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Shao-Ling Zhang and Shin Hiratsuka

Cultivars of the Japanese pear [Pyrus pyrifolia (Burm.) Nakai] have variable degrees of self-incompatibility (SI) and can be classified into at least three groups: strong, intermediate, or weak SI; as shown by the extent of self-pollen tube growth in the style, and the percentage of fruit set following self-pollination. Following self-pollination, the elongation of pollen tubes in the detached styles of `Kosui' and `Kikusui' became increasingly suppressed from 4 days before anthesis (–4 DAA) to 2 days after anthesis (2 DAA). Tube growth of `Kosui' was more suppressed than that of `Kikusui' during this period. In `Osa-Nijisseiki', however, the rate of tube growth did not vary with stage of stylar development, from –8 to 2 DAA. Pollen tubes elongated much better after cross-pollination than after self-pollination at all stages tested, and the extent of the elongation increased as the styles matured. The concentration of total S-protein (sum of two S-proteins per buffer-soluble protein) increased with stylar development, but the rate of increase varied with the cultivar. The rate was significantly greater in the strongly self-incompatible `Kosui' than in the moderately self-incompatible `Kikusui', and was slowest in the weakly self-incompatible `Osa-Nijisseiki' at all developmental stages. During stylar maturation, the concentration of S4-protein, which is common in all cultivars, was highest in `Kosui', followed by `Kikusui' and `Osa-Nijisseiki'. Thus, the cultivar differences in SI expression in the Japanese pear are determined about –4 DAA and appear to be regulated, in part, by the concentration of S-proteins produced in the style.

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Jin Jiao, Xing Liu, Juyou Wu, Guohua Xu and Shaoling Zhang

Mitogen-activated protein kinase (MAPK) cascades are universal signal-transduction modules, but the available information is limited in pear (Pyrus). In this study, 87 MAPK genes were identified from five Rosaceae species: chinese white pear (Pyrus ×bretschneideri cv. Dangshansuli), peach (Prunus persica), apple (Malus domestica), strawberry (Fragaria vesca), and plum (Prunus mume), 23 of which came from chinese white pear, designated as PbrMAPK. Based on the phylogenetic analysis and the architectures of conserved protein motifs of these gene sequences, MAPK family genes of five Rosaceae species were classified into two primary types (I and II) or four groups (Classes A–D). We have indicated that both segment and tandem duplications significantly contributed to the expansion of the MAPK family in Rosaceae by analysis of genomic evolution. In chinese white pear pollen, the expression analysis revealed that all PbrMAPKs could respond to temperature stresses (high/low temperature) and phytohormones, except PbrMAPK8 and PbrMAPK19 that displayed lower expressions, which suggested that PbrMAPKs play pivotal roles in signal-transduction pathways. In addition, we determined that PbrMAPK13 is located in the nucleus and plasma membranes. The lengths of pollen tubes became shorter when PbrMAPK13 was silenced by antisense oligonucleotide transfection. Our results provided an evolutionary foundation and functional characterization for MAPK gene families in chinese white pear and other plant species so as to elucidate their biological roles.

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An Qin, Xiaosan Huang, Huping Zhang, Juyou Wu, Jie Yang and Shaoling Zhang

Ascorbic acid (AsA) is a major antioxidant and redox buffer in plants. Dehydroascorbate reductase (DHAR; EC 1.8.5.1) catalyzes the conversion of dehydroascorbate (DHA) to AsA and is crucial for AsA regeneration. In this study, we developed transgenic tomato plants that overexpressed PbDHAR2 to investigate whether PbDHAR2 could limit the deleterious effects of salt and chilling stresses. These transgenic plants contained significantly higher AsA levels than the wild-type (WT) plants. Overexpression of PbDHAR2 increased the expression of the AsA-glutathione (GSH) cycle genes in transgenic lines under salt and chilling stresses. In addition, the transgenic lines subjected to salt and chilling stresses showed higher levels of antioxidant enzyme activity, lower malondialdehyde (MDA) levels, and higher chlorophyll contents than the WT. Thus, our results demonstrate that the regulation of PbDHAR2 during AsA regeneration contributes to enhanced salt and chilling tolerance in tomato.

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Shutian Tao, Danyang Wang, Cong Jin, Wei Sun, Xing Liu, Shaoling Zhang, Fuyong Gao and Shahrokh Khanizadeh

Lignin is the main component of stone cells, and stone cell content is one of the crucial factors for fruit quality in chinese white pear (Pyrus ×bretschneideri). The lignin biosynthesis pathway is complex and involves many enzymatic reactions. Cinnamate-4-hydroxylase [C4H (EC.1.14.13.11)] is an essential enzyme in lignin metabolism. This study was conducted to investigate the effect of bagging on lignin metabolism during fruit development in chinese white pear. The study showed that bagging had little effect on stone cell content, lignin content, C4H activity, and C4H gene expression and that there was a positive correlation between C4H gene expression and lignin content as well as stone cell content. Moreover, a full-length complementary DNA (cDNA) encoding C4H (PbrC4H, GenBank accession number KJ577541.1) was isolated from chinese white pear fruit. The cDNA is 1515 bp long and encodes a protein of 504 amino acids. Sequence alignment suggested that the deduced protein belongs to the P450 gene family and that C4H might be located subcellularly in the cell membrane. The results indicate that bagging cannot change the lignin and stone cell content significantly and that C4H catalyzes a step in lignin biosynthesis. These findings provide certain theoretical references and practical criteria for improving the quality of chinese white pear.