Biosynthesis of vitamins B by the fungus Pleurotus ostreatus in a submerged culture

The intra- and extracellular contents of vitamins were studied in the course of submerged cultivation of the higher basidial mushroom Pleurotus ostreatus (Jacq.: Fr.) Kummer st. IMBF-1300 on liquid nutrient media. This strain was found to be autotrophic in respect of thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B5), pyridoxine (vitamin B6) and biotin (vitamin B7), but it failed to synthesize cyanocobalamin (vitamin B12). The composition and pH of the culture medium, containing such complex biostimulating supplements as maize extract and concentrated potato sap noticeably influence the contents of vitamins B1, B5 and B7 in the mycelium, and to a less degree they change the level of the intracellular biosynthesis of vitamins B2 and B6. Higher excretion of vitamins B5, B7 and especially B6 was observed on the semisynthetic media during the postexponential growth. Under experimental conditions vitamins B1 and B2 were accumulated only in the cells. The dry mycelium of P. ostreatus obtained by submerged cultivation on liquid media is a valuable source of B vitamins and, especially, of niacin. Thus the oyster mushroom and other edible mushrooms can be put at one of the top places among food-stuffs by the content of niacin.     

Plant amino acid-derived vitamins: biosynthesis and function

Vitamins are essential organic compounds for humans, having lost the ability to de novo synthesize them. Hence, they represent dietary requirements, which are covered by plants as the main dietary source of most vitamins (through food or livestock’s feed). Most vitamins synthesized by plants present amino acids as precursors (B₁, B₂, B₃, B₅, B₇, B₉ and E) and are therefore linked to plant nitrogen metabolism. Amino acids play different roles in their biosynthesis and metabolism, either incorporated into the backbone of the vitamin or as amino, sulfur or one-carbon group donors. There is a high natural variation in vitamin contents in crops and its exploitation through breeding, metabolic engineering and agronomic practices can enhance their nutritional quality. While the underlying biochemical roles of vitamins as cosubstrates or cofactors are usually common for most eukaryotes, the impact of vitamins B and E in metabolism and physiology can be quite different on plants and animals. Here, we first aim at giving an overview of the biosynthesis of amino acid-derived vitamins in plants, with a particular focus on how this knowledge can be exploited to increase vitamin contents in crops. Second, we will focus on the functions of these vitamins in both plants and animals (and humans in particular), to unravel common and specific roles for vitamins in evolutionary distant organisms, in which these amino acid-derived vitamins play, however, an essential role.     

代谢工程在维生素生产中的应用及研究进展

维生素是维持人体生命活动必需的一类有机物质,机体本身一般不能合成或合成量不足,因此需经食物或其他强化产品获取。目前,维生素产品已广泛应用于医药、食品添加剂、饲料添加剂、化妆品等领域,而且全球对维生素的需求也是呈逐年增长态势。维生素的生产方法主要包括化学合成法和生物合成法。化学合成法通常安全隐患大、反应条件严苛、废物污染严重,相比之下,代谢工程生产维生素绿色环保安全、能耗低,因此建立微生物细胞工厂具有重大的科学意义和应用需求。文中回顾了近30年来代谢工程在维生素生产领域的研究进展,详细阐述了水溶性维生素(维生素B1、B2、B3、B5、B6、B7、B9、B12和维生素C的前体)和脂溶性维生素(维生素A、维生素D的前体、维生素E和维生素K)的生物合成研究现状,并对其发酵生产的瓶颈进行了探讨,最后对合成生物技术创建维生素生产菌种进行了展望。     

Vitamin transport and homeostasis in mammalian brain: focus on Vitamins B and E

With the application of genetic and molecular biology techniques, there has been substantial progress in understanding how vitamins are transferred across the mammalian blood-brain barrier and choroid plexus into brain and CSF and how vitamin homeostasis in brain is achieved. In most cases (with the exception of the sodium-dependent multivitamin transporter for biotin, pantothenic acid, and lipoic acid), the vitamins are transported by separate carriers through the blood-brain barrier or choroid plexus. Then the vitamins are accumulated by brain cells by separate, specialized systems. This review focuses on six vitamins (B(1), B(3), B(6), pantothenic acid, biotin, and E) and the newer genetic information including relevant 'knockdown' or 'knockout' models in mice and humans. The overall objective is to integrate this newer information with previous physiological and biochemical observations to achieve a better understanding of vitamin transport and homeostasis in brain. This is especially important in view of the newly described non-cofactor vitamin roles in brain (e.g. of B(1), B(3), B(6), and E) and the potential roles of vitamins in the therapy of brain disorders.     

Mitochondrial function and toxicity: role of the B vitamin family on mitochondrial energy metabolism

The B vitamins are water-soluble vitamins required as coenzymes for enzymes essential for cell function. This review focuses on their essential role in maintaining mitochondrial function and on how mitochondria are compromised by a deficiency of any B vitamin. Thiamin (B1) is essential for the oxidative decarboxylation of the multienzyme branched-chain ketoacid dehydrogenase complexes of the citric acid cycle. Riboflavin (B2) is required for the flavoenzymes of the respiratory chain, while NADH is synthesized from niacin (B3) and is required to supply protons for oxidative phosphorylation. Pantothenic acid (B5) is required for coenzyme A formation and is also essential for alpha-ketoglutarate and pyruvate dehydrogenase complexes as well as fatty acid oxidation. Biotin (B7) is the coenzyme of decarboxylases required for gluconeogenesis and fatty acid oxidation. Pyridoxal (B6), folate and cobalamin (B12) properties are reviewed elsewhere in this issue. The experimental animal and clinical evidence that vitamin B therapy alleviates B deficiency symptoms and prevents mitochondrial toxicity is also reviewed. The effectiveness of B vitamins as antioxidants preventing oxidative stress toxicity is also reviewed.     

Comparative evaluation of the vitamin composition of unicellular algae and higher plants grown under artificial conditions

The vitamin composition of representatives of green (Chlorella vulgaris, Platimonas viridis), blue-green (Synechococcus elongatus, Coccopedia, Spirulina platensis, Cyanidium caldarium), red (Porphyridium cruentum) unicellular algae and higher plants (wheat, chufa, beet, carrot, turnip, radish, cucumber, dill, Welsh onion, potato) grown under artificial conditions was examined. The content of B complex vitamins (thiamine, riboflavine, nicotinic and folic acids), ascorbic acid and carotene was measured. Among the algae studied Chlorella vulgaris and Spirulina platensis showed the highest vitamin activity. The red alga Porphyridium cruentum contained the lowest quantity of thiamine, riboflavine and carotene and larger amounts of nicotinic acid. Comparison of the content of vitamins C, B1, B2, PP, folic acid and carotene in unicellular algae and higher plants, that are natural and traditional sources of the vitamins, demonstrated that the above green and blue-green algae contain greater than higher plants amounts of thiamine, riboflavine, folic acid and carotene, when calculated per g dry matter. All algae, except for Platimonas viridis and Cyanidium caldarium, are superior to beet and carrot in their content of ascorbic acid and inferior to green vegetables (radish, cabbage, dill and Welsh onion) in that parameter.     

Comparative investigation of vitamins and their analogues on terminal differentiation, from preadipocytes to adipocytes, of 3T3-L1 cells

1. The effects of 20 kinds of vitamins or their analogues on the growth rate of preadipocytes and the terminal differentiation of preadipocytes to adipocytes was systematically compared in 3T3-L1 cells. 2. The addition of vitamin C markedly increased the growth rate of preadipocytes at over 50 microM. 3. The addition of vitamin K3 slowed down the growth rate at over 0.1 microM. 4. In water soluble vitamins and their analogues tested, the vitamin B6 group and vitamin C significantly stimulated the differentiation, and consequently increased the glycerophosphate dehydrogenase activity and triglyceride accumulation, to a concentration of over 10 microM. 5. Many fat soluble vitamins and their analogues (the vitamin A group, including beta-carotene, the vitamin D group, vitamin E and the vitamin K group) strongly inhibited the adipose conversion of 3T3-L1 cells at microM level.     

The methylenetetrahydrofolate reductase 677C-->T polymorphism as a modulator of a B vitamin network with major effects on homocysteine metabolism

Folates are carriers of one-carbon units and are metabolized by 5,10-methylenetetrahydrofolate reductase (MTHFR) and other enzymes that use riboflavin, cobalamin, or vitamin B6 as cofactors. These B vitamins are essential for the remethylation and transsulfuration of homocysteine, which is an important intermediate in one-carbon metabolism. We studied the MTHFR 677C-->T polymorphism and B vitamins as modulators of one-carbon metabolism in 10,601 adults from the Norwegian Colorectal Cancer Prevention (NORCCAP) cohort, using plasma total homocysteine (tHcy) as the main outcome measure. Mean concentrations of plasma tHcy were 10.4 micromol/liter, 10.9 micromol/liter, and 13.3 micromol/liter in subjects with the CC (51%), CT (41%), and TT (8%) genotypes, respectively. The MTHFR 677C-->T polymorphism, folate, riboflavin, cobalamin, and vitamin B6 were independent predictors of tHcy in multivariate models (P<.001), and genotype effects were strongest when B vitamins were low (P相似文献   

Vitamin and cofactor biosynthesis pathways in Plasmodium and other apicomplexan parasites

Vitamins are essential components of the human diet. By contrast, the malaria parasite Plasmodium falciparum and related apicomplexan parasites synthesize certain vitamins de novo, either completely or in parts. The various biosynthesis pathways are specific to different apicomplexan parasites and emphasize the distinct requirements of these parasites for nutrients and growth factors. The absence of vitamin biosynthesis in humans implies that inhibition of the parasite pathways might be a way to interfere specifically with parasite development. However, the roles of biosynthesis and uptake of vitamins in the regulation of vitamin homeostasis in parasites needs to be established first. In this article, the procurement of vitamins B(1), B(5) and B(6) by Plasmodium and other apicomplexan parasites is discussed.     

Plants need their vitamins too

Over recent years, the pathways for the biosynthesis of many vitamins have been elucidated at the molecular level in plants, and several unique features are emerging. One is that the mitochondrion plays an important role in the synthesis of folate (vitamin B9), biotin (B7), pantothenate (B5), ascorbate (C), and possibly thiamin (B1). Second, the production of some of these cofactors is regulated by developmental cues, and perhaps more surprisingly, by environmental signals such as high light and salinity. Moreover, the biosynthesis of thiamin in Arabidopsis may be negatively regulated by a riboswitch, a novel method of gene regulation that is characteristic of cofactor biosynthesis in bacteria. Vitamin B12 is unique in that it is not found in vascular plants, but is abundant in algae; recent molecular work has revealed that algae do not synthesise the vitamin but instead obtain it from bacteria.     

Life's utilization of B vitamins on early Earth

Coenzymes are essential across all domains of life. B vitamins (B₁‐thiamin, B₂‐riboflavin, B₃‐niacin, B₅‐pantothenate, B₆‐pyridoxine, B₇‐biotin, and B₁₂‐cobalamin) represent the largest class of coenzymes, which participate in a diverse set of reactions including C₁‐rearrangements, DNA repair, electron transfer, and fatty acid synthesis. B vitamin structures range from simple to complex heterocycles, yet, despite this complexity, multiple lines of evidence exist for their ancient origins including abiotic synthesis under putative early Earth conditions and/or meteorite transport. Thus, some of these critical coenzymes likely preceded life on Earth. Some modern organisms can synthesize their own B vitamins de novo while others must either scavenge them from the environment or establish a symbiotic relationship with a B vitamin producer. B vitamin requirements are widespread in some of the most ancient metabolisms including all six carbon fixation pathways, sulfate reduction, sulfur disproportionation, methanogenesis, acetogenesis, and photosynthesis. Understanding modern metabolic B vitamin requirements is critical for understanding the evolutionary conditions of ancient metabolisms as well as the biogeochemical cycling of critical elements such as S, C, and O.     

Vitamer levels, stress response, enzyme activity, and gene regulation of Arabidopsis lines mutant in the pyridoxine/pyridoxamine 5'-phosphate oxidase (PDX3) and the pyridoxal kinase (SOS4) genes involved in the vitamin B6 salvage pathway

PDX3 and SALT OVERLY SENSITIVE4 (SOS4), encoding pyridoxine/pyridoxamine 5'-phosphate oxidase and pyridoxal kinase, respectively, are the only known genes involved in the salvage pathway of pyridoxal 5'-phosphate in plants. In this study, we determined the phenotype, stress responses, vitamer levels, and regulation of the vitamin B(6) pathway genes in Arabidopsis (Arabidopsis thaliana) plants mutant in PDX3 and SOS4. sos4 mutant plants showed a distinct phenotype characterized by chlorosis and reduced plant size, as well as hypersensitivity to sucrose in addition to the previously noted NaCl sensitivity. This mutant had higher levels of pyridoxine, pyridoxamine, and pyridoxal 5'-phosphate than the wild type, reflected in an increase in total vitamin B(6) observed through HPLC analysis and yeast bioassay. The sos4 mutant showed increased activity of PDX3 as well as of the B(6) de novo pathway enzyme PDX1, correlating with increased total B(6) levels. Two independent lines with T-DNA insertions in the promoter region of PDX3 (pdx3-1 and pdx3-2) had decreased PDX3 activity. Both also had decreased activity of PDX1, which correlated with lower levels of total vitamin B(6) observed using the yeast bioassay; however, no differences were noted in levels of individual vitamers by HPLC analysis. Both pdx3 mutants showed growth reduction in vitro and in vivo as well as an inability to increase growth under high light conditions. Increased expression of salvage and some of the de novo pathway genes was observed in both the pdx3 and sos4 mutants. In all mutants, increased expression was more dramatic for the salvage pathway genes.     

Efficacy of Supplementation with B Vitamins for Stroke Prevention: A Network Meta-Analysis of Randomized Controlled Trials

Background

Supplementation with B vitamins for stroke prevention has been evaluated over the years, but which combination of B vitamins is optimal for stroke prevention is unclear. We performed a network meta-analysis to assess the impact of different combinations of B vitamins on risk of stroke.

Methods

A total of 17 trials (86 393 patients) comparing 7 treatment strategies and placebo were included. A network meta-analysis combined all available direct and indirect treatment comparisons to evaluate the efficacy of B vitamin supplementation for all interventions.

Results

B vitamin supplementation was associated with reduced risk of stroke and cerebral hemorrhage. The risk of stroke was lower with folic acid plus vitamin B6 as compared with folic acid plus vitamin B12 and was lower with folic acid plus vitamin B6 plus vitamin B12 as compared with placebo or folic acid plus vitamin B12. The treatments ranked in order of efficacy for stroke, from higher to lower, were folic acid plus vitamin B6 > folic acid > folic acid plus vitamin B6 plus vitamin B12 > vitamin B6 plus vitamin B12 > niacin > vitamin B6 > placebo > folic acid plus vitamin B12.

Conclusions

B vitamin supplementation was associated with reduced risk of stroke; different B vitamins and their combined treatments had different efficacy on stroke prevention. Folic acid plus vitamin B6 might be the optimal therapy for stroke prevention. Folic acid and vitamin B6 were both valuable for stroke prevention. The efficacy of vitamin B12 remains to be studied.     

Functional analysis of PDX2 from Arabidopsis, a glutaminase involved in vitamin B6 biosynthesis

Vitamin B6 is an essential metabolite in all organisms, being required as a cofactor for a wide variety of biochemical reactions. De novo biosynthesis of the vitamin occurs in microorganisms and plants, but animals must obtain it from their diet. Two distinct and mutually exclusive de novo pathways have been identified to date, namely deoxyxylulose 5-phosphate dependent, which is restricted to a subset of eubacteria, and deoxyxylulose 5-phosphate independent, present in archaea, fungi, plants, protista, and most eubacteria. In these organisms, pyridoxal 5'-phosphate (PLP) formation is catalyzed by a single glutamine amidotransferase (PLP synthase) composed of a glutaminase domain, PDX2, and a synthase domain, PDX1. Despite plants being an important source of vitamin B6, very little is known about its biosynthesis. Here, we provide information for Arabidopsis thaliana. The functionality of PDX2 is demonstrated, using both in vitro and in vivo analyses. The expression pattern of PDX2 is assessed at both the RNA and protein level, providing insight into the spatial and temporal pattern of vitamin B6 biosynthesis. We then provide a detailed biochemical analysis of the plant PLP synthase complex. While the active sites of PDX1 and PDX2 are remote from each other, coordination of catalysis is much more pronounced with the plant proteins than its bacterial counterpart, Bacillus subtilis. Based on a model of the PDX1/PDX2 complex, mutation of a single residue uncouples enzyme coordination and in turn provides tangible evidence for the existence of the recently proposed ammonia tunnel through the core of PDX1.     

EFFECTS OF CERTAIN LIMITING CONDITIONS ON THE SYNTHESIS OF B VITAMINS BY YEAST

In yeast crops which were grown in the presence of various inhibitors, there was considerable variation in content of the various B vitamins. A higher degree of parallelism in variation in content was found to exist between thiamine and niacin than between any other pair of vitamins; this has been interpreted as indicating that the predominant functions of these two vitamins are their established rôles in fermentation. Values for inositol indicate that it may be involved in fermentation processes, but this is not the case for other members of the B complex. Biotin appears to be unique since in no case did the biotin content of yeast grown in the presence of an inhibitor fall below that of the control yeast. There was some evidence of synthesis of biotin, or a material with biotin activity, in the presence of certain inhibitors, the most striking instance being with sulfaguanidine. An exogenous supply of biotin was essential for extensive proliferation of F. B. yeast, and yeast grown in a medium to which biotin was the only added vitamin contained the B vitamins in amounts very similar to those found in the control yeast, the most marked differences being in increased vitamin B₆ and p-aminobenzoic acid contents. In the absence of biotin, significant amounts of all of the B vitamins except biotin were synthesized, both in the presence and absence of certain other members of the B complex. The addition of thiamine, pyridoxine, inositol, and β-alanine to the culture medium caused a reduction in the amounts of vitamin B₆ and p-aminobenzoic acid synthesized. F. B. yeast was able to grow in a xylose medium only when certain of the B vitamins were present, and even then growth was limited. Evidence was obtained for some synthesis of all of the vitamins investigated except biotin and vitamin B₆. The most significant differences in vitamin content between galac yeast and the parent F. B. strain were in folic acid and vitamin B₆, the former being considerably reduced in amount, the latter being increased.     

Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade

It is now well established that vitamins D, E, and K and carotenoids are not absorbed solely through passive diffusion. Broad-specificity membrane transporters such as SR-BI (scavenger receptor class B type I), CD36 (CD36 molecule), NPC1L1 (Niemann Pick C1-like 1) or ABCA1 (ATP-binding cassette A1) are involved in the uptake of these micronutrients from the lumen to the enterocyte cytosol and in their secretion into the bloodstream. Recently, the existence of efflux pathways from the enterocyte back to the lumen or from the bloodstream to the lumen, involving ABCB1 (P-glycoprotein/MDR1) or the ABCG5/ABCG8 complex, has also been evidenced for vitamins D and K. Surprisingly, no membrane proteins have been involved in dietary vitamin A uptake so far. After an overview of the metabolism of fat-soluble vitamins and carotenoids along the gastrointestinal tract (from the mouth to the colon where interactions with microbiota may occur), a focus is placed on the identified and candidate proteins participating in the apical uptake, intracellular transport, basolateral secretion and efflux back to the lumen of fat-soluble vitamins and carotenoids in enterocytes. This review also highlights the mechanisms that remain to be identified to fully unravel the pathways involved in fat-soluble vitamin and carotenoid intestinal absorption.     

Effects of vitamins and common drugs on reduction of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in rat microsomes.

4-(Methylnitrosamino)-1-(3-pyridyl)-butanone (NNK) is a tobacco-specific nitrosamino that requires metabolic activation by cytochrome P450 enzymes. The activation of NNK by cytochrome P450 enzymes leads to the formation of different metabolites. Detoxification of NNK usually occurs via carbonyl reduction to its hydroxyl product, 4-(methylnitrosamino)-1-(3-pyridyl)-butanol (NNAL). In the present study, the influences of common vitamins and P450 modulators on the reduction of NNK by rat microsomes were studied. The formation of NNAL but not other metabolites was detected by the described HPLC method. Among the vitamins tested, vitamins E, A (retinol), B6 and B5 were found to be marginal effective upon reduction of NNK while vitamins A (cis-acid), A (trans-acid), D2, D3, K1, K3, B1 and A (crocetin) increased the formation of NNAL from 3 to 21%. The effect of vitamin C-palmitate (<10 microM) was most pronounced followed by crocetin upon reduction of NNK. Clonidine, tolbutamide and atropine slightly increased the reduction of NNK while cimetidine showed no effects. The modulation of NNK reduction could reduce the carcinogenic potential of NNK, since the main detoxification pathway of NNK involves carbonyl reduction.     

Mitomycin C-activity effected by vitamins B1, C,E and beta-carotene under irradiation with gamma-rays

Vitamin B1 (thiamine) can essentially effect the activity of mitomycin C (MMC), added individually or in combination with antioxidant vitamins (C, E-acetate, beta-carotene) as found in experiments in vitro (Escherichia coli bacteria, AB 1157) under irradiation with gamma-rays. The environment plays a crucial role. In airfree media vitamin B1 leads to a 2-fold increase of the MMC-efficiency, but adding vitamin C it decreases. In the presence of all vitamins (B1, C, E-ac., and beta-carotene) the MMC-action increases about 1.8-fold. In aerated media vitamin B1 causes an about 4-times increase of the MMC-efficiency, but by adding vitamin B1 and C the MMC-activity decreases by a factor of two, whereas in the presence of B1, C, E-ac., and beta-carotene it rises again to 2.6-fold. In environment saturated with N2O (conversion of e(-)aq into OH radicals) a different picture is observed. The presence of vitamin B1 or vitamin B1 + C causes a strong decrease of the MMC-efficiency, but the addition of all vitamins (B1, C, E-ac., and beta-car.) leads to a small increase of the cytostatic action. The results demonstrate the influence of vitamin B1 used individually or in combination with other antioxidants on the MMC-efficiency and the strong effect of the environment. The results are of interest for the application of MMC in radiotherapy.     

盐水球菌Salinicoccus ventosaetal B2-3-5的代谢产物分析及其抑制酪氨酸酶活性的机制

[背景] 酪氨酸酶是黑色素合成过程中的关键酶,也是引起人体色素障碍性疾病和产生果蔬酶促褐变的主要原因。目前,酪氨酸酶抑制剂的开发已引起广泛关注,但一些酪氨酸酶抑制剂如熊果苷、曲酸等均存在一定的安全隐患。微生物资源丰富且具有许多优点,从微生物中寻找特异性强、高效的酪氨酸酶抑制剂已成为该领域研究的热点。[目的] 通过测定分离自新疆乌鲁木齐达坂城盐湖的盐水球菌Salinicoccus ventosaetal B2-3-5和B6-1-4代谢物提取物对酪氨酸酶活性的影响,比较2株菌发酵过程中代谢物的差异,了解所筛选菌株B2-3-5抑制酪氨酸酶活性的机制。[方法] 以曲酸为阳性对照分别测定B2-3-5和B6-1-4这2个菌株发酵产生的代谢物提取物对蘑菇酪氨酸酶的抑制活性;应用LC-MS代谢组学方法检测2株菌在相同条件下产生的所有代谢物质;采用单变量、多元变量、正交偏最小二乘判别分析（Orthogonal Partial Least Squares-Discrimination Analysis,OPLS-DA）法识别差异代谢物;利用层次聚类分析（Hierarchial Cluster Analysis,HCA）法对识别的差异物进行聚类分析;通过Kyoto Encyclopedia of Genes and Genomes （KEGG）代谢通路对比法分析这些差异代谢物主要参与的代谢途径。[结果] 菌株B2-3-5代谢物提取物对蘑菇酪氨酸酶二酚酶活性的抑制率为67%,其IC₅₀为0.277 mg/mL,同属菌株B6-1-4代谢物提取物则对酪氨酸酶无抑制活性。采用代谢组学的检测方法从2株菌的代谢物中筛选出63个差异代谢物,其中氨基酸类化合物、维生素类化合物和羧酸类化合物的种类及相对含量均是B2-3-5菌株明显高于B6-1-4菌株。通过代谢途径分析发现这些差异代谢物主要参与15个代谢通路,其中维生素B6生物合成通路的影响较为显著。[结论] 推测B2-3-5菌株可能是通过增加一些氨基酸类、维生素类及羧酸类等小分子化合物的含量来抑制酪氨酸酶活性。维生素B6代谢途径的上调也表明菌体细胞可通过产生维生素B6与酪氨酸酶中的必需氨基作用或清除酶催化循环过程中产生的活性氧自由基（reactive oxygen species,ROS）来抑制酪氨酸酶活性。     

植物内生菌促进宿主氮吸收与代谢研究进展

内生菌与植物共生能够提高宿主的氮吸收与氮代谢水平,这可能是由于内生菌在植物体内引发的多种效应的综合结果.植物内生菌能够通过促进植物根系发育和固氮作用为宿主植物提供更多的无机氮素;能够通过分泌多种胞外酶系如漆酶、蛋白水解酶等使宿主植物更好地利用有机氮素;能够提高宿主氮代谢关键酶如硝酸还原酶(NR)、谷氨酰胺合成酶(GS)等酶的活性;能够提高宿主植物激素水平和维生素含量从而促进宿主氮代谢;能够通过影响宿主植物氮代谢促进宿主植物分蘖、提高宿主植物叶绿素含量和光合速率等等.综述了国内外关于植物内生菌促进宿主氮代谢的相关报道,归纳了植物内生菌影响宿主氮素吸收与代谢的可能机制,并展望了关于植物内生菌促进宿主氮代谢机制方面的研究方向.