Yang-Seok Lee

OsCOL4 is a constitutive flowering repressor upstream of Ehd1 and downstream of OsphyB

Plants recognize environmental factors to determine flowering time. CONSTANS (CO) plays a central role in the photoperiod flowering pathway of Arabidopsis, and CO protein stability is modulated by photoreceptors. In rice, Hd1, an ortholog of CO, acts as a flowering promoter, and phytochromes repress Hd1 expression. Here, we investigated the functioning of OsCOL4, a member of the CONSTANS-like (COL) family in rice. OsCOL4 null mutants flowered early under short or long days. In contrast, OsCOL4 activation-tagging mutants (OsCOL4-D) flowered late in either environment. Transcripts of Ehd1, Hd3a, and RFT1 were increased in the oscol4 mutants, but reduced in the OsCOL4-D mutants. This finding indicates that OsCOL4 is a constitutive repressor functioning upstream of Ehd1. By comparison, levels of Hd1, OsID1, OsMADS50, OsMADS51, and OsMADS56 transcripts were not significantly changed in oscol4 or OsCOL4-D, suggesting that OsCOL4 functions independently from previously reported flowering pathways. In osphyB mutants, OsCOL4 expression was decreased and osphyB oscol4 double mutants flowered at the same time as the osphyB single mutants, indicating OsCOL4 functions downstream of OsphyB. We also present evidence for two independent pathways through which OsPhyB controls flowering time. These pathways are: (i) night break-sensitive, which does not need OsCOL4; and (ii) night break-insensitive, in which OsCOL4 functions between OsphyB and Ehd1.

Trithorax Group Protein Oryza sativa Trithorax1 Controls Flowering Time in Rice via Interaction with Early heading date3

Histone methyltransferase promotes flowering time predominantly under long-day conditions in rice. Trithorax group proteins are chromatin-remodeling factors that activate target gene expression by antagonistically functioning against the Polycomb group. In Arabidopsis (Arabidopsis thaliana), Arabidopsis Trithorax protein1 (ATX1) regulates flowering time and floral organ identity. Here, we observed that suppression of Oryza sativa Trithorax1 (OsTrx1), an ortholog of ATX1, delayed flowering time in rice (Oryza sativa). Because the delay occurred only under long-day conditions, we evaluated the flowering signal pathways that specifically function under long-day conditions. Among them, the OsMADS50 and Heading date1 pathways were not affected by the mutation. However, the Grain number, plant height, and heading date7 (Ghd7) pathway was altered in ostrx1. Transcript levels of OsGI, phytochrome genes, and Early heading date3 (Ehd3), which function upstream of Ghd7, were unchanged in the mutant. Because Trx group proteins form a complex with other proteins to modify the chromatin structure of target genes, we investigated whether OsTrx1 interacts with a previously identified protein that functions upstream of Ghd7. We demonstrated that the plant homeodomain motif of OsTrx1 binds to native histone H3 from the calf thymus and that OsTrx1 binds to Ehd3 through the region between the plant homeodomain and SET domains. Finally, we showed that the SET domain at the C-terminal end of OsTrx1 has histone H3 methyltransferase activity when incubated with oligonucleosomes. Our results suggest that OsTrx1 plays an important role in regulating flowering time in rice by modulating chromatin structure.

Rice miR172 induces flowering by suppressing OsIDS1 and SNB, two AP2 genes that negatively regulate expression of Ehd1 and florigens

BackgroundRice is a facultative short-day plant that flowers under long days (LD) after a lengthy vegetative phase. Although several inhibitors that delay flowering have been identified, the process by which rice eventually flowers under non-permissive LD conditions is not well understood.ResultsOverexpression of miR172 reduced flowering time significantly, suggesting its role as an inducer. Levels of miR172 increased as plants aged, further supporting our findings. Transcripts of SNB and OsIDS1, two members of the AP2 family that have the miR172 target site, were reduced in older plants as the level of miR172 rose. Overexpression of those AP2 genes delayed flowering; overexpression of miR172-resistant forms of SNB or OsIDS1 further delayed this process. This demonstrated that the AP2 genes function downstream of miR172. Two florigen genes -- Hd3a and RFT1 -- and their immediate upstream regulator Ehd1 were suppressed in the AP2 overexpression plants. This suggested that the AP2 genes are upstream repressors of Ehd1. In phytochrome mutants, miR172d levels were increased whereas those of SNB and OsIDS1 were decreased. Thus, it appears that phytochromes inhibit miR172d, an AP2 suppresser.ConclusionsWe revealed that miR172d developmentally induced flowering via repressing OsIDS1 and SNB, which suppressed Ehd1. We also showed that phytochromes negatively regulated miR172.

Defective Tapetum Cell Death 1 (DTC1) Regulates ROS Levels by Binding to Metallothionein during Tapetum Degeneration

Timely production of superoxides is essential for initiation of tapetum degeneration in rice. After meiosis, tapetal cells in the innermost anther wall layer undergo program cell death (PCD)-triggered degradation. This step is essential for microspore development and pollen wall maturation. We identified a key gene, Defective Tapetum Cell Death 1 (DTC1), that controls this degeneration by modulating the dynamics of reactive oxygen species (ROS) during rice male reproduction. Mutants defective in DTC1 exhibit phenotypes of an enlarged tapetum and middle layer with delayed degeneration, causing male sterility. The gene is preferentially expressed in the tapetal cells during early anther development. In dtc1 anthers, expression of genes encoding secretory proteases or lipid transporters is significantly reduced, while transcripts of PCD regulatory genes, e.g. UDT1, TDR1, and EAT1/DTD, are not altered. Moreover, levels of DTC1 transcripts are diminished in udt1, tdr, and eat1 anthers. These results suggest that DTC1 functions downstream of those transcription factor genes and upstream of the genes encoding secretory proteins. DTC1 protein interacts with OsMT2b, a ROS scavenger. Whereas wild-type plants accumulate large amounts of ROS in their anthers at Stage 9 of development, those levels remain low during all stages of development in dtc1 anthers. These findings indicate that DTC1 is a key regulator for tapetum PCD by inhibiting ROS-scavenging activity.

Genome-wide identification and analysis of Catharanthus roseus RLK1-like kinases in rice.

Main conclusionA genome-wide survey ofCatharanthus roseusreceptor-like kinase1-like kinases (CrRLK1Ls) in rice revealed that the pattern of expression by some CrRLK1Ls is controlled by drought or circadian rhythms. This is probably accomplished through the functioning ofGigantea(OsGI). Such findings provide a novel angle for using CrRLK1Ls to study the drought-stress response and circadian regulation.AbstractThe 17 CrRLK1L members of a novel RLK family have been identified in Arabidopsis. Each carries a putative extracellular carbohydrate-binding malectin-like domain. However, their roles in rice, a widely consumed staple food, are not well understood. To investigate the functions of CrRLK1Ls in rice, we utilized phylogenomics data obtained through anatomical and diurnal meta-expression analyses. This information was integrated with a large set of public microarray data within the context of the rice CrRLK1L family phylogenic tree. Chromosomal locations indicated that 3 of 16 genes were tandem-duplicated, suggesting possible functional redundancy within this family. However, integrated diurnal expression showed functional divergence between two of three genes, i.e., peak expression was detected during the day for OsCrRLK1L2, but during the night for OsCrRLK1L3. We found it interesting that OsCrRLK1L2 expression was repressed in osgigantea (osgi) mutants, which suggests that it could function downstream of OsGI. Network analysis associated with OsCrRLK1L2 and OsGI suggested a novel circadian regulation mechanism mediated by OsGI. In addition, two of five OsCrRLK1Ls preferentially expressed in the roots were stimulated by drought, suggesting a potential role for this family in water-use efficiency. This preliminary identification of CrRLK1Ls and study of their expression in rice will facilitate further functional classifications and applications in plant production.

OsGI controls flowering time by modulating rhythmic flowering time regulators preferentially under short day in rice

Day length is one of the most important factors that determine flowering time. In this study, we report that a mutation in OsGI reduced the photoperiod sensitivity in rice, resulting in flowering at 91 d after germination (DAG) under short days (SD) and 103 DAG under long days (LD). Transcript levels for flowering regulators that show diurnal rhythmic expression were significantly decreased in the osgi mutants under SD, but chromatin remodeling factor genes were not significantly changed. However, expression by those flowering regulators was not significantly altered in the osgi mutants under LD. Transcript levels for Ghd7 were lower in the mutants compared with the WT before 35 DAG, but higher afterward. These results suggested that OsGI differentially modulates Ghd7 as plants develop. The temporal expression pattern of OsGI was similar under SD and LD. Transcript levels for OsGI peaked at 30 DAG before gradually declining to a minimum at 55 DAG, where they then remained constant until flowering occurred. This developmental regulation of OsGI appears to be the main reason for its affecting photoperiod sensitivity strongly under SD but only weakly under LD.

The rice gene DEFECTIVE TAPETUM AND MEIOCYTES 1 (DTM1) is required for early tapetum development and meiosis

Tapetum development and meiosis play crucial roles in anther development. Here we identified a rice gene, DEFECTIVE TAPETUM AND MEIOCYTES 1 (DTM1), which controls the early stages of that development. This gene encodes for an endoplasmic reticulum (ER) membrane protein that is present only in cereals. Our T-DNA insertion mutations gave rise to abnormal tapetal formation. Cellular organelles, especially the ER, were underdeveloped, which led to hampered differentiation and degeneration of the tapetum. In addition, the development of pollen mother cells was arrested at the early stages of meiotic prophase I. RNA in-situ hybridization analyses showed that DTM1 transcripts were most abundant in tapetal cells at stages 6 and 7, and moderately in the pollen mother cells and meiocytes. Transcripts of UDT1, which functions in tapetum development during early meiosis, were reduced in dtm1 anthers, as were those of PAIR1, which is involved in chromosome pairing and synapsis during meiosis. However, expression of MSP1 and MEL1, which function in anther wall specification and germ cell division, respectively, was not altered in the dtm1 mutant. Moreover, transcripts of DTM1 were reduced in msp1 mutant anthers, but not in udt1 and pair1 mutants. These results, together with their mutant phenotypes, suggest that DTM1 plays important roles in the ER membrane during early tapetum development, functioning after MSP1 and before UDT1, and also in meiocyte development, after MEL1 and before PAIR1.

Regulation of flowering time in rice

Rice flowers after a lengthy vegetative growth. During the vegetative growth period flowering is inhibited by several independent pathways. Whereas Grain number, plant height, and heading date 7 (Ghd7), Heading date 1 (Hd1), Heading date 5 (Hd5), Heading date 6 (Hd6), and Heading date 16 (Hd16) preferentially function to delay flowering under long day conditions, Oryza sativa Phytochrome B (OsPhyB), Oryza sativa CONSTANS-like 4 (OsCOL4), SUPERNUMERARY BRACT (SNB) and Oryza sativa INDETERMINATE SPIKELET 1 (OsIDS1) independently inhibit flowering regardless of day length. After sufficient vegetative growth, flowering signals are produced in the leaves due to reduced expression of the inhibitors. In addition, Hd1 becomes a flowering promoter when the day length becomes shorter. Long-day specific activators OsMADS50 and OsDof12, and a constitutive activators Oryza sativa INDETERMINATE 1 (OsId1), Early heading date 4 (Ehd4), and miR172, are accumulated in the leaves when plants are grown sufficiently. Several circadian clock genes are also involved in floral transition, including Oryza sativa GIGANTEA (OsGI), Heading date 2 (Hd2), and Heading date 17 (Hd17). Floral transition is also controlled by photoreceptors and chromatin remodeling factors. Most of the upstream signals are transferred to Early heading date 1 (Ehd1) that is a positive regulator of Heading data 3a (Hd3a) and Rice FT 1 (RFT1), which are transferred to the shoot apical meristem to induce the reproductive transition.

OsMPK6 plays a critical role in cell differentiation during early embryogenesis in Oryza sativa

Highlight The MAP kinase cascade controls early embryogenesis prior to axis formation.

Oral administration of squid lecithin-transphosphatidylated phosphatidylserine improves memory impairment in aged rats

Recently, lecithin-derived phosphatidylserine (PS), which originates from marine life, has received much attention as a viable alternative to bovine cerebral cortex PS. In this study, the use of squid phosphatidylcholine-transphosphatidylated PS (SQ-PS) was evaluated through examination of its ameliorating effects on age-associated learning and memory deficits in rats. Aged rats were orally administered SQ-PS (10, 20, or 50 mg/kg per day) once a day for seven days 30 min prior to behavioral assessment in a Morris water maze. SQ-PS administration produced significant dose-dependent improvements in escape latency for finding the platform in the Morris water maze in the aged rats even though Soy-PS administration also exhibited comparable improvements with SQ-PS. Biochemical alterations in the hippocampal cholinergic system, including changes in choline acetyltransferase and acetylcholinesterase immunoreactivity, were consistent with the behavioral results. In addition, SQ-PS treatment significantly restored age-associated decreases of choline transporter and muscarinic acetylcholine receptor type 1 mRNA expression in the hippocampus. These results demonstrate that orally administered SQ-PS dose-dependently aids in the improvement of memory deficits that occur during normal aging in rats. This suggests that SQ-PS may be a useful therapeutic agent in the treatment of diminished memory function in elderly people.