- Email: [email protected]
Cosmic signals?
research news outward rectifier with a steep Ca2~ dependency, EMBO J. (in press) 4 Assmann, S.M. and Haubrick, L.L. (1996) Transport proteins of the plant plasma membrane, Curr. Opin. Cell Biol. 8, 458-467 5 Mtiller-R6ber,B. et al. (1995) Cloning and electrophysiologicalanalysis of KST1, an inward rectifying K÷channel expressed in potato guard cells, EMBO J. 14, 2409-2416 6 Nakamura, R.L. et al. (1995) Expression of an Arabidopsis potassium channel gene in guard cells, Plant Physiol. 109, 371-374 7 Jan, L.Y. and Jan, Y.N. (1994) Potassium channels and their evolving gates, Nature 371, 119-122 8 Catterall, W.A. (1995) Structure and function of voltage-gated ion channels, Annu. Rev. Biochem. 64, 493-531 9 Kubo, Y. et aI. (1993) Primary structure and functional expression of a mouse inward rectifier potassium channel, Nature 362, 127-133 10 Kubo, Y. et al. (1993) Primary structure and functional expression of a rat G-proteincoupled muscarinic potassium channel, Nature 364, 802-806 11 Ketchum, K.A.et al. (1995) A new family of outwardly rectifying potassium channel proteins with 2 pore domains in tandem, Nature 376, 690-695 12 Fink, M. et al. (1996) Cloning, functional expression and brain localization of a novel unconventional outward rectifier K÷channel, EMBO J. 15, 6854-6862 13 Lesage, F. et al. (1996) TWIK-1, a ubiquitous human weakly inward rectifying K÷channel with a novel structure, EMBO J. 15, 1004-1011 14 Vergani, P. et al. (1997) Extracellular K+and Ba2÷mediate voltage-dependent inactivation of the outward-rectifying K÷channel encoded by the yeast gene TOK1, FEBS Lett. 405, 337-344 15 Roelfsema, M.R.G. and Prins, H.B.A. Ion channels in the guard cells of Arabidopsis thaliana (L.) Heynh., Planta (in press) 16 Stoeckel, H. and Takeda, K. (1995) Calcinmsensitivity of the plasmalemmal delayed rectifier potassium current suggests that calcium influx in pulvinar protoplasts from Mimosa pudica can be revealed by hyperpolarization, J. Membr. Biol. 146, 201-209 17 Ketchum, K.A. and Peele, R.J. (1991) Cytosoliccalcium regulates a potassium current in corn (Zea mays) protoplasts, J. Membr. Biol. 119, 277-288
Coming in September! The Trends Guide to the Internet (1997) T o s u b s c r i b e to Trends in Plant Science a n d r e c e i v e y o u r free guide, use the order f o r m b o u n d in this issue.
246
July1997,Vol.2, No.7
Cosmic signals? As the organizers of the recent symposium on information processing in plants* tried to take credit for a lunar eclipse and the appearance of the comet Hale-Bopp, it became clear that the interpretation of phenomena in space and in plants have at least one thing in common: everyone interprets them differently. The symposium provided an integrated view of cell signaling in diverse areas of plant biology, with the sessions coordinated to introduce four main themes: a comparison of signaling mechanisms across kingdoms; the coordination of plant cell differentiation; the integration of signals from the environment; and signal processing during symbiotic and pathogenic interactions. Throughout the symposium, speakers emphasized the common aspects of diverse communication systems and speculated on the evolution of these pathways.
Signaling across kingdoms To introduce the topic of signaling in nonplant kingdoms, Jeff Williams [University of London (University College), UK] described cell communication in Dictyostelium, and enthralled the audience with a video display of individual cells acting in concert to form a multicellular slug. The topic of environmental sensing and the integration of signals via crosstalk between protein kinase cascades in yeast (Saccharomyces cerevisiae) was discussed by Beverly Errede [University of North Carolina (Chapel Hill), USA]. Peter Walter [University of California (San Francisco), USA] reported on the unfolded-protein response pathway in yeast, with an elegant story of how his lab established the involvement of a transmembrane kinase and regulation via splicing in this unique endoplasmic reticulum-to-nucleus signaling system. When the intricate signaling described in these lower organisms is considered, it is clear that extremely complex mechanisms will be uncovered as related pathways in plants are dissected. An overview of responses coordinated by signal integration in both higher and lower plants was provided by Ralph Quatrano [University of North Carolina (Chapel Hill)]. As an example, he described how Fucus embryos set up asymmetric cell division by establishing an actin network on one side of the embryo, so that the secretion of signals into the cell wall establishes rhizoid-like development. This system is reminiscent of secreted signals used by mammalian epithelial cells and budding yeast, and illustrates how basic cellular mechanisms can be both conserved and elaborated on to produce features unique to *Information ProcessingSystems in Plants: Their Evolution and Function, University of California (Davis), USA, 21-26 March 1997
plants. As another example, Glenn Hicks [Michigan State University (East Lansing), USA] presented some unexpected features of protein import into the plant nucleus. He reported that the nuclear pore complex is modified by a novel carbohydrate moiety, and that the translocation mechanism uses GTP exclusively as its energy source, and is unaffected by temperature. Other highlights of this session included Steve Clouse's [North Carolina State University (Raleigh)] talk on brassinosteroid signaling. A role for steroids in plant development has only recently been established, but it now seems that many dwarf and de-etiolation mutants in Arabidopsis (i.e. DET-2) and tomato have defects in the brassinosteroid biosynthesis pathway. Joe Ecker [University of Pennsylvania (Philadelphia), USA], in his talk on ethylene signal transduction, revealed that a Raf-like protein kinase is involved, indicating that even this plantspecific pathway includes components that are conserved in other organisms. By using Arabidopsis mutants defective in nitrogen metabolism, Gloria Coruzzi (New York University, USA) is beginning to uncover how plants can integrate environmental signals to control metabolic activity: she impressed the audience with a model for the regulation of nitrogen assimilation by light and the level of free amino acids in the phloem.
Coordinated plant development The talks on Sunday again started on a supernatural note - the coordination of plant differentiation was introduced by Alan Bennet [University of California (Davis)[ as a replacement for 'worship services'. Bill Lucas [University of California (Davis)[ opened a session on the evolution of short- and long-distance communication networks with a 'sermon' on the importance of plasmodesmata during plant growth and development. June Nasrallah (Cornell University, Ithaca, USA) then discussed the evolution of the S-locus and cell recognition during pollination. Bob Goldberg [University of California (Los Angeles)] reported on the 'mystical' abilities of cells in the developing anther to coordinate their differentiation with that of the rest of the flower. He discussed evidence based on targeted ablation studies that indicate that stomium cells, which form longitudinal slits between the two pollen sacs, must be living .for dehiscence to occur; this suggests that dehiscence is not a simple matter of cell death, but involves more complex signal processing. The 'fire and brimstone' was provided by Dave Gilchrist [University of California (Davis)], in his talk on apoptosis, who stressed the similarities of apoptotic pathways in plant and animal systems. He also spoke briefly
© 1997 ElsevierScienceLtd
research
about his lab's successful use of FACS (fluorescence-activated cell sorting) for screening populations of protoplasts, and encouraged others to use the technique. Rounding up this impressive session, Mike Grusak (Baylor College of Medicine, Houston, USA) described grafting experiments on pea mutants that hyperaccumulate iron; these have demonstrated homeostatic regulation of the root iron reductase by a phloem-mobile signal. The talks that followed centered on coordinated cell differentiation during flower development. The evolution of MADS-box genes was examined by Heinz Saedler (Max Planck Institute for Breeding Research, Cologne, Germany), who discussed the interesting observation that ferns lack floral homeotic-type MADS-box genes. Rich Jorgensen [University of California (Davis)] explored the complexity of epigenetic influences during development using cosuppression in petunia flowers as a model system. Rick Amasino (University of Wisconsin, Madison, USA) presented a collection of Arabidopsis mutants for flower induction, classifying them either as responsive or nonresponsive to photoperiod and vernalization, and described a model revealing the intricate interactions of these flowering-time genes.
Environmental sensing One of the major unresolved aspects of plant biology centers on how light is translated into regulatory signals. In an excellent session on this topic, Tony Cashmore [University of Pennsylvania (Philadelphia)] and Peter Quail [University of California (Berkeley)] spoke about their contributions to this field, highlighting the complex interdependence between components of the blue-light and red-light signaling pathways. Clark Lagarius [University of California (Davis)] reported on the phosphokinase activity of phytochrome. The recent discovery that cyanobacteria (Synechocystis sp.) carry a phytochrome-like protein with a transmitter kinase domain (S6803phyl, ORF1) led Lagarius' group to search for a likely receiver protein in the cyanobacterial phototransduction pathway. In experiments with recombinant oat phytochrome, the higher plant photoreceptor was able to phosphorylate both itself and a receiver protein recently found in Synechocystis (S6803phyl, ORF2), providing what Quail called 'the best clue yet' about the mode of action of phytochromes. Emphasizing the point that phototransduction researchers must be very aware of the plant's internal clock, Steve Kay (The Scripps Research Institute, La Jolla, USA) spoke about the downregulation of light responses in Arabidopsis during specific phases of their circadian rhythms. In addition, he reported on the fluorescence resonance energy transfer from blue fluorescent
protein to green fluorescent protein to demonstrate protein interactions in living tissues. He predicted that as new, more stable fluorescent proteins become available, this technique may prove to be a major advance in investigations of protein activities in vivo. Of course, in addition to sensing light, plants must integrate many other types of signals originating from interactions with the environment. Lewis Feldman [University of California (Berkeley)] described a model showing how gravitropism may have evolved from chemotactic pathways in unicellular organisms. Elisabeth Vierling (University of Arizona, Tucson, USA) provided new insights into the heat shock response in plants, and a detailed analysis of the role of the small heat shock protein PsHSP18.1 in preventing irreversible aggregation of unfolded proteins. A session on the plant extracellular matrix included discussions on the evolution of the plant cell wall, the cuticle and phenylpropanoid synthesis, with talks by Debby Delmer [University of California (Davis)], Paul Kolattukudy (Ohio State University, Columbus, USA) and Norman Lewis (Washington State University, Pullman, USA). Their presentations highlighted the many roles that these elements play in structural support, protection from the environment and signal transduction processes.
Signaling and species recognition The final day focused on the interactions of plants and microbes in both symbiotic and pathogenic relationships. Calcium signaling during the initial stages of nodulation was discussed by David Ehrhardt (Carnegie Institute of Washington, Stanford, USA), whose studies involve the microinjection of calcium indicators into root-hair cells. The talk was illustrated with beautiful images of a cellautonomous, calcium-spiking response, which begins 9 rain after roots are treated with a nod factor, leaving open the question of what happens during this short delay. Marilynn Etzler [University of California (Davis)] described a lectin (DB46), isolated from Dolichos biflorus roots and expressed on the surface of root hairs, which may be a nod-factor receptor. Yoram Kalpulnik (The Volcani Center, Bet Dagan, Israel) presented evidence that symbiotic fungi that form mycorrhizas suppress the hypersensitive response in their hosts by producing cytokinins. These data fit nicely with work reported by Ann Hirsch [University of California (Los Angeles)], who showed that cytokinins can induce expression of the plant gene ENOD40, which is active during both nodule development and the establishment of arbuscular mycorrhizas. This led to speculation that the programmed pattern of nodule development might have evolved from mycorrhizal symbiosis.
news
The symposium concluded with a session on plant responses to pathogen attack, an area of plant cell signaling research that is currently progressing very rapidly. Focusing on the salicylic acidmediated response, Dan Klessig (Rutgers University, Piscataway, USA) reported the recent identification of a protein [SABP2 ('salicylic acid-binding protein')] that binds salicylic acid with 100-fold greater affinity than catalase, suggesting that SABP2 may be the primary target for salicylic acid during the induction of systemic acquired resistance. He also discussed a salicylic acid-induced MAP kinase, SIP, which appears to interact with a recently identified transcription factor, SARB1 ('salicylic acid-responsive binding protein'), to induce expression of the defense gene, PR-2d ('pathogen-related'). In addition, Arabidopsis mutants that are either salicylic acid-insensitive or constitutively express pathogen-related genes have been isolated. Talks by other leading researchers in this field provided additional evidence that plants appear to have adapted existing signaling pathways to their evolving needs, rather than inventing entirely novel mechanisms. Bud Ryan (Washington State University, Pullman) examined the systemic response of plants to wounding, and discussed the multiple parallels between this and peptide hormone signaling in animals and yeast. The cloning of an IK-B-like receptor from Arabidopsis was reported by Terrence Delaney (Cornell University, Ithaca). The gene was identified by screening for mutants unable to establish systemic acquired resistance upon exposure to an inducing chemical. Barbara Baker ]University of California (Berkeley)] and Pamela Ronald [University of California (Davis)] discussed the pathogen-specific receptors N (tobacco) and Xa21 (rice), further highlighting similarities between receptor-driven induction of defense responses and mechanisms described in animal systems.
Future challenges By the conclusion of the symposium, it had become evident that the signals involved in plant communication, while having little in common with cosmic phenomena, can use mechanisms that are conserved across kingdoms. The challenge now is to determine how these conserved pathways have been modified to suit the unique information processing requirements of higher plants. Friedrich Kragler* and Laurel Mezitt Sectionof Plant Biology, Dept of Biological Sciences, University of California (Davis), California,95616, USA. *Author for correspondence (tel +1 916 752 7737; fax +1 916 752 5410; e-mail ffkragler @ucdavis.edu).
July 1997,Vol.2, No. 7
247
Coming in September! The Trends Guide to the Internet (1997) T o s u b s c r i b e to Trends in Plant Science a n d r e c e i v e y o u r free guide, use the order f o r m b o u n d in this issue.
246
July1997,Vol.2, No.7
Cosmic signals? As the organizers of the recent symposium on information processing in plants* tried to take credit for a lunar eclipse and the appearance of the comet Hale-Bopp, it became clear that the interpretation of phenomena in space and in plants have at least one thing in common: everyone interprets them differently. The symposium provided an integrated view of cell signaling in diverse areas of plant biology, with the sessions coordinated to introduce four main themes: a comparison of signaling mechanisms across kingdoms; the coordination of plant cell differentiation; the integration of signals from the environment; and signal processing during symbiotic and pathogenic interactions. Throughout the symposium, speakers emphasized the common aspects of diverse communication systems and speculated on the evolution of these pathways.
Signaling across kingdoms To introduce the topic of signaling in nonplant kingdoms, Jeff Williams [University of London (University College), UK] described cell communication in Dictyostelium, and enthralled the audience with a video display of individual cells acting in concert to form a multicellular slug. The topic of environmental sensing and the integration of signals via crosstalk between protein kinase cascades in yeast (Saccharomyces cerevisiae) was discussed by Beverly Errede [University of North Carolina (Chapel Hill), USA]. Peter Walter [University of California (San Francisco), USA] reported on the unfolded-protein response pathway in yeast, with an elegant story of how his lab established the involvement of a transmembrane kinase and regulation via splicing in this unique endoplasmic reticulum-to-nucleus signaling system. When the intricate signaling described in these lower organisms is considered, it is clear that extremely complex mechanisms will be uncovered as related pathways in plants are dissected. An overview of responses coordinated by signal integration in both higher and lower plants was provided by Ralph Quatrano [University of North Carolina (Chapel Hill)]. As an example, he described how Fucus embryos set up asymmetric cell division by establishing an actin network on one side of the embryo, so that the secretion of signals into the cell wall establishes rhizoid-like development. This system is reminiscent of secreted signals used by mammalian epithelial cells and budding yeast, and illustrates how basic cellular mechanisms can be both conserved and elaborated on to produce features unique to *Information ProcessingSystems in Plants: Their Evolution and Function, University of California (Davis), USA, 21-26 March 1997
plants. As another example, Glenn Hicks [Michigan State University (East Lansing), USA] presented some unexpected features of protein import into the plant nucleus. He reported that the nuclear pore complex is modified by a novel carbohydrate moiety, and that the translocation mechanism uses GTP exclusively as its energy source, and is unaffected by temperature. Other highlights of this session included Steve Clouse's [North Carolina State University (Raleigh)] talk on brassinosteroid signaling. A role for steroids in plant development has only recently been established, but it now seems that many dwarf and de-etiolation mutants in Arabidopsis (i.e. DET-2) and tomato have defects in the brassinosteroid biosynthesis pathway. Joe Ecker [University of Pennsylvania (Philadelphia), USA], in his talk on ethylene signal transduction, revealed that a Raf-like protein kinase is involved, indicating that even this plantspecific pathway includes components that are conserved in other organisms. By using Arabidopsis mutants defective in nitrogen metabolism, Gloria Coruzzi (New York University, USA) is beginning to uncover how plants can integrate environmental signals to control metabolic activity: she impressed the audience with a model for the regulation of nitrogen assimilation by light and the level of free amino acids in the phloem.
Coordinated plant development The talks on Sunday again started on a supernatural note - the coordination of plant differentiation was introduced by Alan Bennet [University of California (Davis)[ as a replacement for 'worship services'. Bill Lucas [University of California (Davis)[ opened a session on the evolution of short- and long-distance communication networks with a 'sermon' on the importance of plasmodesmata during plant growth and development. June Nasrallah (Cornell University, Ithaca, USA) then discussed the evolution of the S-locus and cell recognition during pollination. Bob Goldberg [University of California (Los Angeles)] reported on the 'mystical' abilities of cells in the developing anther to coordinate their differentiation with that of the rest of the flower. He discussed evidence based on targeted ablation studies that indicate that stomium cells, which form longitudinal slits between the two pollen sacs, must be living .for dehiscence to occur; this suggests that dehiscence is not a simple matter of cell death, but involves more complex signal processing. The 'fire and brimstone' was provided by Dave Gilchrist [University of California (Davis)], in his talk on apoptosis, who stressed the similarities of apoptotic pathways in plant and animal systems. He also spoke briefly
© 1997 ElsevierScienceLtd
research
about his lab's successful use of FACS (fluorescence-activated cell sorting) for screening populations of protoplasts, and encouraged others to use the technique. Rounding up this impressive session, Mike Grusak (Baylor College of Medicine, Houston, USA) described grafting experiments on pea mutants that hyperaccumulate iron; these have demonstrated homeostatic regulation of the root iron reductase by a phloem-mobile signal. The talks that followed centered on coordinated cell differentiation during flower development. The evolution of MADS-box genes was examined by Heinz Saedler (Max Planck Institute for Breeding Research, Cologne, Germany), who discussed the interesting observation that ferns lack floral homeotic-type MADS-box genes. Rich Jorgensen [University of California (Davis)] explored the complexity of epigenetic influences during development using cosuppression in petunia flowers as a model system. Rick Amasino (University of Wisconsin, Madison, USA) presented a collection of Arabidopsis mutants for flower induction, classifying them either as responsive or nonresponsive to photoperiod and vernalization, and described a model revealing the intricate interactions of these flowering-time genes.
Environmental sensing One of the major unresolved aspects of plant biology centers on how light is translated into regulatory signals. In an excellent session on this topic, Tony Cashmore [University of Pennsylvania (Philadelphia)] and Peter Quail [University of California (Berkeley)] spoke about their contributions to this field, highlighting the complex interdependence between components of the blue-light and red-light signaling pathways. Clark Lagarius [University of California (Davis)] reported on the phosphokinase activity of phytochrome. The recent discovery that cyanobacteria (Synechocystis sp.) carry a phytochrome-like protein with a transmitter kinase domain (S6803phyl, ORF1) led Lagarius' group to search for a likely receiver protein in the cyanobacterial phototransduction pathway. In experiments with recombinant oat phytochrome, the higher plant photoreceptor was able to phosphorylate both itself and a receiver protein recently found in Synechocystis (S6803phyl, ORF2), providing what Quail called 'the best clue yet' about the mode of action of phytochromes. Emphasizing the point that phototransduction researchers must be very aware of the plant's internal clock, Steve Kay (The Scripps Research Institute, La Jolla, USA) spoke about the downregulation of light responses in Arabidopsis during specific phases of their circadian rhythms. In addition, he reported on the fluorescence resonance energy transfer from blue fluorescent
protein to green fluorescent protein to demonstrate protein interactions in living tissues. He predicted that as new, more stable fluorescent proteins become available, this technique may prove to be a major advance in investigations of protein activities in vivo. Of course, in addition to sensing light, plants must integrate many other types of signals originating from interactions with the environment. Lewis Feldman [University of California (Berkeley)] described a model showing how gravitropism may have evolved from chemotactic pathways in unicellular organisms. Elisabeth Vierling (University of Arizona, Tucson, USA) provided new insights into the heat shock response in plants, and a detailed analysis of the role of the small heat shock protein PsHSP18.1 in preventing irreversible aggregation of unfolded proteins. A session on the plant extracellular matrix included discussions on the evolution of the plant cell wall, the cuticle and phenylpropanoid synthesis, with talks by Debby Delmer [University of California (Davis)], Paul Kolattukudy (Ohio State University, Columbus, USA) and Norman Lewis (Washington State University, Pullman, USA). Their presentations highlighted the many roles that these elements play in structural support, protection from the environment and signal transduction processes.
Signaling and species recognition The final day focused on the interactions of plants and microbes in both symbiotic and pathogenic relationships. Calcium signaling during the initial stages of nodulation was discussed by David Ehrhardt (Carnegie Institute of Washington, Stanford, USA), whose studies involve the microinjection of calcium indicators into root-hair cells. The talk was illustrated with beautiful images of a cellautonomous, calcium-spiking response, which begins 9 rain after roots are treated with a nod factor, leaving open the question of what happens during this short delay. Marilynn Etzler [University of California (Davis)] described a lectin (DB46), isolated from Dolichos biflorus roots and expressed on the surface of root hairs, which may be a nod-factor receptor. Yoram Kalpulnik (The Volcani Center, Bet Dagan, Israel) presented evidence that symbiotic fungi that form mycorrhizas suppress the hypersensitive response in their hosts by producing cytokinins. These data fit nicely with work reported by Ann Hirsch [University of California (Los Angeles)], who showed that cytokinins can induce expression of the plant gene ENOD40, which is active during both nodule development and the establishment of arbuscular mycorrhizas. This led to speculation that the programmed pattern of nodule development might have evolved from mycorrhizal symbiosis.
news
The symposium concluded with a session on plant responses to pathogen attack, an area of plant cell signaling research that is currently progressing very rapidly. Focusing on the salicylic acidmediated response, Dan Klessig (Rutgers University, Piscataway, USA) reported the recent identification of a protein [SABP2 ('salicylic acid-binding protein')] that binds salicylic acid with 100-fold greater affinity than catalase, suggesting that SABP2 may be the primary target for salicylic acid during the induction of systemic acquired resistance. He also discussed a salicylic acid-induced MAP kinase, SIP, which appears to interact with a recently identified transcription factor, SARB1 ('salicylic acid-responsive binding protein'), to induce expression of the defense gene, PR-2d ('pathogen-related'). In addition, Arabidopsis mutants that are either salicylic acid-insensitive or constitutively express pathogen-related genes have been isolated. Talks by other leading researchers in this field provided additional evidence that plants appear to have adapted existing signaling pathways to their evolving needs, rather than inventing entirely novel mechanisms. Bud Ryan (Washington State University, Pullman) examined the systemic response of plants to wounding, and discussed the multiple parallels between this and peptide hormone signaling in animals and yeast. The cloning of an IK-B-like receptor from Arabidopsis was reported by Terrence Delaney (Cornell University, Ithaca). The gene was identified by screening for mutants unable to establish systemic acquired resistance upon exposure to an inducing chemical. Barbara Baker ]University of California (Berkeley)] and Pamela Ronald [University of California (Davis)] discussed the pathogen-specific receptors N (tobacco) and Xa21 (rice), further highlighting similarities between receptor-driven induction of defense responses and mechanisms described in animal systems.
Future challenges By the conclusion of the symposium, it had become evident that the signals involved in plant communication, while having little in common with cosmic phenomena, can use mechanisms that are conserved across kingdoms. The challenge now is to determine how these conserved pathways have been modified to suit the unique information processing requirements of higher plants. Friedrich Kragler* and Laurel Mezitt Sectionof Plant Biology, Dept of Biological Sciences, University of California (Davis), California,95616, USA. *Author for correspondence (tel +1 916 752 7737; fax +1 916 752 5410; e-mail ffkragler @ucdavis.edu).
July 1997,Vol.2, No. 7
247