Use of gaseous 13NH3 administered to intact leaves of Nicotiana tabacum to study changes in nitrogen utilization during defence induction

Leachates from plants recently infected by root-feeding nematodes cause increased biomass allocation to roots in neighbouring plants

Plants can adjust defence strategies in response to signals from neighbouring plants attacked by aboveground herbivores. Whether similar responses exist to belowground herbivory remains less studied, particularly regarding the spatiotemporal dynamics of such belowground signalling. We grew the grass Agrostis stolonifera with or without root-feeding nematodes (Meloidogyne minor). Leachates were extracted at different distances from these plants and at different times after inoculation. The leachates were applied to receiver A. stolonifera plants, of which root, shoot, and total biomass, root/shoot ratio, shoot height, shoot branch number, maximum rooting depth and root number were measured 3 weeks after leachate application. Receiver plants allocated significantly more biomass to roots when treated with leachates from nematode-inoculated plants at early infection stages. However, receiver plants’ root/shoot ratio was similar when receiving leachates collected at later stages from nematode-infected or control plants. Overall, early-collected leachates reduced growth of receiver plants significantly. Plants recently infected by root-feeding nematodes can thus induce increased root proliferation of neighbouring plants through root-derived compounds. Possible explanations for this response include a better tolerance of anticipated root damage by nematodes or the ability to grow roots away from the nematode-infected soil. Further investigations are still needed to identify the exact mechanisms.

Seasonal nitrogen remobilization and the role of auxin transport in poplar trees

Seasonal nitrogen (N) cycling in Populus, involves bark storage proteins (BSPs) that accumulate in bark phloem parenchyma in the autumn and decline when shoot growth resumes in the spring. Little is known about the contribution of BSPs to growth or the signals regulating N remobilization from BSPs. Knockdown of BSP accumulation via RNAi and N sink manipulations were used to understand how BSP storage influences shoot growth. Reduced accumulation of BSPs delayed bud break and reduced shoot growth following dormancy. Further, 13N tracer studies also showed that BSP accumulation is an important factor in N partitioning from senescing leaves to bark. Thus, BSP accumulation has a role in N remobilization during N partitioning both from senescing leaves to bark and from bark to expanding shoots once growth commences following dormancy. The bark transcriptome during BSP catabolism and N remobilization was enriched in genes associated with auxin transport and signaling, and manipulation of the source of auxin or auxin transport revealed a role for auxin in regulating BSP catabolism and N remobilization. Therefore, N remobilization appears to be regulated by auxin produced in expanding buds and shoots that is transported to bark where it regulates protease gene expression and BSP catabolism.

Defense Priming in Nicotiana tabacum Accelerates and Amplifies 'New' C/N Fluxes in Key Amino Acid Biosynthetic Pathways

In the struggle to survive herbivory by leaf-feeding insects, plants employ multiple strategies to defend themselves. One mechanism by which plants increase resistance is by intensifying their responsiveness in the production of certain defense agents to create a rapid response. Known as defense priming, this action can accelerate and amplify responses of metabolic pathways, providing plants with long-lasting resistance, especially when faced with waves of attack. In the work presented, short-lived radiotracers of carbon administered as 11CO2 and nitrogen administered as 13NH3 were applied in Nicotiana tabacum, to examine the temporal changes in 'new' C/N utilization in the biosynthesis of key amino acids (AAs). Responses were induced by using topical application of the defense hormone jasmonic acid (JA). After a single treatment, metabolic partitioning of recently fixed carbon (designated 'new' carbon and reflected as 11C) increased through the shikimate pathway, giving rise to tyrosine, phenylalanine and tryptophan. Amplification in 'new' carbon fluxes preceded changes in the endogenous (12C) pools of these AAs. Testing after serial JA treatments revealed that fluxes of 'new' carbon were accelerated, amplified and sustained over time at this higher rate, suggesting a priming effect. Similar results were observed with recently assimilated nitrogen (designated 'new' nitrogen reflected as 13N) with its partitioning into serine, glycine and glutamine, which play important roles supporting the shikimate pathway and downstream secondary metabolism. Finally, X-ray fluorescence imaging revealed that levels of the element Mn, an important co-factor for enzyme regulation in the shikimate pathway, increased within JA treated tissues, suggesting a link between plant metal ion regulation and C/N metabolic priming.

A metabolomic strategy revealed the role of JA and SA balance in Clematis terniflora DC. Response to UVB radiation and dark

Clematis terniflora DC. is a valuable resource with potential high pharmaceutical value. Proteomic, transcriptomic and metabolomic analyses of C. terniflora that has been exposed to high levels of UVB irradiation and dark conditions (HUVB + D) have revealed the mechanisms underlying its medicinal potential. However, the signal transduction pathways and the mechanisms of regulation for the accumulation of secondary metabolites remain unclear. In this study, we show that the jasmonic acid (JA) and salicylic acid (SA) signals were activated in C. terniflora in response to HUVB + D. Metabolomic analysis demonstrated that the perturbation in JA and SA balance led to additional reallocation of carbon and nitrogen resources. Evaluating the fold change ratios of differentially changed metabolites proved that JA signal enhanced the transformation of nitrogen to carbon through the 4-aminobutyric acid (GABA) shunt pathway, which increased the carbon reserve to be utilized in the production of secondary metabolites. However, SA signal induced the synthesis of proline, while avoiding the accumulation of secondary metabolites. Over all, the results indicate that the co-increase of JA and SA reconstructed the dynamic stability of transformation from nitrogen to carbon, which effectively enhanced the oxidative defense to HUVB + D in C. terniflora by increasing the secondary metabolites.

Three NPF genes in Arabidopsis are necessary for normal nitrogen cycling under low nitrogen stress

Internal nitrogen (N) cycling is crucial to N use efficiency. For example, N may be remobilized from older, shaded leaves to young leaves near the apex that receive more direct sunlight, where the N can be used more effectively for photosynthesis. Yet our understanding of the mechanisms and regulation of N transport is limited. To identify relevant transporters in Arabidopsis, fifteen transporter knockout mutants were screened for defects in leaf N export using nitrogen-13 (¹³N) administered as ¹³NH₃ gas to leaves. We found that three nitrate/peptide transporter family (NPF) genes were necessary for normal leaf N export under low N but not adequate soil N availability, including AtNPF7.1, which has not been previously characterized. High-throughput phenotyping revealed altered leaf area and chlorophyll fluorescence relative to wild-type plants. High AtNPF7.1 expression in flowers and large flower stalks of Atnpf7.1 mutants in low N suggests that AtNPF7.1 influences leaf N export via sink-to-source feedback, perhaps via a role in sensing plant internal N-status. We also identified previously unreported phenotypes for the mutants of the other two NPF transporters that indicate possible roles in N sensing networks.

Positron-emitting radionuclides for applications, with special emphasis on their production methodologies for medical use

A survey of the positron-emitting radionuclides over the whole mass range of the Periodic Table of Elements was carried out. As already known, positrons are preferably emitted from light mass neutron deficient radionuclides. Their emission from heavier mass nuclides is rather rare. The applications of positron annihilation in three areas, namely materials research, plant physiology and medical diagnosis, are reported. The methods of production of positron emitters are discussed, with emphasis on radionuclides presently attracting more attention in theranostics and multimodal imaging. Some future perspectives of radionuclide development technologies are considered.

The roles of jasmonate signalling in nitrogen uptake and allocation in rice (Oryza sativa L.)

Herbivore damage by chewing insects activates jasmonate (JA) signalling that can elicit systemic defense responses in rice. Few details are known, however, concerning the mechanism, whereby JA signalling modulates nutrient status in rice in response to herbivory. (¹⁵ NH₄ )₂ SO₄ labelling experiments, proteomic surveys, and RT-qPCR analyses were used to identify the roles of JA signalling in nitrogen (N) uptake and allocation in rice plants. Exogenous applications of methyl jasmonate (MeJA) to rice seedlings led to significantly reduced N uptake in roots and reduced translocation of recently-absorbed ¹⁵ N from roots to leaves, likely occurring as a result of down-regulation of glutamine synthetase cytosolic isozyme 1-2 and ferredoxin-nitrite reductase. Shoot MeJA treatment resulted in a remobilization of endogenous unlabelled ¹⁴ N from leaves to roots, and root MeJA treatment also increased ¹⁴ N accumulation in roots but did not affect ¹⁴ N accumulation in leaves of rice. Additionally, proteomic and RT-qPCR experiments showed that JA-mediated plastid disassembly and dehydrogenases GDH2 up-regulation contribute to N release in leaves to support production of defensive proteins/compounds under N-limited condition. Collectively, our results indicate that JA signalling mediates large-scale systemic changes in N uptake and allocation in rice plants.

COI1-dependent jasmonate signalling affects growth, metabolite production and cell wall protein composition in arabidopsis

BACKGROUND AND AIMS

Cultured cell suspensions have been the preferred model to study the apoplast as well as to monitor metabolic and cell cycle-related changes. Previous work showed that methyl jasmonate (MeJA) inhibits leaf growth in a CORONATINE INSENSITIVE 1 (COI1)-dependent manner, with COI1 being the jasmonate (JA) receptor. Here, the effect of COI1 overexpression on the growth of stably transformed arabidopsis cell cultures is described.

METHODS

Time-course experiments were carried out to analyse gene expression, and protein and metabolite levels.

KEY RESULTS

Both MeJA treatment and the overexpression of COI1 modify growth, by altering cell proliferation and expansion. DNA content as well as transcript patterns of cell cycle and cell wall remodelling markers were altered. COI1 overexpression also increases the protein levels of OLIGOGALACTURONIDE OXIDASE 1, BETA-GLUCOSIDASE/ENDOGLUCANASES and POLYGALACTURONASE INHIBITING PROTEIN2, reinforcing the role of COI1 in mediating defence responses and highlighting a link between cell wall loosening and growth regulation. Moreover, changes in the levels of the primary metabolites alanine, serine and succinic acid of MeJA-treated Arabidopsis cell cultures were observed. In addition, COI1 overexpression positively affects the availability of metabolites such as β-alanine, threonic acid, putrescine, glucose and myo-inositol, thereby providing a connection between JA-inhibited growth and stress responses.

CONCLUSIONS

This study contributes to the understanding of the regulation of growth and the production of metabolic resources by JAs and COI1. This will have important implications in dissecting the complex relationships between hormonal and cell wall signalling in plants. The work also provides tools to uncover novel mechanisms co-ordinating cell division and post-mitotic cell expansion in the absence of organ developmental control.

Seasonal nitrogen cycling in temperate trees: Transport and regulatory mechanisms are key missing links

Nutrient accumulation, one of the major ecosystem services provided by forests, is largely due to the accumulation and retention of nutrients in trees. This review focuses on seasonal cycling of nitrogen (N), often the most limiting nutrient in terrestrial ecosystems. When leaves are shed during autumn, much of the N may be resorbed and stored in the stem over winter, and then used for new stem and leaf growth in spring. A framework exists for understanding the metabolism and transport of N in leaves and stems during winter dormancy, but many of the underlying genes remain to be identified and/or verified. Transport of N during seasonal N cycling is a particularly weak link, since the physical pathways for loading and unloading of amino N to and from the phloem are poorly understood. Short-day photoperiod followed by decreasing temperatures are the environmental cues that stimulate dormancy induction, and nutrient remobilization and storage. However, beyond the involvement of phytochrome, very little is known about the signal transduction mechanisms that link environmental cues to nutrient remobilization and storage. We propose a model whereby nutrient transport and sensing plays a major role in source-sink transitions of leaves and stems during seasonal N cycling.

Using 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) to study carbon allocation in plants after herbivore attack

BACKGROUND

Although leaf herbivory-induced changes in allocation of recently assimilated carbon between the shoot and below-ground tissues have been described in several species, it is still unclear which part of the root system is affected by resource allocation changes and which signalling pathways are involved. We investigated carbon partitioning in root tissues following wounding and simulated leaf herbivory in young Nicotiana attenuata plants.

RESULTS

Using 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG), which was incorporated into disaccharides in planta, we found that simulated herbivory reduced carbon partitioning specifically to the root tips in wild type plants. In jasmonate (JA) signalling-deficient COI1 plants, the wound-induced allocation of [(18)F]FDG to the roots was decreased, while more [(18)F]FDG was transported to young leaves, demonstrating an important role of the JA pathway in regulating the wound-induced carbon partitioning between shoots and roots.

CONCLUSIONS

Our data highlight the use of [(18)F]FDG to study stress-induced carbon allocation responses in plants and indicate an important role of the JA pathway in regulating wound-induced shoot to root signalling.

Induced carbon reallocation and compensatory growth as root herbivore tolerance mechanisms

Upon attack by leaf herbivores, many plants reallocate photoassimilates below ground. However, little is known about how plants respond when the roots themselves come under attack. We investigated induced resource allocation in maize plants that are infested by the larvae Western corn rootworm Diabrotica virgifera virgifera. Using radioactive (11) CO(2), we demonstrate that root-attacked maize plants allocate more new (11) C carbon from source leaves to stems, but not to roots. Reduced meristematic activity and reduced invertase activity in attacked maize root systems are identified as possible drivers of this shoot reallocation response. The increased allocation of photoassimilates to stems is shown to be associated with a marked thickening of these tissues and increased growth of stem-borne crown roots. A strong quantitative correlation between stem thickness and root regrowth across different watering levels suggests that retaining photoassimilates in the shoots may help root-attacked plants to compensate for the loss of belowground tissues. Taken together, our results indicate that induced tolerance may be an important strategy of plants to withstand belowground attack. Furthermore, root herbivore-induced carbon reallocation needs to be taken into account when studying plant-mediated interactions between herbivores.

MAPK-dependent JA and SA signalling in Nicotiana attenuata affects plant growth and fitness during competition with conspecifics

BACKGROUND

Induced defense responses to herbivores are generally believed to have evolved as cost-saving strategies that defer the fitness costs of defense metabolism until these defenses are needed. The fitness costs of jasmonate (JA)-mediated defenses have been well documented. Those of the early signaling units mediating induced resistance to herbivores have yet to be examined. Early signaling components that mediate herbivore-induced defense responses in Nicotiana attenuata, have been well characterized and here we examine their growth and fitness costs during competition with conspecifics. Two mitogen-activated protein kinases (MAPKs), salicylic acid (SA)-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK) are rapidly activated after perception of herbivory and both kinases regulate herbivory-induced JA levels and JA-mediated defense metabolite accumulations. Since JA-induced defenses result in resource-based trade-offs that compromise plant productivity, we evaluated if silencing SIPK (irSIPK) and WIPK (irWIPK) benefits the growth and fitness of plants competiting with wild type (WT) plants, as has been shown for plants silenced in JA-signaling by the reduction of Lipoxygenase 3 (LOX3) levels.

RESULTS

As expected, irWIPK and LOX3-silenced plants out-performed their competing WT plants. Surprisingly, irSIPK plants, which have the largest reductions in JA signaling, did not. Phytohormone profiling of leaves revealed that irSIPK plants accumulated higher levels of SA compared to WT. To test the hypothesis that these high levels of SA, and their presumed associated fitness costs of pathogen associated defenses in irSIPK plants had nullified the JA-deficiency-mediated growth benefits in these plants, we genetically reduced SA levels in irSIPK plants. Reducing SA levels partially recovered the biomass and fitness deficits of irSIPK plants. We also evaluated whether the increased fitness of plants with reduced SA or JA levels resulted from increased nitrogen or CO₂ assimilation rates, and found no evidence that greater intake of these fitness-limiting resources were responsible.

CONCLUSIONS

Signaling mediated by WIPK, but not SIPK, is associated with large fitness costs in competing N. attenuata plants, demonstrating the contrasting roles that these two MAPKs play in regulating the plants' growth-defense balance. We discuss the role of SIPK as an important regulator of plant fitness, possibly by modulating SA-JA crosstalk as mediated through ethylene signaling.

Novel application of 2-[(18)F]fluoro-2-deoxy-D-glucose to study plant defenses

INTRODUCTION

Since its first use in humans in 1976, 2-[¹⁸F]fluoro-2-deoxy-d-glucose (¹⁸FDG) continues to serve as a tracer to measure tissue glucose metabolism in medical imaging. Here we demonstrate a novel use for this tracer to study glycoside biosynthesis in plants as a measure of plant response to defense induction.

METHODS

Coupling autoradiography with radio high-performance liquid chromatography analysis of tissue extracts, we examined the combined effects of leaf wounding and treatment using the potent plant defense hormone, methyl jasmonate (MeJA), to measure tracer distribution and tracer use in secondary defense chemistry in Arabidopsis thaliana. We hypothesized that competing sinks like roots and reproductive tissues, as well as vascular architecture, would impact the induction of phenolic defenses of the plant that make use of glucose in glycoside formation by altering distribution and metabolic utilization of ¹⁸FDG.

RESULTS

Our studies showed that leaf orthostichy defined the major route of ¹⁸FDG transport in both vegetative and reproductive plants when a single petiole was cut as the entry point for tracer introduction. However, when nonorthostichous leaves were damaged and treated with MeJA, ¹⁸FDG was transported in its intact form to these leaves 3 h later, where it was incorporated into phenolic glycosides.

CONCLUSIONS

Our work demonstrates a new use for ¹⁸FDG in plant science with insights into carbohydrate allocation that contradict conclusions of previous studies showing transport of resources away from damaged sites.