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behavior. Many of the retinal circuitries have been discovered — or at least studied in detail — with the help of specific mouse lines in which the appropriate neurons are labeled [7–10]. Genetic targeting could therefore also be used to selectively ablate specific neurons. Similar methodology — i.e., specific ablation — has successfully been applied in the case of starburst amacrine cells, an interneuron that had long been suspected to be involved in direction-selectivity. And indeed, after killing the starburst cells, direction-selective cells lost their specificity and responded to movement in all directions [11]. Similarly, one could in the future test the escape reflex in mice that have specific retinal circuitries knocked out, and thus move away from educated speculation to intervention-based evidence. Many interesting questions are looming: will the responsible cell type only be
present in the ventral retina (observing the sky), or throughout the retina? What is its role then in the dorsal retina? In the end, it may also turn out that the escape reflex is initiated by combining the information from many different retinal cells in the brain, and that ablating any one ganglion cell type does not suffice to eliminate the reflex. References 1. Lin, J.Y., Franconeri, S., and Enns, J.T. (2008). Objects on a collision path with the observer demand attention. Psychol. Sci. 19, 686–692. 2. Yilmaz, M., and Meister, M. (2013). Rapid innate defensive responses of mice to looming visual stimuli. Curr. Biol. 23, 2011–2015. 3. Wallace, D.J., Greenberg, D.S., Sawinski, J., Rulla, S., Notaro, G., and Kerr, J.N. (2013). Rats maintain an overhead binocular field at the expense of constant fusion. Nature 498, 65–69. 4. Werblin, F., and Roska, B. (2007). The movies in our eyes. Sci. Am. 296, 72–79. 5. Gollisch, T., and Meister, M. (2010). Eye smarter than scientists believed: neural computations in circuits of the retina. Neuron 65, 150–164. 6. Barlow, H.B., and Levick, W.R. (1965). The mechanism of directionally selective units in rabbit’s retina. J. Physiol. 178, 477–504.
Halotropism: Turning Down the Salty Date Plants have a deep-rooted trust in gravity, but it is not unconditional. A new study shows that, if plant roots sense high doses of salt coming up from below, they dump gravity responses and grow away from the salt contamination. Michel Ruiz Rosquete and Ju¨rgen Kleine-Vehn Higher plants are sessile and have fundamentally different life strategies compared with animals. This ‘otherness’ might be the reason why we show such fascination for turning (‘‘tropos’’ in Greek) response in plants [1]. In this issue of Current Biology, Galva´n-Ampudia and colleagues [2] illustrate how plant expansion is coordinated to allow root growth away from salt-contaminated soil and define a new tropistic growth paradigm (Figure 1). Tropisms are associated with but not restricted to plants and are centrally important for plant performance and survival. Hence, tropism is not only a spectacular example of directional growth regulation, but is also a crucial adaptive response to integrate external abiotic stimuli into plant architecture and accordingly has enormous
agronomical importance. In plants, multiple tropisms, such as growth in response to gravity (geo- or gravitropism), water (hydrotropism), light (phototropism), and contact (thigmotropism), have been well documented and many others, such as turning due to sound (sonotropism), electric field (electrotropism), chemicals (chemotropism), temperature (thermotropism), or salt (halotropism) have been suggested (reviewed in [3–6]). Gravitropism is most central since the gravity stimulus is everlasting on earth and plants strongly relate to this up-and-down information. All the other tropisms have to challenge or modify the plant response to gravity. If we want to approach plant tropisms, we have to understand gravitropism and how the phytohormone auxin steers this response. Auxin mediates many, if not all, of the differential and asymmetric growth responses in plants [7]. On a
7. Mu¨nch, T.A., da Silveira, R.A., Siegert, S., Viney, T.J., Awatramani, G.B., and Roska, B. (2009). Approach sensitivity in the retina processed by a multifunctional neural circuit. Nat. Neurosci. 12, 1308–1316. 8. Yonehara, K., Balint, K., Noda, M., Nagel, G., Bamberg, E., and Roska, B. (2011). Spatially asymmetric reorganization of inhibition establishes a motion-sensitive circuit. Nature 469, 407–410. 9. Siegert, S., Scherf, B.G., Del Punta, K., Didkovsky, N., Heintz, N., and Roska, B. (2009). Genetic address book for retinal cell types. Nat. Neurosci. 12, 1197–1204. 10. Kim, I.J., Zhang, Y., Yamagata, M., Meister, M., and Sanes, J.R. (2008). Molecular identification of a retinal cell type that responds to upward motion. Nature 452, 478–482. 11. Amthor, F.R., Keyser, K.T., and Dmitrieva, N.A. (2002). Effects of the destruction of starburst-cholinergic amacrine cells by the toxin AF64A on rabbit retinal directional selectivity. Vis. Neurosci. 19, 495–509.
Werner Reichardt Centre for Integrative Neuroscience, Eberhard Karls Universita¨t Tu¨bingen, 72076 Tu¨bingen, Germany. E-mail:
[email protected]
http://dx.doi.org/10.1016/j.cub.2013.08.039
cellular level, auxin controls elongation and division rates and its tissue distribution is central to symmetry breaking [4,7]. During gravitropism, auxin levels get redefined in the responding tissues, ultimately leading to asymmetric auxin signalling and organ expansion [5,7–10]. Auxin shows enhanced circulation in some parts of the organ at the expense of less circulation in other parts. The model of primary root gravitropism provides a picture-perfect illustration of this mechanism, where opposite flanks of the root epidermis display coordinated asymmetric auxin levels and consequently differential growth towards gravity [8–11]. ‘Circulation’ of auxin is facilitated by intercellular transport and a vast array of auxin transporters has been described [12]. Most prominently, the PIN-FORMED (PIN) auxin carriers determine the rate and directionality of cellular auxin efflux [12]. According to their importance for polar auxin transport, relocalization of PINs to specific cell membrane domains and regulation of PIN protein abundance at the plasma membrane are the two most important mechanisms underlying asymmetric fluxes of auxin during two distinct phases of the gravitropic response [8,10]. In the first
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Figure 1. Auxin transport-dependent regulation of halotropism. Schematic depiction of a root tip grown in a salt gradient. Cell-to-cell auxin transport (coloured arrows) defines asymmetric auxin signalling (green cells). High salt induces clathrin-dependent (grey C) PIN2 (depicted in red) internalisation at the lower root flank, ultimately providing for asymmetric auxin flux, differential organ growth and a directional halotropic growth response.
phase, changes in the gravity vector induce displacements of starch-loaded amyloplasts (statolith) within columella cells [9], which in turn, and by a so far unknown mechanism, triggers a redistribution of the auxin effluxers PIN3 and PIN7 [8]. The gravity-induced polarization of PIN3 and PIN7 at the ‘new’ bottom side of the cell will trigger more auxin flux to the ‘lower’ epidermal flank, initiating a symmetry-breaking cascade. The following events take place in the epidermis (outermost cell layer), which we refer to as second/ effector phase. PIN2 auxin efflux carriers localize at the upper (shootward) cell side of root epidermal cells, transmitting auxin to the elongation zone (Figure 1). Hence, PIN2 is central to hand over the asymmetric PIN3-dependent auxin flux to the responsive tissues and to initiate differential growth [10,11]. Gravistimulation also leads to enhanced lytic PIN2 turnover at the upper epidermal flank [10,11]. Therefore, PIN2 not only transmits the asymmetric auxin signal, but (due to
this differential degradation mechanism) it furthermore reduces the auxin flow in this upper flank and consequently enhances differential growth responses [11]. Gravity is definitely not the only stimulus relevant to plant tropisms, but it is always there. This raises the question — how do plants react in a ‘directional’ way to the other tropistic stimuli? And furthermore, how do plant organs actually disregard the gravitropic pressure in order to reorient their growth? For growth response to salt, the road map seems within reach. In 2008, Sun and collaborators reported what they called a ‘salt-avoidance behavior’, where Arabidopsis seedlings exposed to high saline concentrations showed dramatically randomized, agravitropic root growth [13]. The authors observed extensive reduction in the starch content of amyloplasts in the columella cells from salt-stressed plants [13]. Alleviating statolith capacity could in principle explain the disrupted gravitropism. Nevertheless, sos mutants, in which salt induced
amyloplast degradation is impaired, are still able to exert ‘salt-avoidance’ responses [13]. These results suggest additional mechanism for salt-induced, agravitropic growth besides damping statolith function. An elegant study in this current issue revisits root growth direction in the presence of high salt and unmasks a type of response, which depicts more than just ‘salt avoidance’ [2]. By using well-defined salt gradients, the authors show how Arabidopsis roots mount not a random but a directional growth response away from salt (halotropism). The suppression of statolith-dependent gravitropism in columella cells [13] cannot explain such a directional response. Mechanistically, the authors found increased PIN2 auxin carrier internalisation in the epidermal flank facing higher salt concentrations (Figure 1), suggesting that this provides the symmetry-breaking step for halotropism. Asymmetric PIN2 internalisation eventually causes asymmetric auxin fluxes, coinciding with higher auxin signalling, and thus less growth, in the opposite epidermal flank [2] (Figure 1). PIN protein internalisation largely depends on clathrin-coated vesicle formation [14] and the authors provide evidence that halotropism specifically triggers PIN2 endocytosis via clathrin-dependent mechanisms. A possible scenario points at regulatory mechanisms involving phospholipids in the plasma membrane (Figure 1). Salt-induced activation of a phospholipase D eventually catalyzes the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). PA plays a regulatory role in clathrin-mediated endocytosis [15] and could link salt perception with PIN2 internalisation. Interestingly, salt induces PIN2 internalisation, but not its lytic degradation. Moreover, salt could even block gravity-induced PIN2 degradation. This finding indicates the existence of a cross-talk between components of the salinity- and the gravity-sensing pathways, possibly consisting of inhibitory signals. This study did not assess salt-induced changes in amyloplast content or its potential importance for halotropism. Nevertheless, it is likely that both the salt effect on statolith [13] and asymmetric PIN2 internalisation are required for halotropism. The initial
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reduction in statolith function might at least partially suppress gravitropic root growth. This weakened response could be further attenuated by the symmetry-breaking PIN2 internalisation, allowing for growth response away from salt and gravity. Amyloplast degradation has also been observed in seedlings subjected to drought stimulation [16]. Under certain conditions, roots neglect gravity and orient their growth towards moisture (hydrotropism) [17]. Also here the ‘commanding’ role of gravitropism has to be challenged. Kobayashi and coworkers isolated in 2007 a hydrotropism-defective miz1 mutant that shows normal gravitropic competence [18]. Based on this, one could think of a scenario where gravitropism, instead of being commanding, represents a ‘default’ pathway. Interestingly, the MIZ1 promoter is in the root tip only active in columella cells [18], suggesting once more a central role for these gravity-sensing cells in root architecture [19]. Remarkably, the miz1 mutant also shows generally wavier root growth and defects in phototropism [18]. This phenotype indicates extensive cross-talk or common mediators in at least some distinct tropism pathways. In this light, it would be interesting to assess how miz1 mutants perform in salt gradients. Moreover, phospholipase D has been not only linked to salt response, but also to other abiotic stresses, such as drought [20], and a potential role in hydrotropism remains to be tested. In conclusion, this pioneering work on halotropism broadened our
mechanistic understanding of how auxin flow in columella and epidermal cells gets instructed to circumvent salty clashes, but it remains to be seen whether distinct tropisms utilize common molecular mechanisms to distract plants from gravitropism. References 1. Darwin, C. (1880). The Power of Movement in Plants (Stuttgart, Germany: Darwins gesammelte Werke, Schweizer-bart’sche Verlagsbuchhandlung), Bd. 13. 2. Galva´n-Ampudia, C.S., Julkowska, M.M., Darwish, E., Gandullo, J., Korver, R.A., Brunoud, G., Haring, M.A., Munnik, T., Vernoux, T., and Testerink, C. (2013). Halotropism is a response of plant roots to avoid a saline environment. Curr. Biol. 23, 2044–2050. 3. Toyota, M., and Gilroy, S. (2013). Gravitropism and mechanical signaling in plants. Am. J. Bot. 100, 111–125. 4. Spalding, E.P. (2013). Diverting the downhill flow of auxin to steer growth during tropisms. Am. J. Bot. 100, 203–214. 5. Baldwin, K.L., Strohm, A.K., and Masson, P.H. (2013). Gravity sensing and signal transduction in vascular plant primary roots. Am. J. Bot. 100, 126–142. 6. Christie, J.M., and Murphy, A.S. (2013). Shoot phototropism in higher plants: new light through old concepts. Am. J. Bot. 100, 35–46. 7. Sauer, M., Robert, S., and Kleine-Vehn, J. (2013). Auxin: simply complicated. J. Exp. Bot. 64, 2565–2577. 8. Kleine-Vehn, J., Ding, Z., Jones, A.R., Tasaka, M., Morita, M.T., and Friml, J. (2010). Gravity-induced PIN transcytosis for polarization of auxin fluxes in gravity-sensing root cells. Proc. Natl. Acad. Sci. USA 107, 22344–22349. 9. Blancaflor, E.B., Fasano, J.M., and Gilroy, S. (1998). Mapping the functional roles of cap cells in the response of Arabidopsis primary roots to gravity. Plant Physiol. 116, 213–222. 10. Abas, L., Benjamins, R., Malenica, N., niewska, J., Paciorek, T., Wis Moulinier-Anzola, J.C., Sieberer, T., Friml, J., and Luschnig, C. (2006). Intracellular trafficking and proteolysis of the Arabidopsis auxin-efflux facilitator PIN2 are involved in root gravitropism. Nat. Cell Biol. 8, 249–256. 11. Baster, P., Robert, S., Kleine-Vehn, J., Vanneste, S., Kania, U., Grunewald, W., De Rybel, B., Beeckman, T., and Friml, J. (2013). SCF(TIR1/AFB)-auxin signalling regulates PIN
Gustatory Receptors: Not Just for Good Taste A recent study has found that a Drosophila gustatory receptor is required for thermotaxis. With other fly gustatory receptors having been shown to act in the detection of CO2, nutrients in the brain, and light, the roles of the so-called ‘gustatory receptors’ clearly go way beyond peripheral detection of non-volatile chemicals. Craig Montell There is a long and celebrated tradition of using fanciful, even irreverent names for Drosophila genes; some of my
favorites include methuselah, rutabaga, van gogh and cheap date. This custom is not simply to show off the uncanny humor that abounds in the Drosophila research community. The
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vacuolar trafficking and auxin fluxes during root gravitropism. EMBO J. 32, 260–274. Zazı´malova´, E., Murphy, A.S., Yang, H., Hoyerova´, K., and Hosek, P. (2010). Auxin transporters–why so many? Cold Spring Harb. Perspect. Biol. 2, a001552. Sun, F., Zhang, W., Hu, H., Li, B., Wang, Y., Zhao, Y., Li, K., Liu, M., and Li, X. (2008). Salt modulates gravity signaling pathway to regulate growth direction of primary roots in Arabidopsis. Plant Physiol. 146, 178–188. Dhonukshe, P., Aniento, F., Hwang, I., Robinson, D.G., Mravec, J., Stierhof, Y.D., and Friml, J. (2007). Clathrin-mediated constitutive endocytosis of PIN auxin efflux carriers in Arabidopsis. Curr. Biol. 17, 520–527. McLoughlin, F., Arisz, S.A., Dekker, H.L., Kramer, G., de Koster, C.G., Haring, M.A., Munnik, T., and Testerink, C. (2013). Identification of novel candidate phosphatidic acid-binding proteins involved in the salt-stress response of Arabidopsis thaliana roots. Biochem. J. 450, 573–581. Takahashi, N., Yamazaki, Y., Kobayashi, A., Higashitani, A., and Takahashi, H. (2003). Hydrotropism interacts with gravitropism by degrading amyloplasts in seedling roots of Arabidopsis and radish. Plant Physiol. 132, 805–810. Jaffe, M.J., Takahashi, H., and Biro, R.L. (1985). A pea mutant for the study of hydrotropism in roots. Science 230, 445–447. Kobayashi, A., Takahashi, A., Kakimoto, Y., Miyazawa, Y., Fujii, N., Higashitani, A., and Takahashi, H. (2007). A gene essential for hydrotropism in roots. Proc. Natl. Acad. Sci. USA 104, 4724–4729. Rosquete, M.R., von Wangenheim, D., Marhavy´, P., Barbez, E., Stelzer, E.H., Benkova´, E., Maizel, A., and Kleine-Vehn, J. (2013). An auxin transport mechanism restricts positive orthogravitropism in lateral roots. Curr. Biol. 23, 817–822. Taniguchi, Y.Y., Taniguchi, M., Tsuge, T., Oka, A., and Aoyama, T. (2010). Involvement of Arabidopsis thaliana phospholipase Dzeta2 in root hydrotropism through the suppression of root gravitropism. Planta 231, 491–497.
Department of Applied Genetics and Cell Biology, University of Natural Resources and Life Sciences, Vienna (BOKU), 1190 Vienna, Austria. E-mail:
[email protected]
http://dx.doi.org/10.1016/j.cub.2013.08.020
somewhat cryptic names seem less short-sighted when additional, unexpected gene functions are uncovered. Unfortunately, it is not always practical to apply levity to nomenclature. A case in point is the collection of 60 Drosophila ‘gustatory receptor’ genes encoding 68 proteins, which were identified on the basis of sequence homology [1–3]. The first few members of this gene family that were molecularly characterized appeared to be expressed exclusively in gustatory receptor neurons, so the family name (‘gustatory receptors’) made sense. Indeed, many of the gustatory receptor
