The colonization of land by plants was a key event in the evolution of life. Here we report the draft genome sequence of the filamentous terrestrial alga Klebsormidium flaccidum (Division Charophyta, Order Klebsormidiales) to elucidate the early transition step from aquatic algae to land plants. Comparison of the genome sequence with that of other algae and land plants demonstrate that K. flaccidum acquired many genes specific to land plants. We demonstrate that K. flaccidum indeed produces several plant hormones and homologues of some of the signalling intermediates required for hormone actions in higher plants. The K. flaccidum genome also encodes a primitive system to protect against the harmful effects of high-intensity light. The presence of these plant-related systems in K. flaccidum suggests that, during evolution, this alga acquired the fundamental machinery required for adaptation to terrestrial environments.
Endogenous JA production is not necessary for wound-induced expression of JA-biosynthetic lipase genes such as DAD1 in Arabidopsis. However, the JA-Ile receptor COI1 is often required for their JA-independent induction. Wounding is a serious event in plants that may result from insect feeding and increase the risk of pathogen infection. Wounded plants produce high amounts of jasmonic acid (JA), which triggers the expression of insect and pathogen resistance genes. We focused on the transcriptional regulation of DEFECTIVE IN ANTHER DEHISCENCE1 and six of its homologs including DONGLE (DGL) in Arabidopsis, which encode lipases involved in JA biosynthesis. Plants constitutively expressing DAD1 accumulated a higher amount of JA than control plants after wounding, indicating that the expression of these lipase genes contributes to determining JA levels. We found that the expression of DAD1, DGL, and other DAD1-LIKE LIPASE (DALL) genes is induced upon wounding. Some DALLs were also expressed in unwounded leaves. Further experiments using JA-biosynthetic and JA-response mutants revealed that the wound induction of these genes is regulated by several distinct pathways. DAD1 and most of its homologs other than DALL4 were fully induced without relying on endogenous JA-Ile production and were only partly affected by JA deficiency, indicating that positive feedback by JA is not necessary for induction of these genes. However, DAD1 and DGL required CORONATINE INSENSITIVE1 (COI1) for their expression, suggesting that a molecule other than JA might act as a regulator of COI1. Wound induction of DALL1, DALL2, and DALL3 did not require COI1. This differential regulation of DAD1 and its homologs might explain their functions at different time points after wounding.
Strigolactones are host factors that stimulate seed germination of parasitic plant species such as Striga and Orobanche. This hormone is also important in shoot branching architecture and photomorphogenic development. Strigolactone biosynthetic and signaling mutants in model systems, unlike parasitic plants, only show seed germination phenotypes under limited growth condition. To understand the roles of strigolactones in seed germination, it is necessary to develop a tractable experimental system using model plants such as Arabidopsis. Here, we report that thermoinhibition, which involves exposing seeds to high temperatures, uncovers a clear role for strigolactones in promoting Arabidopsis seed germination. Both strigolactone biosynthetic and signaling mutants showed increased sensitivity to seed thermoinhibition. The synthetic strigolactone GR24 rescued germination of thermoinbibited biosynthetic mutant seeds but not a signaling mutant. Hormone analysis revealed that strigolactones alleviate thermoinhibition by modulating levels of the two plant hormones, GA and ABA. We also showed that GR24 was able to counteract secondary dormancy in Arabidopsis ecotype Columbia (Col) and Cape Verde island (Cvi). Systematic hormone analysis of germinating Striga helmonthica seeds suggested a common mechanism between the parasitic and non-parasitic seeds with respect to how hormones regulate germination. Thus, our simple assay system using Arabidopsis thermoinhibition allows comparisons to determine similarities and differences between parasitic plants and model experimental systems for the use of strigolactones.
Parasitic weeds of the genera Striga and Orobanche are considered the most damaging agricultural agents in the developing world. An essential step in parasitic seed germination is sensing a group of structurally related compounds called strigolactones that are released by host plants. Although this makes strigolactone synthesis and action a major target of biotechnology, little fundamental information is known about this hormone. Chemical genetic screening using Arabidopsis thaliana as a platform identified a collection of related small molecules, cotylimides, which perturb strigolactone accumulation. Suppressor screens against select cotylimides identified light-signaling genes as positive regulators of strigolactone levels. Molecular analysis showed strigolactones regulate the nuclear localization of the COP1 ubiquitin ligase, which in part determines the levels of light regulators such as HY5. This information not only uncovers new functions for strigolactones but was also used to identify rice cultivars with reduced capacity to germinate parasitic seed.
Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class or their biosynthetic precursors in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.
Diacylglycerol modulates cell functions primarily through activation of protein kinase C (PKC). In a previous study, however, we found that a diacylglycerol analogue, 1-oleoyl-2-acetylglycerol (OAG), accelerated desensitization of neuronal nicotinic acetylcholine receptors (nAchRs) independently of PKC activation in PC12 cells. In the present study, we investigated whether other analogues and endogenous diacylglycerol exert similar effects on neuronal nAchRs and characterized the modulation by diacylglycerol. We measured the nicotine-induced whole-cell current in the absence and presence of diacylglycerol analogues in PC12 cells. We also investigated the effects of a blockade of metabolic pathways of diacylglycerol by inhibiting diacylglycerol lipase and kinase. We found that all four diacylglycerol analogues studied promoted desensitization and depressed the nondesensitized component of the nicotine-induced current. These effects seemed independent of PKC activation because they were not antagonized by the PKC inhibitors staurosporine or bisindolylmaleimide I; one analogue that lacks the PKC-stimulating action was also effective. The effects of diacylglycerol analogues were not antagonized by high doses of nicotine and were independent of the membrane potential. Similar modulatory effects were observed by treatment with RHC80267, a blocker of diacylglycerol lipase, and R59949, an inhibitor of diacylglycerol kinase, in the presence of staurosporine. These results suggest that diacylglycerol, both exogenously applied and endogenously produced, modulates neuronal nAchRs independently of PKC activation in PC12 cells; further, these effects seemed consistent with a noncompetitive and voltage-independent block. They raised the possibility that PKC-independent inhibition of neuronal nAchRs by diacylglycerol may be a novel modulatory process.
Title: Mutations in genes for acetylcholinesterase intensify lethality by acrylamide in Caenorhabditis elegans Kamiya Y, Harada S, Yamamoto H, Hosono R Ref: Neuroscience Letters, 145:37, 1992 : PubMed
Acrylamide inhibits growth and results in death in the nematode Caenorhabditis elegans. The lethargic effect is marked in the mutants defective in genes for acetylcholinesterase (AChE) and the effect is approximately parallel with the decrease in AChE activity by mutations. Although neither the activity nor the localization of the enzyme is affected by acrylamide, the acetylcholine level was significantly elevated.
Title: Additional genes which result in an elevation of acetylcholine levels by mutations in Caenorhabditis elegans Hosono R, Kamiya Y Ref: Neuroscience Letters, 128:243, 1991 : PubMed
Four mutant genes (unc-17, unc-18, unc-41 and unc-13) have been identified that result in abnormal accumulation of acetylcholine (ACh). We have now identified 3 more such genes (unc-63, unc-11 and unc-64). In addition to the abnormal accumulation of ACh, mutants in these 7 genes possess common phenotypes in locomotion, resistance to inhibitors of acetylcholinesterase (AChE) and in post-embryonic development. These results suggest that the 7 genes are involved in some related functions.
