), to study the effects on viral load when a non‐vector herbivore feeds on the infected plants. We observed that virus‐infected plants emitted odours that attracted adult whiteflies, contained three times more amino acids in the phloem than mock‐inoculated controls and supported higher whitefly reproduction as compared to controls. However, viral load decreased almost 100‐fold in whitefly‐carrying plants, which was associated with a depletion of phloem amino acids. Synthesis. We show that a plant virus can suffer from a reduced within‐host reproduction rate when virus‐induced alterations of the plant cause a “fatal attraction” of a non‐vector insect that exploits the altered plant phenotype at its own benefits. The resulting fitness costs might represent a force that can select against the evolution of host manipulation by parasites.

Sulphur deficiency inhibits nitrogen assimilation and recycling in barley plants

Sulphur (S) is incorporated into diverse primary and secondary metabolites that play important roles in proper growth and development of plants. In cereals, a fraction of the nitrogen (N) accumulated in developing grains is guaranteed by amino acid remobilization from vegetative tissues, a contribution that becomes critical when soil nutrients are deficient. Glutamine synthetase (GS) and amino acid transporters (AAT) are key components involved in N assimilation and recycling. The aim of the present study was to evaluate the effect of S availability on the expressions of HvGS and several selected HvAAT genes in barley plants and on the phloem exudation rate of amino acids. To this end, two independent experiments were designed to impose low S availability conditions to barley plants. Low S availability caused a decrease in the phloem exudation rate of amino acids as well as in the gene expression of all the HvGS genes and five of the six HvAAT genes analyzed. The strong correlation found between the phloem amino acid exudation rate and HvGS1-1, HvGS1-2, HvAAP7, and HvProT1 gene expression may indicate the participation of these genes in the regulation of amino acid remobilization through the phloem.

Colonization by Phloem-Feeding Herbivore Overrides Effects of Plant Virus on Amino Acid Composition in Phloem of Chili Plants.

The 'adaptive host manipulation' hypothesis predicts that parasites can enhance their transmission rates via manipulation of their host's phenotype. For example, many plant pathogens alter the nutritional quality of their host for herbivores that serve as their vectors. However, herbivores, including non-vectors, might cause additional alterations in the plant phenotype. Here, we studied changes in the amino acid (AA) content in the phloem of chilli (Capsicum annuum) plants infected with Pepper golden mosaic virus (PepGMV) upon subsequent colonization with a non-vector, the phloem-feeding whitefly (Trialeurodes vaporariorum). Virus infection alone caused an almost 30-fold increase in overall phloem AAs, but colonization by T. vaporariorum completely reversed this effect. At the level of individual AAs, contents of proline, tyrosine, and valine increased, and histidine and alanine decreased in PepGMV -infected as compared to control plants, whereas colonization by T. vaporariorum caused decreased contents of proline, tyrosine, and valine, and increased contents of histidine and alanine. Overall, the colonization by the whitefly had much stronger effects on phloem AA composition than virus infection. We conclude that the phloem composition of a virus-infected host plant can rapidly change upon arrival of an herbivore and that these changes need to be monitored to predict the nutritional quality of the plant in the long run.

Water Stress Modulates Soybean Aphid Performance, Feeding Behavior, and Virus Transmission in Soybean.

Little is known about how water stress including drought and flooding modifies the ability of plants to resist simultaneous attack by insect feeding and transmission of insect-vectored pathogen. We analyzed insect population growth, feeding behaviors, virus transmission, and plant amino acid profiles and defense gene expression to characterize mechanisms underlying the interaction between water stress, soybean aphid and aphid-transmitted, Soybean mosaic virus, on soybean plants. Population growth of non-viruliferous aphids was reduced under drought stress and saturation, likely because the aphids spent less time feeding from the sieve element on these plants compared to well-watered plants. Water stress did not impact population growth of viruliferous aphids. However, virus incidence and transmission rate was lowest under drought stress and highest under saturated conditions since viruliferous aphids took the greatest amount time to puncture cells and transmit the virus under saturated conditions and lowest time under drought stress. Petiole exudates from drought-stressed plants had the highest level of total free amino acids including asparagine and valine that are critical for aphid performance. Aphids did not benefit from improved phloem sap quality as indicated by their lower densities on drought-stressed plants. Saturation, on the other hand, resulted in low amino acid content compared to all of the other treatments. Drought and saturation had significant and opposing effects on expression of marker genes involved in abscisic acid (ABA) signaling. Drought alone significantly increased expression of ABA marker genes, which likely led to suppression of salicylic acid (SA)- and jasmonic acid (JA)-related genes. In contrast, ABA marker genes were down-regulated under saturation, while expression of SA- and JA-related genes was up-regulated. We propose that the apparent antagonism between ABA and SA/JA signaling pathways contributed to an increase in aphid densities under drought and their decrease under saturation. Taken together, our findings suggests that plant responses to water stress is complex involving changes in phloem amino acid composition and signaling pathways, which can impact aphid populations and virus transmission.

The amino acid distribution in rachis xylem sap and phloem exudate of Vitis vinifera ‘Cabernet Sauvignon’ bunches

Amino acids are essential to grape berry and seed development and they are transferred to the reproductive structures through the phloem and xylem from various locations within the plant. The diurnal and seasonal dynamics of xylem and phloem amino acid composition in the leaf petiole and bunch rachis of field-grown Cabernet Sauvignon are described to better understand the critical periods for amino acid import into the berry. Xylem sap was extracted by the centrifugation of excised leaf petioles and rachises, while phloem exudate was collected by immersing these structures in an ethylenediaminetetraacetic acid (EDTA) buffer. Glutamine and glutamic acid were the predominant amino acids in the xylem sap of both grapevine rachises and petioles, while arginine and glycine were the principal amino acids of the phloem exudate. The amino acid concentrations within the xylem sap and phloem exudate derived from these structures were greatest during anthesis and fruit set, and a second peak occurred within the rachis phloem at the onset of ripening. The concentrations of the amino acids within the phloem and xylem sap of the rachis were highest just prior to or after midnight while the flow of sugar through the rachis phloem was greatest during the early afternoon. Sugar exudation rates from the rachis was greater than that of the petiole phloem between anthesis and berry maturity. In summary, amino acid and sugar delivery through the vasculature to grape berries fluctuates over the course of the day as well as through the season and is not necessarily related to levels near the source.

Towards an understanding of how phloem amino acid composition shapes elevated CO2‐induced changes in aphid population dynamics

1. The performance of foliage feeders tends to decrease under elevated CO2, but the responses of phloem‐feeding insects have been much more equivocal. As phloem tissues are less accessible than whole‐plant tissues, much less is known about how phloem composition is altered under elevated CO2 and the mechanisms driving changes in aphid performance.2. In this study, the plant mechanisms underlying the performance of Rhopalosiphum padi aphids on Hordeum vulgare (barley) grown under ambient (390 ppm) and elevated (700 ppm) CO2 were examined. We used aphid stylectomy to sample pure phloem from plants in CO2‐controlled conditions and high‐performance liquid chromatography to analyse phloem samples for amino acid concentrations.3. Aphid abundance significantly increased by 127% under elevated CO2. Consequently, plant biomass decreased under elevated CO2 in trials with herbivores present, possibly due to the increased herbivore load, but increased when aphids were absent. The intrinsic rate of population increase (rm) was significantly higher under elevated CO2; however, there were no statistically significant effects on aphid fecundity or development time. The concentration of individual amino acids tended to increase, although these increases were statistically significant in only a few cases. A principal components analysis revealed that the relative abundance (mol %) of those amino acids considered essential for aphids tended to increase under elevated CO2.4. These results indicate that CO2 may affect nutrient translocation in plants in ways that are contrary to predictions about nitrogen metabolite responses to CO2. Such plant biochemical responses may underlie observations of improved phloem feeder performance under elevated CO2.

Early physiological consequences of fire as an abiotic stressor in metabolic source and sink of young Brutian pine (Pinus brutia Ten.).

Climatic change causes gradual deforestation, partly through forest fires. However, fire has not been seen as an oxidative stressor on surviving forest trees. In addition, discrimination of stress-induced responses from acclimation steps cannot be examined under prolonged stress. Thus, four young Brutian pine (Pinus brutia Ten.) trees, a fire-related species, were subjected to a simulation of a crown-fire event to evaluate its impact on the availability of soluble carbon (C) and nitrogen (N) and the redox status near fire-afflicted tissue. Total soluble sugars, amino acids and non-structural (NS) proteins in needles and phloem, the antioxidant ascorbic acid (AsA) and reactive oxygen species (ROS) in needles were investigated together with the phloem transport velocity. To examine the temporal progress of these parameters, samples were obtained prior to fire (pre-fire), 2 h after fire, the following day (Day 1) and the following week (Week 1). Findings were categorized into shock reactions (2 h) and acclimation steps. Phloem transport accelerated 2 h postfire by almost 30% and correlated negatively to phloem sugars. At the same time the phloem ratio of sugars/amino acids correlated negatively to needle ROS. The trees' main response at 2 h and particularly on Day 1 was a massive increase in phloem NS proteins. The acclimation process involved also significant increases in needle NS proteins and AsA, as well as significant depletion of phloem amino acids by 65% by Week 1. The highest availability of soluble C and N was recorded on Day 1 in the phloem. Regression models explained significantly the variability of most soluble compounds postfire. Our findings suggest sink control over the source and an advanced role of phloem transport in defense processes.

Post-anthesis N and P dynamics and its impact on grain yield and quality in mycorrhizal barley plants.

An essential goal for modern agriculture is the simultaneous improvement of productivity efficiency and nutrient use efficiency. One way to achieve this goal in crops is to enhance nitrogen (N) and phosphorus (P) acquisition through the mycorrhizal association. This study examined the effect of mycorrhization on post-anthesis N and P dynamics and its impact on grain yield and quality in barley. In addition, the efficiency of both N and P utilization and remobilization was evaluated. With those purposes, barley plants inoculated or not with Rhizophagus intraradices were grown in a soil poor in N and P under greenhouse conditions. Inoculation with R. intraradices in barley enhanced both N and P content in grain and vegetative tissue and reduced phloem amino acid export rate. On the other hand, both N and P vegetative tissue content and phloem amino acid and P export rates decreased during grain filling, whereas N and P grain content increased in both treatments according to the senescence process. However, whereas N grain concentration decreased during grain filling, P grain concentration did not vary, thus suggesting a differential regulation on grain filling. Inoculation with R. intraradices improved the yield and grain quality, thus demonstrating that inoculation with R. intraradices in barley is beneficial, but mycorrhization caused a diminution in nutrient utilization efficiency. As the phloem remobilization rate of amino acids and P did not decrease during grain filling in R. intraradices-inoculated plants compared to non-inoculated ones, these results suggest that nutrient utilization efficiency is most probably regulated by sink strength rather by a mycorrhizal effect.

Phloem sugars and amino acids as potential regulators of hordein expression in field grown malting barley (Hordeum vulgare L.)

The commercial quality of malting barley is dependent on the content and composition of grain proteins which are subjected to nitrogen (N) and sulfur (S) control. In nutrient deficient soils, grain protein content is mainly a consequence of the remobilization efficiency. In order to evaluate the effect of N and S soil availability on phloem amino acid and sugar export rates and B- and C-hordein gene expression during grain filling, a factorial combination of N and S fertilization assay was carried out under field conditions. Besides, several carbon (C) and N metabolites were analyzed in leaves and spikes. We observed that, even under soil N and S availabilities that do not limit yield, N and S fertilization induced significant changes in N and C metabolism. N phloem remobilization was promoted by S fertilization independently of N fertilization and C remobilization was also promoted by S fertilization but only in N fertilized plants. The B- and C-hordein gene expression correlated positively with sugar and amino acid exudation rate, respectively. Our results suggest an important role of the export rate of sugars and amino acids in the regulation of grain prolamine expression.

Phloem phytochemistry and aphid responses to elevated CO2, nitrogen fertilization and endophyte infection

Abstract Rising atmospheric CO2 has been shown to alter plant nitrogen metabolism, growth and secondary chemistry. We hypothesized that altered aphid performance under elevated CO2 is linked to phloem nitrogen chemistry. Rhopalosiphum padi performance on endophyte‐infected or uninfected tall fescue was examined under three levels of CO2 (ambient, 800 and 1000 p.p.m.) and high and low nitrogen fertilization. Ethylenediaminetetracetic acid‐facilitated exudation was used to sample phloem sap, followed by quantification of relative amino acid concentrations using reverse‐phase high‐performance liquid chromatography. Aphid abundance was reduced at 800 p.p.m. relative to ambient CO2 but returned to baseline at 1000 p.p.m. The density of aphids was reduced in both the elevated CO2 treatments. Aphids were unsuccessful at colonizing endophyte‐infected plants, possibly as a result of the presence of loline alkaloids. Multivariate analysis showed that certain groups of phloem amino acids were altered by nitrogen fertilization and CO2. We found that four amino acids (valine, arginine, glutamine and aspartate) were correlated with aphid performance. These findings partially explained the effect of plant nitrogen fertilization and elevated CO2 on aphids. The present study represents a first step toward providing a mechanistic explanation of the aphid performance changes that may result from rising atmospheric CO2.

Comparative analysis of genome sequences from four strains of the Buchnera aphidicola Mp endosymbion of the green peach aphid, Myzus persicae

BackgroundMyzus persicae, the green peach aphid, is a polyphagous herbivore that feeds from hundreds of species of mostly dicot crop plants. Like other phloem-feeding aphids, M. persicae rely on the endosymbiotic bacterium, Buchnera aphidicola (Buchnera Mp), for biosynthesis of essential amino acids and other nutrients that are not sufficiently abundant in their phloem sap diet. Tobacco-specialized M. persicae are typically red and somewhat distinct from other lineages of this species. To determine whether the endosymbiotic bacteria of M. persicae could play a role in tobacco adaptation, we sequenced the Buchnera Mp genomes from two tobacco-adapted and two non-tobacco M. persicae lineages.ResultsWith a genome size of 643.5 kb and 579 predicted genes, Buchnera Mp is the largest Buchnera genome sequenced to date. No differences in gene content were found between the four sequenced Buchnera Mp strains. Compared to Buchnera APS from the well-studied pea aphid, Acyrthosiphon pisum, Buchnera Mp has 21 additional genes. These include genes encoding five enzymes required for biosynthesis of the modified nucleoside queosine, the heme pathway enzyme uroporphyrinogen III synthase, and asparaginase. Asparaginase, which is also encoded by the genome of the aphid host, may allow Buchnera Mp to synthesize essential amino acids from asparagine, a relatively abundant phloem amino acid.ConclusionsTogether our results indicate that the obligate intracellular symbiont Buchnera aphidicola does not contribute to the adaptation of Myzus persicae to feeding on tobacco.

Elevated CO2 decreases the response of the ethylene signaling pathway in Medicago truncatula and increases the abundance of the pea aphid

The performance of herbivorous insects is greatly affected by plant nutritional quality and resistance, which are likely to be altered by rising concentrations of atmospheric CO2 . We previously reported that elevated CO2 enhanced biological nitrogen (N) fixation of Medicago truncatula, which could result in an increased supply of amino acids to the pea aphid (Acyrthosiphon pisum). The current study examined the N nutritional quality and aphid resistance of sickle, an ethylene-insensitive mutant of M. truncatula with supernodulation, and its wild-type control A17 under elevated CO2 in open-top field chambers. Regardless of CO2 concentration, growth and amino acid content were greater and aphid resistance was lower in sickle than in A17. Elevated CO2 up-regulated N assimilation and transamination-related enzymes activities and increased phloem amino acids in both genotypes. Furthermore, elevated CO2 down-regulated expression of 1-amino-cyclopropane-carboxylic acid (ACC), sickle gene (SKL) and ethylene response transcription factors (ERF) genes in the ethylene signaling pathway of A17 when infested by aphids and decreased resistance against aphids in terms of lower activities of superoxide dismutase (SOD), peroxidase (POD), and polyphenol oxidase (PPO). Our results suggest that elevated CO2 suppresses the ethylene signaling pathway in M. truncatula, which results in an increase in plant nutritional quality for aphids and a decrease in plant resistance against aphids.

Differential utilization of ash phloem by emerald ash borer larvae: ash species and larval stage effects

Two experiments were performed to determine the extent to which ash species (black, green and white) and larval developmental stage (second, third and fourth instar) affect the efficiency of phloem amino acid utilization by emerald ash borer (EAB) Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) larvae. EAB larvae generally utilized green ash amino acids more efficiently than those of the other two species. For example, the concentrations of only six (two essential) and seven (two essential) amino acids were lower in frass from EAB that fed upon black and white ash than in the corresponding phloem, respectively. By contrast, concentrations of 16 (eight essential) amino acids were lower in the frass from EAB that fed upon green ash than in the phloem. In addition, in green ash, the frass : phloem ratios of 13 amino acids were lower than their counterparts in black and white ash. The concentrations of non‐essential amino acids glycine and hydroxylproline were greater in frass than in phloem when EAB fed on black ash, although not when EAB fed on green or white ash. The concentration of total phenolics (a group of putative defensive compounds to EAB, expressed as antioxidant activity of acetone extraction) was high in EAB frass but even higher in the phloem samples when the data were pooled across ash species and EAB larval stages. This suggests EAB larvae may eliminate phenolics through a combination of direct excretion and enzymatic conversion of phenolics to nonphenolics before excretion. Because the ratio of frass total phenolics to phloem total phenolics in white ash was lower than the ratios in black and green ash, the ability to destroy phenolics or convert them to nonphenolics was greater when EAB larvae fed on white ash. Fourth‐instar EAB extracted phloem amino acids, including threonine, more efficiently than third‐instar EAB. The different larval developmental stages of EAB did not differ in their apparent ability to destroy phenolics or convert them to nonphenolics.

Effects of Selected Fertilizers on the Life History of Bemisia tabaci (Hemiptera: Aleyrodidae) Biotype B

We tested the effects among a purportedly sustainable water-soluble fertilizer, a conventional water-soluble fertilizer, an alternation of these, a controlled-release fertilizer, and a clear water control on the life-history traits of sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae; =Bemisia argentifolii Bellows & Perring) biotype B reared on poinsettia (Euphorbia pulcherrima Willdenow ex Klotzch). Free amino acids in petioles were measured to estimate plant nutrient assimilation and phloem nutritional quality for B. tabaci biotype B. The sustainable fertilizer produced plants with the highest concentration of amino acids. In contrast, fecundity of whiteflies was lowest in plants treated with the sustainable fertilizer and the water control. The relationship between total amino acids in phloem and survival was significantly quadratic, with the highest survival at intermediate levels. Fecundity, however, was negatively correlated with total amino acid content of the maternal host plant. Variation in total amino acid concentration in petioles of plants treated within fertilizer treatments makes it difficult to predict whether a particular fertilizer will produce plants with enough amino acids to deleteriously affect both survivorship and fecundity and yet yield a plant of good quality. Despite this limitation, we can conclude that the use of this sustainable fertilizer will not cause increases in whitefly populations relative to plants fertilized with water-soluble and slow-release fertilizers that deliver the same level of nitrogen to the plant.

Asparagine as a major factor in the N-feedback regulation of N2 fixation in Medicago truncatula

The objective of this study was to assess whether a whole plant N-feedback regulation impact on nitrogen fixation in Medicago truncatula would manifest itself in shifts of the composition of the amino acid flow from shoots to nodules. Detected shifts in the phloem amino acid composition were supposed to be mimicked through artificial phloem feeding and concomitant measurement of nodule activity. The amino acid composition of the phloem exudates was analyzed from plants grown under the influence of treatments (limiting P supply or application of combined nitrogen) known to reduce nodule nitrogen fixation activity. Plants in nutrient solution were supplied with sufficient (9 microM) control, limiting (1 microM) phosphorus or 3 mM NH(4)NO(3) (downregulated nodule activity). Low phosphorus and the application of NH(4)NO(3) reduced per plant and specific nitrogenase activity (H(2) evolution). At day 64 of growth, phloem exudates were collected from cuts of the shoot base. The amount of amino acids was strongly increased in both phloem exudates and nodules of the treatments with downregulated nodule activity. The increase in the downregulated treatments was almost exclusively the result of a higher proportion of asparagine in both phloem exudates and nodules. Leaf labeling with (15)N showed that nitrogen from the leaves is retranslocated to nodules. An artificial phloem feeding with asparagine resulted in an increased concentration of asparagine in nodules and a decreased nodule activity. A possible role of asparagine in an N-feedback regulation of nitrogen fixation in M. truncatula is discussed.

Ustilago maydis infection strongly alters organic nitrogen allocation in maize and stimulates productivity of systemic source leaves.

The basidiomycete Ustilago maydis is the causal agent of corn smut disease and induces tumor formation during biotrophic growth in its host maize (Zea mays). We have conducted a combined metabolome and transcriptome survey of infected leaves between 1 d post infection (dpi) and 8 dpi, representing infected leaf primordia and fully developed tumors, respectively. At 4 and 8 dpi, we observed a substantial increase in contents of the nitrogen-rich amino acids glutamine and asparagine, while the activities of enzymes involved in primary nitrogen assimilation and the content of ammonia and nitrate were reduced by 50% in tumors compared with mock controls. Employing stable isotope labeling, we could demonstrate that U. maydis-induced tumors show a reduced assimilation of soil-derived (15)NO(3)(-) and represent strong sinks for nitrogen. Specific labeling of the free amino acid pool of systemic source leaves with [(15)N]urea revealed an increased import of organic nitrogen from systemic leaves to tumor tissue, indicating that organic nitrogen provision supports the formation of U. maydis-induced tumors. In turn, amino acid export from systemic source leaves was doubled in infected plants. The analysis of the phloem amino acid pool revealed that glutamine and asparagine are not transported to the tumor tissue, although these two amino acids were found to accumulate within the tumor. Photosynthesis was increased and senescence was delayed in systemic source leaves upon tumor development on infected plants, indicating that the elevated sink demand for nitrogen could determine photosynthetic rates in source leaves.