The detection of sweet-tasting compounds is mediated in large part by a heterodimeric receptor comprised of T1R2؉T1R3. Lactisole, a broad-acting sweet antagonist, suppresses the sweet taste of sugars, protein sweeteners, and artificial sweeteners. Lactisole's inhibitory effect is specific to humans and other primates; lactisole does not affect responses to sweet compounds in rodents. By heterologously expressing interspecies combinations of T1R2؉T1R3, we have determined that the target for lactisole's action is human T1R3. From studies with mouse/ human chimeras of T1R3, we determined that the molecular basis for sensitivity to lactisole depends on only a few residues within the transmembrane region of human T1R3. Alanine substitution of residues in the transmembrane region of human T1R3 revealed 4 key residues required for sensitivity to lactisole. In our model of T1R3's seven transmembrane helices, lactisole is predicted to dock to a binding pocket within the transmembrane region that includes these 4 key residues.Taste is a primal sense that enables diverse organisms to identify and ingest sweet-tasting nutritious foods and to reject bitter-tasting environmental poisons (1). Taste perception can be categorized into five distinct qualities: salty, sour, bitter, umami (amino acid taste), and sweet (1). Salty and sour depend on the actions of ion channels. Bitter, umami, and sweet depend on G-protein-coupled receptors (GPCRs) 1 and coupled signaling pathways. Sweet taste in large part depends on a heterodimeric receptor comprised of T1R2ϩT1R3 (2-5).The T1R taste receptors (T1R1, T1R2, and T1R3) are most closely related to metabotropic glutamate receptors (mGluRs), Ca 2ϩ -sensing receptors (CaSRs), and some pheromone receptors (6 -10). All of these receptors are class-C GPCRs, with the large clam shell-shaped extracellular amino-terminal domain (ATD) characteristic of this family. Following the ATD is a cysteine-rich region that connects the ATD to the heptahelical transmembrane domain (TMD); following the TMD is a short intracellular carboxyl-terminal tail. The solved crystal structure of the ATD of mGluR1 identifies a "Venus flytrap module" (VFTM) involved in ligand binding (11). The canonical agonist glutamate binds within the VFTM in a cleft formed by the two lobes of this module to stabilize a closed active conformation of the mGluR1 ATD. In contrast, several positive and negative allosteric modulators of class-C GPCRs have been identified and shown to act via binding not within the VFTM but instead within the TMD (12-16).Over the past few decades, multiple models of the sweet receptor's hypothetical ligand binding site have been generated based on the structures of existing sweeteners but without direct knowledge of the nature of the sweet receptor itself. A consensus feature of these models is the presence of A-H-B groups, in which the AH group is a hydrogen donor and the B group is an electronegative center. These models have explanatory and predictive value for some, but not all sweeteners, suggesting th...

The size and shape of the plant leaf is an important agronomic trait. To understand the molecular mechanism governing plant leaf shape, we characterized a classic rice (Oryza sativa) dwarf mutant named narrow leaf1 (nal1), which exhibits a characteristic phenotype of narrow leaves. In accordance with reduced leaf blade width, leaves of nal1 contain a decreased number of longitudinal veins. Anatomical investigations revealed that the culms of nal1 also show a defective vascular system, in which the number and distribution pattern of vascular bundles are altered. Map-based cloning and genetic complementation analyses demonstrated that Nal1 encodes a plant-specific protein with unknown biochemical function. We provide evidence showing that Nal1 is richly expressed in vascular tissues and that mutation of this gene leads to significantly reduced polar auxin transport capacity. These results indicate that Nal1 affects polar auxin transport as well as the vascular patterns of rice plants and plays an important role in the control of lateral leaf growth.

The phytohormone cytokinin (CK) positively regulates the activity and function of the shoot apical meristem (SAM), which is a major parameter determining seed production. The rice (Oryza sativa L.) Gn1a/OsCKX2 (Grain number 1a/Cytokinin oxidase 2) gene, which encodes a cytokinin oxidase, has been identified as a major quantitative trait locus contributing to grain number improvement in rice breeding practice. However, the molecular mechanism of how the expression of OsCKX2 is regulated in planta remains elusive. Here, we report that the zinc finger transcription factor DROUGHT AND SALT TOLERANCE (DST) directly regulates OsCKX2 expression in the reproductive meristem. DST-directed expression of OsCKX2 regulates CK accumulation in the SAM and, therefore, controls the number of the reproductive organs. We identify that DST(reg1), a semidominant allele of the DST gene, perturbs DST-directed regulation of OsCKX2 expression and elevates CK levels in the reproductive SAM, leading to increased meristem activity, enhanced panicle branching, and a consequent increase of grain number. Importantly, the DST(reg1) allele provides an approach to pyramid the Gn1a-dependent and Gn1a-independent effects on grain production. Our study reveals that, as a unique regulator of reproductive meristem activity, DST may be explored to facilitate the genetic enhancement of grain production in rice and other small grain cereals.

The artificial sweetener cyclamate tastes sweet to humans, but not to mice. When expressed in vitro, the human sweet receptor (a heterodimer of two taste receptor subunits: hT1R2 ؉ hT1R3) responds to cyclamate, but the mouse receptor (mT1R2 ؉ mT1R3) does not. Using mixed-species pairings of human and mouse sweet receptor subunits, we determined that responsiveness to cyclamate requires the human form of T1R3. Using chimeras, we determined that it is the transmembrane domain of hT1R3 that is required for the sweet receptor to respond to cyclamate. Using directed mutagenesis, we identified several amino acid residues within the transmembrane domain of T1R3 that determine differential responsiveness to cyclamate of the human versus mouse sweet receptors. Alanine-scanning mutagenesis of residues predicted to line a transmembrane domain binding pocket in hT1R3 identified six residues specifically involved in responsiveness to cyclamate. Using molecular modeling, we docked cyclamate within the transmembrane domain of T1R3. Our model predicts substantial overlap in the hT1R3 binding pockets for the agonist cyclamate and the inverse agonist lactisole. The transmembrane domain of T1R3 is likely to play a critical role in the interconversion of the sweet receptor from the ground state to the active state.Taste is a primal sense that is essential for humans and other organisms to detect the nutritive quality of a potential food source while avoiding environmental toxins (1-3). Taste perception can be categorized into five distinct qualities: sweet, bitter, salty, sour, and umami (amino acid taste) (1-3). Sweet, bitter, and umami tastes are mediated in large part by G-protein-coupled receptors (GPCRs) 2 and their linked signaling pathways. Sour and salty tastes are thought to be mediated by direct effects on specialized ion channels (1-3).The detection of sweet taste is mediated by two GPCR subunits, T1R2 and T1R3, which are specifically expressed in taste receptor cells (4 -13). When expressed in vitro, T1R2 ϩ T1R3 heterodimer responds to a broad spectrum of chemically diverse sweeteners, ranging from natural sugars (sucrose, fructose, glucose, and maltose), sweet amino acids (D-tryptophan, D-phenylalanine, and D-serine), and artificial sweeteners (acesulfame-K, aspartame, cyclamate, saccharin, and sucralose) to sweet tasting proteins (monellin, thaumatin, and brazzein) (10, 11, 14 -16). To date, all sweeteners tested in vitro activate the T1R2 ϩ T1R3 heterodimer. In vivo, genetic ablation in mice of T1R2, T1R3, or both either reduces or eliminates responses to sweet compounds (12, 13). Thus, the T1R2 ϩ T1R3 heterodimer is broadly tuned and functions as the principal or sole sweet taste receptor in vivo.T1R2 and T1R3 are class C GPCR subunits (4 -9), members of a group that also includes T1R1 (a component of the umami taste receptor), metabotropic glutamate receptors, the calcium-sensing receptor, ␥-aminobutyric acid type B receptors, and vomeronasal receptors. Like most other GPCRs, each class C receptor has a heptahelic...

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Rice grain with excessive cadmium (Cd) is a major source of dietary Cd intake and a serious threat to health for people who consume rice as a staple food. The development of elite rice cultivars with consistently low Cd content is challenging for conventional breeding approaches, and new strategies urgently need to be developed. Here, we report the development of new indica rice lines with low Cd accumulation and no transgenes by knocking out the metal transporter gene OsNramp5 using CRISPR/Cas9 system. Hydroponic culture showed that Cd concentrations in shoots and roots of osnramp5 mutants were dramatically decreased, resulting in rescue of impaired growth in high Cd condition. Cd-contaminated paddy field trials demonstrated that Cd concentration in osnramp5 grains was consistently less than 0.05 mg/kg, in contrast to high Cd concentrations from 0.33 mg/kg to 2.90 mg/kg in grains of Huazhan (the wild-type indica rice). In particular, the plant yield was not significantly affected in osnramp5 mutants. Furthermore, we developed promising hybrid rice lines with extremely low Cd content in grains. Our work supplies a practical approach to developing Cd pollution-safe indica rice cultivars that minimizes Cd contamination risk in grains.

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Cold stress is a major factor limiting production and geographic distribution of rice (Oryza sativa). Although the growth range of japonica subspecies has expanded northward compared to modern wild rice (O. rufipogon), the molecular basis of the adaptation remains unclear. Here we report bZIP73, a bZIP transcription factor-coding gene with only one functional polymorphism (+511 G>A) between the two subspecies japonica and indica, may have facilitated japonica adaptation to cold climates. We show the japonica version of bZIP73 (bZIP73Jap) interacts with bZIP71 and modulates ABA levels and ROS homeostasis. Evolutionary and population genetic analyses suggest bZIP73 has undergone balancing selection; the bZIP73Jap allele has firstly selected from standing variations in wild rice and likely facilitated cold climate adaptation during initial japonica domestication, while the indica allele bZIP73Ind was subsequently selected for reasons that remain unclear. Our findings reveal early selection of bZIP73Jap may have facilitated climate adaptation of primitive rice germplasms.