Spatiogenetic characterization of S receptor kinase (SRK) alleles in naturalized populations of Raphanus sativus L. var. raphanistroides on Yakushima island.

In various coastal areas of Japan, naturalized radish populations are observed. Radish is a cruciferous plant and exhibits self-incompatibility, involving a system controlled by a single locus with multiple S alleles. Although the S allele diversity of radish cultivars and wild radishes has been characterized, the S allele distribution in naturalized populations has not yet been analyzed in relation to the positions of the plants in situ. Here, we show the S allele distribution in naturalized radish populations of Yakushima, a small island in the East China Sea, with positions of the plants. Radish plants were sampled in coastal areas in Yakushima, and their S alleles were detected and characterized. Most of the S alleles had been previously identified in radish cultivars. However, four novel S alleles, which may be unique to Yakushima, were also found. Moreover, seeds in siliques from plants growing in the study areas were sampled, and S allele determination in DNA extracted from these seeds suggested that the plants had exchanged their pollen among their close neighbors. There was also a problem in that the PCR amplification of some SRK alleles was difficult because of their sequence diversity in the naturalized populations, as occurs in cultivars. Our results suggest that the exchange of S alleles between cultivars and naturalized populations occurs and that S alleles in naturalized populations are highly diverse. The methodology established in our study should be applicable to other self-incompatible species to dissect the diversity of S allele distribution in naturalized populations.

Local boron concentrations in tuberous roots of Japanese radish (Raphanus sativus L.) negatively correlate with distribution of brown heart.

Internal browning (or brown heart) in radish is a physiological disorder, manifested as a reddish pigmentation in the central part of the tuberous root. Boron deficiency has been known to induce brown heart, but the relationship between B tissue concentration and the development of brown heart has not been tested. Here, we examined the relationship between these variables. Dissected root tissues of two inbred lines (i.e., cultivars) of East Asian big long radish exhibiting different severity of brown heart were submitted to inductively coupled plasma mass spectrometry (ICP-MS) analysis to reveal the spatial distribution of 19 chemical elements. Statistical analysis revealed that only B correlated negatively with the severity of brown heart. There was no significant difference in the average B concentration between the two cultivars, suggesting that differences in the efficient use of local B may be responsible for the variation in brown heart resistance between the two cultivars.

Physiological and genetic analysis of CO2-induced breakdown of self-incompatibility in Brassica rapa.

Self-incompatibility (SI) of the Brassicaceae family can be overcome by CO2 gas treatment. This method has been used for decades as an effective means to obtain a large amount of inbred seeds which can then be used for F1 hybrid seed production; however, the molecular mechanism by which CO2 alters the SI pathway has not been elucidated. In this study, to obtain new insights into the mechanism of CO2-induced SI breakdown, the focus was on two inbred lines of Brassica rapa (syn. campestris) with different CO2 sensitivity. Physiological examination using X-ray microanalysis suggested that SI breakdown in the CO2-sensitive line was accompanied by a significant accumulation of calcium at the pollen-stigma interface. Pre-treatment of pollen or pistil with CO2 gas before pollination showed no effect on the SI reaction, suggesting that some physiological process after pollination is necessary for SI to be overcome. Genetic analyses using F1 progeny of a CO2-sensitive × CO2-insensitive cross suggested that CO2 sensitivity is a semi-dominant trait in these lines. Analysis of F2 progeny suggested that CO2 sensitivity could be a quantitative trait, which is controlled by more than one gene. Quantitative trait locus (QTL) analyses identified two major loci, BrSIO1 and BrSIO2, which work additively in overcoming SI during CO2 treatment. No QTL was detected at the loci previously shown to affect SI stability, suggesting that CO2 sensitivity is determined by novel genes. The QTL data presented here should be useful for determining the responsible genes, and for the marker-assisted selection of desirable parental lines with stable but CO2-sensitive SI in F1 hybrid breeding.

Mapping of the genes for dioecism and monoecism in Spinacia oleracea L.: evidence that both genes are closely linked.

Spinach is basically a dioecious species, with occasional monoecious plants in some populations. Sexual dimorphism in dioecious spinach plants is controlled by an allelic pair termed X and Y located on the short arm of the longest chromosome (x = 6). Ten AFLP markers, closely linked to the X/Y locus, were identified using bulked segregant analysis, four of which were revealed to co-segregate with Y in the present mapping population. We mapped the AFLP markers and two known male-specific DNAs to a 13.4 cM region encompassing the locus. These markers will be the basis for positional cloning of the sex-determination gene. We also showed that a single, incompletely dominant gene is responsible for the highly staminate monoecious character. The gene was found to be located at a distance of 4.3 cM from microsatellite marker SO4, which mapped 1.6 cM from the X/Y locus. This indicates that the monoecious gene seems not to be allelic to but closely linked to the X/Y gene pair. SO4 will enable breeders to efficiently select highly male monoecious plants for preferential use as the pollen parent for hybrid seed production.

Mapping of quantitative trait loci for high level of self-incompatibility in Brassica rapa L.

The level of self-incompatibility (SI) is important to the purity of F1 seeds produced using the SI system of Brassica vegetables. To analyze the genetic basis of the level of SI, we generated an F2 population derived from a cross between a turnip inbred line showing a high level of SI and a Chinese cabbage inbred line showing a low level, and evaluated the level of SI under insect pollination in two years. We constructed a detailed linkage map of Brassica rapa from the F2 progeny, consisting of SSR, SNP, indel, and CAPS loci segregating into 10 linkage groups covering approximately 700 cM. Five quantitative trait loci (QTL) for high-level SI were identified. The phenotypic variation explained by the QTL ranged between 7.2% and 23.8%. Two QTL were detected in both years. Mapping of SI-related genes revealed that these QTL were co-localized with SLG on R07 and MLPK on R03. This is the first report of QTL for high-level SI evaluated under insect pollination in a Brassica vegetable. Our results could be useful for the marker-assisted selection of parental lines with a stable SI.

Novel self-compatible lines of Brassica rapa L. isolated from the Japanese bulk-populations.

Self-incompatibility (SI) in Brassicaceae is sporophytically controlled by a single S-locus with multi allelic variety. The male S determinant, SP11/SCR (S-locus protein 11/S-locus cysteine-rich protein), is a small cysteine-rich protein, and the female S determinant, SRK (S-locus receptor kinase), functions as a receptor for SP11 at the surface of stigma papilla cells. Although a few of the following downstream factors in the SP11-SRK signaling cascade have been identified, a comprehensive understanding of the SI mechanism still remains unexplained in Brassicaceae. Analysis of self-compatible (SC) mutants is significant for understanding the molecular mechanism in SI reactions, thus we screened SC lines from a variety of Japanese bulk-populations of B. rapa vegetables. Two lines, TSC4 and TSC28, seem to have disruptions in the SI signaling cascade, while the other line, TSC2, seems to have a deficiency in a female S determinant, SRK. In TSC4 and TSC28, known SI-related factors, i.e. SRK, SP11, MLPK (M-locus protein kinase), THL (thioredoxin-h-like), and ARC1 (arm repeat containing 1), were expressed normally, and their expression levels were comparable with those in SI lines. On a B. rapa genetic linkage map, potential SC genes in TSC4 and TSC28 were mapped on linkage groups A3 and A1, respectively, whereas MLPK, ARC1, and THL were mapped on A3, A4, and A6, respectively. Although potential SC genes of TSC4 and MLPK were on the same linkage group, their positions were apparently independent. These results indicate that the SC genes of TSC4 and TSC28 are independent from the S-locus or known SI-related genes. Thus, the SC lines selected here have mutations in novel factors of the SI signaling cascade, and they will contribute to fill pieces in a signal transduction pathway of the SI system in Brassicaceae.