PRpnp, a novel dual activity PNP family protein improves plant vigour and confers multiple stress tolerance in Citrus aurantifolia

Under field conditions, plants are often simultaneously exposed to several abiotic and biotic stresses resulting in significant reductions in growth and yield; thus, developing a multi-stress tolerant variety is imperative. Previously, we reported the neofunctionalization of a novel PNP family protein, Putranjiva roxburghii purine nucleoside phosphorylase (PRpnp) to trypsin inhibitor to cater to the needs of plant defence. However, to date, no study has revealed the potential role and mechanism of either member of this protein group in plant defence. Here, we overexpressed PRpnp in Citrus aurantifolia which showed nuclear-cytoplasmic localization, where it functions in maintaining the intracellular purine reservoir. Overexpression of PRpnp significantly enhanced tolerance to salt, oxidative stress, alkaline pH, drought and two pests, Papilio demoleus and Scirtothrips citri in transgenic plants. Global gene expression studies revealed that PRpnp overexpression up-regulated differentially expressed genes (DEGs) related to ABA- and JA-biosynthesis and signalling, plant defence, growth and development. LC-MS/MS analysis validated higher endogenous ABA and JA accumulation in transgenic plants. Taken together, our results suggest that PRpnp functions by enhancing the endogenous ABA and JA, which interact synergistically and it also inhibits trypsin proteases in the insect gut. Also, like other purine salvage genes, PRpnp also regulates CK metabolism and increases the levels of CK-free bases in transgenic Mexican lime. We also suggest that PRpnp can be used as a potential candidate to develop new varieties with improved plant vigour and enhanced multiple stress resistance.

Potential effects of a high CO2 future on leguminous species

Legumes provide an important source of food and feed due to their high protein levels and many health benefits, and also impart environmental and agronomic advantages as a consequence of their ability to fix nitrogen through their symbiotic relationship with rhizobia. As a result of our growing population, the demand for products derived from legumes will likely expand considerably in coming years. Since there is little scope for increasing production area, improving the productivity of such crops in the face of climate change will be essential. While a growing number of studies have assessed the effects of climate change on legume yield, there is a paucity of information regarding the direct impact of elevated CO₂ concentration (e[CO₂]) itself, which is a main driver of climate change and has a substantial physiological effect on plants. In this review, we discuss current knowledge regarding the influence of e[CO₂] on the photosynthetic process, as well as biomass production, seed yield, quality, and stress tolerance in legumes, and examine how these responses differ from those observed in non-nodulating plants. Although these relationships are proving to be extremely complex, mounting evidence suggests that under limiting conditions, overall declines in many of these parameters could ensue. While further research will be required to unravel precise mechanisms underlying e[CO₂] responses of legumes, it is clear that integrating such knowledge into legume breeding programs will be indispensable for achieving yield gains by harnessing the potential positive effects, and minimizing the detrimental impacts, of CO₂ in the future.

Adaptive responses of amino acid metabolism to the combination of desiccation and low nitrogen availability in Sporobolus stapfianus

Depending on nitrogen availability, S. stapfianus uses different amino acid metabolism strategies to cope with desiccation stress. The different metabolic strategies support essential processes for the desiccation tolerance phenotype. To provide a comprehensive assessment of the role played by amino acids in the adaptation of Sporobolus stapfianus to a combination of desiccation and nitrogen limitation, we used an absolute quantification of free and protein-bound amino acids (FAAs and PBAAs) as well as their gamma-glutamyl (gg-AA) derivatives in four different tissues grown under high- and low-nitrogen regimes. We demonstrate that although specific FAAs and gg-AAs increased in desiccating immature leaves under both nitrogen regimes, the absolute change in the total amount of either is small or negligible, negating their proposed role in nitrogen storage. FAAs and PBAAs decrease in underground tissues during desiccation, when nitrogen is abundant. In contrast, PBAAs are drastically reduced from the mature leaves, when nitrogen is limiting. Nevertheless, the substantial reduction in PBAA and FAA fractions in both treatments is not manifested in the immature leaves, which strongly suggests that these amino acids are further metabolized to fuel central metabolism or other metabolic adjustments that are essential for the acquisition of desiccation tolerance (DT).

Effects of Eutrophication and Different Water Levels on Overwintering of Eichhornia crassipes at the Northern Margin of Its Distribution in China

When exotic species are introduced into new areas, establishment is a vital step in their invasion process. Therefore, overwintering strategies determine whether an exotic species from low latitudes can successfully invade middle- and high-latitude areas. In this study, we investigated the effects of nutrient and water drawdown on overwintering in an exotic aquatic plant from the tropical zone, Eichhornia crassipes, at the northern margin of its distribution in China. The population density, size of individuals, and the size and nitrogen concentration of overwintering organs (stem base) of E. crassipes that grew in high-nutrition water were greater than those that grew in low-nutrient water before winter. The overwinter survival rate of E. crassipes was significantly affected by the water level and nutrient. The thick and dense floating mat of E. crassipes can increase the temperature of water bodies; therefore, the overwinter survival rate of E. crassipes was higher in constant-water-level and high-nutrient treatment. In contrast, due to the loss of heat preservation provided by the floating mats and the low nitrogen concentration in the stem base, all individuals of E. crassipes died in constant-water-level and low-nutrient treatment. In the water-drawdown treatments, the stem base of E. crassipes was directly exposed to low-temperature air; therefore, the overwinter survival rate of E. crassipes was lower. Our results reveal that eutrophication can not only improve the competitiveness of E. crassipes but can also improve the survival rate of overwintering plants in temperate regions. Our study also suggests that removing nutrients from the water and regulating the water level can limit the invasion of E. crassipes in temperate and subtropical regions.

Improving the Yield and Nutritional Quality of Forage Crops

Despite being some of the most important crops globally, there has been limited research on forages when compared with cereals, fruits, and vegetables. This review summarizes the literature highlighting the significance of forage crops, the current improvements and some of future directions for improving yield and nutritional quality. We make the point that the knowledge obtained from model plant and grain crops can be applied to forage crops. The timely development of genomics and bioinformatics together with genome editing techniques offer great scope to improve forage crops. Given the social, environmental and economic importance of forage across the globe and especially in poorer countries, this opportunity has enormous potential to improve food security and political stability.

Source and sink mechanisms of nitrogen transport and use

Contents Summary 35 I. Introduction 35 II. Nitrogen acquisition and assimilation 36 III. Root-to-shoot transport of nitrogen 38 IV. Nitrogen storage pools in vegetative tissues 39 V. Nitrogen transport from source leaf to sink 40 VI. Nitrogen import into sinks 42 VII. Relationship between source and sink nitrogen transport processes and metabolism 43 VIII. Regulation of nitrogen transport 43 IX. Strategies for crop improvement 44 X. Conclusions 46 Acknowledgements 47 References 47 SUMMARY: Nitrogen is an essential nutrient for plant growth. World-wide, large quantities of nitrogenous fertilizer are applied to ensure maximum crop productivity. However, nitrogen fertilizer application is expensive and negatively affects the environment, and subsequently human health. A strategy to address this problem is the development of crops that are efficient in acquiring and using nitrogen and that can achieve high seed yields with reduced nitrogen input. This review integrates the current knowledge regarding inorganic and organic nitrogen management at the whole-plant level, spanning from nitrogen uptake to remobilization and utilization in source and sink organs. Plant partitioning and transient storage of inorganic and organic nitrogen forms are evaluated, as is how they affect nitrogen availability, metabolism and mobilization. Essential functions of nitrogen transporters in source and sink organs and their importance in regulating nitrogen movement in support of metabolism, and vegetative and reproductive growth are assessed. Finally, we discuss recent advances in plant engineering, demonstrating that nitrogen transporters are effective targets to improve crop productivity and nitrogen use efficiency. While inorganic and organic nitrogen transporters were examined separately in these studies, they provide valuable clues about how to successfully combine approaches for future crop engineering.

Nitrogen Reserve Pools in Two Miscanthus × giganteus Genotypes under Contrasting N Managements

Nitrogen (N) reserves in vegetative tissues contribute N to regrowth of Miscanthus × giganteus shoots in spring, but our understanding of how N fertilization and plant genotype affect this process is incomplete. Our specific objectives were to: (1) determine how N fertilizer management impacts accumulation of dry matter and N among aboveground and belowground tissues and organs; (2) understand how changes in N management and tissue N concentration influence seasonal fluctuations in concentrations of buffer-soluble proteins and amino acids in putative storage organs including rhizomes and roots; and (3) characterize genotypic variability and genotype × N interactions for N reserve accumulation and use among Miscanthus × giganteus genotypes. Established plots of the IL Clone and Nagara-sib population were fertilized with 0-0, 0-150, 75-75, 150-0, and 150-150 kg N ha^-1 where the first numeral denotes the N rate applied in 2011 (Year 1) and the second number denotes the N rate applied in 2012 (Year 2). Rhizomes, roots, stembases, and shoots were sampled at 6-week intervals between March and August and then in November at dormancy. Concentrations of N, soluble protein and amino-N increased in all tissues with fertilizer N application. With the exception of rhizome amino-N, concentrations of these N pools in roots and rhizomes declined as plants resumed growth in spring and increased sharply between August and November as growth slowed. Losses in shoot and stembase N mass between August and November were similar to total N accumulation in roots and rhizomes during this interval. Compared to the unfertilized control, specific N managements enhanced growth of above- and belowground tissues. The IL Clone generally had greater biomass yield of all organs than the Nagara-sib; the exception being shoot biomass in November when extensive leaf senescence reduce yield of the IL Clone. High biomass yields were obtained with 75 kg N ha^-1 applied annually rather than semi-annual N applications of 150 kg N^-1 ha that depended on N recycling from roots/rhizomes as a supplemental N source.

Drought-Induced Responses of Physiology, Metabolites, and PR Proteins in Triticum aestivum

The impact of severe drought stress (13% soil moisture) on the physiological responses, metabolic profile, and pathogenesis-related (PR) proteins in wheat above- and below-ground biomass after 20 days of treatment was studied. Drought depleted growth, assimilation pigments, and majority of free amino acids in the shoots (but proline increased considerably, +160%). On the contrary, root growth parameters were elevated, and free amino acids did not decrease, indicating investment of metabolites into the growth of roots under water deficiency. Mineral nutrients were only slightly influenced. Profiling of pathogenesis-related (PR) proteins revealed that chitinases (EC 3.2.1.14) and glucanases (EC 3.2.1.39) were activated in wheat by drought. Individual isoforms and their activity were rather stimulated under drought, especially in shoots. The expression of selected genes is in agreement with enzymatic data and suggests an organ (tissue) specific- and opposing behavior of these two types of defense components in drought-stressed wheat. Metabolic analyses at the level of phenolics showed an increase in the free and bound fraction of phenolic acids almost exclusively in the shoots and flavonoid isoorientin increased considerably: protective action against oxidative stress and dehydration of the leaves seems to be the main reason for this finding. The role of PR proteins and phenolics in drought-stressed tissue is discussed.

Effect of shoot removal on remobilization of carbon and nitrogen during regrowth of nitrogen-fixing alfalfa

The contribution of carbon and nitrogen reserves to regrowth following shoot removal has been studied in the past. However, important gaps remain in understanding the effect of shoot cutting on nodule performance and its relevance during regrowth. In this study, isotopic labelling was conducted at root and canopy levels with both (15) N2 and (13) C-depleted CO2 on exclusively nitrogen-fixing alfalfa plants. As expected, our results indicate that the roots were the main sink organs before shoots were removed. Seven days after regrowth the carbon and nitrogen stored in the roots was invested in shoot biomass formation and partitioned to the nodules. The large depletion in nodule carbohydrate availability suggests that root-derived carbon compounds were delivered towards nodules in order to sustain respiratory activity. In addition to the limited carbohydrate availability, the upregulation of nodule peroxidases showed that oxidative stress was also involved during poor nodule performance. Fourteen days after cutting, and as a consequence of the stimulated photosynthetic and N2 -fixing machinery, availability of Cnew and Nnew strongly diminished in the plants due to their replacement by C and N assimilated during the post-labelling period. In summary, our study indicated that during the first week of regrowth, root-derived C and N remobilization did not overcome C- and N-limitation in nodules and leaves. However, 14 days after cutting, leaf and nodule performance were re-established.

Growth, photosynthetic acclimation and yield quality in legumes under climate change simulations: an updated survey

Continued emissions of CO2, derived from human activities, increase atmospheric CO2 concentration. The CO2 rise stimulates plant growth and affects yield quality. Effects of elevated CO2 on legume quality depend on interactions with N2-fixing bacteria and mycorrhizal fungi. Growth at elevated CO2 increases photosynthesis under short-term exposures in C3 species. Under long-term exposures, however, plants generally acclimate to elevated CO2 decreasing their photosynthetic capacity. An updated survey of the literature indicates that a key factor, perhaps the most important, that characteristically influences this phenomenon, its occurrence and extent, is the plant source-sink balance. In legumes, the ability of exchanging C for N at nodule level with the N2-fixing symbionts creates an extra C sink that avoids the occurrence of photosynthetic acclimation. Arbuscular mycorrhizal fungi colonizing roots may also result in increased C sink, preventing photosynthetic acclimation. Defoliation (Anthyllis vulneraria, simulated grazing) or shoot cutting (alfalfa, usual management as forage) largely increases root/shoot ratio. During re-growth at elevated CO2, new shoots growth and nodule respiration function as strong C sinks that counteracts photosynthetic acclimation. In the presence of some limiting factor, the legumes response to elevated CO2 is weakened showing photosynthetic acclimation. This survey has identified limiting factors that include an insufficient N supply from bacterial strains, nutrient-poor soils, low P supply, excess temperature affecting photosynthesis and/or nodule activity, a genetically determined low nodulation capacity, an inability of species or varieties to increase growth (and therefore C sink) at elevated CO2 and a plant phenological state or season when plant growth is stopped.

Nodule performance within a changing environmental context

Global climate models predict that future environmental conditions will see alterations in temperature, water availability and CO2 concentration ([CO2]) in the atmosphere. Climate change will reinforce the need to develop highly productive crops. For this purpose it is essential to identify target traits conditioning plant performance in changing environments. N2 fixing plants represent the second major crop of agricultural importance worldwide. The current review provides a compilation of results from existing literature on the effects of several abiotic stress conditions on nodule performance and N2 fixation. The environmental factors analysed include water stress, salinity, temperature, and elevated [CO2]. Despite the large number of studies analysing [CO2] effects in plants, frequently they have been conducted under optimal growth conditions that are difficult to find in natural conditions where different stresses often occur simultaneously. This is why we have also included a section describing the current state of knowledge of interacting environmental conditions in nodule functioning. Regardless of the environmental factor considered, it is evident that some general patterns of nodule response are observed. Nodule carbohydrate and N compound availability, together with the presence of oxygen reactive species (ROS) have proven to be the key factors modulating N2 fixation at the physiological/biochemical levels. However, with the exception of water availability and [CO2], it should also be considered that nodule performance has not been characterised in detail under other limiting growth conditions. This highlights the necessity to conduct further studies considering these factors. Finally, we also observe that a better understanding of these metabolic effects of changing environment in nodule functioning would require an integrated and synergistic investigation based on widely used and novel protocols such as transcriptomics, proteomics, metabolomics and stable isotopes.

Influence of short-term drought conditions and subsequent re-watering on the physiology and proteome of Lolium multiflorum/Festuca arundinacea introgression forms, with contrasting levels of tolerance to long-term drought

Festuca arundinacea is a drought tolerant species. Lolium multiflorum has better forage quality but lower tolerance to abiotic stresses. Their hybrids offer an opportunity to perform research on the molecular basis of tolerance to drought. The aim of this work was to recognise the mechanisms of response to short-term drought (11 days) in a glasshouse in two L. multiflorum/F. arundinacea introgression forms with distinct levels of tolerance to long-term drought (14 weeks) in the field. Measurements of physiological parameters, analyses of protein accumulation profiles using two-dimensional gel electrophoresis, and mass spectrometry identification of proteins, which were accumulated differentially between the selected genotypes during short-term drought, were performed. Genotype 7/6, with lower yield potential during 14 weeks of drought, and lower ability to re-grow after watering, had a higher capacity for photosynthesis during 11 days of drought. Genotype 4/10, more tolerant to long-term drought, was able to repair damaged cell membranes after watering and was also characterised by lower transpiration during short-term drought. A total of 455 proteins were analysed, and the 17 that were differentially accumulated between the two genotypes were identified. The results of physiological and proteomic research led to a hypothesis that the higher photosynthetic capacity of genotype 7/6 could be due to a more efficient Calvin cycle, supported by higher accumulation of crucial proteins involving chloroplast aldolase.

Photosynthesis, N(2) fixation and taproot reserves during the cutting regrowth cycle of alfalfa under elevated CO(2) and temperature

Future climatic conditions, including rising atmospheric CO(2) and temperature may increase photosynthesis and, consequently, plant production. A larger knowledge of legume performance under the predicted growth conditions will be crucial for safeguarding crop management and extending the area under cultivation with these plants in the near future. N(2) fixation is a key process conditioning plant responsiveness to varying growth conditions. Moreover, it is likely to increase under future environments, due to the higher photosynthate availability, as a consequence of the higher growth rate under elevated CO(2). However, as described in the literature, photosynthesis performance is frequently down-regulated (acclimated) under long-term exposure to CO(2), especially when affected by stressful temperature and water availability conditions. As growth responses to elevated CO(2) are dependent on sink-source status, it is generally accepted that down-regulation occurs in situations with insufficient plant C sink capacity. Alfalfa management involves the cutting of shoots, which alters the source-sink relationship and thus the photosynthetic behaviour. As the growth rate decreases at the end of the pre-cut vegetative growth period, nodulated alfalfa plants show photosynthetic down-regulation, but during regrowth following defoliation, acclimation to elevated CO(2) disappears. The shoot harvest also leads to a drop in mineral N uptake and C translocation to the roots, resulting in a reduction in N(2) fixation due to the dependence on photosynthate supply to support nodule function. Therefore, the production of new shoots during the first days following cutting requires the utilization of reduced C and N compounds that have been stored previously in reserve organs. The stored reserves are mediated by phytohormones such as methyl jasmonate and abscisic acid and in situations where water stress reduces shoot production this potentially enables the enhancement of taproot protein levels in nodulated alfalfa, which may lead to these plants being in better condition in the following cut/regrowth cycle. Furthering our knowledge of legume performance under predicted climate change conditions will be crucial for the development of varieties with better adaptation that will achieve greater and more efficient production values. Furthermore, for this purpose it will be necessary to improve existing methodologies and create new ones for phenotype characterization. Such knowledge will provide key information for future plant breeding programs.

Nitrogen nutrition of poplar trees

Many forest ecosystems have evolved at sites with growth-limiting nitrogen (N) availability, low N input from external sources and high ecosystem internal cycling of N. By contrast, many poplar species are frequent constituents of floodplain forests where they are exposed to a significant ecosystem external supply of N, mainly nitrate, in the moving water table. Therefore, nitrate is much more important for N nutrition of these poplar species than for many other tree species. We summarise current knowledge of nitrate uptake and its regulation by tree internal signals, as well as acquisition of ammonium and organic N from the soil. Unlike herbaceous plants, N nutrition of trees is sustained by seasonal, tree internal cycling. Recent advances in the understanding of seasonal storage and mobilisation in poplar bark and regulation of these processes by temperature and daylength are addressed. To explore consequences of global climate change on N nutrition of poplar trees, responses of N uptake and metabolism to increased atmospheric CO(2) and O(3) concentrations, increased air and soil temperatures, drought and salt stress are highlighted.

Biomass partitioning, morphology and water status of four alfalfa genotypes submitted to progressive drought and subsequent recovery

The predicted worldwide increase of arid areas and water stress episodes will strongly affect crop production. Numerous plants have developed specific morphological and physiological mechanisms as a means to increase their tolerance to drought. Water stress modifies dry matter partitioning and morphological components such as leaf area ratio (LAR), specific leaf area (SLA) and leaf weight ratio (LWR). Alfalfa has a wide-ranging distribution and is thus expected to show differing levels of drought tolerance. The aim of our study was to determine the effect of progressive drought and subsequent recovery in four alfalfa genotypes differing in drought sensitivity: three cultivars adapted to a Mediterranean climate, Tafilalet (TA), Tierra de Campos (TC) and Moapa (MO), and another representative of an oceanic climate, Europe (EU). Mild drought did not affect biomass production or water status in the studied varieties. Under moderate drought conditions, TA and MO showed decreased leaf production, which may help them to maintain relative water content (RWC). Despite observations that water stress did not affect root growth, after the recovery period, TA increased its root biomass, making higher water soil prospecting possible. Mediterranean cultivars modified LAR and SLA depending on water availability, whereas EU alters LWR. At the end of the experiment, TC was the most productive cultivar, but severe drought did not predict differences among cultivars. Severe water stress increased the root/shoot ratio in order to diminish water consumption and increase absorption of water. In spite of all cultivars showing a decreased LWR, TA also decreased SLA, which may suggest higher drought resistance. Morphological traits from Mediterranean cultivars, including the ability to alter SLA or LAR may be used for drought-tolerant cultivar improvement.

Kinetics of drought-induced pathogenesis-related proteins and its physiological significance in white clover leaves

To investigate the responses of pathogenesis-related (PR) proteins to the intensity of drought stress and their physiological significance in white clover (Trifolium repens L.), the change of enzyme activity and its relationship with some physiological parameters were assessed for 28 days under well-watered (control) and water-deficit conditions. Water-deficit treatment gradually decreased leaf water potential (Psi(w)) to -2.33 MPa at day 28. Dry matter significantly decreased from 21 days of water-deficit treatment, while proline and ammonia concentration increased within 7 days. The increase in PR-protein activity was closely related with the decrease in Psi(w). The beta-1,3-glucanase (EC 3.2.1.39) activity in water-deficit leaves rapidly increased for the first 14 days (Psi(w) >or= -1.67) and then slightly decreased, while the chitinase (EC 3.2.1.14) and cellulase (EC 3.2.1.4) activity continued to increase throughout the experimental period. The enhanced activation of beta-1,3-glucanase, chitinase and cellulase for the period of days 0-14 was significantly (P <or= 0.01) related to the increase of proline and ammonia concentrations. The results indicate that the enhanced activity of beta-1,3-glucanase, cellulase and chitinase for the early period might be an act of transient tolerance to drought stress, but the activation of these enzymes during terminal stress might be a drought-stress-induced injurious symptom.

Elevated atmospheric CO2 and strain of rhizobium alter freezing tolerance and cold-induced molecular changes in alfalfa (Medicago sativa)

BACKGROUND AND AIMS

The objective of the study was to assess the impact of elevated CO2 in interaction with rhizobial strains on freezing tolerance and cold-induced molecular changes in alfalfa.

METHODS

Alfalfa inoculated with two different strains of rhizobium (A2 and NRG34) was grown and cold acclimated (2 weeks at 2 degrees C) under either 400 (ambient) or 800 micromol mol(-1) (elevated) CO2.

KEY RESULTS

Plants acclimated under 400 micromol mol(-1) CO2 were more freezing tolerant than those maintained under 800 micromol mol(-1). Cryoprotective sugars typically linked with the acquisition of freezing tolerance such as sucrose, stachyose and raffinose increased in roots in response to low temperature but did not differ between CO2 treatments. Similarly high CO2 did not alter the expression of many cold-regulated (COR) genes although it significantly increased the level of transcripts encoding a COR gene homologous to glyceraldehyde-3-phosphate-dehydrogenase (GAPDH). A significant effect of rhizobial strain was observed on both freezing tolerance and gene expression. Plants of alfalfa inoculated with strain A2 were more freezing tolerant than those inoculated with strain NRG34. Transcripts of COR genes homologous to a pathogenesis-related protein (PR-10) and to a nuclear-targeted protein were markedly enhanced in roots of alfalfa inoculated with strain A2 as compared with strain NRG34. Transcripts encoding the vegetative storage proteins (VSPs) beta-amylase and chitinase were more abundant in roots of non-acclimated plants inoculated with strain NRG34 than with strain A2.

CONCLUSIONS

Taken together, the results suggest that elevated CO2 stimulates plant growth and reduces freezing tolerance. The acquisition of cold tolerance is also influenced by the rhizobial strain, as indicated by lower levels of expression of COR genes and sustained accumulation of VSP-encoding transcripts in alfalfa inoculated with strain NRG34 as compared with strain A2.

Nitrogen reserves, spring regrowth and winter survival of field-grown alfalfa (Medicago sativa) defoliated in the autumn

AIMS

The objective of the study was to characterize variations in proline, arginine, histidine, vegetative storage proteins, and cold-inducible gene expression in overwintering roots of field-grown alfalfa, in response to autumn defoliation, and in relation to spring regrowth and winter survival.

METHODS

Field trials, established in 1996 in eastern Canada, consisted of two alfalfa cultivars ('AC Caribou' and 'WL 225') defoliated in 1997 and 1998 either only twice during the summer or three times with the third defoliation taken 400, 500 or 600 growing degree days (basis 5 degrees C) after the second summer defoliation.

KEY RESULTS

The root accumulation of proline, arginine, histidine and soluble proteins of 32, 19 and 15 kDa, characterized as alfalfa vegetative storage proteins, was reduced the following spring by an early autumn defoliation at 400 or 500 growing degree days in both cultivars; the 600-growing-degree-days defoliation treatment had less or no effect. Transcript levels of the cold-inducible gene msaCIA, encoding a glycine-rich protein, were markedly reduced by autumn defoliation in 'WL 225', but remained unaffected in the more winter-hardy cultivar 'AC Caribou'. The expression of another cold-inducible gene, the dehydrin homologue msaCIG, was not consistently affected by autumn defoliation. Principal component analyses, including components of root organic reserves at the onset of winter, along with yield and plant density in the following spring, revealed that (a) amino acids and soluble proteins are positively related to the vigour of spring regrowth but poorly related to winter survival and (b) winter survival, as indicated by plant density in the spring, is associated with higher concentrations of cryoprotective sugars in alfalfa roots the previous autumn.

CONCLUSIONS

An untimely autumn defoliation of alfalfa reduces root accumulation of specific N reserves such as proline, arginine, histidine and vegetative storage proteins that are positively related to the vigour of spring regrowth but poorly related to winter survival.

Protein, leucine aminopeptidase, esterase, acid phosphatase and photosynthetic responses of oleander (Nerium oleander L.) during cold acclimation and freezing treatments

Six-month-old oleander (Nerium oleander L.) pot plants, derived from vegetative propagation by cuttings, were tested for their ability to cold hardening. Damage of the non-acclimated (NA) plants was visible when treated by low freezing temperatures (below -2 degrees C). The responses of total proteins, leucine aminopeptidase (LAP), esterase (EST) and acid phosphatase (ACP) isoforms of NA and cold-acclimated (CA; 4 degrees C for 14 days) plants were compared using polyacrylamide gel electrophoresis. These molecular markers were also compared in NA and CA plants which received for 2h temperatures of 0, -2, -4, -6 and -8 degrees C. A new 38-kDa polypeptide appeared from day 7 to 14 during the acclimation treatment in the bark extracts and on day 14 in the leaf extracts. The above-mentioned polypeptide band (38 kDa) strongly appeared in all freezing treatments (0, -2, -4, -6 and -8 degrees C) in both bark and leaf extracts of the CA plants. Alterations in the number and the intensity of LAP and EST isoforms as well as in the intensity of ACP isoforms were observed in both bark and leaf of the CA oleander plants. A newly expressed EST isoform is proposed as biochemical marker for the cold acclimation treatment. CO2 assimilation rates (A) as well as transpiration rates (E) in NA plants were positive in 0 degrees C and negative in all temperatures below zero in the freezing treatments. In contrast, CO2 assimilation rates (A) and transpiration rates (E) were positive in CA plants in all temperatures of freezing treatment. A significant decrease (P<0.05) in chlorophyll (Chl) a, Chl a+b concentration and Chl a/b ratio were noticed in oleander plants during the acclimation treatment (from day 0 to 14), while Chl b concentration was unchanged at the respective time. On the other hand, no significant (P<0.05) differences were observed in the freezing treatments.

Nitrogen storage and seasonal nitrogen cycling in Populus: bridging molecular physiology and ecophysiology

While both annual and perennial plants store nitrogen resources during the growing season, seasonal N cycling is a hallmark of the perennial habit. In Populus the vegetative storage proteins BSP, WIN4 and PNI288 all play a role in N storage during active growth, whereas BSP is the major form of reduced N storage during winter dormancy. In this review we explore cellular and molecular events implicated in seasonal N cycling in Populus, as well as environmental cues that modulate both the phenology of seasonal N cycling, and the efficiency and proficiency of autumn N resorption. We highlight recent advances that have been made using Populus genomics resources to address processes germane to seasonal N cycling. Genetic and genetological studies are enabling us to connect our understanding of seasonal N cycling at molecular and cellular levels with that at ecophysiological levels. With the genomics resources and foundational knowledge that are now in place, Populus researchers are poised to build an integrative understanding of seasonal N cycling that spans from genomes to ecosystems.