Seasonally fluctuating bark proteins are a potential form of nitrogen storage in three temperate hardwoods

Plant lectins: Handymen at the cell surface

Lectins are carbohydrate-binding proteins and are involved in a multitude of biological functions. Lectins at the surface of plant cells often occur as lectin receptor-like kinases (LecRLK) anchored to the plasma membrane. These LecRLKs are part of the plant's pattern-recognition receptor (PRR) system enabling the plant to perceive threats and respond adequately. Furthermore, plant lectins also occur as secreted proteins, which are associated with stress signalling and defence. The aim of this short review is to provide a general perspective on plant lectins and their role at the cell surface.

Major episodes of horizontal gene transfer drove the evolution of land plants

How horizontal gene transfer (HGT) has contributed to the evolution of animals and plants remains a major puzzle. Despite recent progress, defining the overall scale and pattern of HGT events in land plants has been largely elusive. In this study, we performed systematic analyses for acquired genes in different plant groups and throughout land plant evolution. We found that relatively recent HGT events occurred in charophytes and all major land plant groups, but their frequency declined rapidly in seed plants. Two major episodes of HGT events occurred in land plant evolution, corresponding to the early evolution of streptophytes and the origin of land plants, respectively. Importantly, a vast majority of the genes acquired in the two episodes have been retained in descendant groups, affecting numerous activities and processes of land plants. We analyzed some of the acquired genes involved in stress responses, ion and metabolite transport, growth and development, and specialized metabolism, and further assessed the cumulative effects of HGT in land plants.

Characterization of the bark storage protein gene (JcBSP) family in the perennial woody plant Jatropha curcas and the function of JcBSP1 in Arabidopsis thaliana

BACKGROUND

Bark storage protein (BSP) plays an important role in seasonal nitrogen cycling in perennial deciduous trees. However, there is no report on the function of BSP in the perennial woody oil plant Jatropha curcas.

METHODS

In this study, we identified six members of JcBSP gene family in J. curcas genome. The patterns, seasonal changes, and responses to nitrogen treatment in gene expression of JcBSPs were detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Overexpression of JcBSP1 in transgenic Arabidopsis thaliana was driven by a constitutive cauliflower mosaic virus (CaMV) 35S RNA promoter.

RESULTS

JcBSP members were found to be expressed in various tissues, except seeds. The seasonal changes in the total protein concentration and JcBSP1 expression in the stems of J. curcas were positively correlated, as both increased in autumn and winter and decreased in spring and summer. In addition, the JcBSP1 expression in J. curcas seedlings treated with different concentrations of an NH₄NO₃ solution was positively correlated with the NH₄NO₃ concentration and application duration. Furthermore, JcBSP1 overexpression in Arabidopsis resulted in a phenotype of enlarged rosette leaves, flowers, and seeds, and significantly increased the seed weight and yield in transgenic plants.

Review: The multiple roles of plant lectins

For decades, the biological roles of plant lectins remained obscure and subject to speculation. With the advent of technological and scientific progress, researchers have compiled a vast amount of information regarding the structure, biological activities and functionality of hundreds of plant lectins. Data mining of genomes and transcriptome sequencing and high-throughput analyses have resulted in new insights. This review aims to provide an overview of what is presently known about plant lectins, highlighting their versatility and the importance of plant lectins for a multitude of biological processes, such as plant development, immunity, stress signaling and regulation of gene expression. Though lectins primarily act as readers of the glycocode, the multiple roles of plant lectins suggest that their functionality goes beyond carbohydrate-recognition.

The plant axis as the command centre for (re)distribution of sucrose and amino acids

Along with the increase in size required for optimal colonization of terrestrial niches, channels for bidirectional bulk transport of materials in land plants evolved during a period of about 100 million years. These transport systems are essentially still in operation - though perfected over the following 400 million years - and make use of hydrostatic differentials. Substances are accumulated or released at the loading and unloading ends, respectively, of the transport channels. The intermediate stretch between the channel termini is bifunctional and executes orchestrated release and retrieval of solutes. Analyses of anatomical and physiological data demonstrate that the release/retrieval zone extends deeper into sources and sinks than is commonly thought and covers usually much more than 99% of the translocation stretch. This review sketches the significance of events in the intermediate stretch for distribution of organic materials over the plant body. Net leakage from the channels does not only serve maintenance and growth of tissues along the pathway, but also diurnal, short-term or seasonal storage of reserve materials, and balanced distribution of organic C- and N-compounds over axial and terminal sinks. Release and retrieval are controlled by plasma-membrane transporters at the vessel/parenchyma interface in the contact pits along xylem vessels and by plasma-membrane transporters at the interface between companion cells and phloem parenchyma along sieve tubes. The xylem-to-phloem pathway vice versa is a bifacial, radially oriented system comprising a symplasmic pathway, of which entrance and exit are controlled at specific membrane checkpoints, and a parallel apoplasmic pathway. A broad range of specific sucrose and amino-acid transporters are deployed at the checkpoint plasma membranes. SUCs, SUTs, STPs, SWEETs, and AAPs, LTHs, CATs are localized to the plasma membranes in question, both in monocots and eudicots. Presence of Umamits in monocots is uncertain. There is some evidence for endo- and exocytosis at the vessel/parenchyma interface supplementary to the transporter-mediated uptake and release. Actions of transporters at the checkpoints are equally decisive for storage and distribution of amino acids and sucrose in monocots and eudicots, but storage and distribution patterns may differ between both taxa. While the majority of reserves is sequestered in vascular parenchyma cells in dicots, lack of space in monocot vasculature urges "outsourcing" of storage in ground parenchyma around the translocation path. In perennial dicots, specialized radial pathways (rays) include the sites for seasonal alternation of storage and mobilization. In dicots, apoplasmic phloem loading and a correlated low rate of release along the path would favour supply with photoassimilates of terminal sinks, while symplasmic phloem loading and a correlated higher rate of release along the path favours supply of axial sinks and transfer to the xylem. The balance between the resource acquisition by terminal and axial sinks is an important determinant of relative growth rate and, hence, for the fitness of plants in various habitats. Body enlargement as the evolutionary drive for emergence of vascular systems and mass transport propelled by hydrostatic differentials.

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.

Nutrient Storage in the Perennial Organs of Deciduous Trees and Remobilization in Spring - A Study in Almond (Prunus dulcis) (Mill.) D. A. Webb

The annual dynamics of whole mature almond tree nutrient remobilization in spring and the accumulation of nutrients in perennial tissues during the year were determined by sequential coring, tissue sampling, nutrient analysis, whole tree excavation and biomass estimation for trees grown under four nitrogen rate treatments 140 kg ha-1 N (N140), 224 kg ha-1 N (N224), 309 kg ha-1 N (N309), and 392 kg ha-1 N (N392) over 2 years. Whole tree perennial organ N content was greatest in dormancy then declined through bud swell, flowering and fruit set, achieving the lowest total whole tree nutrient content of perennial organs by March 12 [12-14 days after full bloom (DAFB)] coincident with 60-70% leaf expansion. During this period no net increment in whole tree N content (annual plus perennial N) was observed indicating that tree demand for N for bud break, flowering, fruit set and leaf out was met by remobilized stored N and that there was no net N uptake from soil. Remobilizable N increased with increasing N application up to N309 and was maximal at 44.4 ± 4 kg ha-1 and 37.5 ± 5.7 kg ha-1 for the optimally fertilized N309 in 2012 and 2013 respectively. Net increases in perennial organ N (stored N) commenced 41 DAFB and continued through full leaf abscission at 249 DAFB. Total annual N increment in perennial organs varied from 25 to 60 kg ha-1 and was strongly influenced by N rate and tree yield. N remobilized from senescing leaves contributed from 11 to 15.5 ± 0.6 kg ha-1 to perennial stored N. Similar patterns of nutrient remobilization and storage were observed for P, K, and S with maximal whole tree perennial storage occurring during dormancy and remobilization of that stored P, K, S to support annual tree demands through to fruit set and 70-100% leaf development. Net annual increment in perennial organ P, K, S commenced 98 DAFB and continued through full leaf abscission at 249 DAFB. Organ specific contribution to remobilizable and stored nutrients changes over the growing season are presented. Details of the pattern of perennial organ nutrient allocation, storage, and remobilization provides a framework for the optimal management of nutrients in almond with relevance for other deciduous tree species.

Metabolome and Lipidome Profiles of Populus × canescens Twig Tissues During Annual Growth Show Phospholipid-Linked Storage and Mobilization of C, N, and S

The temperate climax tree species Fagus sylvatica and the floodplain tree species Populus × canescens possess contrasting phosphorus (P) nutrition strategies. While F. sylvatica has been documented to display P storage and mobilization (Netzer et al., 2017), this was not observed for Populus × canescens (Netzer et al., 2018b). Nevertheless, changes in the abundance of organic bound P in gray poplar trees indicated adaptation of the P nutrition to different needs during annual growth. The present study aimed at characterizing seasonal changes in metabolite and lipid abundances in gray poplar and uncovering differences in metabolite requirement due to specific needs depending on the season. Seasonal variations in the abundance of (i) sugar-Ps and phospholipids, (ii) amino acids, (iii) sulfur compounds, and (iv) carbon metabolites were expected. It was hypothesized that seasonal changes in metabolite levels relate to N, S, and C storage and mobilization. Changes in organic metabolites binding Pi (Porg) are supposed to support these processes. Variation in triacylglycerols, in sugar-phosphates, in metabolites of the TCA cycle and in the amino acid abundance of poplar twig buds, leaves, bark, and wood were found to be linked to changes in metabolite abundances as well as to C, N, and S storage and mobilization processes. The observed changes support the view of a lack of any P storage in poplar. Yet, during dormancy, contents of phospholipids in twig bark and wood were highest probably due to frost-hardening and to its function in extra-plastidic membranes such as amyloplasts, oleosomes, and protein bodies. Consistent with this assumption, in spring sugar-Ps increased when phospholipids declined and poplar plants entering the vegetative growth period and, hence, metabolic activity increases. These results indicate that poplar trees adopt a policy of P nutrition without P storage and mobilization that is different from their N- and S-nutrition strategies.

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.

Effects of changes in micro-weather conditions on structural features, total protein and carbohydrate content in leaves of the Atlantic rain forest tree golden trumpet (Tabebuia chrysotricha)

Golden trumpet, Tabebuia chrysotricha, is a native tree from the Brazilian Atlantic rain forest, with a broad latitudinal distribution. In this study, we investigated the potential effects of short-term changes in micro-weather conditions on structural features, and total protein and carbohydrate content of golden trumpet leaves, using structural and histochemical approaches. Leaves were harvested in four different micro-weather conditions: 1. Afternoon, after a hot, sunny day; 2. at dawn, after a previously hot, sunny day; 3. at noon, of a hot, sunny day; and 4. at noon, of a cold, cloudy day. Leaflets exposed to low light irradiance showed flattened chloroplasts, uniformly distributed within the cells, throughout the palisade parenchyma. Conversely, leaflets exposed to high light irradiance presented flattened and rounded chloroplasts, in the upper and lower palisade parenchyma cells, respectively. The strongest protein staining was found for leaves harvested at the coldest period, whereas the weakest protein staining was found for leaves harvested after a hot, sunny day. The largest and most numerous starch grains were found for leaves harvested in the afternoon, after a hot, sunny day. Conversely, the smallest and less numerous starch grains were found for leaves harvested at dawn. Analysis of the data reported herein suggests that the leaflet responses to transient changes in micro-weather conditions are likely to contribute to the golden trumpet successful establishment in the broad latitudinal distribution in which the species is found.

The Pokeweed Leaf mRNA Transcriptome and Its Regulation by Jasmonic Acid

The American pokeweed plant, Phytolacca americana, is recognized for synthesizing pokeweed antiviral protein (PAP), a ribosome inactivating protein (RIP) that inhibits the replication of several plant and animal viruses. The plant is also a heavy metal accumulator with applications in soil remediation. However, little is known about pokeweed stress responses, as large-scale sequencing projects have not been performed for this species. Here, we sequenced the mRNA transcriptome of pokeweed in the presence and absence of jasmonic acid (JA), a hormone mediating plant defense. Trinity-based de novo assembly of mRNA from leaf tissue and BLASTx homology searches against public sequence databases resulted in the annotation of 59 096 transcripts. Differential expression analysis identified JA-responsive genes that may be involved in defense against pathogen infection and herbivory. We confirmed the existence of several PAP isoforms and cloned a potentially novel isoform of PAP. Expression analysis indicated that PAP isoforms are differentially responsive to JA, perhaps indicating specialized roles within the plant. Finally, we identified 52 305 natural antisense transcript pairs, four of which comprised PAP isoforms, suggesting a novel form of RIP gene regulation. This transcriptome-wide study of a Phytolaccaceae family member provides a source of new genes that may be involved in stress tolerance in this plant. The sequences generated in our study have been deposited in the SRA database under project # SRP069141.

Differences in cold hardiness, carbohydrates, dehydrins and related gene expressions under an experimental deacclimation and reacclimation in Prunus persica

To boost our understanding of a recent outbreak of freezing injury, we sought to confirm distinctive features between the shoot tissues of the peach (Prunus persica) cultivars Daewol and Kiraranokiwami by mimicking unseasonable changes of temperatures that occur in the early spring through repeated deacclimation and reacclimation treatments. Patterns of cold hardiness declined dramatically during the deacclimation and rose during the reacclimation in both cultivars. Our results indicated that 'Daewol' possessed higher capacity in response to repeated deacclimation and reacclimation treatments than 'Kiraranokiwami'. 'Daewol' showed more sensitive changes in the carbohydrates in response to warm and low temperatures compared with 'Kiraranokiwami'. 'Daewol' indicated almost similar repeated down- and up-patterns in soluble sugar content in response to repeated deacclimation and reacclimation, whereas it indicated repeated up- and down-patterns in starch content. However, 'Kiraranokiwami' showed a progressive increase in the soluble sugar content and a progressive decrease in starch content. Notably, patterns of accumulation of a 60-kDa dehydrin protein encoded by the PpDhn1 gene were confirmed through western blotting and paralleled fluctuations of cold hardiness in both cultivars. Expression of this dehydrin was weak in both cultivars during deacclimation but its band intensity increased during reacclimation. Changes in related genes (β-amylase, PpDhn1, PpDhn2 and PpDhn3) were positively correlated with changes in cold hardiness throughout the experiment. Our results indicate that recent repeated warm periods may cause premature deacclimation in the early spring, and that more cold-tolerant cultivar may be more resilient to freezing injury caused by unstable temperature conditions.

Scaling of xylem and phloem transport capacity and resource usage with tree size

Xylem and phloem need to maintain steady transport rates of water and carbohydrates to match the exchange rates of these compounds at the leaves. A major proportion of the carbon and nitrogen assimilated by a tree is allocated to the construction and maintenance of the xylem and phloem long distance transport tissues. This proportion can be expected to increase with increasing tree size due to the growing transport distances between the assimilating tissues, i.e., leaves and fine roots, at the expense of their growth. We formulated whole tree level scaling relations to estimate how xylem and phloem volume, nitrogen content and hydraulic conductance scale with tree size, and how these properties are distributed along a tree height. Xylem and phloem thicknesses and nitrogen contents were measured within varying positions in four tree species from Southern Finland. Phloem volume, nitrogen amount and hydraulic conductance were found to be concentrated toward the branch and stem apices, in contrast to the xylem where these properties were more concentrated toward the tree base. All of the species under study demonstrated very similar trends. Total nitrogen amount allocated to xylem and phloem was predicted to be comparable to the nitrogen amount allocated to the leaves in small and medium size trees, and to increase significantly above the nitrogen content of the leaves in larger trees. Total volume, hydraulic conductance and nitrogen content of the xylem were predicted to increase faster than that of the phloem with increasing tree height in small trees (<~10 m in height). In larger trees, xylem sapwood turnover to heartwood, if present, would maintain phloem conductance at the same level with xylem conductance with further increases in tree height. Further simulations with a previously published xylem-phloem transport model demonstrated that the Münch pressure flow hypothesis could explain phloem transport with increasing tree height even for the tallest trees.

Elucidating the evolutionary history and expression patterns of nucleoside phosphorylase paralogs (vegetative storage proteins) in Populus and the plant kingdom

BACKGROUND

Nucleoside phosphorylases (NPs) have been extensively investigated in human and bacterial systems for their role in metabolic nucleotide salvaging and links to oncogenesis. In plants, NP-like proteins have not been comprehensively studied, likely because there is no evidence of a metabolic function in nucleoside salvage. However, in the forest trees genus Populus a family of NP-like proteins function as an important ecophysiological adaptation for inter- and intra-seasonal nitrogen storage and cycling.

RESULTS

We conducted phylogenetic analyses to determine the distribution and evolution of NP-like proteins in plants. These analyses revealed two major clusters of NP-like proteins in plants. Group I proteins were encoded by genes across a wide range of plant taxa while proteins encoded by Group II genes were dominated by species belonging to the order Malpighiales and included the Populus Bark Storage Protein (BSP) and WIN4-like proteins. Additionally, we evaluated the NP-like genes in Populus by examining the transcript abundance of the 13 NP-like genes found in the Populus genome in various tissues of plants exposed to long-day (LD) and short-day (SD) photoperiods. We found that all 13 of the Populus NP-like genes belonging to either Group I or II are expressed in various tissues in both LD and SD conditions. Tests of natural selection and expression evolution analysis of the Populus genes suggests that divergence in gene expression may have occurred recently during the evolution of Populus, which supports the adaptive maintenance models. Lastly, in silico analysis of cis-regulatory elements in the promoters of the 13 NP-like genes in Populus revealed common regulatory elements known to be involved in light regulation, stress/pathogenesis and phytohormone responses.

CONCLUSION

In Populus, the evolution of the NP-like protein and gene family has been shaped by duplication events and natural selection. Expression data suggest that previously uncharacterized NP-like proteins may function in nutrient sensing and/or signaling. These proteins are members of Group I NP-like proteins, which are widely distributed in many plant taxa. We conclude that NP-like proteins may function in plants, although this function is undefined.

Low temperatures counteract short-day induced nitrogen storage, but not accumulation of bark storage protein transcripts in bark of grey poplar (Populus × canescens) trees

According to climate change scenarios, the seasonal course of temperature will change in most regions of the world, raising the question of how this will influence seasonal nitrogen (N) storage in deciduous trees. The key to this question is a detailed understanding of the underlying regulatory mechanisms, which was addressed in this study by analysing (i) the effects of low temperatures (13-1 °C) on bark storage protein (BSP) transcription, BSP and total protein accumulation and amino acid metabolism; (ii) the effects of interactions between low temperatures and photoperiod on these processes; and (iii) the regulatory role of amino acids in the bark. For this purpose, we exposed grey poplar trees (Populus × canescens) to three different treatments of changing photoperiod at constant temperature, changing temperature at constant photoperiod, and both changing photoperiod and temperature. Under a shortened photoperiod, a substantial increase of BSP transcripts was observed that was correlated with the accumulation of bark proteins, indicating a metabolic shift to promote long-term N storage. Irrespective of the applied photoperiod, exposure to low temperatures (5 or 1 °C) caused a strong increase of BSP transcripts, which was not paralled by significant increases of BSP and total bark proteins. We conclude that the interaction between effects of photoperiod and temperature is dependent on the carbon status of the trees, and reflects a metabolic adjustment of reduced carbon consumption for BSP synthesis. These results demonstrate the differential temperature sensitivity of processes involved in seasonal N storage, implying vulnerability to changing environmental conditions.

Effects of jasmonic acid, branching and girdling on carbon and nitrogen transport in poplar

• Here, we examined the impact of jasmonate (JA) treatment, branching and phloem girdling on ¹³C and ¹⁵N import, invertase activity and polyphenol accumulation in juvenile tissues of unbranched and branched hybrid poplar saplings (Populus nigra × P. deltoides). • The import of ¹³C to juvenile tissues was positively correlated with invertase activity at the treatment site and enhanced by JA. Both invertase activity and ¹³C import were greater in shorter, younger branches and smaller, younger leaves. By contrast, JA treatments, branching and girdling had little or no impact on ¹⁵N import. • In poplar saplings with multiple lateral branches, we observed almost no ¹³C movement from subtending source leaves into lateral branches above them, with or without JA treatment. The presence of potentially competing branches, treated with JA or not, girdled or not, had no impact on carbohydrate (CHO) import or polyphenol accumulation in target branches. • We conclude that poplar branches comprise modules that are relatively independent from each other and from the stem below in terms of CHO movement, carbon-based defence production and response to elicitors. By contrast, branches are closely linked modules in terms of nitrogen movement. This should produce trees that are highly heterogeneous in quality for herbivores.

Seasonal nitrogen cycling in the bark of field-grown Grey poplar is correlated with meteorological factors and gene expression of bark storage proteins

Seasonal tree-internal nitrogen cycling is an important strategy for trees to achieve high efficiency in the use of nitrogen (N). Key processes of this N redistribution are autumnal leaf senescence and storage of released N as bark storage proteins (BSP) in perennial tissues. While the regulation of leaf senescence has been intensively analysed in trees, the coordination of the complementary storage processes is still poorly understood. Therefore, we ascertained relationships between physiological-level and molecular-level processes and environmental factors under natural conditions in the bark of Populus x canescens. We analysed amino-N concentrations, total soluble protein concentration and transcript abundances of BSP genes in the bark of field-grown P. x canescens harvested during two annual growth cycles. By correlation analysis and linear modelling, we assessed interactions between biological data and meteorological conditions. Day length correlated with BSP expression, and air temperature correlated strongly with total protein concentration (r = -0.92), gamma-aminobutyric acid (GABA; r = 0.76) and arginine (r = -0.70). GABA and arginine also correlated significantly with total protein concentration and transcript abundances of BSP genes. We conclude that GABA and arginine potentially contribute to adjust storage processes in the bark of poplar trees to seasonal changes in environmental conditions.

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.

Changes in carbohydrates, ABA and bark proteins during seasonal cold acclimation and deacclimation in Hydrangea species differing in cold hardiness

Cold injury is frequently seen in the commercially important shrub Hydrangea macrophylla but not in Hydrangea paniculata. Cold acclimation and deacclimation and associated physiological adaptations were investigated from late September 2006 to early May 2007 in stems of field-grown H. macrophylla ssp. macrophylla (Thunb.) Ser. cv. Blaumeise and H. paniculata Sieb. cv. Kyushu. Acclimation and deacclimation appeared approximately synchronized in the two species, but they differed significantly in levels of mid-winter cold hardiness, rates of acclimation and deacclimation and physiological traits conferring tolerance to freezing conditions. Accumulation patterns of sucrose and raffinose in stems paralleled fluctuations in cold hardiness in both species, but H. macrophylla additionally accumulated glucose and fructose during winter, indicating species-specific differences in carbohydrate metabolism. Protein profiles differed between H. macrophylla and H. paniculata, but distinct seasonal patterns associated with winter acclimation were observed in both species. In H. paniculata concurrent increases in xylem sap abscisic acid (ABA) concentrations ([ABA](xylem)) and freezing tolerance suggests an involvement of ABA in cold acclimation. In contrast, ABA from the root system was seemingly not involved in cold acclimation in H. macrophylla, suggesting that species-specific differences in cold hardiness may be related to differences in [ABA](xylem). In both species a significant increase in stem freezing tolerance appeared long after growth ceased, suggesting that cold acclimation is more regulated by temperature than by photoperiod.

Vegetative storage protein in Litchi chinensis, a subtropical evergreen fruit tree, possesses trypsin inhibitor activity

BACKGROUND AND AIMS

Vegetative storage proteins (VSPs) are commonly bioactive in herbaceous plants but few VSPs with bioactivity have been identified in trees. In addition, information on the characterization of VSPs in evergreen trees is limited. The objective of this study was to characterize the VSPs with bioactivity in evergreen trees. Methods The VSP in lychee (Litchi chinensis), an evergreen fruit tree, was characterized by a combination of cytological, biochemical and molecular biological techniques.

KEY RESULTS

The VSP in lychee was a 22-kDa protein. It accumulated in the large central vacuoles of protein-storing cells (PSCs) in two distinguishable forms, granular and floccular. The PSCs were of a novel type. The 22-kDa protein is distributed in mature leaves, bark tissues of branches, trunk and large roots, paralleling the distribution of PSCs. Its homologues were present in mature seed. During young shoot development and fruiting, the 22-kDa protein decreased apparently, suggesting a nitrogen-storage function. The 22-kDa protein had several isoforms encoded by a small multigene family. One gene member, LcVSP1, was cloned. The LcVSP1 had no intron and contained a 675 bp open reading frame encoding a putative protein of 225 amino acids. LcVSP1 was homologous to Kunitz trypsin inhibitors. The 22-kDa protein inhibited trypsin and chymotrypsin, but had no inhibitory effect on subtilisin.

CONCLUSIONS

Lychee is rich in a 22-kDa VSP with trypsin inhibitor activity. The VSP plays an important role in nitrogen storage while its possible defensive function remains to be elucidated.

Mobilisation of nutrients and transport via the xylem sap in a shrub (Ligustrum ovalifolium) during spring growth: N and C compounds and interactions

In open-field soilless culture there can be great deal of leaching, particularly in rainy springs. Ligneous plants have the capacity to store large quantities of nutrients in perennial organs. Knowledge of the plant's internal nutrient mobilisation during spring to supply growing organs could lead to reduction of fertiliser application. To quantify the fraction of storage mobilisation available for growth of new organs during spring, Ligustrum ovalifolium shrubs were grown for 2 years with or without fertilisation in the second spring. Nitrogen (N) absorption and N and carbon (C) mobilisation from storage were followed during spring growth via the sap quality. A mathematical combination of the sap composition with flow velocity provided the transported quantities of N and C. Nitrogen and C mobilisation towards new shoots took place during all the spring growth from bud break onwards. In unfertilised plants, C was mobilised primarily as sugars (stachyose, mannose and sucrose) and starch. In fertilised plants, the same sugars were transported in the xylem sap, but at lower concentrations. Stachyose concentration was lower in fertilised than in unfertilised plants and decreased during spring growth. Nitrogen was transported in the xylem sap mainly as amino acids in both fertilisation treatments. Glutamine was the predominant form at bud break and during shoot elongation. In fertilised plants, arginine became predominant after shoot elongation, and was related to low C availability. The interactions of N with C are discussed; specifically, insufficient availability of N limits the use of C, more of which is directed to aerial organs by sap flow.

Environmental change and carbon limitation in trees: a biochemical, ecophysiological and ecosystem appraisal

As C(3) photosynthesis is not yet CO(2)-saturated, forests offer the possibility of enhanced growth and carbon (C) sequestration with rising atmospheric CO(2). However, at an ecosystem scale, increased photosynthetic rates are not always translated into faster tree growth, and in free air carbon enrichment (FACE) experiments with trees, the stimulation in above-ground growth often declines with time. So is tree growth C-limited? The evidence is reviewed here at three different scales. First, at the biochemical scale, the role of Rubisco is discussed by considering its evolution and role as a nitrogen (N) storage protein. Second, at the ecophysiological scale, C allocation to gain nutrients from the soil is considered and it is argued that any C limitation is only through a limitation to soil nutrient cycling. Finally, the response of forest ecosystems to rising atmospheric CO(2) concentrations is considered and evidence from FACE experiments is discussed. From the three lines of evidence we conclude that the growth of trees is not C-limited, with the key to understanding future responses to climate change being turnover of soil organic matter and nutrient cycling.

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.

Improved growth in a field trial of transgenic hybrid poplar overexpressing glutamine synthetase

•  In the present work the performance of transgenic poplars expressing a pine glutamine synthetase (GS) transgene was studied in natural conditions. •  A field study of eight independent transgenic lines and control plants was carried out for 3 yr in the province of Granada (Spain). •  Transgenic poplars reached average heights that were 21, 36 and 41% greater than control plants after the first, second and third year of growth, respectively. Transgene expression affected plant features with time resulting in increased protein, total GS and ferredoxin-dependent glutamate synthase (Fd-GOGAT) in leaves. However, neither differences in the large subunit of Rubisco (LSU) abundance nor water content were detected between lines. Furthermore, no significant differences were found in total polysaccharide and lignin content in tree trunks. •  The analyses of stem diameter, and protein contents in the bark suggest that higher levels of nitrogen reserves accumulated in the stem of transgenics. Our results suggest that modification of GS1 expression may be a useful strategy to complement traditional tree breeding in short rotation plantations.

Protein cryoprotective activity of a cytosolic small heat shock protein that accumulates constitutively in chestnut stems and is up-regulated by low and high temperatures

Heat shock, and other stresses that cause protein misfolding and aggregation, trigger the accumulation of heat shock proteins (HSPs) in virtually all organisms. Among the HSPs of higher plants, those belonging to the small HSP (sHSP) family remain the least characterized in functional terms. We analyzed the occurrence of sHSPs in vegetative organs of Castanea sativa (sweet chestnut), a temperate woody species that exhibits remarkable freezing tolerance. A constitutive sHSP subject to seasonal periodic changes of abundance was immunodetected in stems. This protein was identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry and internal peptide sequencing as CsHSP17.5, a cytosolic class I sHSP previously described in cotyledons. Expression of the corresponding gene in stems was confirmed through cDNA cloning and reverse transcription-PCR. Stem protein and mRNA profiles indicated that CsHSP17.5 is significantly up-regulated in spring and fall, reaching maximal levels in late summer and, especially, in winter. In addition, cold exposure was found to quickly activate shsp gene expression in both stems and roots of chestnut seedlings kept in growth chambers. Our main finding is that purified CsHSP17.5 is very effective in protecting the cold-labile enzyme lactate dehydrogenase from freeze-induced inactivation (on a molar basis, CsHSP17.5 is about 400 times more effective as cryoprotectant than hen egg-white lysozyme). Consistent with these observations, repeated freezing/thawing did not affect appreciably the chaperone activity of diluted CsHSP17.5 nor its ability to form dodecameric complexes in vitro. Taken together, these results substantiate the hypothesis that sHSPs can play relevant roles in the acquisition of freezing tolerance.

Vegetative storage proteins in the tropical tree Swietenia macrophylla: seasonal fluctuation in relation to a fundamental role in the regulation of tree growth

The protein-storing cells in Swietenia macrophylla King were investigated. They were found to be of the Populus type, i.e., ordinary parenchyma cells containing both vacuole protein inclusion and starch grains. Vegetative storage proteins with molecular masses of 18 and 21 kDa were separated by SDS–PAGE (sodium dodecyl sulfate – polyacrylamide gel electrophoresis). Immunoblotting with the 21-kDa protein antiserum showed that the 18- and 21-kDa proteins shared common epitopes. The 21-kDa protein and presumably the 18-kDa protein were demonstrated by immunogold labeling to be the main components of the vacuole protein inclusion of the protein-storing cells. At the late stage of an annual growth cycle, vegetative storage proteins were found in the branchlets, trunk, large roots, and small roots. They were stored in large amounts in the secondary phloem of these organs and also in the secondary xylem of the terminal branchlets and small roots. In a new growth cycle, the consumption of the previously accumulated vegetative storage proteins began in the terminal branchlets of the last growth cycle. The vegetative storage proteins in the branchlets were exhausted completely when the new shoot leaves matured, while the storage proteins in the trunk and large roots had no detectable changes in abundance. On the other hand, the tree started to accumulate the two proteins in the stem of the new shoots as early as 1 week after the new shoot leaves matured. These results suggested that the previously accumulated vegetative storage proteins were used for new shoot growth and cambial activity in preference to the newly assimilated nitrogen and that vegetative storage proteins existed in considerable amounts in the stems throughout an annual growth cycle. This seasonal fluctuating pattern of vegetative storage proteins in the whole tree may be an important mechanism by which the tree regulates its growth.Key words: vegetative storage proteins, nitrogen metabolism, Populus-type of protein-storing cells, tropical hardwoods, Swietenia macrophylla King.

Coupling sap flow velocity and amino acid concentrations as an alternative method to (15)N labeling for quantifying nitrogen remobilization by walnut trees

The temporal dynamics of N remobilization was studied in walnut (Juglans nigra x regia) trees growing in sand culture. Trees were fed with labeled N ((15)N) during 1999 and unlabeled N in 2000. Total N and (15)N contents in different tree compartments were measured during 80 d after bud burst and were used to estimate N remobilization for spring growth. The seasonal (and occasionally diurnal) dynamics of the concentration and (15)N enrichment of the major amino acids in xylem sap were determined concurrently. Sap flow velocity was also measured for sample trees. A new approach coupling amino acid concentrations to sap flow velocity for quantifying N remobilization was tested. A decrease of the labeled N contents of medium roots, tap roots, and trunk was observed concurrently to the increase in the labeled N content of new shoots. Remobilized N represented from previous year storage 54% of N recovered in new shoots. Arginine, citruline, gamma-amino butyric acid, glutamic acid, and aspartic acid always represented around 80% of total amino acid and amide N in xylem sap and exhibited specific seasonal trends and significant diurnal trends. N translocation was mainly insured by arginine during the first 15 d after bud burst, and then by glutamic acid and citruline. The pattern of N remobilization estimated by the new approach was consistent with that measured by the classical labeling technique. Implications for quantifying N remobilization for large, field-growing trees are discussed.

Root storage proteins, with particular reference to taproots

The presence of storage proteins has been reported in roots of several perennial and biennial weed and crop species, and particularly in members of the Compositae, Euphorbiaceae, and Leguminosae. In some species the amount of these root proteins fluctuates seasonally, increasing in the fall and winter months and declining in the spring and early summer. Also, the root proteins may decline during regrowth of decapitated plants. The evidence that these proteins play a role as storage proteins is frequently only circumstantial; moreover, they are usually only a relatively minor component of the total nitrogen pool within the root. Only one root protein, that from the dandelion taproot, has been extensively characterized, and it has no properties in common with known vegetative storage proteins. The literature on root proteins is reviewed, with particular emphasis on those present in taproots. The paucity of definitive data allows few conclusions to be reached, and more research is required to establish the role, nature, and importance of root proteins.Key words: taproots, perennial weeds, root proteins, nitrogen pools, storage proteins.

Protein Alterations in Tall Fescue in Response to Drought Stress and Abscisic Acid

Drought stress may alter protein synthesis in turfgrasses. The objectives of this study were to investigate physiological changes associated with the synthesis of dehydrin and a cytosolic-heat shock protein (HSC 70) in response to drought stress in two tall fescue (Festuca arundinacea L.) cultivars, 'Southeast' and 'Rebel Jr.'. The effects of abscisic acid (ABA) application on the drought tolerance of the cultivars also were evaluated. The cultivars were subjected to three treatments in growth chambers: well-watered control, drought stress, and drought stress following ABA treatment. Turf quality and leaf relative water content (RWC) decreased and electrolyte leakage (EL) increased during drought stress for both cultivars. The ABA-treated plants maintained higher turf quality and RWC, and lower EL than untreated plants under drought stress conditions. Levels of 20- and 29-kDa polypeptides increased during drought stress, and a 35-kDa polypeptide was noted in both cultivars only when subjected to drought stress either with or without ABA treatment. Immunoblot analysis indicated that dehydrin-like polypeptides of about 23-60 kDa were induced by progressive water deficit in both cultivars. The 53 kDa dehydrin polypeptide was present in Southeast with or without ABA treatment at 10 d of drought stress, whereas the 40 kDa dehydrin polypeptide accumulated in Rebel Jr. in both treatment. The 23- and 27-kDa dehydrin polypeptides were present at 10 d in drought-stressed and ABA-treated plants in both cultivars, but were more pronounced in the drought-stressed plants without ABA. A cytosolic-heat shock protein (HSC 70) was detected in plants in all treatments including well-watered plants of both cultivars, but its levels were higher in drought-stressed and ABA-treated plants. No single dehydrin polypeptide was induced by ABA treatment under drought stress, however, the promotive effects of ABA on the reduced drought stress paralleled the delayed induction of protein synthesis in tall fescue.

Phytochrome-mediated photoperiod perception, shoot growth, glutamine, calcium, and protein phosphorylation influence the activity of the poplar bark storage protein gene promoter (bspA)

In poplars (Populus), bspA encodes a 32-kD bark storage protein that accumulates in the inner bark of plants exposed to either short-day (SD) photoperiods or elevated levels of nitrogen. In this study, poplars transformed with a chimeric gene consisting of the bspA promoter fused to beta-glucuronidase (uidA) were used to investigate the transcriptional regulation of the bspA promoter. Photoperiodic activation of the bspA promoter was shown to involve perception by phytochrome and likely involves both a low fluence response and a parallel very low fluence response pathway. Activity of the bspA promoter was also influenced by shoot growth. High levels of bspA expression usually occur in the bark of plants during SD but not long day or SD with a night break. When growth was inhibited under growth permissive photoperiods (SD with night break) levels of bark beta-glucuronidase (GUS) activity increased. Stimulating shoot growth in plants treated with SD inhibited SD-induced increases in bark GUS activity. Because changes in photoperiod and growth also alter carbon and nitrogen partitioning, the role of carbon and nitrogen metabolites in modulating the activity of the bspA promoter were investigated by treating excised stems with amino acids or NH4NO3 with or without sucrose. Treatment with either glutamine or NH4NO3 resulted in increased stem GUS activity. The addition of sucrose with either glutamine or NH4NO3 resulted in synergistic induction of GUS, whereas sucrose alone had no effect. Glutamine plus sucrose induction of GUS activity was inhibited by EGTA, okadaic acid, or K-252A. Inhibition by EGTA was partially relieved by the addition of Ca2+. The Ca2+ ionophore, ionomycin, also induced GUS activity in excised shoots. These results indicate that transcriptional activation of bspA is complex. It is likely that SD activation of bspA involves perception by phytochrome coupled to changes in growth. These growth changes may then alter carbon and nitrogen partitioning that somehow signals bspA induction by a yet undefined mechanism that involves carbon and nitrogen metabolites, Ca2+, and protein phosphorylation/dephosphorylation.

Isolation and molecular cloning of a novel type 2 ribosome-inactivating protein with an inactive B chain from elderberry (Sambucus nigra) bark

One of the predominant proteins in the bark of elderberry (Sambucus nigra) has been identified as a novel type 2 ribosome-inactivating protein that exhibits a normal RNA N-glycosidase activity, but is devoid of carbohydrate binding activity. Sequence analysis of the corresponding cDNA clones revealed a striking homology to the previously cloned bark lectins from elderberry, suggesting that the new protein is a lectin-related protein. Molecular modeling of the protein confirmed that its A chain is fully active, whereas its B chain contains two functionally inactive carbohydrate-binding sites. These findings not only demonstrate for the first time the occurrence of a type 2 ribosome-inactivating protein with an inactive B chain, but also offer interesting perspectives for the synthesis of immunotoxins with an improved selectivity.

A lectin and a lectin-related protein are the two most prominent proteins in the bark of yellow wood (Cladrastis lutea)

Using a combination of cDNA cloning and protein purification it is demonstrated that bark of yellow wood (Cladrastis lutea) contains two mannose/glucose binding lectins and a lectin-related protein which is devoid of agglutination activity. One of the lectins (CLAI) is the most prominent bark protein. It is built up of four 32 kDa monomers which are post-translationally cleaved into a 15 kDa and a 17 kDa polypeptide. The second lectin (CLAII) is a minor protein, which strongly resembles CLAI except that its monomers are not cleaved into smaller polypeptides. Molecular cloning of the Cladrastis lectin family revealed also the occurrence of a lectin-related protein (CLLRP) which is the second most prominent bark protein. Although CLLRP shows sequence homology to the true lectins, it is devoid of carbohydrate binding activity. Molecular modelling of the three Cladrastis proteins has shown that their three-dimensional structure is strongly related to the three-dimensional models of other legume lectins and, in addition, revealed that the presumed carbohydrate binding site of CLLRP is disrupted by an insertion of three extra amino acids. Since it is demonstrated for the first time that a lectin and a non-carbohydrate binding lectin-related protein are the two most prominent proteins in the bark of a tree, the biological meaning of their simultaneous occurrence is discussed.

A family of wound-induced genes in Populus shares common features with genes encoding vegetative storage proteins

Two wound-inducible cDNAs from poplar leaves show sequence identity to vegetative storage proteins (VSP) that accumulate seasonally in poplar bark tissues. We have compared the genomic organization, cDNA sequences and expression of the genes encoding the wound-inducible cDNAs (win4) with that of a bark VSP (called bark storage protein, or BSP). There appear to be several win4 genes in the poplar genome which segregate as a single locus and are therefore likely to be clustered. The same is true of the BSP genes. The win4 locus is linked (map distance of 5 cM) to the BSP locus, consistent with a common evolutionary origin of the genes. A near full-length win4 cDNA shows 75% sequence identity to BSP cDNAs. Both win4 and BSP are systemically wound-inducible; win4 transcripts accumulate in leaves and stems, whereas BSP transcripts accumulate almost exclusively in stems. A phloem transport-dependent signaling mechanism appears to be involved in systemic win4 expression after wounding. In contrast to BSP gene expression, win4 genes are not expressed in response to short day conditions. The data indicate win4 and BSP genes are differentially regulated, and their products may play important roles in the storage and reallocation of nitrogen in perennial plants.

Seasonal changes in the concentration of the major storage protein and its mRNA in xylem ray cells of poplar trees

In the autumn large amounts of a major storage protein accumulate in the woody stem of poplar trees. This protein is stored in xylem ray cells during the winter season and is degraded in late spring. The accumulation of this protein is preceded by a dramatic but transient appearance of the corresponding mRNA. Thus, the seasonal change in the mRNA content appears to be a crucial event for the storage of nitrogen in the stem. The amino acid sequence of the storage protein has been deduced from the nucleotide sequence of a full-length cDNA. The cDNA shows a nucleotide sequence similarity of approximately 75% with two published cDNA sequences of poplar which represent transcripts that accumulate systemically in leaves of poplar trees in response to wounding.

Proteins in the roots of the perennial weeds chicory (Cichorium intybus L.) and dandelion (Taraxacum officinale Weber) are associated with overwintering

Roots are the overwintering structures of herbaceous perennial weeds growing in temperate climates. During the fall they accumulated reserves which are remobilized when growth resumes in the spring. An 18kDa (kilodalton) protein increases in both chicory and dandelion roots during the fall months. The proteins in both species are antigenically similar, and are recognized also by an antibody to a storage-protein deposited in Jerusalem artichoke (Helianthus tuberosus) tubers. In chicory, the protein is root-specific, but in dandelion it is detectable in the flowers, vestigial stem and the seed. Electrophoretic characterization of the 18-kDa protein shows that it is a single polypeptide, without subunits, with charge isomers of pI values close to pH 6.5. The major protein present in chicory and dandelion roots is unlike the vegetative storage proteins recently found in soybean or the storage proteins in the bark of trees.