An optimized LC-MS/MS method as a pivotal tool to steer thiamine biofortification strategies in rice

Simultaneous quantification of seven B vitamins from wheat grains using UHPLC-MS/MS

B-group vitamins are important micronutrients for maintaining human health; nevertheless, B vitamin deficiency is a globally widespread issue. Thus, it is relevant to accurately assess the B-vitamin content in staple crop products such as wheat grains. Here, we developed a multi-enzyme extraction method allowing accurate quantification of seven B vitamins in wheat using ultra high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). Free forms of thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7) and folates (B9) were determined with recoveries ranging from 81 to 118% and accuracy below 15% bias. The precision was below 20% relative standard deviation and the internal standards adequately compensated for matrix effects. The method was applied to determine the B vitamin stabilities in wheat grains stored at different temperatures and periods. The results provide an important basis in future studies aiming at understanding nutritional availability of B vitamins.

A UPLC-MS/MS method for quantification of metabolites in the ethylene biosynthesis pathway and its biological validation in Arabidopsis

The plant hormone ethylene is of vital importance in the regulation of plant development and stress responses. Recent studies revealed that 1-aminocyclopropane-1-carboxylic acid (ACC) plays a role beyond its function as an ethylene precursor. However, the absence of reliable methods to quantify ACC and its conjugates malonyl-ACC (MACC), glutamyl-ACC (GACC), and jasmonyl-ACC (JA-ACC) hinders related research. Combining synthetic and analytical chemistry, we present the first, validated methodology to rapidly extract and quantify ACC and its conjugates using ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). Its relevance was confirmed by application to Arabidopsis mutants with altered ACC metabolism and wild-type plants under stress. Pharmacological and genetic suppression of ACC synthesis resulted in decreased ACC and MACC content, whereas induction led to elevated levels. Salt, wounding, and submergence stress enhanced ACC and MACC production. GACC and JA-ACC were undetectable in vivo; however, GACC was identified in vitro, underscoring the broad applicability of the method. This method provides an efficient tool to study individual functions of ACC and its conjugates, paving the road toward exploration of novel avenues in ACC and ethylene metabolism, and revisiting ethylene literature in view of the recent discovery of an ethylene-independent role of ACC.

A protocol for a turbidimetric assay using a Saccharomyces cerevisiae thiamin biosynthesis mutant to estimate total vitamin B1 content in plant tissue samples

BACKGROUND

Understanding thiamin (thiamine; vitamin B1) metabolism in plants is crucial, as it impacts plant nutritional value as well as stress tolerance. Studies aimed at elucidating novel aspects of thiamin in plants rely on adequate assessment of thiamin content. Mass spectrometry-based methods provide reliable quantification of thiamin as well as closely related biomolecules. However, these techniques require expensive equipment and expertise. Microbiological turbidimetric assays can evaluate the presence of thiamin in a given sample, only requiring low-cost, standard lab equipment. Although these microbiological assays do not reach the accuracy provided by mass spectrometry-based methods, the ease with which they can be deployed in an inexpensive and high-throughput manner, makes them a favorable method in many circumstances. However, the thiamin research field could benefit from a detailed step-by-step protocol to perform such assays as well as a further assessment of its potential and limitations.

RESULTS

Here, we show that the Saccharomyces cerevisiae thiamin biosynthesis mutant thi6 is an ideal candidate to be implemented in a turbidimetric assay aimed at assessing the content of thiamin and its phosphorylated equivalents (total vitamer B1). An optimized protocol was generated, adapted from a previously established microbiological assay using the thi4 mutant. A step-by-step guidance for this protocol is presented. Furthermore, the applicability of the assay is illustrated by assessment of different samples, including plant as well as non-plant materials. In doing so, our work provides an extension of the applicability of the microbiological assay on top of providing important considerations upon implementing the protocol.

CONCLUSIONS

An inexpensive, user-friendly protocol, including step-by-step guidance, which allows adequate estimation of vitamer B1 content of samples, is provided. The method is well-suited to screen materials to identify altered vitamer B1 content, such as in metabolic engineering or screening of germplasm.

Identification of Metabolite Interference Is Necessary for Accurate LC-MS Targeted Metabolomics Analysis

Targeted metabolomics has been broadly used for metabolite measurement due to its good quantitative linearity and simple metabolite annotation workflow. However, metabolite interference, the phenomenon where one metabolite generates a peak in another metabolite's MRM setting (Q1/Q3) with a close retention time (RT), may lead to inaccurate metabolite annotation and quantification. Besides isomeric metabolites having the same precursor and product ions that may interfere with each other, we found other metabolite interferences as the result of inadequate mass resolution of triple-quadruple mass spectrometry and in-source fragmentation of metabolite ions. Characterizing the targeted metabolomics data using 334 metabolite standards revealed that about 75% of the metabolites generated measurable signals in at least one other metabolite's MRM setting. Different chromatography techniques can resolve 65-85% of these interfering signals among standards. Metabolite interference analysis combined with the manual inspection of cell lysate and serum data suggested that about 10% out of ∼180 annotated metabolites were mis-annotated or mis-quantified. These results highlight that a thorough investigation of metabolite interference is necessary for accurate metabolite measurement in targeted metabolomics.

A novel panel of yeast assays for the assessment of thiamin and its biosynthetic intermediates in plant tissues

Thiamin (or thiamine), known as vitamin B1, represents an indispensable component of human diets, being pivotal in energy metabolism. Thiamin research depends on adequate vitamin quantification in plant tissues. A recently developed quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is able to assess the level of thiamin, its phosphorylated entities and its biosynthetic intermediates in the model plant Arabidopsis thaliana, as well as in rice. However, their implementation requires expensive equipment and substantial technical expertise. Microbiological assays can be useful in deter-mining metabolite levels in plant material and provide an affordable alternative to MS-based analysis. Here, we evaluate, by comparison to the LC-MS/MS reference method, the potential of a carefully chosen panel of yeast assays to estimate levels of total vitamin B1, as well as its biosynthetic intermediates pyrimidine and thiazole in Arabidopsis samples. The examined panel of Saccharomyces cerevisiae mutants was, when implemented in microbiological assays, capable of correctly assigning a series of wild-type and thiamin biofortified Arabidopsis plant samples. The assays provide a readily applicable method allowing rapid screening of vitamin B1 (and its biosynthetic intermediates) content in plant material, which is particularly useful in metabolic engineering approaches and in germplasm screening across or within species.

Metabolic engineering of rice endosperm towards higher vitamin B1 accumulation

Rice is a major food crop to approximately half of the human population. Unfortunately, the starchy endosperm, which is the remaining portion of the seed after polishing, contains limited amounts of micronutrients. Here, it is shown that this is particularly the case for thiamin (vitamin B1). Therefore, a tissue-specific metabolic engineering approach was conducted, aimed at enhancing the level of thiamin specifically in the endosperm. To achieve this, three major thiamin biosynthesis genes, THIC, THI1 and TH1, controlled by strong endosperm-specific promoters, were employed to obtain engineered rice lines. The metabolic engineering approaches included ectopic expression of THIC alone, in combination with THI1 (bigenic) or combined with both THI1 and TH1 (trigenic). Determination of thiamin and thiamin biosynthesis intermediates reveals the impact of the engineering approaches on endosperm thiamin biosynthesis. The results show an increase of thiamin in polished rice up to threefold compared to WT, and stable upon cooking. These findings confirm the potential of metabolic engineering to enhance de novo thiamin biosynthesis in rice endosperm tissue and aid in steering future biofortification endeavours.