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Background Kiwifruit (Actinidia Lindl.) is considered an important fruit species worldwide. Due to its temperate origin, this species is highly vulnerable to freezing injury while under low-temperature stress. To obtain further knowledge of the mechanism underlying freezing tolerance, we carried out a hybrid transcriptome analysis of two A. arguta (Actinidi arguta) genotypes, KL and RB, whose freezing tolerance is high and low, respectively. Both genotypes were subjected to − 25 °C for 0 h, 1 h, and 4 h. Results SMRT (single-molecule real-time) RNA-seq data were assembled using the de novo method, producing 24,306 unigenes with an N50 value of 1834 bp. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of DEGs showed that they were involved in the ‘starch and sucrose metabolism’, the ‘mitogen-activated protein kinase (MAPK) signaling pathway’, the ‘phosphatidylinositol signaling system’, the ‘inositol phosphate metabolism’, and the ‘plant hormone signal transduction’. In particular, for ‘starch and sucrose metabolism’, we identified 3 key genes involved in cellulose degradation, trehalose synthesis, and starch degradation processes. Moreover, the activities of beta-GC (beta-glucosidase), TPS (trehalose-6-phosphate synthase), and BAM (beta-amylase), encoded by the abovementioned 3 key genes, were enhanced by cold stress. Three transcription factors (TFs) belonging to the AP2/ERF, bHLH (basic helix-loop-helix), and MYB families were involved in the low-temperature response. Furthermore, weighted gene coexpression network analysis (WGCNA) indicated that beta-GC, TPS5, and BAM3.1 were the key genes involved in the cold response and were highly coexpressed together with the CBF3, MYC2, and MYB44 genes. Conclusions Cold stress led various changes in kiwifruit, the ‘phosphatidylinositol signaling system’, ‘inositol phosphate metabolism’, ‘MAPK signaling pathway’, ‘plant hormone signal transduction’, and ‘starch and sucrose metabolism’ processes were significantly affected by low temperature. Moreover, starch and sucrose metabolism may be the key pathway for tolerant kiwifruit to resist low temperature damages. These results increase our understanding of the complex mechanisms involved in the freezing tolerance of kiwifruit under cold stress and reveal a series of candidate genes for use in breeding new cultivars with enhanced freezing tolerance.

BackgroundFreezing injury, which is an important abiotic stress in horticultural crops, influences the growth and development and the production area of kiwifruit (ActinidiaLind1). AmongActinidiaspecies,Actinidia argutahas excellent cold resistance, but knowledge relevant to molecular mechanisms is still limited. Understanding the mechanism underlying cold resistance in kiwifruit is important for breeding cold resistance.ResultsIn our study, a population resulting from the cross ofA. arguta‘Ruby-3’ × ‘Kuilv’ male was generated for kiwifruit hardiness study, and 20 cold-tolerant and 20 cold-sensitive populations were selected from 492 populations according to their LT50. Then, we performed bulked segregant RNA-seq combined with single-molecule real-time sequencing to identify differentially expressed genes that provide cold hardiness. We found that the content of soluble sucrose and the activity of β-amylase were higher in the cold-tolerant population than in the cold-sensitive population. Upon − 30 °C low-temperature treatment, 126 differentially expressed genes were identify; the expression of 59 genes was up-regulated and that of 67 genes was down-regulated between the tolerant and sensitive pools, respectively. KEGG pathway analysis showed that the DEGs were primarily related to starch and sucrose metabolism, amino sugar and nucleotide sugar metabolism. Ten major key enzyme-encoding genes and two regulatory genes were up-regulated in the tolerant pool, and regulatory genes of theCBFpathway were found to be differentially expressed. In particular, a14–3-3gene was down-regulated and anEBFgene was up-regulated.To validate the BSR-Seq results, 24 DEGs were assessed via qRT-PCR, and the results were consistent with those obtained by BSR-Seq.ConclusionOur research provides valuable insights into the mechanism related to cold resistance inActinidiaand identified potential genes that are important for cold resistance in kiwifruit.

Additional file 8 Table S5. Primers used for quantitative real-time RT-PCR analysis.

BackgroundThe freezing injury, which is one of the important abiotic stresses in horticultural crops, can influences the growth and development and the production area of kiwifruit (Actinidia Lind1). Actinidia arguta has excellent cold resistance in Actinidia species, but knowledge relevant to molecular mechanisms is still limited so far. Understanding the mechanism of cold resistance in kiwifruit is important for cold-resistant breeding. ResultsIn our study, a cross of ‘Ruby-3’בKuilv’ male was built for the A. arguta hardiness study, and 20 cold-tolerant and 20 cold-sensitive populations were selected from 492 populations according to LT50. Then, we performed Bulked segregant RNA-seq combined with single-molecule real-time sequencing to identify differentially expressed genes with cold hardiness. We found that the content of soluble sucrose and the activity of β-amylase were higher in the cold tolerant population pool than in the cold sensitive population pool. Upon -30°C low temperature treatment, 126 differentially expressed genes were found, and 59 genes were upregulated and 67 genes were downregulated when comparing the tolerant and sensitive pools, respectively. KEGG pathway analysis showed that the DEGs mainly belonged to starch and sucrose metabolism and amino sugar and nucleotide sugar metabolism. There were main 10 key enzyme encoding genes and two regulatory genes were up regulated in tolerant pool, regulated genes of CBF pathway were found to be different expressed, especially, 14-3-3 gene was down regulated and EBF gene was up regulated. To validate the BSR-Seq results, 24 DEGs were determined by qRT-PCR, and the results were consistent with BSR-Seq.ConclusionOur research provides valuable insights into the mechanism related to cold resistance in Actinidia and identified potential genes that are important for cold resistance in kiwifruit.

Kiwifruit bacterial canker, caused by the bacterial pathogen Pseudomonas syringae pv. actinidiae (Psa), is a destructive disease in the kiwifruit industry globally. Consequently, understanding the mechanism of defense against pathogens in kiwifruit could facilitate the development of effective novel protection strategies. The Non-expressor of Pathogenesis-Related genes 1 (NPR1) is a critical component of the salicylic acid (SA)-dependent signaling pathway. Here, a novel kiwifruit NPR1-like gene, designated AeNPR1a, was isolated by using PCR and rapid amplification of cDNA ends techniques. The full-length cDNA consisted of 1952 base pairs with a 1,746-bp open-reading frame encoding a 582 amino acid protein. Homology analysis showed that the AeNPR1a protein is significantly similar to the VvNPR1 of grape. A 2.0 Kb 5'-flanking region of AeNPR1a was isolated, and sequence identification revealed the presence of several putative cis-regulatory elements, including basic elements, defense and stress response elements, and binding sites for WRKY transcription factors. Real-time quantitative PCR results demonstrated that AeNPR1a had different expression patterns in various tissues, and its transcription could be induced by phytohormone treatment and Psa inoculation. The yeast two-hybrid assay revealed that AeNPR1a interacts with AeTGA2. Constitutive expression of AeNPR1a induced the expression of pathogenesis-related gene in transgenic tobacco plants and enhanced tolerance to bacterial pathogens. In addition, AeNPR1a expression could restore basal resistance to Pseudomonas syringae pv. tomato DC3000 (Pst) in Arabidopsis npr1-1 mutant. Our data suggest that AeNPR1a gene is likely to play a pivotal role in defense responses in kiwifruit.

Study on kiwifruit (Actinidia chinensis and A. deliciosa) color mainly concentrated in green and yellow-fleshed cultivars, less about molecular mechanism of red-fleshed trait formation, rarely in all red-fleshed fruit. Using ‘Tianyuanhong’ and ‘Yongfengyihao’ (‘TY’, a kind of all red-fleshed cultivar, from Actinidia arguta; ‘YF’, a kind of all green-fleshed cultivar, also from Actinidia arguta) as experimental material, we performed RNA-seq to obtain 202,742 unigenes with an average length of 603 bp and N50 of 873 bp via transcriptome data analysis. Of these unigenes, 72,508 (35.76%) were annotated and 997 were assigned to secondary metabolic pathways, of which 104 unigenes were involved in flavonoid and anthocyanin biosynthesis. According to the parameter log2fold-change and p-adjusted, 12 differentially expressed structural genes were selected for performing expression profiles and cluster analysis. Physiological traits including color ration, hue angle, and anthocyanin content were also investigated. From the results, we concluded AaLDOX (genes encoding leucoanthocyanidin dioxygenase) maybe the key gene controlling anthocyanin biosynthesis in flesh of ‘TY’ kiwifruit, which promoted accumulation of anthocyanin, finally leading to the red flesh coloration.

Actinidia arguta ‘Tianyuanhong’ is a new kiwifruit variety with an all-red pericarp and pulp, in contrast to the all-green pulp of A. arguta ‘Yongfengyihao’. Transcriptome profile analysis of fruit color has been reported, however, the metabolic mechanisms producing red flesh remain unknown, and it is unclear why the pulp of ‘Tianyuanhong’ is red rather than green. Herein, we identified differences between the proteomes of two A. arguta cultivars with different fruit color by using iTRAQ-based quantitative proteomic methods during the stage of color change. In total, 2310 differentially abundant proteins were detected between the two cultivars at 70 and 100 days after flowering, and the protein functions were analyzed based on KEGG and GO. The largest group of differentially expressed proteins were related to photosynthesis, glyoxylate metabolism, N metabolism, and anthocyanin biosynthesis. Finally, to verify the iTRAQ data, 12 representative genes encoding differentially expressed proteins were analyzed via quantitative real-time PCR, and these genes differed in transcriptional and translational expression levels. Our proteomic study contributes to understanding the metabolic pathways and biological processes involved in fruit color changes in different cultivars of A. arguta. These data and analyses will provide new insight into the development of kiwifruit flesh color.

Actinidia arguta is the most basal species in a phylogenetically and economically important genus in the family Actinidiaceae. To better understand the molecular basis of the Actinidia arguta chloroplast (cp), we sequenced the complete cp genome from A. arguta using Illumina and PacBio RS II sequencing technologies. The cp genome from A. arguta was 157,611 bp in length and composed of a pair of 24,232 bp inverted repeats (IRs) separated by a 20,463 bp small single copy region (SSC) and an 88,684 bp large single copy region (LSC). Overall, the cp genome contained 113 unique genes. The cp genomes from A. arguta and three other Actinidia species from GenBank were subjected to a comparative analysis. Indel mutation events and high frequencies of base substitution were identified, and the accD and ycf2 genes showed a high degree of variation within Actinidia. Forty-seven simple sequence repeats (SSRs) and 155 repetitive structures were identified, further demonstrating the rapid evolution in Actinidia. The cp genome analysis and the identification of variable loci provide vital information for understanding the evolution and function of the chloroplast and for characterizing Actinidia population genetics.