Quantification of indole-3-acetic acid and amino acid conjugates in rice by liquid chromatography-electrospray ionization-tandem mass spectrometry

A method for quantifying indole-3-acetic acid (IAA) and its conjugates with the six amino acids, Ala, -Asp, -Ile, -Glu, -Phe and -Val, in rice (Oryza sativa) by using high-performance liquid chromatography coupled with electrospray ionization and tandem mass spectrometry (HPLC-ESI-MS/MS) is described. Samples from the rice plant or callus were treated with 80% acetone in water containing 2.5 mM diethyl dithiocarbamate. Each extract was partially purified in C18 cartridge column for solid-phase extraction (SPE) and subjected to HPLC-ESI-MS/MS without converting the product. The detection limit was 3.8 fmol for IAA, and 0.4-2.9 fmol for the IAA amino acid conjugates. The method was applied to the analysis of IAA and its conjugates in rice seedlings, dehulled rice and calli, using 20-100 mg tissue samples.     

Metabolic changes in Arabidopsis thaliana expressing the feedback-resistant anthranilate synthase alpha subunit gene OASA1D

Anthranilate synthase (AS) is a key enzyme in tryptophan (Trp) biosynthesis. Metabolic changes in transgenic Arabidopsis plants expressing the feedback-resistant anthranilate synthase alpha subunit gene OASA1D were investigated with respect to Trp synthesis and effects on secondary metabolism. The Trp content varied depending on the transgenic line, with some lines showing an approximately 200-fold increase. The levels of AS activity in crude extracts from the transgenic lines were comparable to those in the wild type. On the other hand, the enzyme prepared from the lines accumulating high levels of Trp showed a relaxed feedback sensitivity. The AS activity, determined in the presence of 50 microM L-Trp, correlated well with the amount of free Trp in the transgenic lines, indicating the important role of feedback inhibition in control of Trp pool size. In Arabidopsis, Trp is a precursor of multiple secondary metabolites, including indole glucosinolates and camalexin. The amount of indol-3-ylmethyl glucosinolate (I3 M) in rosette leaves of the high-Trp accumulating lines was 1.5- to 2.1-fold greater than that in wild type. The treatment of the leaves with jasmonic acid resulted in a more pronounced accumulation of I3 M in the high-Trp accumulating lines than in wild type. The induction of camalexin formation after the inoculation of Alternaria brassicicola was not affected by the accumulation of a large amount of Trp. The accumulation of constitutive phenylpropanoids and flavonoids was suppressed in high-Trp accumulating lines, while the amounts of Phe and Tyr increased, thereby indicating an interaction between the Trp branch and the Phe and Tyr branch in the shikimate pathway.     

Induction of serotonin accumulation by feeding of rice striped stem borer in rice leaves

Tryptophan (Trp)-related secondary metabolism has been implicated in the defense against pathogen infection and insect feeding in various gramineous species. Recently, we also reported that rice plant accumulated serotonin and tryptamine as well as their amide compounds coupled with phenolic acids in response to the infection by fungal pathogen. These compounds were likely to play an important role in the formation of physical barrier to the invading pathogens. To extend our study to elucidate the defensive role of Trp-derived secondary metabolism in gramineous plants, we examined in this study whether it is activated in response to herbivore attack as well. Third leaves of rice plant were fed on by third instar larvae of rice striped stem borer for 24 h or 48 h. The analysis of four Trp-derived metabolites including tryptamine, serotonin feruloyltryptamine (FerTry) and p-coumaroylserotonin (CouSer) by liquid chromatography coupled with tandem mass spectrometry revealed that their contents clearly increased in response to the larvae feeding. The respective amounts of tryptamine, serotonin, FerTry and CouSer in the larvae-fed leaves were 12-, 3.5-, 33- and 140-fold larger than those in control leaves 48 h after the start of feeding.Key words: rice, Oryza sativa, Gramineae, serotonin, secondary metabolism, rice striped stem borer, Chilo suppressalisPlants defend themselves from environmental stresses by utilizing secondary metabolism. One of major biological stresses that plants have to cope with is attack by herbivorous insects. In the interactions with herbivorous insects, various secondary metabolites that are derived from tryptophan (Trp) pathway have been shown to play defensive roles in plants including gramineous species. For example, benzoxazinone glucosides in wheat (Triticum aestivum), rye (Secale sereale) and maize (Zea mays) express toxic and antifeeding effects on herbivorous insects.^1,2 Benzoxazinones are biosynthesized from indole-3-glycerol phosphate, an intermediate of Trp synthesis.^3,4 Another example of those compounds is gramine in barley (Hordeum vulgare). Gramine is a Trp-rerived indole amine,⁵ and has been received attention in the resistance mainly against aphids on the basis of its toxicity and deterrence.⁶We recently found that Trp-derived secondary metabolism is also involved in defense responses of rice (Oryza sativa) leaves to infection by brown spot fungus (Bipolaris oryzae).⁷ The infection of the fungus activates Trp biosynthesis and accumulation of serotonin and of smaller amounts of tryptamine, feruloyltryptamine (FerTry) and p-coumaroylserotonin (CouSer). In addition, the enhancement of serotonin peroxidase activity and incorporation of serotonin in the cell walls were detected. Thus, it is very likely that that serotonin-derived materials deposit in cell walls after oxidative polymerization to constitute a part of physical defense system of rice, which may be reminiscent of the wound sloughing in animals. These findings prompted us to investigate whether Trp-related secondary metabolism is also involved in the defense of rice plant against the attack by insects, as in the cases of other gramineous plants mentioned above. While the response of plants to pathogenic infection is generally different from that to insect herbivory, Trp-derived secondary metabolites have occasionally been implicated in both responses.^8–10 Here, we report the results of our study to examine the effects of herbivory by rice striped stem borer (Chilo suppressalis) on the Trp derived secondary metabolism in rice leaves.Rice (cv. Nipponbare) leaves were incubated with larvae of C. suppressalis in a feeding tube assembled according to Oikawa et al.,⁸ Aerial parts of two 12-day-old rice seedlings were excised, and their cutting ends were immersed in distilled water in a vial. Three third instar larvae of C. suppressalis were put on the leaves, and the leaves with larvae were covered by a plastic tube. For comparison, the control leaves were wounded by razor blade at the start of the incubation. After incubation for 24 h or 48 h with 16/8 h LD cycle at 28°C, the leaves were extracted with 10 volumes of 80% methanol, and analyzed by liquid chromatography coupled with tandem mass spectrometry in multiple reaction monitoring mode.As shown in Figure 1, the contents of tryptamine and serotonin increased along with time in the larvae-fed leaves. The respective contents of tryptamine and serotonin in the leaves were 12- and 3.5-fold larger than those in control leaves 48 h after the start of feeding. The accumulation of FerTry and CouSer was also observed after larvae feeding with the contents being 33- and 140-fold larger than those in control leaves, respectively. Their contents, however, were approximately 10-fold smaller than the corresponding amines.Open in a separate windowFigure 1Accumulation of Trp derived metabolites in the leaves attacked by rice striped stem borer. Chemical structures of analyzed compounds (A). The contents of tryptamine (B), serotonin (B), FerTry (C) and CouSer (D) were determined by LC-MS/MS analysis. The third leaves of 12-d-old rice seedlings were fed on by rice striped stem borer (brack bars) or wounded by razor blade as control (white bars). After incubation, the leaves were extracted by 80% methanol. The contents of metabolites at time 0 are represented as gray bars.In the interaction of rice plant with B. oryzae, serotonin was shown to be incorporated into cell walls as a part of physical defense system.⁷ In an analogous way, modification of cell walls by serotonin might function in sealing the sites injured by insect feeding to protect the leaves from desiccation, and opportunistic and insect-mediated infection by microorganisms. Indeed, at the cutting edge of the leaves, the formation of brown materials was observed. In addition, since serotonin is a neurotransmitter of insects and tryptamine has been indicated to be a neuroactive substance, their accumulation might directly affect behavior and physiology of some insects. High concentrations of tryptamine have been shown to express anti-oviposition activity toward Bemisia tabaci¹¹ and anti-feeding activities toward Malacosoma disstria and Manduca sexta.¹²The low levels of serotonin, tryptamine and their amides in the control leaves suggest that these compounds are induced in response to some components produced during the interaction between the plant and the herbivore. In this relation, it has been shown that elicitors are present in the saliva of some herbivous insects, which induce volatile emission from the plant to attracts their natural enemies.^13,14 Induction of Trp-derived secondary metabolites by the herbivore attack may likely be a result of recognition of some insect-derived molecules by rice leaves, similarly to the induction of volatile emission.The induced accumulation of indole amines and their hydroxycinnamic acid amides in the rice leaves attacked by C. suppressalis suggests that a common signaling pathway might be involved in the responses to pathogen infection and insect feeding. However, the composition of induced compounds was different between the responses to the two biological stresses. The content of tryptamine in the larvae-fed leaves was comparable to that reported in the B. oryzae-infected leaves, whereas the amount of serotonin (approximately 35 nmol/gFW) was much smaller than that in the infected leaves (approximately 250 nmol/gFW). This characteristic was similar to the response of rice leaves to methyl jasmonate (MeJA), which also induced accumulation of these Trp-derived secondary metabolites.⁷ The strong activation of the conversion of tryptamine to serotonin may require infection-specific signals.The serotonin accumulation in rice appears to be similar to the accumulation of gramine in barley in several aspects. Gramine accumulation has been demonstrated to be induced by either infection by pathogens⁹ or infestation by the aphid Schizaphis graminum.¹⁰ In addition, the gene encoding N-methyltransferase that catalyzes the final reaction in the gramine biosynthetic pathway is upregulated by MeJA, suggesting gramine synthesis is at least partly under the control of jasmonate signaling pathway.^15,16 The inducible serotonin production may be an archetypal form of the biosynthesis of more complicated indole amine in barley.     

Deposition of a recombinant peptide in ER-derived protein bodies by retention with cysteine-rich prolamins in transgenic rice seed

A 7Crp peptide composed of seven major human T cell epitopes derived from the Japanese cedar pollen allergens Cry j 1 and Cry j 2 is an ideal tolerogen for peptide immunotherapy against Japanese cedar pollinosis. To maximize the accumulation level of the 7Crp peptide in transgenic rice seed, we tested endosperm specific promoters and intracellular localizations suitable for stable accumulation. A 7Crp peptide carrying the KDEL ER retention signal directed by the 2.3-kb promoter of the glutelin GluB-1, which contains a signal peptide, accumulated at the highest level of about 60 μg/grain. Notably, the 7Crp peptide predominantly accumulated in ER-derived protein bodies irrespective of the presence of various sorting signals or expression as a fusion protein with glutelin. We attribute this abnormal pattern of accumulation to the formation of disulfide bonds between the 7Crp peptide and cysteine-rich (Cys-rich) prolamin storage proteins. Furthermore, the formation of these aggregates induced the chaperone proteins BiP and PDI as an ER stress response.     

Evaluation of amino acid content and nutritional quality of transgenic soybean seeds with high-level tryptophan accumulation

Anthranilate synthase (AS) is a key regulatory enzyme in tryptophan (Trp) biosynthesis and is subject to feedback inhibition by Trp. The gene encoding a mutated feedback-resistant α subunit of rice AS (OASA1D) under the control of either a soybean glycinin gene promoter or the 35S promoter of cauliflower mosaic virus for seed-specific or constitutive expression, respectively, was introduced into soybean [Glycine max (L.) Merrill] by particle bombardment. A total of seven different transgenic lines that showed markedly increased accumulation of free Trp in their seeds were developed. The overproduction of free Trp was stably inherited in subsequent generations without any apparent detrimental effect on plant growth or reproduction. The total Trp content of transgenic seeds was also about twice that of nontransgenic seeds, whereas the amount of protein-bound Trp was not substantially affected by OASA1D expression. In spite of the marked increase in free Trp content, metabolic profiling by high-performance liquid chromatography coupled with mass spectrometry revealed little change in the amounts of other aromatic compounds in the transgenic seeds. We developed a rapid and feasible system based on farmed rainbow trout to evaluate the nutritional quality of a limited quantity of transgenic soybean seeds. Supplementation of fish food with OASA1D transgenic soybean seeds or with nontransgenic seeds plus crystalline Trp increased the growth rate of the farmed fish. These results indicate transformation with OASA1D is a reliable approach to improve the nutritional quality of soybean (or of other grain legumes) for human and animal food.     

Analysis of ER stress in developing rice endosperm accumulating β‐amyloid peptide

The common neurodegenerative disorder known as Alzheimer’s disease is characterized by cerebral neuritic plaques of amyloid β (Aβ) peptide. Plaque formation is related to the highly aggregative property of this peptide, because it polymerizes to form insoluble plaques or fibrils causing neurotoxicity. Here, we expressed Aβ peptide as a new causing agent to endoplasmic reticulum (ER) stress to study ER stress occurred in plant. When the dimer of Aβ_1–42 peptide was expressed in maturing seed under the control of the 2.3‐kb glutelin GluB‐1 promoter containing its signal peptide, a maximum of about 8 μg peptide per grain accumulated and was deposited at the periphery of distorted ER‐derived PB‐I protein bodies. Synthesis of Aβ peptide in the ER lumen severely inhibited the synthesis and deposition of seed storage proteins, resulting in the generation of many small and abnormally appearing PB bodies. This ultrastructural change was accounted for by ER stress leading to the accumulation of aggregated Aβ peptide in the ER lumen and a coordinated increase in ER‐resident molecular chaperones such as BiPs and PDIs in Aβ‐expressing plants. Microarray analysis also confirmed that expression of several BiPs, PDIs and OsbZIP60 containing putative transmembrane domains was affected by the ER stress response. Aβ‐expressing transgenic rice kernels exhibited an opaque and shrunken phenotype. When grain phenotype and expression levels were compared among transgenic rice grains expressing several different recombinant peptides, such detrimental effects on grain phenotype were correlated with the expressed peptide causing ER stress rather than expression levels.     

Discovery of genome-wide DNA polymorphisms in a landrace cultivar of Japonica rice by whole-genome sequencing

Molecular breeding approaches are of growing importance to crop improvement. However, closely related cultivars generally used for crossing material lack sufficient known DNA polymorphisms due to their genetic relatedness. Next-generation sequencing allows the identification of a massive number of DNA polymorphisms such as single nucleotide polymorphisms (SNPs) and insertions-deletions (InDels) between highly homologous genomes. Using this technology, we performed whole-genome sequencing of a landrace of japonica rice, Omachi, which is used for sake brewing and is an important source for modern cultivars. A total of 229 million reads, each comprising 75 nucleotides of the Omachi genome, was generated with 45-fold coverage and uniquely mapped to 89.7% of the Nipponbare genome, a closely related cultivar. We identified 132,462 SNPs, 16,448 insertions and 19,318 deletions between the Omachi and Nipponbare genomes. An SNP array was designed to validate 731 selected SNPs, resulting in validation rates of 95 and 88% for the Omachi and Nipponbare genomes, respectively. Among the 577 SNPs validated in both genomes, 532 are entirely new SNP markers not previously reported between related rice cultivars. We also validated InDels on a part of chromosome 2 as DNA markers and successfully genotyped five japonica rice cultivars. Our results present the methodology and extensive data on SNPs and InDels available for whole-genome genotyping and marker-assisted breeding. The polymorphism information between Omachi and Nipponbare is available at NGRC_Rice_Omachi (http://www.nodai-genome.org/oryza_sativa_en.html).