Clubroot disease is one of the major diseases affecting Brassicaceae crops. For example, Chinese cabbage (Brassica rapa L. ssp. pekinensis) is known to be highly susceptible to clubroot disease. For protection from this disease, genes for resistance to clubroot were introduced from the European turnip. CRa is a gene that confers specific resistance to the clubroot pathogen Plasmodiophora brassicae isolate M85. Fine mapping of the CRa locus using synteny to the Arabidopsis thaliana genome and partial genome sequences of B. rapa revealed a candidate gene encoding a TIR-NBS-LRR protein. There were several structural differences in this candidate gene between susceptible and resistant lines, and CRa expression was observed only in the resistant line. Additionally, four mutant lines lacking clubroot resistance were obtained by the UV irradiation of pollen from a resistant line. All of these mutant lines carried independent mutations in the candidate TIR-NBS-LRR gene. This genetic and molecular evidence strongly suggests that the revealed gene is CRa. This is the first 2 report, on the identification of a clubroot Resistance gene in Brassicaceae, and on the identification of the disease resistance gene in B. rapa.

Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC1 locus, which is involved in a higher K /Na ratio in shoots, corresponds to the OsHKT1;5 gene encoding a Na -selective transporter. However, physiological roles of OsHKT1;5 in rice exposed to salt stress remain elusive, and no OsHKT1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T-DNA insertional OsHKT1;5 mutants. Measurements of ion contents in tissues and Na tracer imaging experiments showed that loss-of-function of OsHKT1;5 in salt-stressed rice roots triggers massive Na accumulation in shoots. Salt stress-induced increases in the OsHKT1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno-staining using an anti-OsHKT1;5 peptide antibody indicated that OsHKT1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of Na tracer to leaf sheaths also demonstrated the involvement of OsHKT1;5 in xylem Na unloading in leaf sheaths. Furthermore, OsHKT1;5 was indicated to be present in the plasma membrane and found to localize also in the phloem of diffuse vascular bundles in basal nodes. Together with the characteristic Na allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of OsHKT1;5 in mediating Na exclusion in the phloem to prevent Na transfer to young leaf blades. Our findings further demonstrate that the function of OsHKT1;5 is crucial over growth stages of rice, including the protection of the next generation seeds as well as of vital leaf blades under salt stress.

Many clubroot resistant (CR) cultivars of Chinese cabbage (Brassica rapa ssp. pekinensis) have been bred so far, but their usage is limited because the capacity for resistance breaks down with time. This degradation is caused by a pathogenic variation in the causal fungus Plasmodiophora brassicae. We attempted to accumulate 3 CR genes, CRa, CRk, and CRc, through marker-assisted selection. Five doubled haploid CR lines with an individual CR locus were used as breeding materials. The CR lines were crossed with each other. A subsequent selection for resistance was performed using sequence characterized amplified region markers in segregating generations. As a result, 4 homozygous lines for 3 resistance genes and the F 1 hybrids between them were developed. CR pyramiding lines were inoculated with 6 field isolates of P. brassicae. The homozygous lines for 3 CR genes, whether selfed or crossed, exhibited exceedingly high resistance against all of the isolates. Morphological characters of F 1 hybrids were comparable to those of a control cultivar, but the degree of heterosis was less than expected, which is probably because of genetic similarity of the parents. The results of this study prove that clubroot resistance can be reinforced through the accumulation of varied resistance genes in B. rapa.

Papaya (Carica papaya) is a trioecious plant species that has male, female and hermaphrodite flowers on different plants. The primitive sex chromosomes genetically determine the sex of the papaya. Although draft sequences of the papaya genome are already available, the genes for sex determination have not been identified, likely due to the complicated structure of its sex-chromosome sequences. To identify the candidate genes for sex determination, we conducted a transcriptome analysis of flower samples from male, female and hermaphrodite plants using high-throughput SuperSAGE for digital gene expression analysis. Among the short sequence tags obtained from the transcripts, 312 unique tags were specifically mapped to the primitive sex chromosome (X or Yh) sequences. An annotation analysis revealed that retroelements are the most abundant sequences observed in the genes corresponding to these tags. The majority of tags on the sex chromosomes were located on the X chromosome, and only 30 tags were commonly mapped to both the X and Yh chromosome, implying a loss of many genes on the Yh chromosome. Nevertheless, candidate Yh chromosome-specific female determination genes, including a MADS-box gene, were identified. Information on these sex chromosome-specific expressed genes will help elucidating sex determination in the papaya.