In order to quantitatively study the jasmonate biosynthetic pathway, we chemically synthesized a pair of isotope mass probes and established a labeling protocol. The pair of mass probes used in our work were ω-bromoacetonylpyridinium bromide (BPB) and d5-ω-bromoacetonylpyridinium bromide (d5-BPB), which contain carboxylic acid reactive groups, isotopically labeled groups and permanent positive charges. High performance liquid chromatography (HPLC) and electrospray ionization quadrupole-time of flight mass spectrometry (ESI-QTOF-MS) were used for the detection of labeled standard mixtures and plant samples. In comparison to negative mode electrospray ionization detection of unlabeled analytes, the ESI signal of reverse charge labeled compounds was shown to improve by 20- to 80-fold. Accurate relative quantification was achieved as no isotopic effects of the different isotope labeled phytohormones during RP/SCX mixed-mode liquid chromatographic separation were observed. A data analysis method was established for analyzing metabolic pathways using our labeling strategy. We then applied our method and examined the jasmonate biosynthetic pathway of rice under salt stress and the premature senescence mutant. Here we found that under salt stress conditions, rice showed up-regulation in (13S)-hydroperoxyoctadecatrienoic acid (HOPT), cis-(+)-12-oxophytodienoic acid (OPDA), 3-oxo-2-(2′-pentenyl)-cyclopentane-1-octanoic acid (OPC-8) and jasmonoyl-valine (JA-Val) levels, while α-linolenic acid (LA) and jasmonic acid (JA) showed down-regulation, and three components (HPOT, OPC-8 and JA-Val) were accumulated. The premature senescence mutant showed up-regulation in all major components of the jasmonate biosynthetic pathway with the exception of LA, and an accumulation of HPOT, OPC-6 and JA-Val. This study demonstrates that our chemical stable isotope labeling strategy can be used as a powerful tool for metabolic pathway analysis of phytohormones in plants.
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