APETALA2/ethylene-responsive factor superfamily (AP2/ERF) is a transcription factor involved in abiotic stresses, for instance, cold, drought, and low oxygen. In this study, a novel ethylene-responsive transcription factor named AdRAP2.3 was isolated from Actinidia deliciosa ‘Jinkui’. AdRAP2.3 transcription levels in other reproductive organs except for the pistil were higher than those in the vegetative organs (root, stem, and leaf) in kiwi fruit. Plant hormones (Salicylic acid (SA), Methyl-jasmonate acid (MeJA), 1-Aminocyclopropanecarboxylic Acid (ACC), Abscisic acid (ABA)), abiotic stresses (waterlogging, heat, 4 °C and NaCl) and biotic stress (Pseudomonas Syringae pv. Actinidiae, Psa) could induce the expression of AdRAP2.3 gene in kiwi fruit. Overexpression of the AdRAP2.3 gene conferred waterlogging stress tolerance in transgenic tobacco plants. When completely submerged, the survival rate, fresh weight, and dry weight of transgenic tobacco lines were significantly higher than those of wile type (WT). Upon the roots being submerged, transgenic tobacco lines grew aerial roots earlier. Overexpression of AdRAP2.3 in transgenic tobacco improved the pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) enzyme activities, and improved the expression levels of waterlogging mark genes NtPDC, NtADH, NtHB1, NtHB2, NtPCO1, and NtPCO2 in roots under waterlogging treatment. Overall, these results demonstrated that AdRAP2.3 might play an important role in resistance to waterlogging through regulation of PDC and ADH genes in kiwi fruit.

Mitogen activated protein kinase (MAPK) cascades are universal signal transduction modules that play crucial roles in various biotic and abiotic stresses, hormones, cell division, and developmental processes in plants. Mitogen activated protein kinase (MAPK/MPK), being a part of this cascade, performs an important function for further appropriate cellular responses. Although MAPKs have been investigated in several model plants, no systematic analysis has been conducted in kiwifruit (Actinidia chinensis). In the present study, we identified 18 putative MAPKs in the kiwifruit genome. This gene family was analyzed bioinformatically in terms of their chromosome locations, sequence alignment, gene structures, and phylogenetic and conserved motifs. All members possess fully canonical motif structures of MAPK. Phylogenetic analysis indicated that AcMAPKs could be classified into five subfamilies, and these gene motifs in the same group showed high similarity. Gene structure analysis demonstrated that the number of exons in AcMAPK genes ranged from 2 to 29, suggesting large variation among kiwifruit MAPK genes. The expression profiles of these AcMAPK genes were further investigated using quantitative real-time polymerase chain reaction (qRT-PCR), which demonstrated that AcMAPKs were induced or repressed by various biotic and abiotic stresses and hormone treatments, suggesting their potential roles in the biotic and abiotic stress response and various hormone signal transduction pathways in kiwifruit. The results of this study provide valuable insight into the putative physiological and biochemical functions of MAPK genes in kiwifruit.

Kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) has brought about a severe threat to the kiwifruit industry worldwide since its first outbreak in 2008. Studies on other pathovars of P. syringae are revealing the pathogenesis of these pathogens, but little about the mechanism of kiwifruit bacterial canker is known. In order to explore the species-specific interaction between Psa and kiwifruit, we analyzed the transcriptomic profile of kiwifruit infected by Psa. After 48 h, 8255 differentially expressed genes were identified, including those involved in metabolic process, secondary metabolites metabolism and plant response to stress. Genes related to biosynthesis of terpens were obviously regulated, indicating terpens may play roles in suppressing the growth of Psa. We identified 283 differentially expressed resistant genes, of which most U-box domain containing genes were obviously up regulated. Expression of genes involved in plant immunity was detected and some key genes showed differential expression. Our results suggest that Psa induced defense response of kiwifruit, including PAMP (pathogen/microbe-associated molecular patterns)-triggered immunity, effector-triggered immunity and hypersensitive response. Metabolic process was adjusted to adapt to these responses and production of secondary metabolites may be altered to suppress the growth of Psa.

Nucleotide-binding site and leucine-rich repeat (NBS-LRR) genes represent the most important disease resistance genes in plants. The genome sequence of kiwifruit (Actinidia chinensis) provides resources for the characterization of NBS-LRR genes and identification of new R-genes in kiwifruit. In the present study, we identified 100 NBS-LRR genes in the kiwifruit genome and they were grouped into six distinct classes based on their domain architecture. Of the 100 genes, 79 are truncated non-regular NBS-LRR genes. Except for 37 NBS-LRR genes with no location information, the remaining 63 genes are distributed unevenly across 18 kiwifruit chromosomes and 38.01% of them are present in clusters. Seventeen families of cis-acting elements were identified in the promoters of the NBS-LRR genes, including AP2, NAC, ERF and MYB. Pseudomonas syringae pv. actinidiae (pathogen of the kiwifruit bacterial canker) infection induced differential expressions of 16 detected NBS-LRR genes and three of them are involved in plant immunity responses. Our study provides insight of the NBS-LRR genes in kiwifruit and a resource for the identification of new R-genes in the fruit.

A previous report showed that both Pyruvate decarboxylase (PDC) genes were significantly upregulated in kiwifruit after waterlogging treatment using Illumina sequencing technology, and that the kiwifruit AdPDC1 gene was required during waterlogging, but might not be required during other environmental stresses. Here, the function of another PDC gene, named AdPDC2, was analyzed. The expression of the AdPDC2 gene was determined using qRT-PCR, and the results showed that the expression levels of AdPDC2 in the reproductive organs were much higher than those in the nutritive organs. Waterlogging, NaCl, and heat could induce the expression of AdPDC2. Overexpression of kiwifruit AdPDC2 in transgenic Arabidopsis enhanced resistance to waterlogging and heat stresses in five-week-old seedlings, but could not enhance resistance to NaCl and mannitol stresses at the seed germination stage and in early seedlings. These results suggested that the kiwifruit AdPDC2 gene may play an important role in waterlogging resistance and heat stresses in kiwifruit.