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.
Shao-Ling Zhang and Shin Hiratsuka
Xinhua Zhang, Fujun Li, Nana Ji, Shujun Shao, Dongyang Wang, Ling Li, and Fansheng Cheng
The physiological role of arginase in nitrogen remobilization processes from protein degradation during seed germination has well been described in several species. However, very little is known about its possible roles in plant stress responses. Treatment of tomato fruit (Solanum lycopersicum L.) with 0.05 mm methyl jasmonate (MeJA) enhanced transcription levels of arginase genes, especially LeARG2. Chilling injury (CI) of fruit treated with 0.05 mm MeJA for 12 hours was also effectively alleviated, as manifested by decreases in CI index, electrolyte leakage, and malondialdehyde (MDA) content. To investigate the potential role of arginase in MeJA-induced chilling tolerance, fruit were treated with MeJA or the arginase inhibitor Nω-hydroxy-nor-l-arginine (nor-NOHA) combined with MeJA and then stored at 2 °C for 28 days. MeJA-induced arginase activity was strongly inhibited and the reduction of CI by MeJA was nearly abolished by the inhibitor. In addition, MeJA treatment increased the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX); inhibited peroxidase (POD) activities; and promoted proline and polyamines accumulation. These effects were partially counteracted by nor-NOHA; putrescine accumulation, however, was unaffected by the inhibitor. Our results indicate that arginase may be involved in MeJA-induced chilling tolerance, possibly by ameliorating the antioxidant enzyme system of fruit and increasing proline levels.