Hyunsik Hwang

A rice orthologue of the ABA receptor, OsPYL/RCAR5, is a positive regulator of the ABA signal transduction pathway in seed germination and early seedling growth

Abscisic acid (ABA) is a phytohormone that positively regulates seed dormancy and stress tolerance. PYL/RCARs were identified an intracellular ABA receptors regulating ABA-dependent gene expression in Arabidopsis thaliana. However, their function in monocot species has not been characterized yet. Herein, it is demonstrated that PYL/RCAR orthologues in Oryza sativa function as a positive regulator of the ABA signal transduction pathway. Transgenic rice plants expressing OsPYL/RCAR5, a PYL/RCAR orthologue of rice, were found to be hypersensitive to ABA during seed germination and early seedling growth. A rice ABA signalling unit composed of OsPYL/RCAR5, OsPP2C30, SAPK2, and OREB1 for ABA-dependent gene regulation was further identified, via interaction assays and a transient gene expression assay. Thus, a core signalling unit for ABA-responsive gene expression modulating seed germination and early seedling growth in rice has been unravelled. This study provides substantial contributions toward understanding the ABA signal transduction pathway in rice.

Overexpression of PYL5 in rice enhances drought tolerance, inhibits growth, and modulates gene expression

Abscisic acid (ABA) is a phytohormone that plays important roles in the regulation of seed dormancy and adaptation to abiotic stresses. Previous work identified OsPYL/RCARs as functional ABA receptors regulating ABA-dependent gene expression in Oryza sativa. OsPYL/RCARs thus are considered to be good candidate genes for improvement of abiotic stress tolerance in crops. This work demonstrates that the cytosolic ABA receptor OsPYL/RCAR5 in O. sativa functions as a positive regulator of abiotic stress-responsive gene expression. The constitutive expression of OsPYL/RCAR5 in rice driven by the Zea mays ubiquitin promoter induced the expression of many stress-responsive genes even under normal growth conditions and resulted in improved drought and salt stress tolerance in rice. However, it slightly reduced plant height under paddy field conditions and severely reduced total seed yield. This suggests that, although exogenous expression of OsPYL/RCAR5 is able to improve abiotic stress tolerance in rice, fine regulation of its expression will be required to avoid deleterious effects on agricultural traits.

Classification of rice (oryza sativa l. japonica nipponbare) immunophilins (fkbps, cyps) and expression patterns under water stress

BackgroundFK506 binding proteins (FKBPs) and cyclophilins (CYPs) are abundant and ubiquitous proteins belonging to the peptidyl-prolyl cis/trans isomerase (PPIase) superfamily, which regulate much of metabolism through a chaperone or an isomerization of proline residues during protein folding. They are collectively referred to as immunophilin (IMM), being present in almost all cellular organs. In particular, a number of IMMs relate to environmental stresses.ResultsFKBP and CYP proteins in rice (Oryza sativa cv. Japonica) were identified and classified, and given the appropriate name for each IMM, considering the ortholog-relation with Arabidopsis and Chlamydomonas or molecular weight of the proteins. 29 FKBP and 27 CYP genes can putatively be identified in rice; among them, a number of genes can be putatively classified as orthologs of Arabidopsis IMMs. However, some genes were novel, did not match with those of Arabidopsis and Chlamydomonas, and several genes were paralogs by genetic duplication. Among 56 IMMs in rice, a significant number are regulated by salt and/or desiccation stress. In addition, their expression levels responding to the water-stress have been analyzed in different tissues, and some subcellular IMMs located by means of tagging with GFP protein.ConclusionLike other green photosynthetic organisms such as Arabidopsis (23 FKBPs and 29 CYPs) and Chlamydomonas (23 FKBs and 26 CYNs), rice has the highest number of IMM genes among organisms reported so far, suggesting that the numbers relate closely to photosynthesis. Classification of the putative FKBPs and CYPs in rice provides the information about their evolutional/functional significance when comparisons are drawn with the relatively well studied genera, Arabidopsis and Chlamydomonas. In addition, many of the genes upregulated by water stress offer the possibility of manipulating the stress responses in rice.

Unique Features of Two Potassium Channels, OsKAT2 and OsKAT3, Expressed in Rice Guard Cells

Potassium is the most abundant cation and a myriad of transporters regulate K+ homeostasis in plant. Potassium plays a role as a major osmolyte to regulate stomatal movements that control water utility of land plants. Here we report the characterization of two inward rectifying shaker-like potassium channels, OsKAT2 and OsKAT3, expressed in guard cell of rice plants. While OsKAT2 showed typical potassium channel activity, like that of Arabidopsis KAT1, OsKAT3 did not despite high sequence similarity between the two channel proteins. Interestingly, the two potassium channels physically interacted with each other and such interaction negatively regulated the OsKAT2 channel activity in CHO cell system. Furthermore, deletion of the C-terminal domain recovered the channel activity of OsKAT3, suggesting that the C-terminal region was regulatory domain that inhibited channel activity. Two homologous channels with antagonistic interaction has not been previously reported and presents new information for potassium channel regulation in plants, especially in stomatal regulation.

Differential gene expression of two outward-rectifying shaker-like potassium channels OsSKOR and OsGORK in rice

Shaker-like potassium channels are highly conserved voltage-dependent ion channels in plants. Rice, a monocot model plant, has similar numbers of shaker-like potassium channels as Arabidopsis. Although several inward-rectifying potassium channels have been identified, no outward-rectifying potassium channels have yet been reported for rice. Here, we identified two outward-rectifying shaker-like potassium channels in rice, with high amino acid sequence similarities to Arabidopsis stelar K+ outward rectifier (SKOR) and guard cell outward rectifying K+ channel (GORK). To characterize these channels we monitored their expression patterns in several tissues. qRTPCR and promoter-GUS analysis showed that OsSKOR is expressed in root vascular tissues, flower, and seed scutellum. OsGORK was expressed to some degree in most tissues, such as leaf blade, node, leaf sheath and root, but at high levels in flowers. This is the first report characterizing rice outward-rectifying potassium channels and contributes to understanding the regulation of potassium homeostasis in monocot plants.

OsKAT2 is the prevailing functional inward rectifier potassium channels in rice guard cell

AtKAT1 plays roles as a major channel to uptake K+ in guard cell when stomata open in dicot model plant Arabidopsis. In a recent publication, we isolated 3 KAT-like potassium channels in rice. We expressed them in CHO cell to identify electrophysiological characteristics of the channels. OsKAT2 showed much bigger inwardly rectifying potassium channel activities among them. The histochemical X-glu staining of transgenic rice leaf blades expressing β-glucuronidase fused with OsKAT2 promoter showed that the OsKAT2 is dominantly expressed in rice guard cell. These findings indicate that OsKAT2 may be a functional ortholog of AtKAT1 in rice. Thus this gene will be the prime target for engineering the guard cell movement to improve drought tolerance in monocot plants, including most major crops.

A Dominant Negative OsKAT2 Mutant Delays Light-Induced Stomatal Opening and Improves Drought Tolerance without Yield Penalty in Rice

Stomata are the main gateways for water and air transport between leaves and the environment. Inward-rectifying potassium channels regulate photo-induced stomatal opening. Rice contains three inward rectifying shaker-like potassium channel proteins, OsKAT1, OsKAT2, and OsKAT3. Among these, only OsKAT2 is specifically expressed in guard cells. Here, we investigated the functions of OsKAT2 in stomatal regulation using three dominant negative mutant proteins, OsKAT2(T235R), OsKAT2(T285A) and OsKAT2(T285D), which are altered in amino acids in the channel pore and at a phosphorylation site. Yeast complementation and patch clamp assays showed that all three mutant proteins lost channel activity. However, among plants overexpressing these mutant proteins, only plants overexpressing OsKAT2(T235R) showed significantly less water loss than the control. Moreover, overexpression of this mutant protein led to delayed photo-induced stomatal opening and increased drought tolerance. Our results indicate that OsKAT2 is an inward- rectifying shaker-like potassium channel that mainly functions in stomatal opening. Interestingly, overexpression of OsKAT2(T235R) did not cause serious defects in growth or yield in rice, suggesting that OsKAT2 is a potential target for engineering plants with improved drought tolerance without yield penalty.

Isolation and Characterization of a cDNA Encoding CycD3 Gene from Potato(Solanum tuberosum L.)

D-type cyclins are believed to regulate the G1 to S phase transition in response to nutrient and hormonal signals. We investigated the expression characteristics of the key cell-cycle regulators, mitotic and G1 cyclins in potato (Solanum tuberosum L.). We isolated D-type cyclin gene from potato and it was classified as D3 cyclin by sequence similarities and a phylogenetic analysis, and named as StcycD3;1. The accumulation of transcripts was predominantly associated with mitotically active organs, such as stolons, roots, flowers, leaves, and stems. Transcription of StcycD3;1 can be induced by sucrose.

Characteristics and functions of shaker like potassium channels in rice

Abstract Potassium (K + ) is one of the most abundant cations in higher plant. It comprises about 10% of plant dry weight and it plays roles in numerous functions such as osmo- and turgor regulation, charge balance of plasma mem-brane and control of stomata and organ movement. Several potassium transporters and potassium channels regulate K + homeostasis in response to K + uptake systems. In this review, we describe the biological, biochemical and physiological characteristics of shaker like potassium channels in higher plant. Especially, we searched the rice genome databases and analysized expressed genes, genome structures and protein domain characteristics of shaker like potassium channels. Keywords plant potassium channel, shaker like, rice 서 론 식물 생장에 필수적인 무기 영양소 중 K + (potassium) 이온은 식물체 내에서 가장 풍부한 양이온으로서 조직이나 식물에 따라 다르나 mM 수준의 농도로 식물 세포에 존재한다. K + 이온은 호흡과 광합성에 필요한 효소들의 활성조절, 세포막 전위차 (plasmamembrane potential)의 조절, 그리고 음이온 그룹과의 전기적 중성화 등을 통해 세포의 항상성 유지, 팽압 조절, 세포 신장, 기공 개폐 과정들에서 중요한 역할을 수행한다 (Lebaudy et al. 2007; Gam-bale and Uozumi 2006). 이러한 K