Metabolite analysis of long chain branched fatty acids and capsaicin biosynthesis in Capsicum annuum placenta

Lipoxygenase (LOX) in Sweet and Hot Pepper (Capsicum annuum L.) Fruits during Ripening and under an Enriched Nitric Oxide (NO) Gas Atmosphere

Lipoxygenases (LOXs) catalyze the insertion of molecular oxygen into polyunsaturated fatty acids (PUFA) such as linoleic and linolenic acids, being the first step in the biosynthesis of a large group of biologically active fatty acid (FA)-derived metabolites collectively named oxylipins. LOXs are involved in multiple functions such as the biosynthesis of jasmonic acid (JA) and volatile molecules related to the aroma and flavor production of plant tissues, among others. Using sweet pepper (Capsicum annuum L.) plants as a model, LOX activity was assayed by non-denaturing polyacrylamide gel electrophoresis (PAGE) and specific in-gel activity staining. Thus, we identified a total of seven LOX isozymes (I to VII) distributed among the main plant organs (roots, stems, leaves, and fruits). Furthermore, we studied the FA profile and the LOX isozyme pattern in pepper fruits including a sweet variety (Melchor) and three autochthonous Spanish varieties that have different pungency levels (Piquillo, Padrón, and Alegría riojana). It was observed that the number of LOX isozymes increased as the capsaicin content increased in the fruits. On the other hand, a total of eight CaLOX genes were identified in sweet pepper fruits, and their expression was differentially regulated during ripening and by the treatment with nitric oxide (NO) gas. Finally, a deeper analysis of the LOX IV isoenzyme activity in the presence of nitrosocysteine (CysNO, a NO donor) suggests a regulatory mechanism via S-nitrosation. In summary, our data indicate that the different LOX isozymes are differentially regulated by the capsaicin content, fruit ripening, and NO.

Arabidopsis ACYL-ACTIVATING ENZYME 9 (AAE9) encoding an isobutyl-CoA synthetase is a key factor connecting branched-chain amino acid catabolism with iso-branched wax biosynthesis

Iso-branched wax compounds are well known in plants, but their biosynthetic pathways are still mostly unknown. It has been speculated that branched waxes are derived from branched-chain amino acid (BCAA) catabolism, but the evidence for this is very limited. Gas chromatography-flame ionisation detection (GC-FID) analysis revealed that mutations in two subunits of the branched-chain ketoacid dehydrogenase (BCKDH) complex, a key enzyme complex in the degradation of BCAAs, significantly decreased the amounts of branched wax compounds, indicating that BCAA degradation may be integral to the synthesis of iso-branched wax. Substrate feeding studies further revealed that the metabolic precursor of iso-branched wax compounds is isobutyric acid (iBA), which is derived from valine degradation in Arabidopsis. We also isolated a novel mutant and found that its branched wax deficient phenotype could not be rescued by iBA. Map-based cloning together with complementation analysis revealed that mutation in ACYL-ACTIVATING ENZYME 9 (AAE9) is responsible for this phenotype. Genetic and enzyme activity analysis demonstrated that AAE9 is located downstream of the BCAA degradation pathway, and that it activates iBA to isobutyryl-CoA for use on branched wax synthesis. Taken together, our study demonstrates that AAE9 is a key factor connecting BCAA catabolism with branched wax biosynthesis.

Comparative Analysis of Fruit Metabolites and Pungency Candidate Genes Expression between Bhut Jolokia and Other Capsicum Species

Bhut jolokia, commonly known as Ghost chili, a native Capsicum species found in North East India was recorded as the naturally occurring hottest chili in the world by the Guinness Book of World Records in 2006. Although few studies have reported variation in pungency content of this particular species, no study till date has reported detailed expression analysis of candidate genes involved in capsaicinoids (pungency) biosynthesis pathway and other fruit metabolites. Therefore, the present study was designed to evaluate the diversity of fruit morphology, fruiting habit, capsaicinoids and other metabolite contents in 136 different genotypes mainly collected from North East India. Significant intra and inter-specific variations for fruit morphological traits, fruiting habits and 65 fruit metabolites were observed in the collected Capsicum germplasm belonging to three Capsicum species i.e., Capsicum chinense (Bhut jolokia, 63 accessions), C. frutescens (17 accessions) and C. annuum (56 accessions). The pungency level, measured in Scoville Heat Unit (SHU) and antioxidant activity measured by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assay showed maximum levels in C. chinense accessions followed by C. frutescens accessions, while C. annuum accessions showed the lowest value for both the traits. The number of different fruit metabolites detected did not vary significantly among the different species but the metabolite such as benzoic acid hydroxyl esters identified in large percentage in majority of C. annuum genotypes was totally absent in the C. chinense genotypes and sparingly present in few genotypes of C. frutescens. Significant correlations were observed between fruit metabolites capsaicin, dihydrocapsaicin, hexadecanoic acid, cyclopentane, α-tocopherol and antioxidant activity. Furthermore, comparative expression analysis (through qRT-PCR) of candidate genes involved in capsaicinoid biosynthesis pathway revealed many fold higher expression of majority of the genes in C. chinense compared to C. frutescens and C. annuum suggesting that the possible reason for extremely high pungency might be due to the higher level of candidate gene(s) expression although nucleotide variation in pungency related genes may also be involved in imparting variations in level of pungency.

Metabolomic Characterization of Hot Pepper (Capsicum annuum "CM334") during Fruit Development

Non-targeted metabolomic analysis of hot pepper (Capsicum annuum "CM334") was performed at six development stages [16, 25, 36, 38, 43, and 48 days post-anthesis (DPA)] to analyze biochemical changes. Distinct distribution patterns were observed in the changes of metabolites, gene expressions, and antioxidant activities by early (16-25 DPA), breaker (36-38 DPA), and later (43-48 DPA) stages. In the early stages, glycosides of luteolin, apigenin, and quercetin, shikimic acid, γ-aminobutyric acid (GABA), and putrescine were highly distributed but gradually decreased over the breaker stage. At later stages, leucine, isoleucine, proline, phenylalanine, capsaicin, dihydrocapsaicin, and kaempferol glycosides were significantly increased. Pathway analysis revealed metabolite-gene interactions in the biosynthesis of amino acids, capsaicinoids, fatty acid chains, and flavonoids. The changes in antioxidant activity were highly reflective of alterations in metabolites. The present study could provide useful information about nutrient content at each stage of pepper cultivation.