Kwan Jeong Song

Rapid Shoot Propagation from Micro-Cross Sections of Kiwifruit (Actinidia deliciosa cv. ‘Hayward’)

Kiwifruit shoots can be rapidly propagated through a micro-cross section (MCS) system we established here. Optimal culture conditions were determined for different expiant types, section sizes, and concentrations of inorganic salts and plant growth regulators. Rates of survival and callus formation were higher in half-strength MS salts than in full-strength MS media. Similar performance (i.e., survival and callus formation) was achieved with section sizes of either 800 μm or 1200 μm. Proliferation efficiency was greatest when expiants from stem tissue were cultured on 1/2 MS supplemented with 4.5x10-3 μM 2,4-dichlo-rophenoxyacetic acid and 4.6x10-1 μM zeatin. The number of shoots averaged 2.61 per expiant, representing an efficiency of 94%. RAPD analysis revealed that the regenerated plants from our MCS system were genetically stable. These results show that the culturing of micro-cross sections from stem tissue is a powerful method for kiwifruit propagation.

Characterization of a RAPD fragment unique to species with hairy fruit skin in the genusActinidia

To develop a SCAR primer related to the hairy-fruit trait in the genusActinidia, we took a PCR-RAPD approach using arbitrary 10-mer primers. PCR with the UBC 376 primer generated specific fragments from three species with hairy fruit skin. Those fragments were then cloned to determine their nucleotide sequences. Two SCAR primers were designed from the UBC 376 primer and nucleotide sequences were obtained from the PCR fragments. A SCAR primer, OKC385, specifically amplified a 385-bp fragment from one clone ofActinidia eriantha, four ofActinidia chinensis, and four ofActinidia deliciosa. Deduced amino acid sequences of this fragment showed high sequence homology with plant cellulose synthases, which are involved in the biosynthesis of cellulose, a major cell wall component. The 385-bp fragment was specifically detected only in the seriesPerfectae C.F. Liang of sectionStellatae Li. This type has many hairs on the leaves, fruits, and stems, suggesting that the gene containing the PCR fragment is involved in hair formation in this phylogenetic group. Taken together, our results suggest that the SCAR primer, OKC385, can be used as a specific primer for early selection of the non-hair trait in breeding of the genusActinidia.

Characterization of microspore development and pollen tube growth response to self and cross-pollination in Jeju old local citrus species

This study was conducted to investigate the seed formation potential by the evaluation of microspore development and self- and cross-compatibility through their pollen tube growth response to self and cross pollination in Jeju old local citrus species. Anther and pollen histological analyses were done to observe microspore development for three species including byungkyul, sadoogam, and dangyooza. Pollens of ten citrus species were tested for pollen viability by staining with Lugol’s solution and fluorescein diacetate. Flowers of all species were emasculated and self- and cross-pollinated just before anthesis. Pistils of nine pollinating combinations between the listed three species were collected at 1, 3, 5, 7, and 9 days after pollination for evaluation of a growth pattern of the pollen tube. Pistils of the other combinations were collected at 9 days after pollination for self- and cross-compatibility evaluation. Pistils were prepared by squashing and staining method with aniline blue. Eight pollen developmental stages were recognized and indicated normal development of microspore. These ten citrus species produced high percentage of viable pollens. Pollen tube behavior in compatible and incompatible pollination was distinguished. Pollen tube performance was largely depended on male-female combination. All of these Jeju old local citrus species were male fertile and have the ability to produce enough viable pollens for self- and cross-pollination. All evaluated citrus species were self- and cross-compatible to each other except dangyooza which was self-incompatible.

Current status and prospects of kiwifruit (Actinidia chinensis) genomics

Kiwifruit is a new fruit crop that was commercialized in the late 1970s. Recently, its cultivation and consumption have increased rapidly worldwide. Kiwifruit is a dioecious, deciduous, and climbing plant having fruit with hairs and various flesh colors and a variation in ploidy level; however, the industry consists of very simple cultivars or genotypes. The need for efficient cultivar improvement together with the evolutional and biological perspectives based on unique plant characteristics, have recently encouraged genome analysis and bioinformatics application. The draft genome sequence and chloroplast genome sequence of kiwifruit were released in 2013 and 2015, respectively; and gene annotation has been in progress. Recently, transcriptome analysis has shifted from previous ESTs analysis to the RNA-seq platform for intensive exploration of controlled genetic expression and gene discovery involved in fruit ascorbic acid biosynthesis, flesh coloration, maturation, and vine bacterial canker tolerance. For improving conventional breeding efficiency, molecular marker development and genetic linkage map construction have advanced from basic approaches using RFLP, RAPD, and AFLP to the development of NGS-based SSR and SNP markers linked to agronomically important traits and the construction of highly saturated linkage maps. However, genome and transcriptome studies have been limited in Korea. In the near future, kiwifruit genome and transcriptome studies are expected to translate to the practical application of molecular breeding.

Current status and prospects of citrus genomics

Citrus is an economically important fruit tree with the largest amount of fruit production in the world. It provides important nutrition such as vitamin C and other health-promoting compounds including its unique flavonoids for human health. However, it is classified into the most difficult crops to develop new cultivars through conventional breeding approaches due to its long juvenility and some unique reproductive biological features such as gamete sterility, nucellar embryony, and high level of heterozygosity. Due to global warming and changes in consumer trends, establishing a systematic and efficient breeding programs is highly required for sustainable production of high quality fruits and diversification of cultivars. Recently, reference genome sequences of sweet orange and clementine mandarin have been released. Based on the reference whole-genome sequences, comparative genomics, reference-guided resequencing, and genotyping-by-sequencing for various citrus cultivars and crosses could be performed for the advance of functional genomics and development of traits-related molecular markers. In addition, a full understanding of gene function and gene co-expression networks can be provided through combined analysis of various transcriptome data. Analytic information on whole-genome and transcriptome will provide massive data on polymorphic molecular markers such as SNP, INDEL, and SSR, suggesting that it is possible to construct integrated maps and high-density genetic maps as well as physical maps. In the near future, integrated maps will be useful for map-based precise cloning of genes that are specific to citrus with major agronomic traits to facilitate rapid and efficient marker-assisted selection.

Effect of artificial defoliation on cane regrowth and fruit development in ‘Jecy Gold’ kiwifruit

Leaves of kiwifruit have long petiole and large size, so that they are easily damaged by strong wind. This study was performed to evaluate the effect of leaves loss by typhoon damage on fruit quality and yield and new cane growth with artificial defoliation. Five-year-old ‘Jecy Gold’ kiwifruit vines grown in a plastic house were used in this study. Artificial defoliation was done on August 23 (100 days after anthesis) at the different defoliation levels, such as 0 (control), 50, 75, and 100%. Cane regrowth by axillary bud burst, cane bark carbohydrate reserve, and fruit quality and yield were investigated. Severe defoliation at 75 and 100% level showed significantly higher axillary budburst than the control and the 50% defoliation. Especially, the 100% defoliation induced a few abnormal off-season flowering. Starch and sucrose contents were significantly declined during the first 2 weeks after defoliation at the 75 and 100% defoliation. However, other soluble carbohydrate reserves including fructose, glucose, and myo-inositol were not affected significantly. Fruit drop was promoted and the mean fruit weight, firmness, and soluble solid concentrations were lower by the defoliation. Our results indicated that the loss of leaves above 50% would induce a transient shortage of assimilate supply and reduce fruit quality and yield significantly.

Current status and prospects of molecular marker development for systematic breeding program in citrus

세계적인 과수작물로서의 경제적 중요성에도 불구하고, 감귤 생산은 주로 자연교잡 실생이나 눈 돌연변이로부터의 선발 또는 단순 품종 도입 등을 통해 이루어지고 있는 실정이다. 긴 유년기, 다배성, 자가불화합성과 같은 감귤 고유의 식물학적 특성, 주요 형질들(병저항성, 수량성, 품질 등)의 QTL에 의한 조절 등은 전통...

Defoliation time influences vine regrowth, off-season flowering, and fruit quality in ‘Jecy Gold’ kiwifruit vines

Abrupt and severe loss of leaves caused by a typhoon disrupts physiological development and metabolism in kiwifruit vines. The aim of this study was to evaluate the effects of time of defoliation on off-season flowering, vine regrowth, and fruit quality in ‘Jecy Gold’ kiwifruit. ‘Jecy Gold’ kiwifruit vines were subjected to artificial 100% defoliation on July 28 and August 29, 2014. In October, the occurrence of off-season flowering was observed only on vines that were defoliated on August 29. There was no significant difference in vine regrowth between defoliation on July 28 or August 29. However, defoliation on August 29 showed a significant reduction in soluble solid content in fruits at harvesting time. Furthermore, there was no significant difference in starch and sugar content of cane bark between the different times of defoliation. The results indicate that the floral evocation of ‘Jecy Gold’ kiwifruit might occur in August and that fruit maturation is delayed after defoliation on August 29 in comparison to defoliation on July 28.