The development of new cultivars exhibiting high levels of disease resistance and superior quality is critical for vegetable production. Recent advances in genomics and DNA sequencing technologies have facilitated the identification of sequence variants associated with specific phenotypes. This has enabled the development of novel and improved DNA markers for integration into marker-assisted selection (MAS). The application of MAS has significantly enhanced the efficiency of stacking loci responsible for disease resistance and quality characteristics. This paper provides an overview of recent research findings related to DNA markers and their applications in breeding in Japan, focusing primarily on vegetables from Brassicaceae, Solanaceae, Cucurbitaceae, and Allium, as well as strawberries.

Buckwheat (Fagopyrum esculentum) is a heterostylous self-incompatible crop that requires outcrossing for seed formation. Pollen parents influence the traits of seeds in many plants, but their influence in buckwheat is unknown. Here, we crossed self-incompatible (SI) and self-compatible (SC) lines with different rutin contents. The average rutin contents of SI leading cultivars were all 0.15–0.21 mg/g, and a SI high rutin content line that we had developed was 0.6 mg/g, although it has wide range SDs (0.12–0.41). On the other hand, the average rutin contents of SC lines were 0.01–0.06 mg/g, with stable SDs (0.02–0.03). In crosses between high- and low-rutin parents, the average rutin content of F1 seeds was significantly lower than that of the high-rutin parent and higher than that of the low-rutin parent, indicating that the pollen parent influences the rutin content in seeds of buckwheat. RNA-seq analysis confirmed that alleles of several genes encoding enzymes involved in rutin synthesis derived from pollen parents were expressed during seed formation.

Snap bean leaves are the primary site of photosynthesis. Mutant leaf color provides valuable tools for investigating leaf color changes, their role in photosynthesis, and pigment metabolic pathways. In this study, we found that the seedling leaves of ‘ytg-2’ exhibited a yellow phenotype characterized by reduced chlorophyll content compared with ‘A18’. Blockage of the uroporphyrinogen III (Urogen III) to the fecal porphyrinogen III (Coprogen III) synthesis pathway primarily affected the synthesis of chlorophyll intermediate metabolites. As the plants matured, their leaves transformed from yellow to light green, accompanied by an increase in the total chlorophyll content. Transcriptome analysis revealed that the significantly down-regulated expression of four genes in the HemE gene family (Phvul.008G059400.1, Phvul.010G110900.1, Phvul.008G147200.1, and Phvul.010G069500.1), which blocked the Urogen III to Coprogen III conversion, was the primary cause of the yellow phenotype in ‘ytg-2’ seedlings. Additionally, the down-regulation of POR (Phvul.004G113000.1) and NOL (Phvul.004G163900) genes during the chlorophyll cycle explained the reduced total chlorophyll content in ‘ytg-2’ and the gradual normalization of the chlorophyll a/b ratio in ‘ytg-2’ yellow leaves. The expression of PAO (Phvul.004G163900.1), a key enzyme in chlorophyll degradation, further confirmed that the yellow-to-green transition in ‘ytg-2’ leaves was linked to chlorophyll degradation processes.

“Tanpo”, a Japanese rice landrace widely cultivated approximately 120 years ago in Akita Prefecture, exhibits a shorter, wider, thicker, and heavier grain compared to Akitakomachi. Microscopic analysis has revealed that the epidermal cells of Tanpo spikelet hulls are narrower and shorter, with an increased number of cells in the grain width direction, thus resulting in a distinctive grain shape. In a genetic analysis of an F2 population derived from a cross between Tanpo and Akitakomachi, the Tanpo GW5 allele was found to determine the grain shape in a recessive manner. The GW5 allele in Tanpo is a loss-of-function allele because it generates a stop codon immediately after the start codon with a 100-bp deletion within the first exon. Because the GW5 protein suppresses glycogen synthase kinase 2 (GSK2), a negative regulator of brassinosteroid (BR) signaling, GW5 deficiency in Tanpo results in reduced BR signaling. As a result, the expansion of epidermal cells was suppressed, while the radial cell division was promoted, which led to thicker and shorter spikelet hulls and, ultimately, the characteristic grain shape of Tanpo. The identification of this unique allele in the Tanpo landrace provides a valuable resource for breeding new rice varieties with unique grain characteristics.

One approach to sustainable agricultural production in a changing global environment is the effective utilization of unutilized germplasms. Among these, crop wild relatives (CWRs) represent valuable germplasms that retain the diversity lost during domestication. The genus Oryza has two cultivated species and 22 wild species. One of the cultivated species, Oryza sativa, produces the rice that is the staple food for half of the world's population. We are responsible for the maintenance and distribution of wild Oryza genetic resources held by Japan's National Institute of Genetics (NIG). The NIG has collected the genome sequences of numerous wild Oryza accessions, aiming at understanding and promoting the utilization of Oryza germplasm for both basic and applied sciences, such as breeding. The genome information of many wild Oryza germplasms deciphered by multiple groups is publicly available in databases, allowing for pangenome analysis. This review mainly introduces the wild Oryza genetic resources held by the NIG, discusses the genome diversity revealed through genome sequencing, presents new attempts to utilize wild Oryza germplasm as novel resources enabled by genome sequencing, and discusses the challenges in further effectively utilizing wild Oryza germplasm in breeding.

Japanese morning glory (Ipomoea nil), a short day plant, has been used for studying flowering times. Here, quantitative trait loci (QTL) analysis for days from sowing to flowering (DTF) of F2 between I. nil var. Tokyo Kokei Standard (TKS) and I. hederacea line var. Q65, an early flowering variety, revealed seven QTLs: QTL Ipomoea Flowering 1-7 (qIF1-7). The position of qIF3, which had the most significant effect among the seven QTLs, corresponds with that of I. nil (or I. hederacea) CONSTANS (InCO/IhCO) in the linkage map. There is a single-base InDel in the coding sequence of InCO/IhCO. The single-base deletion (SBD) causes a frame-shift mutation and loss of function in TKS allele (inco-1). I. nil accessions bearing inco-1 tend to flower early, similarly to rice varieties bearing the loss of function allele of CO ortholog, hd1. The TKS allele of qIF3 reduces DTF and corresponds with the inferred effect of inco-1. Based on the distribution of inco-1, a hypothesis was proposed that the SBD in inco-1 might have played an important role in the expansion of Japanese morning glories, originally native to the tropical regions of the Americas, into temperate Asia.

To identify QTLs controlling the eating quality of 'Akidawara', 'Satojiman', and 'Ikuhikari' rice, we performed a genome-wide association mapping analysis using two breeding populations in 2013 and 2014 derived from crosses between these and another parental line. Through sensory tests by a trained panel, we evaluated five components of the eating quality of cooked rice. Fifty-eight QTLs for these components were detected in breeding lines in 2013 (seven regions of chromosome [chr.] 1, 4, and 11) and 2014 (ten of chr. 1, 2, 4, 8, 9, and 11). The Akidawara, Satojiman, or Ikuhikari alleles at these QTLs increased eating quality. QTLs on the short arm of chr. 4, the middle of the long arm of chr. 4, the distal end of the long arm of chr. 4, and the short arm of chr. 11, were identified in both years. The genetic effects of the Satojiman alleles at QTLs on the distal end of the long arm of chr. 4 and on the short arm of chr. 11 were confirmed by analysis of two chromosome segment substitution lines containing a Satojiman segment in the 'Koshihikari' background in 2016 and 2017, in which the Satojiman alleles increased the level of eating quality of Koshihikari.

Combining high-throughput genotyping data with the latest wheat genomic information provided more detailed information on the genetic diversity of the Japanese wheat core collection (JWC). Analysis of genomic population structure divided the JWC accessions into three populations: northeast Japan accessions, native and southwest Japan accessions, and modern accessions showing mixed breeding patterns. This indicates that Japanese wheat varieties have a background of native genomes from southwest Japan incorporating valuable genes from various exotic lines, which is supported by the history of Japanese wheat breeding. Association analyses of several agronomic traits have revealed how genes or alleles have been selected in Japanese wheat breeding and how they differ from those in other regions of the world. This analysis of the JWC collection is expected to contribute not only to the elucidation of genetic diversity in Japanese wheat accessions but also to future wheat breeding by providing a new genetic resource.

Starch properties are the major determinants of grain quality and food characteristics in rice (Oryza sativa L.). Understanding the interactions between genetic regions responsible for starch properties will lead to the development of rice cultivars with desirable characteristics. This study investigated the genetic effect and interaction between qAC9.3, a low-amylose quantitative trait locus (QTL), and the genetic region around Starch branching enzyme IIb (SbeIIb). Both these factors are responsible for the starch properties of the Hokkaido breeding population. The amylose content, pasting temperature, and amylopectin chain-length distribution were compared using F5 lines derived from the cross between the lower amylose content and lower pasting temperature strain 'Hokkai332 (qAC9.3, SbeIIb)' and the higher amylose content and higher pasting temperature variety 'Kitagenki (-, SbeIIbsr )'. The qAC9.3 genotype exhibited low amylose content and reduced the hardness of boiled rice but increased the ratio of amylopectin long chains and did not alter the pasting temperature. In contrast, the SbeIIb genotype was associated with pasting temperature but did not affect the amylose content and hardness of boiled rice. It was suggested that appropriately selecting genotypes of these genetic regions and QTL would allow the fine-tuning of starch properties of cooked rice suitable for future demand.