Light effects on α-amylase activity and carbohydrate content in relation to lipid mobilization during the seedling growth of sunflower

The changes in α-amylase activity and in starch and free sugar content were investigated in correlation with lipid mobilization inHelianthus annuus during the first 15 days of seedling growth in discontinuous light and in darkness. Throughout the seedling development α-amylase activity increased more significantly in light than in darkness. It was always lower in cotyledons than in other tissues of the embryo axis. In both culture conditions, most of the transitory carbohydrates accumulated in germinating cotyledons were very likely synthesized by gluconeogenesis from the stored lipid breakdown. Nevertheless, in light-grown cotyledons, photosynthesis contributes to increase the carbohydrate levels. The study of several soluble sugars indicates that 1) sucrose stored in cotyledons of mature seeds was used at the onset of seedling growth, more rapidly in light than in darkness, 2) galactose and xylose, both involved as precursors of some cell-wall polysaccharides, remained at a very low level throughout the 15 days and 3) glucose, fructose and maltose accumulated in old etiolated cotyledons in contrast to what occurred in the light.     

Salt stress differentially regulates mobilisation of carbon and nitrogen reserves during seedling establishment of Pityrocarpa moniliformis

• Seedling establishment is a critical step in environment colonisation by higher plants that frequently occurs under adverse conditions. Thus, we carried out an integrated analysis of seedling growth, water status, ion accumulation, reserve mobilisation, metabolite partitioning and hydrolase activity during seedling establishment of the native Caatinga species Piptadenia moniliformis (Benth.) Luckow & R.W. Jobson under salinity.
• Two‐day‐old seedlings were cultivated in vitro for 4 days in water agar (control) or supplemented with 50 or 100 mm NaCl. Biochemical determinations were performed according to standard spectrophotometric protocols.
• We found that 100 mm NaCl stimulated starch degradation, amylase activity and soluble sugar accumulation, but limited storage protein hydrolysis in the cotyledons of P. moniliformis seedlings. Although Na⁺ accumulation in the seedling affected K⁺ partitioning between different organs, it was not possible to associate the salt‐induced changes in reserve mobilisation with Na⁺ toxicity, or water status, in the cotyledons. Remarkably, we found that starch content increased in the roots of P. moniliformis seedlings under 100 mm NaCl, probably in response to the toxic effects of Na⁺.
• The mobilisation of carbon and nitrogen reserves is independently regulated in P. moniliformis seedlings under salt stress. The salt‐induced delay in seedling establishment and the resulting changes in the source–sink relationship may lead to storage protein retention in the cotyledons. Possibly, the intensification of starch mobilisation in the cotyledons supported starch accumulation in the root as a potential mechanism to mitigate Na⁺ toxicity.

     

Sulla carnosa modulates root invertase activity in response to the inhibition of long‐distance sucrose transport under magnesium deficiency

• Being the principal product of photosynthesis, sucrose is involved in many metabolic processes in plants. As magnesium (Mg) is phloem mobile, an inverse relationship between Mg shortage and sugar accumulation in leaves is often observed.
• Mg deficiency effects on carbohydrate contents and invertase activities were determined in Sulla carnosa Desf. Plants were grown hydroponically at different Mg concentrations (0.00, 0.01, 0.05 and 1.50 mM Mg) for one month.
• Mineral analysis showed that Mg contents were drastically diminished in shoots and roots mainly at 0.01 and 0.00 mM Mg. This decline was adversely associated with a significant increase of sucrose, fructose and mainly glucose in shoots of plants exposed to severe deficiency. By contrast, sugar contents were severely reduced in roots of these plants indicating an alteration of carbohydrate partitioning between shoots and roots of Mg‐deficient plants. Cell wall invertase activity was highly enhanced in roots of Mg‐deficient plants, while the vacuolar invertase activity was reduced at 0.00 mM Mg. This decrease of vacuolar invertase activity may indicate the sensibility of roots to Mg starvation resulting from sucrose transport inhibition. ¹⁴CO₂ labeling experiments were in accordance with these findings showing an inhibition of sucrose transport from source leaves to sink tissues (roots) under Mg depletion.
• The obtained results confirm previous findings about Mg involvement in photosynthate loading into phloem and add new insights into mechanisms evolved by S. carnosa to cope with Mg shortage in particular the increase of the activity of cell wall invertase.

     

Effects of chilling stress on the accumulation of soluble sugars and their key enzymes in <Emphasis Type="Italic">Jatropha curcas</Emphasis> seedlings

As osmolytes and signaling molecules, soluble sugars participate in the response and adaptation of plants to environmental stresses. In the present study, we measured the effect of chilling (12 °C) stress on the contents of eight soluble sugars in the leaves, cotyledons, stems, and roots of Jatropha curcas seedlings, as well as on the activities of eight rate-limiting enzymes that are critical to the metabolism of those soluble sugars. Chilling stress promoted both starch hydrolysis and soluble sugar accumulation. The soluble sugar contents of the leaves and cotyledons were affected more than that of the stems and roots. Meanwhile, the activities of the corresponding metabolic enzymes (e.g., β-amylase, uridine diphosphate glucose phosphorylase, and sucrose phosphate synthase) also increased in some organs. The gradual increase of soluble neutral alkaline invertase activity in the four studied organs suggested that sucrose catabolic production, such as glucose and fructose, was especially important in determining resistance to chilling stress and hexose signal transduction pathway. In addition, the substantial accumulation of raffinose family oligosaccharides and increase in corresponding metabolic enzyme activity suggested that galactinol and raffinose play an important role in determining the chilling resistance of J. curcas. Together, these findings establish a foundation for determining the relationship between the chilling resistance and soluble sugar accumulation of J. curcas and for investigating the mechanisms underlying sugar signaling transduction and stress responses.     

S‐nitrosylation/denitrosylation as a regulatory mechanism of salt stress sensing in sunflower seedlings

Nitric oxide (NO) and various reactive nitrogen species produced in cells in normal growth conditions, and their enhanced production under stress conditions are responsible for a variety of biochemical aberrations. The present findings demonstrate that sunflower seedling roots exhibit high sensitivity to salt stress in terms of nitrite accumulation. A significant reduction in S‐nitrosoglutathione reductase (GSNOR) activity is evident in response to salt stress. Restoration of GSNOR activity with dithioerythritol shows that the enzyme is reversibly inhibited under conditions of 120 mM NaCl. Salt stress‐mediated S‐nitrosylation of cytosolic proteins was analyzed in roots and cotyledons using biotin‐switch assay. LC‐MS/MS analysis revealed opposite patterns of S‐nitrosylation in seedling cotyledons and roots. Salt stress enhances S‐nitrosylation of proteins in cotyledons, whereas roots exhibit denitrosylation of proteins. Highest number of proteins having undergone S‐nitrosylation belonged to the category of carbohydrate metabolism followed by other metabolic proteins. Of the total 61 proteins observed to be regulated by S‐nitrosylation, 17 are unique to cotyledons, 4 are unique to roots whereas 40 are common to both. Eighteen S‐nitrosylated proteins are being reported for the first time in plant systems, including pectinesterase, phospholipase d ‐alpha and calmodulin. Further physiological analysis of glyceraldehyde‐3‐phosphate dehydrogenase and monodehydroascorbate reductase showed that salt stress leads to a reversible inhibition of both these enzymes in cotyledons. However, seedling roots exhibit enhanced enzyme activity under salinity stress. These observations implicate the role of S‐nitrosylation and denitrosylation in NO signaling thereby regulating various enzyme activities under salinity stress in sunflower seedlings.     

Increased Na+ and Cl− accumulation induced by NaCl salinity inhibits cotyledonary reserve mobilization and alters the source-sink relationship in establishing dwarf cashew seedlings

We studied the NaCl-induced changes in cotyledons and the embryonic axis of establishing dwarf cashew (Anacardium occidentale) seedlings. The salt stress reduced the growth of dwarf cashew seedlings, and this response was related to the inhibition of cotyledonary reserve depletion. Lipid mobilization was inhibited by NaCl due to reduced lipase activity in the emerging and establishing seedlings. Additionally, there was reduced transient starch accumulation in the cotyledons of the salt-stressed seedlings that was associated with lower starch synthase activity at the early developmental stages and inhibited amylolytic and starch phosphorylase activities at the established seedling stage. The NaCl-induced changes in lipid and starch metabolism influenced the soluble sugar content in the cotyledons. Protein mobilization was inhibited by NaCl, and we observed the accumulation of amino acids and the inhibition of proteolytic activity in the cotyledons of the salt-stressed established seedlings. Salinity significantly reduced the free amino acid and reducing sugar contents in the embryonic axes of both emerged and established seedlings, whereas the non-reducing sugar content was affected by this stress only in the established seedlings. The Na⁺ and Cl^? contents progressively increased in the cotyledons and embryonic axis of the seedlings as the salinity increased. We conclude that salt stress inhibits dwarf cashew seedling establishment by inhibiting the mobilization of reserves, an inhibition that was related to increased Na⁺ and Cl^? accumulation in the cotyledons. Additionally, these toxic ions reduced the sink strength of the embryonic axis with regard to the products of cotyledonary reserve mobilization.     

Inter organ comparison of amylases and starch content in mungbean seedlings

During seedling growth of mungbean in dark, depletion of cotyledonary starch is reflected by an increase in starch content of root and shoot. With progress of seedling growth, amylolytic activity increases in all organs i.e. cotyledons, shoots and roots. A rapid turnover of starch in shoots and roots has been proposed. Amylase activity of seedlings was in the order of cotyledons>shoots>roots. Five days after germination (DAG) α-amylase from cotyledons of mungbean seedlings was purified using ammonium sulphate precipitation, DEAE cellulose and sephadex G-150 column chromatography. Phytic acid was a stronger inhibitor of α-amylase than EDTA. Phytic acid, Hg²⁺, Zn²⁺ and Mn²⁺ were non-competitive inhibitors and the corresponding Ki values were 5.0–5.7, 0.36–0.38, 2.6–3.8 and 0.7–0.8 mol·M⁻³. Elution patterns of α-amylases of cotyledons, shoots and roots on sephadex G-100 column showed that cotyledonary α-amylase had a higher molecular mass than that of shoot and root α-amylases which had identical molecular masses. All α-amylases showed the same optimum pH 5.0 whereas optimum temperature was 55 °C for cotyledonary and 45 °C for shoot and root α-amylases. In all these tissues α-amylases were stable to 30 min heat treatment at 50 °C however unlike cereal α-amylases they lost activity at 70 °C. Km for α-amylases from cotyledons, shoots and roots with starch was 1.9, 4.3 and 6.6 mg per cm³, respectively. α-amylase of cotyledons and roots showed activity in reactions with various substrates in the order of starch>amylose>dextrin-I=dextrin-IV>α-cyclodextrin=β-cyclodextrin>amylopectin>pullulan. The shoot α-amylase showed high activity with amylopectin, which was comparable with that obtained with amylose, and the activity with α and β-cyclodextrin was higher in comparison with dextrin-I and IV. The α-amylases from these tissues liberated maltose, maltotriose and higher oligosaccharides from starch. It could be concluded that amylases from different organs of a seedling could have different physical and kinetic properties.     

干旱胁迫下紫花苜蓿幼苗非结构性碳水化合物代谢对NO的响应

以紫花苜蓿（Medicago sativa）为材料,采用盆栽试验方法,用聚乙二醇（PEG-6000）作为渗透介质模拟干旱胁迫,外源喷施NO供体硝普钠,NO清除剂（carboxy-PTIO,cPTIO）,对紫花苜蓿幼苗叶片、根系中非结构性碳水化合物含量及相关酶活性的变化进行研究,探讨NO对紫花苜蓿耐旱机制的作用。结果表明：外源NO促进了紫花苜蓿叶片中淀粉的分解、根系中淀粉的积累,提高叶片及根系中可溶性糖（蔗糖、果糖和葡萄糖）含量,降低了渗透势,促进细胞吸水,缓解干旱造成的损伤。此外,外源NO能提高干旱胁迫下紫花苜蓿叶片中蔗糖合成酶（SS）、酸性转化酶（AI）和中性转化酶（NI）活性,降低了蔗糖磷酸合成酶（SPS）的活性,提高根系中SS、SPS和转化酶活性,使蔗糖的合成与分解处于高水平的动态平衡,增强了紫花苜蓿的抗旱性。而NO清除剂cPTIO则会不同程度的抑制紫花苜蓿幼苗中非结构性碳水化合物（NSC）及其相关酶活性。因此,NO可以通过调控NSC的代谢响应干旱胁迫,缓解干旱胁迫造成的不利影响,在紫花苜蓿的抗旱中扮演着重要的角色。     

Piriformospora indica inoculation alleviates the adverse effect of NaCl stress on growth,gas exchange and chlorophyll fluorescence in tomato (Solanum lycopersicum L.)

• Salinity is now an increasingly serious environmental issue that affects the growth and yield of many plants.
• In the present work, the influence of inoculation with the symbiotic fungus, Piriformospora indica, on gas exchange, water potential, osmolyte content, Na/K ratio and chlorophyll fluorescence of tomato plants under three salinity levels (0, 50, 100 and 150 mm NaCl) and three time periods (5, 10 and 15 days after exposure to salt) was investigated.
• Results indicate that P. indica inoculation improved growth parameters of tomato under salinity stress. This symbiotic fungus significantly increased photosynthetic pigment content under salinity, and more proline and glycine betaine accumulated in inoculated roots than in non‐inoculated roots. P. indica further significantly improved K⁺ content and reduced Na⁺ level under salinity treatment. After inoculation with the endophytic fungus, leaf physiological parameters, such as water potential, net photosynthesis, stomatal conductance and transpiration, were all higher under the salt concentrations and durations compared with controls without P. indica. With increasing salt level and salt treatment duration, values of F₀ and qP increased but F_m, F_v/F_m, F′_v/F′_m and NPQ declined in the controls, while inoculation with P. indica improved these values.
• The results indicate that the negative effects of NaCl on tomato plants were alleviated after P. indica inoculation, probably by improving physiological parameters such as water status and photosynthesis.

     

Excess nickel modulates activities of carbohydrate metabolizing enzymes and induces accumulation of sugars by upregulating acid invertase and sucrose synthase in rice seedlings

The effects of increasing concentrations of nickel sulfate, NiSO₄ (200 and 400 μM) in the growth medium on the content of starch and sugars and activity levels of enzymes involved in starch and sugar metabolism were examined in seedlings of the two Indica rice cvs. Malviya-36 and Pant-12. During a 5–20 day growth period of seedlings in sand cultures, with Ni treatment, no definite pattern of alteration in starch level could be observed in the seedlings. In both roots and shoots of the seedlings Ni treatment led to a significant decrease in activities of starch degrading enzymes α-amylase, β-amylase, whereas starch phosphorylase activity increased. The contents of reducing, non-reducing, and total sugars increased in Ni-treated rice seedlings with a concomitant increase in the activities of sucrose degrading enzymes acid invertase and sucrose synthase. However, the activity of sucrose synthesizing enzyme sucrose phosphate synthase declined. These results suggest that Ni toxicity in rice seedlings causes marked perturbation in metabolism of carbohydrates leading to increased accumulation of soluble sugars. Such perturbation could serve as a limiting factor for growth of rice seedlings in Ni polluted environments and accumulating soluble sugars could serve as compatible solutes in the cells under Ni toxicity conditions.     

Sugar and starch changes in pea cotyledons during germination

Changes in starch and sugar contents in the cotyledons during germination have been compared in a smooth (cv. Alaska) and a wrinkled (cv. Progress) cultivar of the garden pea ( Pisum sativum L.). In both cultivars there was an initial accumulation of sucrose due to the hydrolysis of sucrosyl oligosaccharides, but galactose did not accumulate in the cotyledons. Starch mobilization in the Progress pea was linear with time and started before the rise in α-amylase (EC 3.2.1.1) activity in the cotyledons; sucrose was synthesized in the cotyledons, and their excision from the axis resulted in an additional accumulation of this sugar. In the Alaska pea, the onset of starch hydrolysis coincided with the rise in α-amylase activity; no accumulation of sucrose was found in excised cotyledons, whilst the sucrose content decreased continuously in attached cotyledons.
The same sugars were found in the cotyledons of both cultivars, suggesting a common pathway for starch breakdown. Maltose, maltotriose and linear malto-dextrins were not present and only trace amounts of glucose were detected, suggesting a degradation of starch by phosphorylase after an initial attack by α-amylase. α-Amylase activity in the cotyledons was higher in the presence of the axis, but was influenced by the water content of the cotyledons. Transient changes in α-amylase activity correlated well with changes in the rate of starch hydrolysis, but after 2–3 days starch mobilization was reduced in excised cotyledons probably due to the resynthesis of starch.     

Intrinsic morphology and spatial distribution of non-structural carbohydrates contribute to drought resistance of two mulberry cultivars

• Drought is one of the most adverse environmental stresses limiting plant growth and productivity. However, the underlying mechanisms regarding metabolism of non-structural carbohydrates (NSC) in source and sink organs are still not fully elucidated in woody trees.
• Saplings of mulberry cv Zhongshen1 and Wubu were subjected to a 15-day progressive drought stress. NSC levels and gene expression involved in NSC metabolism were investigated in roots and leaves. Growth performance and photosynthesis, leaf stomatal morphology, and other physiological parameters were also analysed.
• Under well-watered conditions, Wubu had a higher R/S, with higher NSC in leaves than in roots; Zhongshen1 had a lower R/S with higher NSC in roots than leaves. Under drought stress, Zhongshen1 showed decreased productivity and increased proline, abscisic acid, ROS content and activity of antioxidant enzymes, while Wubu sustained comparable productivity and photosynthesis. Interestingly, drought resulted in decreased starch and slightly increased soluble sugars in leaves of Wubu, accompanied by notable downregulation of starch-synthesizing genes and upregulation of starch-degrading genes. Similar patterns in NSC levels and relevant gene expression were also observed in roots of Zhongshen1. Concurrently, soluble sugars decreased and starch was unchanged in roots of Wubu and leaves of Zhongshen1. However, gene expression of starch metabolism in roots of Wubu was unaltered, but in leaves of Zhongshen1 starch metabolism was more activated.
• These findings revealed that intrinsic R/S and spatial distribution of NSC in roots and leaves concomitantly contribute to drought resistance in mulberry.

     

Photomorphogenesis of the root system in developing sunflower seedlings: a role for sucrose

• The domestic sunflower (Helianthus annuus L. cv. ‘Giganteus’) has been used since the 19th century as a model plant for the study of seedling development in darkness and white light (WL) (scoto‐ versus photomorphogenesis). However, most pertinent studies have focused on the developmental patterns of the hypocotyl and cotyledons, whereas the root system has been largely ignored.
• In this study, we analysed entire sunflower seedlings (root and shoot) and quantified organ development in the above‐ and belowground parts of the organism under natural (non‐sterile) conditions.
• We document that seedlings, raised in moist vermiculite, are covered with methylobacteria, microbes that are known to promote root development in Arabidopsis. Quantitative data revealed that during photomorphogenesis in WL, the root system expands by 90%, whereas stem elongation is inhibited, and hook opening/cotyledon expansion occurs. Root morphogenesis may be mediated via imported sucrose provided by the green, photosynthetically active cotyledons. This hypothesis is supported by the documented effect of sucrose on the induction of lateral root initials in sunflower cuttings. Under these experimental conditions, phytohormones (auxin, cytokinin, brassinolide) exerted little effect on root and cotyledon expansion, and no hormone‐induced initiation of lateral roots was observed.
• It is concluded that sucrose not only acts as an energy source to fuel cell metabolism but is also a shoot‐derived signalling molecule that triggers root morphogenesis.

     

Physiological and Biochemical Characteristics of Sugar Beet Plants Grown at an Increased Carbon Dioxide Concentration and at Various Nitrate Doses

Three-week-old sugar beet (Beta vulgaris L.) seedlings were grown for an additional four weeks under controlled conditions: in river sand watered with a modified Knop mixture containing one half-fold (0.5N), standard (1N), and or threefold (3N) nitrate amount, at the irradiance of 90 W/m² PAR, and at the carbon dioxide concentrations of 0.035% (1C treatment) or 0.07% (2C treatment). The increase in the carbon dioxide concentration and in the nitrogen dose resulted in an increase in the leaf area and the leaf and root dry weight per plant. With the increase in the nitrogen dose, morphological indices characterizing leaf growth increased more noticeably in 1C plants than in 2C plants. And vice versa, the effects of increased CO₂ concentration were reduced with the increase in the nitrogen dose. Roots responded to the changes in the CO₂ and nitrate concentrations otherwise than leaves. At a standard nitrate dose (1N), the contents of proteins and nonstructural carbohydrates (sucrose and starch) in leaves depended little on the CO₂ concentration. At a double CO₂ concentration, the content of chlorophyll somewhat decreased, and the net photosynthesis rate (P _n) calculated per leaf area unit increased. An increase in the nitrogen dose did not affect the leaf carbohydrate content of the 1C and 2C plants except the leaves of the 2C-3N plants, where the carbohydrate content decreased. In 1C and 2C plants, an increase in the nitrogen dose caused an increase in the protein and chlorophyll content. Specific P _n values somewhat decreased in 1C-0.5N plants and had hardly any dependence on the nitrate dose in the 2C plants. The carbohydrate content in roots did not depend on the CO₂ concentration, and the content was the highest at 0.5N. Characteristic nitrogen dose-independent acclimation of photosynthesis to an increased carbon dioxide concentration, which was postulated previously [1], was not observed in our experiments with sugar beet grown at doubled carbon dioxide concentration.     

Carbohydrate metabolism in one- and two-year-old spruce needles, and stem carbohydrates from three months before until three months after bud break

Starch and sucrose metabolism of one- and two-year-old needles of Norway spruce (Picea abies [L.] Karst., about 30 years old) was investigated from three months before until three months after bud break at a natural site. We distinguish different metabolic states according to the extractable activities of enzymes (α-amylase [EC 3.2.1.1], ADP-glucose pyrophosphorylase [AGP, EC 2.7.7.27], D-enzyme [EC 2.4.1.25], starch phosphorylase [STP. EC 2.4.1.1]), sucrose phosphate synthase [SPS, EC 2.4.1.14], sucrose syntbase [SS, EC 2.4.1.13]. acid invertase [AI, EC 3.2.1.261) and pool sizes of related metabolites (starch, glucose, fructose, sucrose, raffinose, stachyose, fructose 6-phosphate [F6P], glucose 6-phosphate [G6P], fructose 2,6-bisphosphate [F26BP], and inorganic phosphate [P₁]). The period ending with bud break was characterized by high rates of net photosynthesis, a pronounced decrease in the amount of soluble sugars, and a steep rise in starch (from the detection limit to approximately 600 nmol glycosyl units [mg dry weight]_-1). In parallel, the extractable activity of AGP increased, while D-enzyme was on a relative high level when compared with the period after bud break. With respect to sucrose metabolism, F26BP, an inhibitor of sucrose synthesis, decreased from 1 to 0.4 pmol (mg dry weight)_-1. This was complemented by SPS activity, which was due to both increased protein levels shown by immunoblotting and activation under metabolite control (high levels of G6P and a low P_i/G6P ratio). This indicates a high capacity of synthesis of starch and sucrose in the period before bud break. These observations are in accordance with estimates of photosynthetic carbon gain, which indicate that in early spring large amounts of carbon from current photosynthesis are exported out of the needles. In addition, the content of nonstructural carbohydrates (expressed as hexoses) increased in the bark of the stem. This could also be a consequence of an enhanced carbon export from the needles. After the onset of bud break, starch concentration decreased in all tissues under investigation. In contrast, the level of total nonstructural carbohydrates in the outermost sapwood nearly doubled from bud break until the end of sampling. In the needles, net photosynthesis was reduced by about 75% and a decrease in SPS activity and protein level were found together with lower G6P concentration, and an increased P_i/G6P ratio. These results suggest that during that period sucrose synthesis was reduced in the older needles. In addition, under conditions of reduced photosynthesis, carbon demand of current year needles was in part ensured by the mobilization of starch in the older needles. Taken together our data show that before bud break carbon metabolism of mature leaves is related with the sink demands of storage organs. After bud break the accumulated assimilate pools in needles and stem, mainly the bark, are mobilized and support carbon supply to new tissues.     

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress

• Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown.
• The present study investigated the impact of exogenously applied ACh (10 µm ) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150 mm NaCl).
• Salinity reduced root hydraulic conductivity while ACh‐treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin‐IX, Mg‐photoporphyrin‐IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up‐regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO₂ concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity‐induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio.
• Results of the present study indicate that ACh can be effective in ameliorating NaCl‐induced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

     

Effect of aluminium on metabolism of starch and sugars in growing rice seedlings

The effect of increasing concentrations of Al₂(SO₄)₃ in situ on the content of starch, sugars and activity behaviour of enzymes related to their metabolism were studied in growing seedlings of two rice cvs. Malviya-36 and Pant-12 in sand cultures. Al₂(SO₄)₃ levels of 80 and 160 μM in the growth medium caused an increase in the contents of starch, total sugars as well as reducing sugars in roots as well as shoots of the rice seedlings during a 5–20 days growth period. The activities of the enzymes of starch hydrolysis α-amylase, β-amylase and starch phosphorylase declined in Al-exposed seedlings, whereas the activities of sucrose hydrolyzing enzymes sucrose synthase and acid invertase increased in the seedlings due to Al³⁺ treatment. The enzyme of sucrose synthesis, sucrose phosphate synthase showed decreased activity in Al³⁺ treated seedlings compared to controls. Results suggest that Al³⁺ toxicity in rice seedlings impairs the metabolism of starch and sugars and favours the accumulation of hexoses by enhancing the activities of sucrose hydrolyzing enzymes.     

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.