Shigemi Akita

Expression of α-expansin genes in young seedlings of rice (Oryza sativa L.)

Abstract. Rice is the only cereal in which germination and coleoptile elongation occur in hypoxia or anoxia. Little is known of the molecular basis directly underlying coleoptile cell extension. In this paper, we describe the expression of α-expansin genes in embryos during seed development and young seedlings grown under various oxygen concentrations. The genes Os-EXP2 and Os-EXP1 were predominantly expressed in the developing seeds, mainly in newly developed leaves, coleoptiles, and seminal roots. These expansins expressed in the developing seeds may give cells the potential to expand after seed imbibition begins. In coleoptiles, Os-EXP4 and Os-EXP2 mRNAs were greatly induced by submergence, while they were weakly detected in aerobic or anoxic conditions. Under submerged soil conditions, the signals hybridized with probes Os-EXP4 and Os-EXP2 in coleoptiles were strongest when coleoptiles elongated in the water layer. These data show that expansin gene expression is highly correlated with coleoptile elongation in response to oxygen concentrations. The Os-EXP4 gene was also expressed in leaves, mesocotyls, and coleorhizas of young seedlings. The growth of these tissues was also correlated with the presence of expansins. Therefore, the evidence derived from this study clearly demonstrates that expansins are indispensable for the growing tissues of rice seedlings.

Higher Leaf Area Growth Rate Contributes to Greater Vegetative Growth of F1 Rice Hybrids in the Tropics.

Abstract The early vigor of hybrid rice (Oryza sativa L.) developed in temperate areas has been mainly attributed to its higher tillering rate. The objective of this study is to identify factors that contribute to greater vegetative growth of Fi hybrid rice recently developed in the tropics. We analyzed data from field experiments conducted from 1994 to1997 in both dry and wet seasons at the International Rice Research Institute farm. Each experiment had one to four pairs of comparisons between best hybrids and best conventional varieties. Parameters used for comparison were tillering rate, leaf area growth rate (LAGR), and crop growth rate (CGR) between transplanting (TR) and midtillering (MT) and also between MT and panicle initiation (PI). In most cases, the hybrids exhibited greater CGR than the conventional varieties, either between TR and MT or between MT and PI or both. The difference in CGR was associated with the difference in LAGR between hybrids and conventional varieties. However, the tillering rate of hybrids was significantly lower than or equal to that of conventional varieties. Therefore, the higher LAGR, independent from tillering, contributed to greater CGR of recently developed F1 hybrid rice in the tropics. The difference in LAGR and CGR between hybrids and conventional varietieswas not associated with tillering. The higher LAGR of the Fl hybrids than that of the conventional varieties could be attributed to their thinner leaf blades.

Factors causing the variation in the temperature coefficient of dark respiration in rice (Oryza sativa L.)

Abstract The temperature coefficient (Q10) of dark respiration in rice gradually decreased in continuous darkness, fluctuating as the circadian rhythmic variation in dark respiration. The solar radiation (MJ m-2) on the day of measurement and the nitrogen concentration in the culture solution hardly affected the Q10 value. However, the plants exposed to high-intensity light (260W m-2) for two weeks showed a higher Q10 of dark respiration than those exposed to low-intensity light (70W m-2). The seasonal variation in Q10 value was not observed. The average Q10 value during the growth stage measured at 15 to 25°C, 20 to 30°C, and 25 to 35°C was 2.14, 1.76, and 1.56, respectively. High crop yield in the regions where the day/night temperature difference is large may be partly explained by the high Q10 at low night temperature. Thus, the Q10 may be an important physiological factor determining high crop yield.

Decomposition of (1–3,1–4)-β-Glucan and Expression of the (1–3,1–4) -β-Glucanase Gene in Rice Stems during Ripening

Abstract To elucidate the possible participation of hemicellulose decomposition in lodging resistance, we studied the change of hemicellulose and cellulose content in the stems of rice during the ripening stage by methylation analysis and the expression of related genes by Northern blotting. In the rice stem in ripening stage, content of (1-3,1-4)-β-glucan, a component of hemicellulose, decreased markedly although the content of arabinoxylan, a major component of hemicellulose, and cellulose showed little change during the same growth period. On the other hand, expression of the Gns 1 gene, which may encode (1-3,1-4) -β-glucanase that catalyzes the degradation of (1-3,14) -β-glucan, increased sharply in the stem. The mechanism of decomposition of (1-3,1-4) -β-glucan in rice stem and the possible association with lodging resistance is discussed.

Expression of α-Amylase Isoforms and the RAmy1A Gene in Rice (Oryza sativa L.) during Seed Germination, and its Relationship with Coleoptile Length in Submerged Soil

Summary A significant difference in the seed germinability was observed between the two rice cultivars, ‘Nipponbare’ and ‘Suweon 287’, under anoxia (i.e., during germination in submerged soil at 18°C), although little difference was seen under aerobic (in air) or hypoxic (in water) conditions. The number of α-amylase isoforms synthesized in germinating seeds was inversely proportional to the O2 concentrations at the early germination stage. The formation of isoform B was promoted by oxygen supply, while isoform H was undetectable if the seeds were unable to germinate. The activity of isoform H was highly correlated with the coleoptile length in the submerged soil at 18°C, indicating that isoform H is a critical factor for seed germination under anoxia. The expression of the rice α-amylase RAmylA gene was repressed when the seeds germinated under hypoxia or anoxia. The interactions between oxygen stress, gibberel-lin, and carbon metabolites on the expression of α-amylase in rice are discussed.

Expression of the α-Amylase Gene RAmy3D in Rice(Oryza sativa L.)under Aerobic, Hypoxic and Anoxic Conditions

Abstract Expression of the α-amylase gene RAmy3D was studied in aerobic, hypoxic, and anoxic conditions using the rice cuitivars Suweon 287 and Nipponbare by in situ hybridization. In the aerobic condition, the initial expression of RAmy3D in the scutellar epithelium and its later expression in the aleurone layer were consistent with the previous reports. However, hypoxia delayed the expression of RAmy3D in the scutellar epithelium, and blocked expression in the aleurone layer. In anoxia, RAmy3D expression in cv. Nipponbare was similar to that in hypoxia, but that in cv. Suweon 287 was significantly suppressed 4 d after seeding. These results suggest that the expression of the RAmy3D gene is also affected by anoxia. Considering the results of our previous report together, RAmy3D-encoded isoforms in anoxia may be mainly regulated at the post-transcriptional level. A possible process in the regulation of RAmy3D expression in germinating seeds is discussed.