High-temperature inhibition of biosynthesis and transportation of anthocyanins results in the poor red coloration in red-fleshed Actinidia chinensis.

In plants, the role of anthocyanins trafficking in response to high temperature has been rarely studied, and therefore poorly understood. Red-fleshed kiwifruit has stimulated the world kiwifruit industry owing to its appealing color. However, fruit in warmer climates have been found to have poor flesh coloration, and the factors responsible for this response remain elusive. Partial correlation and regression analysis confirmed that accumulative temperatures above 25 °C (T25) was one of the dominant factors inhibiting anthocyanin accumulation in red-fleshed Actinidia chinensis, 'Hongyang'. Expression of structural genes, AcMRP and AcMYB1 in inner pericarp sampled from the two high altitudes (low temperature area), was notably higher than the low altitude (high temperature area) during fruit coloration. AcMYB1 and structural genes coordinate expression supported the MYB-bHLH (basic helix-loop-helix)-WD40 regulatory complex mediated downregulation of anthocyanin biosynthesis induced by high temperatures in kiwifruit. Moreover, cytological observations using the light and transmission electronic microscopy showed that there were a series of anthocyanic vacuolar inclusion (AVI)-like structures involved in their vacuolization process and dissolution of the pigmented bodies inside cells of fruit inner pericarp. Anthocyanin transport was inhibited by high temperature via retardation of vacuolization or reduction in AIV-like structure formation. Our findings strongly suggested that complex multimechanisms influenced the effects of high temperature on red-fleshed kiwifruit coloration.

Genome assembly of autotetraploid Actinidia arguta highlights adaptive evolution and enables dissection of important economic traits.

Actinidia arguta, the most widely distributed Actinidia species and the second cultivated species in the genus, can be distinguished from the currently cultivated Actinidia chinensis on the basis of its small and smooth fruit, rapid softening, and excellent cold tolerance. Adaptive evolution of tetraploid Actinidia species and the genetic basis of their important agronomic traits are still unclear. Here, we generated a chromosome-scale genome assembly of an autotetraploid male A. arguta accession. The genome assembly was 2.77 Gb in length with a contig N50 of 9.97 Mb and was anchored onto 116 pseudo-chromosomes. Resequencing and clustering of 101 geographically representative accessions showed that they could be divided into two geographic groups, Southern and Northern, which first diverged 12.9 million years ago. A. arguta underwent two prominent expansions and one demographic bottleneck from the mid-Pleistocene climate transition to the late Pleistocene. Population genomics studies using paleoclimate data enabled us to discern the evolution of the species' adaptation to different historical environments. Three genes (AaCEL1, AaPME1, and AaDOF1) related to flesh softening were identified by multi-omics analysis, and their ability to accelerate flesh softening was verified through transient expression assays. A set of genes that characteristically regulate sexual dimorphism located on the sex chromosome (Chr3) or autosomal chromosomes showed biased expression during stamen or carpel development. This chromosome-level assembly of the autotetraploid A. arguta genome and the genes related to important agronomic traits will facilitate future functional genomics research and improvement of A. arguta.

[Construction of cDNA library of 'Hongyang' kiwifruit and analysis of F3H expression].

The SMART switching mechanism at 5' end of the RNA transcript technique was used to construct a cDNA library from inner pericarp of the red flesh kiwifruit Actinidia chinesis cv 'Hongyang'. Construction of cDNA library facilitated cloning of the genes associated with the secondary metabolism, the specific genes in the course of anthocyanin biosynthesis. The titers of the primary library and the amplified library were 6.7x104 cfu/mL and 2.72x108 cfu/mL, respectively. The recom-bination rate was over 99.8%. The lengths of most cDNAs in the library ranged from 700 bp to 1 000 bp. A total of 1 014 clones randomly chosen from the cDNA library were sequenced and these expressed sequence tags (ESTs) were analyzed. A set of 963 sequences were obtained. Clustering and assembly of these cDNA sequences resulted in 632 unigenes, includ-ing 92 contigs and 540 singletons. Among them, 441 EST unigenes were predicted to have known functions. Gene AcF3H, which participated in anthocyanin biosynthesis from sequencing, was obtained. The length of the AcF3H cDNA was 1 369 bp (GenBank accession No: FJ542819). Bioinformatic analysis showed that AcF3H ORF region was 1 101 bp, which en-coded a peptide with 366 amino acids. The amino acid sequences of this gene shared extensive homology to Arabidopsis, Vitis, and Eustoma. The expression of AcF3H was investigated in inner pericarp of 'Hongyang' at six stages during fruit development using RT-PCR. The expression level was high before colour-changed stage, and then decreased at the primary stage of pigmentation.