TPS-b family genes involved in signature aroma terpenes emission in ripe kiwifruit

Abstract

Aroma is a critical factor influencing consumer acceptability of ripe fruit. When fruit are eaten, the aroma travels retronasally from the mouth into the olfactory receptors located in the nose after exhaling. In kiwifruit (Actinidia spp.), terpene volatiles such as α-terpinolene and 1,8-cineole have been shown to contribute to the characteristic aroma of ripe fruit. Notably, 1,8-cineole contributes a key floral/eucalyptus note to the aroma of ripe A. chinensis 'Hort16A' kiwifruit, based on sensory descriptive and discriminant analysis. Emission of α-terpinolene and 1,8-cineole in kiwifruit is induced by ethylene, and production peaks when fruit are at eating ripeness. Two monoterpene synthase TPS-b family genes have been isolated from the fruit of A. arguta and A. chinensis that produce α-terpinolene and 1,8-cineole, respectively. Here we discuss terpene volatiles with respect to fruit aroma and consumer sensory evaluation, analyze the gene structure and conserved motifs of TPS-b genes in published kiwifruit genomes and then construct a transcriptional regulatory network based on Actinidia TPS-b. These data provide further insights into the potential molecular mechanisms underlying signature monoterpene synthesis to improve flavor in kiwifruit.

Keywords: Kiwifruit; TPS; aroma; consumer sensory evaluation; terpenes.

Figure 1.

Model for genetic improvement of aroma quality of horticultural crops. Sensory evaluation of germplasm resources including segregating cross populations can lead to the identification of major genes controlling specific aromas and increases our understanding of the mechanisms for aroma formation and maintenance. With this knowledge, molecular markers can be developed to direct aroma breeding and to toward new varieties with stable flavor. GC-MS, Gas Chromatography-Mass Spectrometry; QTL, Quantitative Trait Locus; GWAS, Genome Wide Association Study; GC-MS-O, GC-MS-Olfactory

Figure 2.

Proportion of terpene volatiles in ripe kiwifruit from different varieties. A, ‘Hongyang’; B, ‘Hort16A’; C, Red5; D, ‘Hayward’; E. ‘Hortgem Tahi’; F, EA01_01. A–C: A. chinensis var. chinensis, D: A. chinensis var. deliciosa, E: A. arguta, F: A. eriantha. The data in B–E were derived from ., ⁶

Figure 3.

Phylogenetic tree of the TPS-b gene family in kiwifruit. Alignment of the deduced amino acid sequences of published AcTPS1, AaTPS1 and AcTPS1a-d ^, and other TPS-b gene models identified in the published kiwifruit genomes from A. chinensis ‘Hongyang’ (marked in red) and Red5 (marked in yellow) and A. eriantha (marked in green) plus the TPS-b genes of several other plant species (tomato is marked in gray; Arabidopsis is marked in blue; grape is marked in purple; poplar is marked in pink).The phylogenetic tree was constructed by the neighbor-joining method using MEGA X software with 500 bootstrap replicates

Figure 4.

Gene structure and conserved motifs of TPS-b genes in published kiwifruit genomes. From top: A. chinensis var. chinensis Red5 (orange), and ‘Hongyang’ (red) and A. eriantha (green). The TPS gene models identified in the kiwifruit genome were aligned using MUSCLE in MEGA X. A, Exon and intron structure. The black lines indicate introns, and the colored boxes indicate the exon structural domains. B, Protein motif composition. Motifs identified by MEME (Multiple EM for Motif Elicitation) analysis are represented by different colored boxes. Detailed information is listed in Supplemental Figure S3

Figure 5.

The alignment of promoter sequences of AaTPS1, AcTPS1a, AcTPS1b and AcTPS1. The black box indicates NAC element, which has been verified to bind to the promoter of AaTPS1.

Figure 6.

Potential transcription factors (TFs) that may interact with TPS-b gene family promoters in kiwifruit. Circles of different sizes indicate the number of TFs contained in each family, and the name of the TF family is marked inside the circle