Discovery of novel unfunctional pAMT allele pamt 10 causing loss of pungency in sweet bell pepper (Capsicum annuum L.)

Genetic analysis of pungency deficiency in Japanese chili pepper 'Shishito' (Capsicum annuum) revealed its unique heredity and brought the discovery of two genetic loci involved with the reduction of pungency

The sensation of pungency generated by capsaicinoids is a characteristic trait of chili peppers (Capsicum spp.), and the presence or absence of pungency is central in determining its usage as a spice or a vegetable. In the present study, we aimed to clarify the heredity and genetic factors involved in the deficiency of pungency (quite low pungency) that is uniquely observed in the Japanese chili pepper 'Shishito' (Capsicum annuum). First, the F₂ population ('Shishito' × pungent variety 'Takanotsume') was used for segregation analysis, and pungency level was investigated using capsaicinoid quantification with high-performance liquid chromatography. Also, restriction site associated DNA sequencing of the F₂ population was performed, and genetic map construction and quantitative trait locus (QTL) mapping were implemented. The results indicated that the F₂ population showed varying capsaicinoid content and two major QTLs were detected, Shql3 and Shql7, which explained 39.8 and 19.7% of the genetic variance, respectively. According to these results, the quite low pungency of 'Shishito' was a quantitative trait that involved at least the two loci. Further, this trait was completely separate from general non-pungent traits controlled by individual recessive genes, as described in previous studies. The present study is the first report to investigate the genetic mechanism of pungency deficiency in Japanese chili peppers, and our results provide new insights into the genetic regulation of pungency in chili pepper.

Genetic mapping revealed that the Pun2 gene in Capsicum chacoense encodes a putative aminotransferase

Several genes regulating capsaicinoid biosynthesis including Pun1 (also known as CS), Pun3, pAMT, and CaKR1 have been studied. However, the gene encoded by Pun2 in the non-pungent Capsicum chacoense is unknown. This study aimed to identify the Pun2 gene by genetic mapping using interspecific (C. chacoense × Capsicum annuum) and intraspecific (C. chacoense × C. chacoense) populations. QTL mapping using the interspecific F₂ population revealed two major QTLs on chromosomes 3 and 9. Two bin markers within the QTL regions on two chromosomes were highly correlated with the capsaicinoid content in the interspecific population. The major QTL, Pun2_PJ_Gibbs_3.11 on chromosome 3, contained the pAMT gene, indicating that the non-pungency of C. chacoense may be attributed to a mutation in the pAMT gene. Sequence analysis revealed a 7 bp nucleotide insertion in the 8^th exon of pAMT of the non-pungent C. chacoense. This mutation resulted in the generation of an early stop codon, resulting in a truncated mutant lacking the PLP binding site, which is critical for pAMT enzymatic activity. This insertion co-segregated with the pungency phenotype in the intraspecific F₂ population. We named this novel pAMT allele pamt¹¹ . Taken together, these data indicate that the non-pungency of C. chacoense is due to the non-functional pAMT allele, and Pun2 encodes the pAMT gene.

Altered capsaicin levels in domesticated chili pepper varieties affect the interaction between a generalist herbivore and its ectoparasitoid

Plant domestication has commonly reduced levels of secondary metabolites known to confer resistance against insects. Chili pepper is a special case because the fruits of different varieties have been selected for lower and higher levels of capsaicin, the main compound associated with defence. This may have important consequences for insect herbivores and their natural enemies. Despite the widespread consumption of chili peppers worldwide, the effects of capsaicin on insects are poorly understood. Here, we investigated the effect of capsaicin on a generalist herbivore, Spodoptera latifascia (Lepidoptera: Noctuidae) and its ectoparasitoid, Euplectrus platyhypenae (Hymenoptera: Eulophidae). Using chili varieties with three pungency levels: non-pungent (Padron), mild (Cayenne) and highly pungent (Habanero), as well as artificial diets spiked with three different levels of synthetic capsaicin, we determined whether higher capsaicin levels negatively affect the performance of these insects. Overall, capsaicin had a negative effect on both herbivore and parasitoid performance, particularly at high concentrations. Caterpillars reared on highly pungent fruits and high-capsaicin diet had longer development time, reduced pupation success, lower adult emergence, but also lower parasitism rates than caterpillars reared on mild or non-capsaicin treatments. In addition, we found that the caterpillars were capable of sequestering capsaicinoids in their haemolymph when fed on the high pungent variety with consequences for parasitoids' performance and oviposition decisions. These results increase our understanding of the role of capsaicin as a chemical defence against insects and its potential implications for pest management.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10340-021-01399-8.

Factors affecting the capsaicinoid profile of hot peppers and biological activity of their non-pungent analogs (Capsinoids) present in sweet peppers

Capsaicinoids are acid amides of C₉-C₁₁ branched-chain fatty acids and vanillylamine and constitute important chemical compounds of Capsicum annuum together with their non-pungent analogs (capsinoids) which have an impressive list of health benefit properties (i.e., analgesia, anti-obesity, thermogenic, cardiovascular, gastrointestinal, antioxidant, anti-bacterial, anti-virulence, anti-inflamatory, anti-diabetic, inhibits angiogenesis, and improves glucose metabolism) . In this review, the state of art on how capsaicinoids are affected by different pre- and postharvest factors is discussed together with their biological activity. For instance, high light intensity and heat treatments may reduce capsaicinoid content in fruits probably due to the loss of activity of capsaicin synthase (CS) and phenylalanine ammonia lyase (PAL). The pungency in peppers varies also with environment, genotype or cultivar, node position, fruiting and maturity stages, nitrogen and potassium contents. As the fruit mature, capsaicinoid levels increase. Fruits from the second node tend to have higher accumulation of pungency than those of other positions and the pungency decreases linearly as the node position increase. Sodium hydroxide treatment reduces the pungency of pepper fruit as it hydrolyzes and modifies one of the features (vanillyl group, the acid-amide linkage and alkyl side chain) of capsaicin molecule. Salt and water stress increase PAL and capsaicin synthase activity and increase the capsaicinoid accumulation in fruit, by negatively regulating peroxidase activity at appropriate levels. Future research must be directed in better understanding the changes of capsinoids during pre and post-harvest management, the causal drivers of the loss of activity of the aminotransferase gene (pAMT) and if possible, studies with genetically modified sweet peppers with functional pAMT. Available data provided in this review can be used in different agricultural programs related to developing new cultivars with specific pungency levels. The contents of capsaicinoids and capsinoids in both fresh fruits and marketed products are also of remarkable importance considering the preferences of certain niches in market where higher added-value products might be commercialized.