Advances in and prospects for Actinidia viruses

Kiwifruit (Actinidia spp.) is an economically important fruit crop worldwide. Before 2010, kiwifruit viruses had not received much attention; since then, more than 20 viruses infecting kiwifruit have been discovered. Some of these viruses cause severe yellowing, mosaic, necrosis, ringspots and other symptoms on leaves, seriously impacting yield and quality. Many of these viruses are widely distributed. This review summarizes recent research advances in the identification, genomic variation, distribution, transmission, detection, incidence, prevention and control of kiwifruit viruses and proposes directions for future research. Using virus-tested propagation material is the most economical and effective method for controlling kiwifruit viruses.

Physiological Responses of Two Contrasting Kiwifruit (Actinidia spp.) Rootstocks against Waterlogging Stress

Rootstocks from Actinidia valvata are much more tolerant to waterlogging stress than those from Actinidia deliciosa, which are commonly used in kiwifruit production. To date, the tolerance mechanism of A. valvata rootstocks’ adaptation to waterlogging stress has not been well explored. In this study, the responses of KR5 (A. valvata) and ‘Hayward’ (A. deliciosa) to waterlogging stress were compared. Results showed that KR5 plants performed much better than ‘Hayward’ during waterlogging by exhibiting higher net photosynthetic rates in leaves, more rapid formation of adventitious roots at the base of stems, and less severe damage to the main root system. In addition to morphological adaptations, metabolic responses of roots including sufficient sucrose reserves, modulated adjustment of fermentative enzymes, avoidance of excess lactic acid and ethanol accumulation, and promoted accumulation of total amino acids all possibly rendered KR5 plants more tolerant to waterlogging stress compared to ‘Hayward’ plants. Lysine contents of roots under waterlogging stress were increased in ‘Hayward’ and decreased in KR5 compared with their corresponding controls. Overall, our results revealed the morphological and metabolic adaptations of two kiwifruit rootstocks to waterlogging stress, which may be responsible for their genotypic difference in waterlogging tolerance.

A molecular protocol for Early Sex Discrimination (ESD) in Actinidia spp

Dioecism and an extended juvenile phase of 3–7 years in kiwifruit hinder the progress in breeding new cultivars. The identification of fruit-bearing females at an early stage of growth is crucial for breeders. Consequently, molecular markers have become a key tool for identifying female and male plants at an early stage of development. Several efforts were made to identify PCR-based sex linked markers in Actinidia; however, those markers are characterized by a highly polymorphic nature affecting the result of the screening reliability, suggesting the need of more suitable, stable markers, characterized by a consistent transferability among genotypes and species. The main goal of this work was to develop a method for the ultimate discrimination of females from male plants at an early stage of growth using sex-linked markers. We developed an Early Sex Discrimination molecular Test (ESD Test) that allows the discrimination of male and female plants using a simple PCR amplification test. We demonstrate that the test could unequivocally identify the gender of an unknown sample both in the most commercially important species A. chinensis and in further 13 Actinidia species tested with the exception of Actinidia latifolia, where markers fail in gender discrimination. Male genotypes could be easily identified and discarded reducing the cost of a breeding program.

Comparative Analysis of Volatile Compounds in Flowers of Different Actinidia Species

Among the actinidia genus (Actinidia spp.) native to China, few species are grown commercially for their edible and healthy fruits. As they are dioecious plants, a lot of interest is paid to effective pollination and to insects as the most efficient pollinators. In this study we have concentrated on the composition of volatile compounds in male flowers of four different actinidia species (A. chinensis var. deliciosa, A. arguta, A. kolomikta and A. polygama) and on the diversity between male and female flower volatiles for the two most winter-hardy species (A. arguta and A. kolomikta) with growing commercial value. Analyses were provided using gas chromatography with mass spectrometry (GC-MS). In total, 120 compounds were found in 15 actinidia genotypes. However, the number of identified compounds varied between species. Different main compounds or groups of volatile compounds characterised flowers of every species. Smaller differences were observed between male and female flowers of the same species. Our results suggest that actinidia flowers could be attractive to pollinating insects.

Genotype identification and diversity analysis in Kiwifruit (Actinidia spp.) using RAPD markers

Kiwifruit (Actinidia spp.) is a significant plantation crop belonging to family Actinidiaceae, having deciduous, dioecious, and scrambling vines with chromosome number 2n=58. Dioecy in kiwifruit forms the basis for several breeding programs. The present study was carried out for diversity analysis in kiwifruit genotypes using RAPD markers. 7 kiwifruit genotypes (2 males viz. Allision & Tomuri and 5 females viz. Hayward, Bruno, Allision, Monty & Abbott) were analysed for molecular polymorphism using 94 RAPD primers, out of which 23 primers amplified the genomic DNA in all the genotypes. RAPD data was analysed using NTSYS-pc software and dendrogram construction was done using UPGMA method. Two separate clusters of male and female genotypes were formed. Similarity matrix indices showed maximum similarity between Tomuri (M) and Allision (M) with a similarity coefficient of 0.719 while Abbott (F) and Allision (M) were found to have least similarity having a similarity coefficient of 0.521. Four RAPD primers amplified unique amplicons in Monty, Hayward, Bruno, Allision (M) and Abbott and two primers amplified unique amplicons in Allision (M) & Tomuri (M) along with the male and female plants of Allision genotype respectively. Therefore, these primers can help in distinguishing the genotypes of kiwifruit and can also be validated as putative markers for the sex identification in kiwifruit.

Occurrence and distribution of Actinidia viruses in Shaanxi Province of China

Kiwifruit (Actinidia spp.) is an economically important fruit crop globally. China is the largest kiwifruit-growing country in the world, and Shaanxi Province is the major kiwifruit-growing region in China. A systematic survey detected various symptoms in kiwifruit plants grown in a commercial kiwifruit field in Shaanxi Province. Samples were collected from kiwifruit plants showing symptoms and used for virus detection by high-throughput sequencing. In addition to ten known kiwifruit viruses, three new viruses were detected and tentatively named Actinidia yellowing ringspot virus (AYRSpV), Actinidia yellowing virus 1 (AcYV1) and Actinidia yellowing virus 2 (AcYV2). The genome sequences of the three new viruses and four known viruses were determined. Based on the demarcation criteria of the International Committee on Taxonomy of Viruses (ICTV), AYRSpV might be a new member of the genus Ilarvirus in the family Bromoviridae, AcYV1 might be a new virus of the genus Waikavirus in the family Secoviridae, and AcYV2 might be a novel virus in the family Tombusviridae. Spherical viral particles were found in the samples infected with AYRSpV, AcYV1 and AcYV2 by transmission electron microscopy. Further analysis showed that all thirteen viruses can infect both A. deliciosa and A. chinensis, but the incidences of these infections vary among different kiwifruit cultivars in different regions. These results provide valuable information for understanding the viriome of kiwifruit in China.