Leaf contents differ depending on the position in a rice leaf sheath during sink-source transition.

Carbohydrate contents varied with position in a leaf sheath, and differed between the flag leaf sheath and the second leaf sheath below the flag leaf (-2 leaf sheath) in rice (Oryza sativa L.). In the -2 leaf sheath before heading, light microscopy revealed differences in the distribution of starch granules depending on position. Leaf sheaths were divided into several parts, and the contents of carbohydrates (starch, sucrose, and hexoses) were measured in each part. Before heading, the content of accumulated starch increased linearly from the top to the bottom in -2 leaf sheaths (r2=0.99, P<0.001), as did the contents of accumulated sucrose and hexoses in flag leaf sheaths (r2=0.94, P<0.01). In flag leaf sheaths, the relative content of sucrose synthase (SuS), which plays a central role in the degradation of sucrose into hexoses, increased from the top to the bottom, consistent with hexose contents. After heading, the accumulated carbohydrates were dramatically decreased. In -2 leaf sheaths, the activity of alpha-amylase (EC 3.2.1.1), the rate-limiting step in starch degradation, was consistent with the degree of starch degradation, but in flag leaf sheaths with little starch before heading. These results show that carbohydrate contents differ, depending on the position in a leaf sheath. In addition, there were big differences in leaf contents between flag leaf sheaths and -2 leaf sheaths.     

Quantitative trait loci for sucrose, starch, and hexose accumulation before heading in rice.

We studied the storage of sucrose, starch, and hexose before heading in rice (Oryza sativa L.) plants by quantitative trait locus (QTL) analysis with a population of backcross inbred lines (BILs) of japonica cv. Nipponbare x indica cv. Kasalath. Carbohydrates are accumulated in the rice plant before heading and are translated to the panicle after heading. A higher capacity for accumulation is thus a main target for improvement in yield. The form of carbohydrate (sucrose, starch, or hexose) differs depending on the organ in which it is stored. There was no correlation between starch and sucrose or hexose contents in BILs, and the positions of QTLs controlling starch differed from those for sucrose and hexose accumulation. These results suggest that the genetic control of accumulation differs between starch and sugars. QTLs that control the ratio of sucrose to starch content were detected, suggesting the existence of a mechanism(s) that determines this ratio. On chromosome 1, sucrose-phosphate synthase 1, the key enzyme in sucrose synthesis was close to the peaks of the likelihood odds ratios in QTLs for sucrose or hexose content. These results suggest that SPS1 is related to conversion of carbohydrate to sucrose as accumulated form in a plant before heading.     

一种水稻酰基辅酶A结合蛋白cDNA的鉴定(英文)

对一水稻ｃＤＮＡ 克隆（Ｒ１９０８） 的分析表明, 其可能编码水稻酰基辅酶Ａ 结合蛋白（ａｃｙｌＣｏＡｂｉｎｄｉｎｇ ｐｒｏｔｅｉｎ,ＡＣＢＰ）。Ｓｏｕｔｈｅｒｎ 杂交显示水稻（ Ｏｒｙｚａ ｓａｔｉｖａ Ｌ．） 基因组中仅有一个该基因的拷贝。Ｎｏｒｔｈｅｒｎ 分析表明水稻的ＡＣＢＰ基因在水稻的根、茎、叶、叶鞘、黄化苗和幼穗中皆表达,而以黄化苗的绿苗叶鞘中的表达强度高于绿苗叶片。     

Culturable bacterial communities on leaf sheaths and panicles of rice plants in Japan

Culturable bacterial communities on rice plants were investigated from 2001 to 2003. In total, 1,394 bacterial isolates were obtained from the uppermost leaf sheaths at 1 month before heading time and from leaf sheaths and panicles at heading time. The average culturable bacterial population on the leaf sheaths was larger at heading time than at 1 month previously. Furthermore, the population was significantly larger on panicles than on leaf sheaths, suggesting that the bacterial population is influenced by the organs of rice plants. Larger proportions of bacteria were obtained from the macerates of leaf sheaths after washing with phosphate buffer, and most culturable bacteria were verified to inhabit the inside or inner surface, rather than the outer surface, of the tissues. Verification of the bacterial composition based on 16S rRNA gene sequences revealed that genera of Sphingomonas, Microbacterium, Methylobacterium, and Acidovorax tended to be dominant colonizers on leaf sheaths, whereas Pseudomonas and Pantoea were isolated mainly from the panicles, indicating that leaf sheaths and panicles harbor distinct communities. Furthermore, the richness of bacterial genera was less on both leaf sheaths and panicles at heading time compared with that observed 1 month before heading time. Phylogenetic analyses using bacterial isolates belonging to the four dominant genera inhabiting leaf sheaths at heading time revealed that particular bacterial groups in each genus colonized the leaf sheaths.     

Coordinated Regulation of the Genes Participating in Starch Biosynthesis by the Rice Floury-2 Locus

The recessive floury-2 (flo-2) locus of rice (Oryza sativa L.), which is located on chromosome 4, causes a strong reduction in expression of the gene encoding an isoform of branching enzyme RBE1 in immature seeds 10 d after flowering. Mapping of the RBE1 gene demonstrated the localization on rice chromosome 6, suggesting that the wild-type Floury-2 (Flo-2) gene regulates RBE1 gene expression in trans. However, reduced expression of the genes encoding some other starch-synthesizing enzymes, including another isoform of branching enzyme RBE3 and granule-bound starch synthase, was also found in the flo-2 seeds. In spite of the low level of RBE1 gene expression in the immature seeds of the flo-2 mutants, the RBE1 gene was equally expressed in the leaves of the wild type and flo-2 mutants. Thus, these results imply that the Flo-2 gene may co-regulate expression of some of the genes participating in starch synthesis possibly in a developing seed-specific manner.     

Molecular regulation of sink–source transition in rice leaf sheaths during the heading period

The upper leaf sheath of rice (Oryza sativa L.) serves as a temporary starch sink before heading, subsequently becoming a carbon source tissue to the growing panicle at the post-heading stage. The time of sink–source transition in upper leaf sheaths is highly correlated to the panicle exsertion. Here, we found that the expression profiles of starch synthesis genes such as ADP-glucose pyrophosphorylase large subunit 2, granule-bound starch synthase II, soluble starch synthase I, starch branching enzyme (SBE) I, SBEIII, and SBEIV were highly correlated with starch content changes during the heading period in the second leaf sheath below the flag leaf. In addition, the α-amylase2A and β-amylase were considered as major genes that were in charge of starch degradation at the post-heading period. Of the five sucrose transporter (OsSUT) genes, OsSUT1 and OsSUT4 appeared to play an important role in sucrose loading into the phloem of source leaf sheaths. Moreover, the microarray-based data implied that the dominant processes associated with functional leaf sheath transition from sink to source were carbohydrate metabolism and the translocation of the carbon and nitrogen sources and inorganic phosphate.     

Expression patterns of genes encoding carbohydrate-metabolizing enzymes and their relationship to grain filling in rice (Oryza sativa L.): comparison of caryopses located at different positions in a panicle

In rice, caryopses located at the base of the panicle have a lower growth rate than those at the tip of the panicle. The former and latter types of caryopses are called inferior and superior caryopses, respectively. Taking the different growth rate into consideration, sugar status and the expression of genes encoding carbohydrate-metabolizing enzymes in inferior caryopses were compared with those in superior caryopses. During the first 5 d after flowering, superior caryopses elongated rapidly, but inferior caryopses did not. At this phase, inferior caryopses had a low ratio of hexose to sucrose, high activity of acid invertase and the absence of the expression of the genes encoding the above enzymes except for two isoforms of cell wall invertase, OsCIN4 and INV1, in comparison with superior caryopses. At the start of caryopsis elongation in both superior and inferior caryopses, the hexose/sucrose ratio increased accompanied by gene expression of vacuolar invertase (INV3), sucrose synthase (RSus1) and ADP-glucose pyrophosphorylase (AGP-L2: D50317). Furthermore, the genes related to endospermal starch accumulation were expressed highly with the decrease in the hexose/sucrose ratio after its peak. Based on the comparison of superior and inferior caryopses, the possible mechanism of grain filling in rice is discussed.     

Starch metabolism in the leaf sheaths and culm of rice

The levels of starch and dextrin, free sugars, soluble protein, and enzymes involved in starch metabolism—α-amylase, β-amylase, phosphorylase, Q-enzyme, R-enzyme, and ADP-glucose starch synthetases—were assayed in the leaf sheaths and culm of the rice plant (Oryza sativa L., variety IR8) during growth.