Virus-induced silencing of Comt, pAmt and Kas genes results in a reduction of capsaicinoid accumulation in chili pepper fruits

Newly mutated putative-aminotransferase in nonpungent pepper (Capsicum annuum) results in biosynthesis of capsinoids, capsaicinoid analogues

Capsinoids make up a group of nonpungent capsaicinoid analogues produced in Capsicum fruits. They have bioactivities similar to those of capsaicinoids such as suppression of fat accumulation and antioxidant activity. Because of their low pungency, they are more palatable ingredients in dietary supplements than capsaicinoids. We recently reported that capsinoid biosynthesis is caused by nonsense mutation in a putative aminotransferase gene (p-AMT) in a nonpungent cultivar CH-19 Sweet. Here we report on the screening of nonpungent germplasm that revealed a nonpungent cultivar Himo, which contains high levels of capsinoids. We have shown that Himo has a recessive allele of p-amt, which contains a mutation different from that of CH-19 Sweet. Sequence analysis of p-amt in Himo revealed that a single-nucleotide substitution results in one amino acid substitution from cysteine to arginine in the pyridoxal 5-phosphate binding domain. Genetic analysis using a cleaved amplified polymorphic sequence marker confirmed that the p-AMT genotype was precisely cosegregated with capsinoid biosynthesis and nonpungency. Himo will provide a new natural source of capsinoids.

Stone formation in peach fruit exhibits spatial coordination of the lignin and flavonoid pathways and similarity to Arabidopsis dehiscence

BACKGROUND

Lignification of the fruit endocarp layer occurs in many angiosperms and plays a critical role in seed protection and dispersal. This process has been extensively studied with relationship to pod shatter or dehiscence in Arabidopsis. Dehiscence is controlled by a set of transcription factors that define the fruit tissue layers and whether or not they lignify. In contrast, relatively little is known about similar processes in other plants such as stone fruits which contain an extremely hard lignified endocarp or stone surrounding a single seed.

RESULTS

Here we show that lignin deposition in peach initiates near the blossom end within the endocarp layer and proceeds in a distinct spatial-temporal pattern. Microarray studies using a developmental series from young fruits identified a sharp and transient induction of phenylpropanoid, lignin and flavonoid pathway genes concurrent with lignification and subsequent stone hardening. Quantitative polymerase chain reaction studies revealed that specific phenylpropanoid (phenylalanine ammonia-lyase and cinnamate 4-hydroxylase) and lignin (caffeoyl-CoA O-methyltransferase, peroxidase and laccase) pathway genes were induced in the endocarp layer over a 10 day time period, while two lignin genes (p-coumarate 3-hydroxylase and cinnamoyl CoA reductase) were co-regulated with flavonoid pathway genes (chalcone synthase, dihydroflavanol 4-reductase, leucoanthocyanidin dioxygen-ase and flavanone-3-hydrosylase) which were mesocarp and exocarp specific. Analysis of other fruit development expression studies revealed that flavonoid pathway induction is conserved in the related Rosaceae species apple while lignin pathway induction is not. The transcription factor expression of peach genes homologous to known endocarp determinant genes in Arabidopsis including SHATTERPROOF, SEEDSTCK and NAC SECONDARY WALL THICENING PROMOTING FACTOR 1 were found to be specifically expressed in the endocarp while the negative regulator FRUITFUL predominated in exocarp and mesocarp.

CONCLUSIONS

Collectively, the data suggests, first, that the process of endocarp determination and differentiation in peach and Arabidopsis share common regulators and, secondly, reveals a previously unknown coordination of competing lignin and flavonoid biosynthetic pathways during early fruit development.

Characterization of putative capsaicin synthase promoter activity

Capsaicin is a very important secondary metabolite that is unique to Capsicum. Capsaicin biosynthesis is regulated developmentally and environmentally in the placenta of hot pepper. To investigate regulation of capsaicin biosynthesis, the promoter (1,537 bp) of pepper capsaicin synthase (CS) was fused to GUS and introduced into Arabidopsis thaliana (Col-0) via Agrobacterium tumefaciens to produce CSPRO::GUS transgenic plants. The CS was specifically expressed in the placenta tissue of immature green fruit. However, the transgenic Arabidopsis showed ectopic GUS expressions in the leaves, flowers and roots, but not in the stems. The CSPRO activity was relatively high under light conditions and was induced by both heat shock and wounding, as CS transcripts were increased by wounding. Exogenous capsaicin caused strong suppression of the CSPRO activity in transgenic Arabidopsis, as demonstrated by suppression of CS expression in the placenta after capsaicin treatment. Furthermore, the differential expression levels of Kas, Pal and pAmt, which are associated with the capsaicinoid biosynthetic pathway, were also suppressed in the placenta by capsaicin treatment. These results support that capsaicin, a feedback inhibitor, plays a pivotal role in regulating gene expression which is involved in the biosynthesis of capsaicinoids.

Virus-induced gene silencing in soybean seeds and the emergence stage of soybean plants with Apple latent spherical virus vectors

Virus-induced gene silencing (VIGS) has great potential as a reverse-genetics tool in plant genomics. In this study, we examined the potential of VIGS in soybean seeds and the emergence stage of soybean plants using Apple latent spherical virus (ALSV) vectors. Inoculation of an ALSV vector (soyPDS-ALSV) carrying a fragment of the soybean phytoene desaturase (soyPDS) gene into soybean seedlings resulted in a highly uniform photo-bleached phenotype, typical of PDS inhibition, on the upper leaves throughout plant growth. The photo-bleached phenotype was also found on all immature pods, all seed coats, and about 50% embryos of seeds on soybean plants infected with soyPDS-ALSV. Infection with an ALSV vector (soyIFS2-ALSV) having a fragment of soybean isoflavone synthase 2 (soyIFS2) gene also led to a reduction of the levels of both soyIFS2- and soyIFS1- mRNAs and an isoflavone content in the cotyledons of about 36% mature seeds of infected soybean plants. Furthermore, VIGS of soyPDS was induced in the next generation plants by the seed transmission of soyPDS-ALSV. Thus ALSV vectors will be useful for studying gene functions in the reproductive stages and early growth stages, such as emergence and cotyledon stages, in addition to the vegetative stages of soybean plants.

Functional loss of pAMT results in biosynthesis of capsinoids, capsaicinoid analogs, in Capsicum annuum cv. CH-19 Sweet

Capsaicinoids are responsible for the spicy flavor of pungent peppers (Capsicum). The cultivar CH-19 Sweet is a non-pungent pepper mutant derived from a pungent pepper strain, Capsicum annuum CH-19. CH-19 Sweet biosynthesizes capsaicinoid analogs, capsinoids. We determined the genetic and metabolic mechanisms of capsinoid biosynthesis in this cultivar. We analyzed the putative aminotransferase (pAMT) that is thought to catalyze the formation of vanillylamine from vanillin in the capsaicinoid biosynthetic pathway. Enzyme assays revealed that pAMT activity catalyzing vanillylamine formation was completely lost in CH-19 Sweet placenta tissue. RT-PCR analysis showed normal mRNA transcription of the pAMT gene; however, SNP analysis of the cDNA sequence showed a T nucleotide insertion at 1291 bp in the pAMT gene of CH-19 Sweet. This insertion formed a new stop codon, TGA, that prevented normal translation of the gene, and the pAMT protein did not accumulate in CH-19 Sweet as determined using Western blot analysis. We developed a dCAPS marker based on the T insertion in the pAMT gene of CH-19 Sweet, and showed that the pAMT genotype co-segregated with the capsinoid or capsaicinoid fruit phenotype in the F(2) population. The T insertion was not found in other pungent and non-pungent Capsicum lines, suggesting that it is specific to CH-19 Sweet. CH-19 Sweet's pAMT gene mutation is an example of a nonsense mutation in a single gene that alters a secondary metabolite biosynthetic pathway, resulting in the biosynthesis of analogs. The dCAPS marker will be useful in selecting lines with capsinoid-containing fruits in pepper-breeding programs.

Chili pepper fruits: content and pattern of capsaicinoids in single fruits of different ages

The content of capsaicinoids differs widely in fruits of an individual plant. This is shown for Capsicum annuum var. Cayenne and var. DeArbol and Capsicum frutescens var. Hot Siberian, respectively. Three age groups, (i) very young, (ii) medium age, and (iii) older fruits, were studied. A consistent dependence on the node position on the plant for fruit weight and capsaicinoid content of the individual fruits was not observed. These traits do not develop concomitantly and are influenced differently by environmental factors. Therefore, the expression as capsaicinoid content per fruit leads to a different conclusion than a comparison of concentration values (mg/kg). This is exemplified for C. frutescens var. Hot Siberian grown in two consecutive years with fruits of lower fruit weight but the same capsaicinoid accumulation in the second year. Higher values for pungency (expressed as mg/kg) would have been the result from the analysis of bulked material. The fatty acid pattern of capsaicinoids is uniform for all fruits from one plant, irrespective of the large variation of total capsaicinoid content.

Virus-induced gene silencing and its application in characterizing genes involved in water-deficit-stress tolerance

Understanding post-transcriptional gene silencing (PTGS) phenomena in plants has provided breakthroughs in advancing plant functional genomics. A recently developed approach based on one of the strategies adopted by plants to defend against viruses, called virus-induced gene silencing (VIGS), is being widely used to enumerate the function of plant genes. Since its discovery, VIGS has been widely used to characterize plant genes involved in metabolic pathways, homeostasis, basic cellular functions, plant-microbe, plant-nematode and plant-herbivore interaction. Recently, the application of this technique has been extended to characterize the genes and cellular processes involved in abiotic-stress tolerance, and in particular drought and oxidative stress. Because abiotic-stress tolerance is multigenic, identification and characterization of genes involved in this process is challenging. VIGS could become one among the several potential tools in understanding the relevance of these stress-responsive genes. Development of VIGS protocols for the use of heterologous gene sequences as VIGS-inducers has extended its applicability to analyze genes of VIGS recalcitrant plant species. This article describes the methodology of VIGS for characterizing the water-deficit-stress-responsive genes, precautions to be taken during the experimentation, and future application of this technology as a fast forwarded as well as a reverse genetics tool to identify and characterize plant genes involved in drought tolerance. We also describe the importance of accurate water-deficit-stress imposition and quantification of stress-induced changes in the silenced plants during the process of screening to identify genes responsible for tolerance. Further, limitations of VIGS in characterizing the abiotic-stress-responsive genes are noted, with suggestions to overcome these limitations.