Infection of maize leaves with Ustilago maydis prevents establishment of C4 photosynthesis

The Basidiomycete fungus Ustilago maydis is the common agent of corn smut and is capable of inducing gall growth on infected tissue of the C4 plant maize (Zea mays). While U. maydis is very well characterized on the genetic level, the physiological changes in the host plant in response to U. maydis infection have not been studied in detail, yet. Therefore, we examined the influence of U. maydis infection on photosynthetic performance and carbon metabolism in maize leaf galls. At all stages of development, U. maydis-induced leaf galls exhibited carbon dioxide response curves, CO2 compensation points and enzymatic activities that are characteristic of C3 photosynthesis, demonstrating that the establishment of C4 metabolism is prevented in infected tissue. Hexose contents and hexose/sucrose ratio of leaf galls remained high at 6 days post infection, while a shift in free sugar metabolism was observed in the uninfected controls at that time point. Concomitantly, transitory starch production and sucrose accumulation during the light period remained low in leaf galls. Given that U. maydis is infectious on young developing tissue, the observed changes in carbohydrate metabolism suggest that the pathogen manipulates the developing leaf tissue to arrest sink-to-source transition in favor of maintaining sink metabolism in the host cells. Furthermore, evidence is presented that carbohydrate supply during the biotrophic phase of the pathogen is assured by a fungal invertase.     

Stimulating natural defenses in poplar clones (OP-367) increases plant metabolism of carbon tetrachloride

Groundwater contamination by carbon tetrachloride (CCl4) presents a health risk as a potential carcinogen and pollutant that is capable of depleting the ozone layer. Although use of poplar trees in a phytoremediation capacity has proven to be cost effective for cleaning contaminated sites, minimizing leaf emission of volatile contaminants remains a pressing issue. We hypothesized that recently fixed carbon plays a key role in CCl4 metabolism in planta yielding nonvolatile trichloroacetic acid (TCA) and that the extent of this metabolism can be altered by heightening plant defenses. Labeling intact leaves with (11)CO2 (t 1/2 20.4 m) can test this hypothesis, because the extremely short half-life of the tracer reflects only those processes involving recently fixed carbon. Using radio-HPLC analysis, we observed [(11)C]TCA from leaf extract from poplar clones (OP-367) whose roots were exposed to a saturated solution of CCl4 (520 ppm). Autoradiography of [(11)C]photosynthate showed increased leaf export and partitioning to the apex within 24 h of CCl4 exposure, suggesting that changes in plant metabolism and partitioning of recently fixed carbon occur rapidly. Additionally, leaf CCl4 emissions were highest in the morning, when carbon pools are low, suggesting a link between contaminant metabolism and leaf carbon utilization. Further, treatment with methyljasmonate, a plant hormone implicated in defense signal transduction, reduced leaf CCl4 emissions two-fold due to the increased formation of TCA.     

Infection of maize leaves with Ustilago maydis prevents establishment of C4 photosynthesis

The Basidiomycete fungus Ustilago maydis is the common agent of corn smut and is capable of inducing gall growth on infected tissue of the C₄ plant maize (Zea mays). While U. maydis is very well characterized on the genetic level, the physiological changes in the host plant in response to U. maydis infection have not been studied in detail, yet.Therefore, we examined the influence of U. maydis infection on photosynthetic performance and carbon metabolism in maize leaf galls.At all stages of development, U. maydis-induced leaf galls exhibited carbon dioxide response curves, CO₂ compensation points and enzymatic activities that are characteristic of C₃ photosynthesis, demonstrating that the establishment of C₄ metabolism is prevented in infected tissue. Hexose contents and hexose/sucrose ratio of leaf galls remained high at 6 days post infection, while a shift in free sugar metabolism was observed in the uninfected controls at that time point. Concomitantly, transitory starch production and sucrose accumulation during the light period remained low in leaf galls. Given that U. maydis is infectious on young developing tissue, the observed changes in carbohydrate metabolism suggest that the pathogen manipulates the developing leaf tissue to arrest sink-to-source transition in favor of maintaining sink metabolism in the host cells.Furthermore, evidence is presented that carbohydrate supply during the biotrophic phase of the pathogen is assured by a fungal invertase.     

The influence of plant carbon dioxide and nutrient supply on susceptibility to insect herbivores

The carbon/nutrient ratio of plants has been hypothesized to be a significant regulator of plant susceptibility of leaf-eating insects. As rising atmospheric carbon dioxide stimulates photosynthesis, host plant carbon supply is increased and the accompanying higher levels of carbohydrates, especially starch, apparently dilute the protein content of the leaf. When host plant nitrogen supply is limited, plant responses include increased carbohydrate accumulation, reduced leaf protein content, but also increased carbon-based defensive chemicals. No change, however, has been observed in the concentration of leaf defensive allelochemicals with elevated carbon dioxide during host plant growth. Insect responses to carbon-fertilized leaves include increased consumption with little change in growth, or alternatively, little change in consumption with decreased growth, as well as enhanced leaf digestibility, reduced nitrogen use efficiency, and reduced fecundity. The effects of plant carbon and nutrient supply on herbivores appear to result, at least in part, from independent processes affecting secondary metabolism.     

NON-PHOTOSYNTHETIC FIXATION OF CARBON DIOXIDE AND POSSIBLE BIOLOGICAL ROLES IN HIGHER PLANTS

1. Non-photosynthetic fixation of CO₂/HCO₃^- occurs both under light and dark conditions and involve the addition of carbon to substrates which in higher plants are derived originally from carbon reduced to carbohydrates during photosynthesis. Despite the endergonic nature of these carboxylations, the advantages offered seem to be sufficient to outweigh the disadvantages of energy loss. 2. Non-photosynthetic carbon incorporation into metabolism is dealt mainly in relation to PEP carboxylase, acetyl-CoA carboxylase, carbamoyl phosphate synthetase and phosphoribosylaminoimidazole carboxylase while other carboxylases await further characterization or discovery. The extent to which a carboxylase participates depends upon the need for products of its activity in metabolism. 3. Non-photosynthetic carbon fixation is intricately involved in several pathways of metabolism throughout the ontogeny of plants. The roles in relation to leaf carbon metabolism, respiratory metabolism, nitrogen metabolism, lipid and isoprenoid biosynthesis, purine and pyrimidine metabolism and metabolism associated with the action of growth regulators have been described. The fixation reactions appear to be largely concerned with the production of intermediary metabolites, circumvention of energy barriers in metabolism and regulation of plant metabolism. In addition, the activity of PEP carboxylase is involved in ionic balance and pH-stat. 4. Malate derived by way of PEP carboxylase and NAD-malate dehydrogenase acts as an effective osmoticum and a counter-ion for K⁺ accumulation in actively growing plant cells. In addition, malate may enter the TCA cycle or can be decarboxylated by cytoplasmic NADP-malic enzyme converting NADH to NADPH. Wherever it has been sought in different plant tissues, some evidence for PEP carboxylase and metabolism of malate has always been found. 5. Almost every plant process spanning from seed development and germination to flowering and fruit-set requires the essential participation of non-photosynthetic carbon fixation in regulating certain metabolic and cellular functions but it does not contribute in a major way to the carbon nutrition of plants. It is largely the tissue type that appears to determine which of the roles is predominant at any one time.     

Implications of nitrogen phloem loading for carbon metabolism and transport during Arabidopsis development

Metabolite transport processes and primary metabolism are highly interconnected. This study examined the importance of source-to-sink nitrogen partitioning, and associated nitrogen metabolism for carbon capture, transport and usage. Specifically, Arabidopsis aap8(AMINO ACID PERMEASE 8) mutant lines were analyzed to resolve the consequences of reduced amino acid phloem loading for source leaf carbon metabolism,sucrose phloem transport and sink development during vegetative and reproductive growth phase. Results showed that decreased amino acid transport had a negative effect on sink development of aap8 lines throughout the life cycle, leading to an overall decrease in plant biomass. During vegetative stage, photosynthesis and carbohydrate levels were decreased in aap8 leaves, while expression of carbon metabolism and transport genes, as well as sucrose phloem transport were not affected despite reduced sink strength.However, when aap8 plants transitioned to reproductive phase, carbon fixation and assimilation as well as sucrose partitioning to siliques were strongly decreased. Overall,this work demonstrates that phloem loading of nitrogen has varying implications for carbon fixation, assimilation and source-to-sink allocation depending on plant growth stage. It further suggests alterations in source-sink relationships, and regulation of carbon metabolism and transport by sink strength in a development-dependent manner.     

Comparative proteomic analysis of leaves between photoperiod-sensitive and photoperiod-insensitive maize inbred seedlings under long day treatments

Day length is an important environmental factor affecting the growth and development of maize (Zea mays), a short day (SD) plant grown in different latitudes. Leaf has been recognized as the light perceiving and signal producing organ. Under long day (LD) conditions, photoperiod-sensitive induction phase in maize begins at the fourth fully expanded leaf stage. However, the changes of maize leaf proteome in response to LD are largely unknown. To reveal maize proteome response to LD, proteins extracted from newly expanded fifth, sixth and seventh leaves from maize inbred line 496-10 (photoperiod sensitive) and Huangzao4 (HZ4, photoperiod insensitive) under LD treatments were compared via gel-based proteomic approach. As a result, eleven differentially expressed proteins were identified between 496-10 and HZ4 by mass spectrometry. This difference in protein accumulation was highly reproducible during the fifth to seventh leaf stages and most obvious at the seventh leaf stage. The identified proteins are mainly involved in circadian clock or iron metabolism, light harvesting and photosynthesis, nucleic acid metabolism and carbon fixation or energy metabolism. This study provides new insight into the influences of LD treatment on SD plants, such as maize, at proteome level.     

Effect of water stress on the photosynthetic apparatus of plants and the protective role of cytokinins: a review

Characteristics of photosynthetic apparatus (the pool of pigments and proteins; the activity of photosystems; the intensities of in vivo photoassimilation of carbon dioxide and in vitro activity of enzymes of carbon metabolism; leaf structure; chloroplast structure), undergoing changes under the conditions of water deficiency, have been reviewed. The protective role of cytokinins is due to their regulatory effects on the renewal of disrupted cellular structures, the condition of the stomata, and de novo synthesis and activation of proteins that are required for increasing plant resistance to water stress.     

Use of mitochondrial electron transport mutants to evaluate the effects of redox state on photosynthesis, stress tolerance and the integration of carbon/nitrogen metabolism

Primary leaf metabolism requires the co-ordinated production and use of carbon skeletons and redox equivalents in several subcellular compartments. The role of the mitochondria in leaf metabolism has long been recognized, but it is only recently that molecular tools and mutants have become available to evaluate cause-and-effect relationships. In particular, analysis of the CMSII mutant of Nicotiana sylvestris, which lacks functional complex I, has provided information on the role of mitochondrial electron transport in leaf function. The essential feature of CMSII is the absence of a major NADH sink, i.e. complex I. This necessitates re-adjustment of whole-cell redox homeostasis, gene expression, and also influences metabolic pathways that use pyridine nucleotides. In air, CMSII is not able to use its photosynthetic capacity as well as the wild type. The mutant shows up-regulation of the leaf antioxidant system, lower leaf contents of reactive oxygen species, and enhanced stress resistance. Lastly, the loss of a major mitochondrial dehydrogenase has important repercussions for the integration of primary carbon and nitrogen metabolism, causing distinct changes in leaf organic acid profiles, and also affecting downstream processes such as the biosynthesis of the spectrum of leaf amino acids.     

Shift in birch leaf metabolome and carbon allocation during long-term open-field ozone exposure

Current and future ozone concentrations have the potential to reduce plant growth and increase carbon demand for defence and repair processes, which may result in reduced carbon sink strength of forest trees in long‐term. Still, there is limited understanding regarding the alterations in plant metabolism and variation in ozone tolerance among tree species and genotypes. Therefore, this paper aims to study changes in birch leaf metabolome due to long‐term realistic ozone stress and to relate these shifts in the metabolism with growth responses. Two European white birch (Betula pendula Roth) genotypes showing different ozone sensitivity were growing under 1.4–1.7 × ambient ozone in open‐field conditions in Central Finland. After seven growing seasons, the trees were analysed for changes in leaf metabolite profiling, based on 339 low molecular weight compounds (including phenolics, polar and lipophilic compounds, and pigments) and related whole‐tree growth responses. Genotype caused most of the variance of metabolite concentrations, while ozone concentration was the second principal component explaining the metabolome profiling. The main ozone caused changes included increases in quercetin‐phenolic compounds and compounds related to leaf cuticular wax layer, whereas several compounds related to carbohydrate metabolism and function of chloroplast membranes and pigments (such as chlorophyll‐related phytol derivatives) were decreasing. Some candidate compounds such as surface wax‐related squalene, 1‐dotriacontanol, and dotriacontane, providing growth‐related tolerance against ozone were demonstrated. This study indicated that current growth‐based ozone risk assessment methods are inadequate, because they ignore ecophysiological impacts due to alterations in leaf chemistry.     

Leaf life span plasticity in tropical seedlings grown under contrasting light regimes

BACKGROUND AND AIMS: The phenotypic plasticity of leaf life span in response to low resource conditions has a potentially large impact on the plant carbon budget, notably in evergreen species not subject to seasonal leaf shedding, but has rarely been well documented. This study evaluates the plasticity of leaf longevity, in terms of its quantitative importance to the plant carbon balance under limiting light. METHODS: Seedlings of four tropical tree species with contrasting light requirements (Alstonia scholaris, Hevea brasiliensis, Durio zibethinus and Lansium domesticum) were grown under three light regimes (full sunlight, 45 % sunlight and 12 % sunlight). Their leaf dynamics were monitored over 18 months. RESULTS: All species showed a considerable level of plasticity with regard to leaf life span: over the range of light levels explored, the ratio of the range to the mean value of life span varied from 29 %, for the least plastic species, to 84 %, for the most. The common trend was for leaf life span to increase with decreasing light intensity. The plasticity apparent in leaf life span was similar in magnitude to the plasticity observed in specific leaf area and photosynthetic rate, implying that it has a significant impact on carbon gain efficiency when plants acclimate to different light regimes. In all species, median survival time was negatively correlated with leaf photosynthetic capacity (or its proxy, the nitrogen content per unit area) and leaf emergence rate. CONCLUSIONS: Longer leaf life spans under low light are likely to be a consequence of slower ageing as a result of a slower photosynthetic metabolism.     

Elevated sucrose-phosphate synthase activity in transgenic tobacco sustains photosynthesis in older leaves and alters development

Constitutive over-expression of a maize sucrose-phosphate synthase (SPS) gene in tobacco (Nicotiana tabacum) had major effects on leaf carbohydrate budgets with consequences for whole plant development. Transgenic tobacco plants flowered earlier and had greater flower numbers than wild-type plants. These changes were not linked to modified source leaf carbon assimilation or carbon export, although sucrose to starch ratios were significantly higher in leaves expressing the transgene. The youngest and oldest leaves of plants over-expressing SPS had up to 10-fold wild-type maximal extractable SPS activity, but source leaf SPS activities were only 2-3 times greater in these lines than in the wild type. In the oldest leaves, where the expression of the transgene led to the most marked enhancement in SPS activity, photosynthesis was also increased. It was concluded that these increases in the capacity for sucrose synthesis and carbon assimilation, particularly in older leaves, accelerate the whole plant development and increase the abundance of flowers without substantial changes in the overall shoot biomass.     

Leaf expansion and cell division are affected by reducing absorbed light before but not after the decline in cell division rate in the sunflower leaf

The effect of absorbed photosynthetic photon flux density (PPFD) on leaf expansion is a key issue for analysing the phenotypic variability between plants and for modelling feedback loops. Expansion and epidermal cell division in leaf 8 of sunflower were analysed in a series of five experiments where absorbed photosynthetic photon flux density (PPFD) was reduced either by shading or by covering part of the leaf area. These treatments were imposed at different times during leaf development. Expansion and cell division were affected by a reduction in absorbed PPFD only in the first part of leaf development, while the leaf area was less than 2% of its final value and while absolute expansion rate was slow. In contrast, it was not affected if imposed later when the leaf was visible and absolute expansion rate was at maximum. A reduction in absorbed PPFD caused the same reduction in expansion and in cell division whether it was due to a reduction in incident PPFD or to a reduction in photosynthetic leaf area, suggesting that carbon metabolism was involved. Relative expansion rate recovered to control levels when relative division rate began to decline, in all experiments and in all zones of a leaf. This was probably linked to the source–sink transition, after which the leaf had such a high priority in carbon allocation that it was largely insensitive to changes in absorbed PPFD. The final leaf area was therefore closely related to the cumulated PPFD absorbed by the plant from leaf initiation to the end of exponential cell division.     

植物海藻糖代谢及海藻糖-6-磷酸信号研究进展

海藻糖代谢和海藻糖-6-磷酸（T6P）信号途径在植物生长和发育过程中具有重要的调控作用。T6P是海藻糖的代谢前体,是植物响应碳元素可用性、调控生长发育的关键信号分子。植物体中除了自身的海藻糖合成途径外,由病原菌产生的海藻糖或T6P能够导致植物代谢和发育的重新编程。植物不同阶段的生长发育,包括胚胎发育、幼苗生长、成花诱导及叶片衰老等,都受T6P的调控。T6P信号的一个关键互作因子是蔗糖非发酵相关激酶1（SnRKl）,T6P能够抑制SnRK1的催化活性,进而调控植物的生长和发育过程。     

内蒙古高原湖滨湿地优势植物功能性状特征及其适应性

受人类活动和气候变化的影响,湖泊湖滨带退化速度显著加快。植物功能性状的方法可以量化植物特征,预测植物对外界环境干扰的反应,有助于理解退化湖滨带湿地植物应对环境变化所表现出的适应机制,对湖泊湖滨湿地生态系统植被的恢复与重建具有重要意义。在内蒙古高原典型湖泊湖滨湿地选取芦苇（Phragmites australis）、赖草（Leymus secalinus）、毛茛（Ranunculus japonicus）、鹅绒委陵菜（Potentilla anserina）、碱蓬（Suaeda glauca）、盐角草（Salicornia europaea）和拂子茅（Calamagrostis epigeios）7种优势植物的叶片和根系作为研究对象,对不同湿地植物的11种功能性状变化规律及其与环境因子的关系进行研究。旨在探究环境变化影响下湖滨带湿地植物的物种分布和功能性状的差异,以及湿地植物在不同湖滨带湿地生境下的适应策略。在评估植物功能性状差异基础上,采用环境矩阵连接性状矩阵（RLQ）结合第四角分析（Fourth-Corner）的方法分析环境因子对植物功能性状的影响。结果表明,内蒙古湖滨带湿地中7种优势植物为了适应不同的环境的影响,植物的功能性状均产生不同程度的种间与种内变异,在湖滨带湿地中植物的植株高度、叶片碳含量、叶片氮含量、叶片碳氮比、比根长、根组织密度、根氮含量对环境变化的响应比较敏感,土壤pH与叶片干物质含量呈显著负相关;土壤盐分与植株高度、叶片碳含量和叶碳氮比显著负相关,与叶片氮含量、根组织密度显著正相关;土壤的总氮含量与植株高度显著正相关,与比根长显著负相关;土壤碳氮比与植株高度和叶片碳含量显著负相关,与植物比根长显著正相关;土壤容重与根氮含量显著负相关。研究表明内蒙古高原湖滨带湿地植物的功能性状受环境的作用强烈,植物采取了不同的性状策略来适应环境。     

Leaf senescence in response to elevated atmospheric CO<Subscript>2</Subscript> concentration and low nitrogen supply

This review reports the physiological and metabolic changes in plants during development under elevated atmospheric carbon dioxide concentration and/or limited-nitrogen supply in order to establish their effects on leaf senescence induction. Elevated CO₂ concentration and nitrogen supply modify gene expression, protein content and composition, various aspects of photosynthesis, sugar metabolism, nitrogen metabolism, and redox state in plants. Elevated CO₂ usually causes sugar accumulation and decreased nitrogen content in plant leaves, leading to imbalanced C/N ratio in mature leaves, which is one of the main factors behind premature senescence in leaves. Elevated CO₂ and low nitrogen decrease activities of some antioxidant enzymes and thus increase H₂O₂ production. These changes lead to oxidative stress that results in the degradation of photosynthetic pigments and eventually induce senescence. However, this accelerated leaf senescence under conditions of elevated CO₂ and limited nitrogen can mobilize nutrients to growing organs and thus ensure their functionality.     

Contribution of chloroplast biogenesis to carbon–nitrogen balance during early leaf development in rice

Chloroplast biogenesis is most significant during the changes in cellular organization associated with leaf development in higher plants. To examine the physiological relationship between developing chloroplasts and host leaf cells during early leaf development, we investigated changes in the carbon and nitrogen contents in leaves at the P4 developmental stage of rice, during which leaf blade structure is established and early events of chloroplast differentiation occur. During the P4 stage, carbon content on a dry mass basis remained constant, whereas the nitrogen content decreased by 30%. Among carbohydrates, sucrose and starch accumulated to high levels early in the P4 stage, and glucose, fructose and cellulose degradation increased during the mid-to-late P4 stage. In the chloroplast-deficient leaves of the virescent-1 mutant of rice, however, the carbon and nitrogen contents, as well as the C/N ratio during the P4 stage, were largely unaffected. These observations suggest that developing rice leaves function as sink organs at the P4 stage, and that chloroplast biogenesis and carbon and nitrogen metabolism in the leaf cell is regulated independently at this stage.     

中国东北典型森林生态系统植物叶片的非结构性碳水化合物研究

植物叶片的非结构性碳水化合物(NSC)不仅可以反应植物的碳供应状况,也能反应植物对外界环境的适应策略。利用传统的蒽酮比色法测定了东北3个典型森林生态系统(呼中、凉水和长白山)242种常见植物叶片的非结构碳水化合物,探讨了温带主要森林植物叶片NSC沿纬度梯度的变化趋势及其在物种-生活型-群落间的分布规律。实验结果表明:3个典型森林生态系统植物叶片可溶性糖、淀粉和NSC含量均呈偏正态分布,多数物种的含量偏中低水平;242种植物叶片可溶性糖、淀粉和NSC的平均含量分别为63.31、65.66和128.96 mg/g。在所调查的森林生态系统中,叶片可溶性糖、淀粉和NSC含量在不同生活型中表现各异。此外,乔木植物叶片的可溶性糖、淀粉和NSC含量从北到南呈递增趋势,呼中最低,凉水次之,长白山最高。乔木淀粉含量均表现为落叶树种大于常绿树种,可溶性糖和NSC含量变化趋势复杂。研究结果不仅为阐明东北主要森林生态系统植被碳代谢和生长适应对策提供数据基础,而且对理解植物对未来气候变化的响应机理提供数据支撑。     

Chemical and isotopic markers detect water deficit and its influence on nutrient allocation in Phaseolus vulgaris

The impact of drought on plant growth and yield has been widely studied and is considered a major limitation to crops reaching yield potential. Less known is the impact of water deficit on the nutritional quality of the resulting yield. This study characterised the impact of water deficit on carbon assimilation, modelled water use efficiency from carbon isotope discrimination and analysed the concentration of mineral nutrients, amino acids and sugars in leaf, phloem and pod pools collected from Phaseolus vulgaris L. (common bean) grown in a controlled environment. Water deficit led to an isohydric response, impacting on carbon isotope abundance in all tissues though not translating to any significant treatment differences in water use efficiency or nutrient content in tissues over the course of plant development. The results obtained in this study demonstrate that nutrient content of P. vulgaris yield was not impacted by the availability of water. The absence of significant changes in the nutrient content of individual seeds highlights the plasticity of developing reproductive tissue to changes in whole plant water availability.     

New insights into the cellular mechanisms of plant growth at elevated atmospheric carbon dioxide concentrations

Rising atmospheric carbon dioxide concentration ([CO₂]) significantly influences plant growth, development, and biomass. Increased photosynthesis rate, together with lower stomatal conductance, has been identified as the key factors that stimulate plant growth at elevated [CO₂] (e[CO₂]). However, variations in photosynthesis and stomatal conductance alone cannot fully explain the dynamic changes in plant growth. Stimulation of photosynthesis at e[CO₂] is always associated with post‐photosynthetic secondary metabolic processes that include carbon and nitrogen metabolism, cell cycle functions, and hormonal regulation. Most studies have focused on photosynthesis and stomatal conductance in response to e[CO₂], despite the emerging evidence of e[CO₂]'s role in moderating secondary metabolism in plants. In this review, we briefly discuss the effects of e[CO₂] on photosynthesis and stomatal conductance and then focus on the changes in other cellular mechanisms and growth processes at e[CO₂] in relation to plant growth and development. Finally, knowledge gaps in understanding plant growth responses to e[CO₂] have been identified with the aim of improving crop productivity under a CO₂ rich atmosphere.