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easily and there was a museum to help. Such circumstances are exceptional. Species also vary geographically so what might initially seem to be different species may, with extensive fieldwork, be seen to belong to continuous variation. Comparison of specimens across the world is a fundamentally hard problem logistically. Appeltans et al. [3] plot the percentage of scientific names that are valid — i.e., not synonyms. The pattern is intuitive: before 1800, the percentage of valid names was high because taxonomists were describing species for the first time. It then fell below 60% until the 1870s as the same species was given two or more different names mistakenly. The percentage of correct names rose above 80% in 1950 and has been steadily improving ever since. The fourth problem is the other side of the taxonomic coin. What is given a single name may be several very similar, or cryptic species. Only very careful inspection may separate them. Perhaps subtle ecological differences uncovered by careful fieldwork provide the clue. Increasingly, of course, genetic studies — especially cheap and easy barcoding [4] — are splitting apart many species. The fifth problem is how many species are missing, that is, still not described. Yes, there are the known unknowns — the species that have already been collected, but not yet described. Then there are the unknown unknowns — species that are still awaiting collection. At its core, the problem of cataloguing biodiversity is that the need for taxonomic and geographic breadth fights the taxonomic depth needed to solve the problems of synonymy, the extent of cryptic species, and estimating how many species wait to be described or even to be collected. These last three problems require extraordinary specialised knowledge of individual taxa. The solution is to assemble a necessarily large team that can cover all the taxa, do so globally, and in the requisite detail. Bringing together such a team is a very significant achievement. Some 80 pages of detailed analysis, taxon by taxon, in the supplemental materials form a compelling summary of existing knowledge. Much credit goes to WoRMS — the World Register of Marine Species, www.marinespecies.org. This
database grew from a few records in 2005, to nearly half a million today. Under the current chair of Mark Costello, a steering committee coordinates the projects of specialists who, in turn, produce the lists of species and assess which names are valid or otherwise. The web page reports: ‘‘The content of WoRMS is controlled by taxonomic experts... WoRMS has an editorial management system where each taxonomic group is represented by an expert who has the authority over the content, and is responsible for controlling the quality of the information. Each of these main taxonomic editors can invite several specialists of smaller groups within their area of responsibility to join them.’’ Over the entire marine data, Appeltans et al. [3] conclude that approximately 226,000 species names are valid, about 170,000 are synonyms, and 70,000 species are awaiting description in collections. Cryptic species will add to the total — and may resurrect some of the synonyms, which may be valid after all. At least a third more species may remain to be discovered. The web has now made such heroic, global compilations of diverse taxa standard. There are similar ones for many groups of terrestrial organisms [5]. Within a few years, taxonomists have become organized globally. Marine scientists are now describing species at unprecedented rates, including 20,000 species within the past decade. Yes, it matters. We are still discovering new species because they
are rare in two senses of that word [2]. They generally have small geographic ranges and are locally uncommon where they do live [6]. In the oceans as on land, recent discoveries are often in places under severe threat from human actions [7]. The most important factor in completing the taxonomic catalogue may well be that it’s shrinking through extinctions faster than we are describing species. Only if we know what we are in danger of missing, and where these species live, can we act prudently to protect biodiversity. References 1. Westwood, J. (1833). On the probable number of species of insects in the creation; together with descriptions of several minute Hymenoptera. The Magazine of Natural History and Journal of Zoology, Botany, Mineralogy. Geology and Meterology 6, 116–123. 2. Sheffers, B.R., Joppa, L.N., Pimm, S.L., and Laurance, W.F. (2012). What we know and don’t know about Earth’s missing biodiversity. Trends Ecol. Evol. 27, 501–510. 3. Appeltans, W., Ahyong, S.T., Anderson, G., Angel, M.V., Artois, T., Bailly, N., Bamber, R., Barber, A., Bartsch, I., Berta, A., et al. (2012). The magnitude of global marine species diversity. Curr. Biol. 22, 2189–2202. 4. Hebert, P.D.N., Penton, E.H., Burns, J.M., Janzen, D.H., and Hallwachs, W. (2004). Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly Astraptes fulgerator. Proc. Natl. Acad. Sci. USA 101, 14812–14817. 5. Joppa, L.N., Roberts, D.L., and Pimm, S.L. (2011). The population ecology and social behaviour of taxonomists. Trends Ecol. Evol. 26, 551–553. 6. Pimm, S.L., Jenkins, C.N., Joppa, L.N., Roberts, D.L., and Russell, G.J. (2010). How many endangered species remain to be discovered in Brazil? Natureza & Conservac¸a˜o 8, 71–77. 7. Joppa, L.N., Roberts, D.L., Myers, N., and Pimm, S.L. (2011). Biodiversity hotspots house most undiscovered plant species. Proc. Natl. Acad. Sci. USA 108, 13171–13176.
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Mycorrhizal Symbiosis: Ancient Signalling Mechanisms Co-opted Mycorrhizal root endosymbiosis is an ancient property of land plants. Two parallel studies now provide novel insight into the mechanism driving this interaction and how it is used by other filamentous microbes like pathogenic oomycetes. Rene´ Geurts1 and Vivianne G.A.A. Vleeshouwers2 The vast majority of today’s land plants is able to establish a symbiosis
between their roots and obligatory biotrophic fungi of the order Glomeromycota (so-called mycorrhizal fungi). As fossil records indicate an early Devonian origin of this symbiosis,
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Figure 1. GRAS-type proteins control endomycorrhizal signalling. Endomycorrhizal fungi (e.g. Glomus species) secrete lipo-chitooligosaccharides (LCOs) that are perceived by specific LysM-type receptors of the host plant [4,20]. Subsequent responses are mediated via the nuclear localized GRAS-type transcriptional regulators RAM1, NSP1 and NSP2 that can form specific heterodimeric complexes [5,9]. NSP1–NSP2 control the expression of DWARF27 (D27), which encodes a key enzyme in the biosynthesis of strigolactones, molecules that act as ex planta stimuli for hyphal branching [10,11]. RAM1, possibly in conjunction with NSP2, is essential for RAM2-mediated biosynthesis of cutin monomers. The latter act as elicitors of appressorium formation. Interestingly, pathogenic Phytophthora species can exploit the RAM2-dependent cutin monomer biosynthetic pathway, though in a RAM1-independent fashion [5,6].
its widespread occurrence today underlines the importance of this symbiosis for plant functioning in ecosystems [1]. The symbiotic relationship begins upon fungal contact and the formation of specialized infection structures, called ‘appressoria’, on root epidermal cells and continues with fungal penetration into the plant root. To enable nutrient exchange between both partners, the fungus forms highly branched intracellular structures called ‘arbuscules’ in inner cortical cells [2]. The fungal mass that remains in the soil dramatically increases the capacity of the plant root to access nutrients, especially immobile phosphates, which the plant receives from the fungus at the expense of carbohydrates. The advantage that the plant gains by this symbiosis is accentuated by the huge investments it is willing to make — up to 20% of its fixed carbon is transferred to the fungus [3].
Despite the ecological importance of endomycorrhizae, the molecular and genetic networks underlying this symbiosis are only partially characterized. Most in-depth insights have been obtained in legumes, especially by exploiting two model species, Medicago truncatula and Lotus japonicus. Legumes are known for their symbiotic relationship with nitrogen-fixing rhizobium bacteria. Rhizobia secrete specific lipochito-oligosaccharides (LCOs or Nod factors) that lead to the formation of root nodules, in which the bacteria are hosted intracellularly as organelle-like structures. In these organs, atmospheric dinitrogen is fixed into ammonia that can be used by the plant. Rhizobium LCOs set in motion root nodule formation and are also essential for bacterial infection. It has long been known that a substantial subset of legume mutants unable to respond to rhizobium are also impaired in
mycorrhizae symbiosis. The genes affected in these mutants encode components that are also essential to transmit rhizobium LCO-induced signalling [2], demonstrating that during evolution rhizobium co-opted the mycorrhizal signalling machinery. These results further suggest that mycorrhizal fungi produce similar LCO-type signalling molecules (named Myc factors), as was indeed recently demonstrated for Glomus intraradices [4]. Now, two novel M. truncatula endomycorrhizal mutants have been identified that are not affected in rhizobium symbiosis [5,6]. These mutants shed light on a universal mechanism that filamentous microbes use to penetrate plant tissues. The newly identified loci — named REQUIRED FOR ARBUSCULAR MYCORRHIZATION (RAM1 and RAM2) — encode a transcriptional regulator of the plant-specific GRAS class and a glycerol3-phosphate acyl-transferase (GPAT), respectively [5,6]. RAM1 is the third symbiotic GRAS transcriptional regulator identified in legumes. Previous studies identified NSP1 and NSP2 to be essential for rhizobium LCO-induced responses [7,8]. Experiments in heterologous systems revealed that NSP2 can heterodimerize with NSP1 as well as RAM1, suggesting a regulatory link between these protein complexes [5,9]. In-depth studies of nsp1 and nsp2 mutant phenotypes showed that both proteins are essential for strigolactone biosynthesis (Figure 1) [10]. Strigolactones fulfill a dual function — they can act as ex planta stimuli for mycorrhizal hyphae [11] and they can also act as plant hormones that interfere with auxin transport [12]. Although strigolactones are not essential for mycorrhization, the absence of their biosynthesis can reduce the endomycorrhizal colonization efficiency [4,10,11]. GRAS transcriptional regulators are highly conserved in plants and homologs of NSP1 and NSP2 can be found basically in all plant species [7,8]. This is in stark contrast to RAM1, which is absent in Brassicaceae species, such as Arabidopsis thaliana [5]. As Brassicaceae is one of the few plant families that have lost mycorrhizal
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symbiosis, the finding that RAM1 is lost in this family suggests a specific function of RAM1 in this interaction (Figure 1) [5]. A primary target of the RAM1 transcriptional regulator is the GPAT encoding gene RAM2 [5]. The RAM2 enzyme is functionally homologous to Arabidopsis GPAT6 — an enzyme that is essential for biosynthesis of cutin-specific monomers [6,13]. In aerial parts of the plant these hydroxylated fatty acids can esterify and form insoluble waxy polymers that are a main component of the plant’s cuticle, whereas roots generally lack a cuticle. These results suggest that the RAM2-synthesized cutin monomers have a different function. From studies on foliar pathogenic fungi it is known that cutin monomers can act as elicitors in appressorium formation [14–16]. Now, Wang and co-workers [6] show that mycorrhizae actively trigger the biosynthesis of cutin monomers in the root, and the presence of these monomers is sufficient to trigger appressorium formation (Figure 1). In analogy to foliar fungal pathogens, the authors speculate that cutin monomers act as signalling molecules that are perceived by the mycorrhizal fungus. This is a likely hypothesis; however, we speculate that stimulating appressorium formation may not be the sole function of cutin monomers in endomycorrhizal symbiosis. As RAM2 activity increases when roots become more extensively colonised [6], together with the fact that the M. truncatula ram2 mutant occasionally forms aberrant arbuscular structures [6], we argue that biosynthesis of cutin monomers is also likely to play a role at later stages of the endosymbiotic engagement. Unraveling these functions in more depth could provide novel insights in the role of cutin monomers in this plant–fungus interaction. RAM2 activity also promotes plant root infection of the pathogenic oomycete Phytophthora palmivora, though in a RAM1-independent manner (Figure 1) [6]. Oomycetes include some of the most notorious pathogens of major crops world-wide. P. palmivora, which colonizes various tissues in a broad range of host plants, shows impaired appressorium formation and decreased infection on
roots of the M. truncatula ram2 mutant [6]. In analogy to endomycorrhizal colonisation, the addition of cutin monomers leads to wild-type levels of disease symptoms. RAM2 of M. truncatula is induced in roots after P. palmivora infection, and in potato leaves, RAM2/GPAT homologs are upregulated upon infection with the late blight pathogen Phytophthora infestans [6]. Interestingly, the increased expression of RAM2/GPAT genes peaks at later stages than appressorium formation and extends beyond the early phase of P. infestans infection [17,18]. This phase is characterized by the formation of haustoria — specialized infection structures that parallel mycorrhizal arbuscules. In analogy to arbuscle formation in endomycorrhizae, it would be particularly interesting to study whether RAM2/GPAT genes also play a role during haustoria formation. Glomeromycota and oomycetes are only remotely related and both their origins predate the birth of land plants [19]. As these filamentous microbes have co-opted the same mechanism to penetrate plant roots, it underlines the occurrence of mechanistic constraints with which the invaders must cope. Unraveling these mechanisms in more depth could possibly lead to novel strategies to control a broad range of plant pathogens. References 1. Remy, W., Taylor, T.N., Hass, H., and Kerp, H. (1994). Four hundred-million-year-old vesicular arbuscular mycorrhizae. Proc. Nat. Acad. Sci. USA 91, 11841–11843. 2. Parniske, M. (2008). Arbuscular mycorrhiza: the mother of plant root endosymbioses. Nat. Rev. Microbiol. 6, 763–775. 3. Bago, B., Pfeffer, P.E., and Shachar-Hill, Y. (2000). Carbon metabolism and transport in arbuscular mycorrhizas. Plant Physiol. 124, 949–958. 4. Maillet, F., Poinsot, V., Andre´, O., Puech-Page`s, V., Haouy, A., Gueunier, M., Cromer, L., Giraudet, D., Formey, D., Niebel, A., et al. (2011). Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza. Nature 469, 58–63. 5. Gobbato, E., Marsh, J.F., Vernie´, T., Wang, E., Maillet, F., Kim, J., Miller, J.B., Sun, J., Bano, S.A., Ratet, P., et al. (2012). A GRAS-type transcription factor with a specific function in mycorrhizal signalling. Curr. Biol. 22, 2236–2241. 6. Wang, E., Schornack, S., Marsh, J., Gobbato, E., Schwessinger, B., Eastmond, P., Schultze, M., Kamoun, S., and Oldroyd, G.E.D. (2012). A common signalling process that promotes mycorrhizal and oomycete colonisation of plants. Curr. Biol. 22, 2242–2246. 7. Smit, P., Raedts, J., Portyanko, V., Debelle´, F., Gough, C., Bisseling, T., and Geurts, R. (2005). NSP1 of the GRAS protein family is essential for rhizobial Nod factor-induced transcription. Science 308, 1789–1791.
8. Kalo´, P., Gleason, C., Edwards, A., Marsh, J., Mitra, R.M., Hirsch, S., Jakab, J., Sims, S., Long, S.R., Rogers, J., et al. (2005). Nodulation signalling in legumes requires NSP2, a member of the GRAS family of transcriptional regulators. Science 308, 1786–1789. 9. Hirsch, S., Kim, J., Munoz, A., Heckmann, A.B., Downie, J.A., and Oldroyd, G.E.D. (2009). GRAS proteins form a DNA binding complex to induce gene expression during nodulation signalling in Medicago truncatula. Plant Cell 21, 545–557. 10. Liu, W., Kohlen, W., Lillo, A., Op den Camp, R., Ivanov, S., Hartog, M., Limpens, E., Jamil, M., Smaczniak, C., Kaufmann, K., et al. (2011). Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2. Plant Cell 23, 3853–3865. 11. Akiyama, K., Matsuzaki, K., and Hayashi, H. (2005). Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature 435, 824–827. 12. Domagalska, M.A., and Leyser, O. (2011). Signal integration in the control of shoot branching. Nat. Rev. Mol. Cell Bio. 12, 211–221. 13. Li-Beisson, Y., Pollard, M., Sauveplane, V., Pinot, F., Ohlrogge, J., and Beisson, F. (2009). Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester. Proc. Nat. Acad. Sci. USA 106, 22008–22013. 14. Dickman, M.B., Ha, Y.-S., Yang, Z., Adams, B., and Huang, C. (2003). A protein kinase from Colletotrichum trifolii is induced by plant cutin and Is required for appressorium formation. Mol. Plant-Microbe Interact. 16, 411–421. 15. Mendoza-Mendoza, A., Berndt, P., Djamei, A., Weise, C., Linne, U., Marahiel, M., Vranes, M., Kamper, J., and Kahmann, R. (2009). Physical-chemical plant-derived signals induce differentiation in Ustilago maydis. Mol. Microbiol. 71, 895–911. 16. Liu, W., Zhou, X., Li, G., Li, L., Kong, L., Wang, C., Zhang, H., and Xu, J.-R. (2011). Multiple plant surface signals are sensed by different mechanisms in the rice blast fungus for appressorium formation. PLOS Pathogens 7, e1001261. 17. Haas, B.J., Kamoun, S., Zody, M.C., Jiang, R.H., Handsaker, R.E., Cano, L.M., Grabherr, M., Kodira, C.D., Raffaele, S., Torto-Alalibo, T., et al. (2009). Genome sequence and analysis of the Irish potato famine pathogen Phytophthora infestans. Nature 461, 393–398. 18. Cooke, D.E., Cano, L.M., Raffaele, S., Cooke, L.R., Etherington, G., Deahl, K.L., Farrer, R.A., Gilroy, E.M., Goss, E.M., Gru¨nwald, N.J., et al. (2012). Genome analyses of an aggressive and invasive lineage of the Irish potato famine pathogen. PLoS Pathogens 8, e1002940. 19. Berbee, M.L., and Taylor, J.W. (2010). Dating the molecular clock in fungi – how close are we? Fung. Biol. Rev. 24, 1–16. 20. Op den Camp, R., Streng, A., De Mita, S., Cao, Q., Polone, E., Liu, W., Ammiraju, J.S.S., Kudrna, D., Wing, R.A., Untergasser, A., et al. (2011). LysM-type mycorrhizal receptor recruited for rhizobium symbiosis in nonlegume Parasponia. Science 331, 909–912.
1Department of Plant Science, Laboratory of Molecular Biology, 2Wageningen UR Plant Breeding, Wageningen University, Droevendaalsesteeg 1, 6709PB, Wageningen, The Netherlands. E-mail:
[email protected],
[email protected]
http://dx.doi.org/10.1016/j.cub.2012.10.021
