Annual Plant Reviews (2015) 48, 289–336 http://onlinelibrary.wiley.com doi: 10.1002/9781118958841.ch11 Chapter 11 METABOLIC ADAPTATIONS OF THE NON-MYCOTROPHIC PROTEACEAE TO SOILS WITH LOW PHOSPHORUS AVAILABILITY Hans Lambers,1 Peta L. Clode,2 Heidi-Jayne Hawkins,3 Etienne Laliberte,´ 1,4 Rafael S. Oliveira,1,5 Paul Reddell,6 Michael W. Shane,1 Mark Stitt7 and Peter Weston8 1School of Plant Biology, University of Western Australia, Crawley (Perth), Australia 2Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley (Perth), Australia 3Department of Biology, University of Cape Town, South Africa; Conservation South Africa, Centre for Biodiversity Conservation, Kirstenbosch National Botanical Gardens, Claremont, South Africa 4Institut de Recherche en Biologie Veg´ etale,´ UniversitedeMontr´ eal,´ Montreal,´ Canada 5Departamento de Biologia Vegetal, Instituto de Biologia, Universidade Estadual de Campinas, Sao˜ Paulo, Brazil 6EcoBiotics Ltd, Yungaburra, Queensland, Australia 7Max Planck Institute of Molecular Plant Physiology, Potsdam-Golm, Germany 8Royal Botanic Gardens and Domain Trust, Sydney, New South Wales, Australia Abstract: Proteaceae are almost all non-mycorrhizal and most species produce pro- teoid (= cluster) roots when grown in low-phosphorus (P) soils. In south-western Australia and the Cape Floristic Region of South Africa, Proteaceae have diversi- fied more than anywhere else, and occur on the most severely P-impoverished soils in the landscape. Several traits related to their P nutrition account for the success of south-western Australian Proteaceae on P-impoverished soils: (i) a P-acquisition strategy based on carboxylate release from ephemeral cluster roots, which allows the species to ‘mine’ P that is ‘sorbed’ to soil particles; (ii) efficient use of P in Annual Plant Reviews Volume 48: Phosphorus Metabolism in Plants, First Edition. Edited by William C. Plaxton and Hans Lambers. C⃝2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd. 289 290 Phosphorus Metabolism in Plants photosynthesis, based on a very low investment in ribosomal RNA, extensive re- placement of phospholipids by lipids that do not contain P, and allocation of P to photosynthetic cells and not epidermal cells; (iii) a very high P-remobilisation efficiency; and (iv) a high seed P content. Proteaceae in southern South Amer- ica do have a P-acquisition strategy based on carboxylate release, but lack the other P-efficiency traits. They occur on soils that contain vast amounts ofP,but with a very low P availability, and invest less biomass in cluster roots. However, these ephemeral structures live somewhat longer and release far more carboxy- lates when compared with Proteaceae from south-western Australia. The various aspects of P nutrition in Proteaceae across the world are discussed in a phyloge- netic context. Keywords: Cluster roots, delayed greening, phospholipids, phosphorus- acquisition efficiency, photosynthetic phosphorus-use efficiency, phosphorus toxi- city, ribosomal RNA, seed phosphorus. 11.1 Introduction The flowering plant family Proteaceae is considered of Gondwanan age,with a fossil record that dates back to the mid-Cretaceous, 120–90 million years ago (Johnson & Briggs, 1975; Dettmann & Jarzen, 1998; Doyle & Donoghue, 1992; Sauquet et al., 2009a), when the major Gondwanan continental blocks were still connected (Ali & Krause, 2011). Both, modern and fossil lineages are asso- ciated with all southern continents, but the present distribution of Proteaceae can only partly be explained by the break-up of Gondwana. Indeed, some sister groups post-date the break-up of Gondwana, indicating transoceanic dispersal (Barker et al., 2007). Proteaceae diversified faster in both the Cape Floristic Region of Southern Africa and in south-western Australia than in any other area (Sauquet et al., 2009a). These regions are severely nutrient- impoverished (Lambers et al., 2010), and Sauquet and coworkers (2009a) spec- ulated that higher soil fertility in other regions may have led to local extinc- tions of Proteaceae, partly due to strong competition for soil resources with species from other families. Shifts in climate are probably also involved. For example, large-scale extinctions of a hyperdiverse sclerophyll flora in east- ern Australia occurred when the climate in this region markedly changed from a high-rainfall summer-wet climate in the Early Pleistocene to a much drier climate, whereas the climate was more stable in south-western Aus- tralia (Sniderman et al., 2013), at least along its coastal margin (Wyrwoll et al., 2014). The high diversity of Proteaceae in both south-western Aus- tralia (about 700 species) and the Cape Floristic Region in South Africa (>350 species) is likely the result of rapid speciation or slow rates of extinction, determined by low soil fertility, habitat fragmentation, and a relatively stable climate (Hopper, 2009; Sniderman et al., 2013). In the northern hemisphere, about 110 species of Proteaceae occur in southern India, Sri Lanka, Japan, south-east Asia, northern South America, northern Africa, and Central Metabolic adaptations of the non-mycotrophic Proteaceae 291 America (Venkata Rao, 1967; Pate et al., 2001; Weston, 2007). Since little phys- iological and biochemical research has been conducted on those taxa, this chapter focuses primarily on species of Proteaceae from the southern hemi- sphere, some of which have been studied in great detail. Proteaceae are basal eudicots, belonging to the Proteales, which also includes the sister group of the Proteaceae, the Platanaceae (including the London plane tree), and sister to those two families, the Nelumbonaceae (including the sacred lotus) (Gu et al., 2013). However, despite their phylo- genetic proximity, the Platanaceae do not share many of the traits that are typical of the Proteaceae and which will be explored in this chapter. In par- ticular, most species of Proteaceae are non-mycorrhizal (Brundrett, 2002), whereas Platanus forms symbiotic associations with arbuscular mycorrhizal fungi (Pope, 1980; Tisserant et al., 1996). Nelumbo is probably not mycorrhizal, despite records to the contrary (Koul et al., 2012), which appear to have relied on insufficiently rigorous criteria for recognising the presence of arbuscu- lar mycorrhizas (see Brundrett, 2009). Given its aquatic habitat, Nelumbo is unlikely to be the host of mycorrhizal fungi, which typically require aerobic conditions (Smith & Read, 2008). Almost all species of Proteaceae produce specialised ‘proteoid’ or cluster roots when P is limiting (Figure 11.1; Purnell, 1960; Shane & Lambers, 2005a), but cluster roots have never been found in either Platanaceae or Nelumbonaceae. The functioning of these cluster roots is discussed next. 11.2 Phosphorus nutrition of Proteaceae, with a focus on south-western Australia 11.2.1 Phosphorus acquisition by non-mycorrhizal roots: cluster roots Cluster roots were first noted on a species of Proteaceae growing in the botan- ical gardens in Leipzig, Germany (cited in Purnell, 1960). The first detailed study on proteoid roots was by Purnell (1960), who coined the term ‘proteoid’ roots, being “ ... dense clusters of rootlets of limited growth along the lateral roots of many members of the family Proteaceae”. Since then, the term ‘clus- ter roots’ has been used for similar structures on root systems in several other families (Dinkelaker et al., 1995; Lamont, 1981; Shane & Lambers, 2005a). Phosphorus (P) deprivation generally favours lateral over primary root growth, especially in nutrient-rich soil patches, a response termed ‘topsoil foraging’ (Lynch & Brown, 2001). Likewise, cluster-root formation is most pronounced when plants are grown at a very low supply of P (Figure 11.1a, b), and the formation of cluster roots is suppressed when plants are grown at a higher P supply (Lamont, 1972; Shane et al., 2003; Zu´ niga-Feest˜ et al., 2010). Clusters may comprise a single ‘simple’ cluster root, as in Australian Hakea and the southern South American Embothrium species (Figures 11.1a, c, e, f), or 292 Phosphorus Metabolism in Plants Figure 11.1 Cluster roots of Proteaceae species from plants grown at ≤1 μM phosphorus (P) in nutrient solution (a–c, f) or P-impoverished soils (d, e). (a) Compound cluster roots of the south-western Australian Banksia repens. (b) Simple cluster roots of south-western Australian Hakea prostrata. (c) Time course of development in H. prostrata from rootlet initiation at day 1 (far left) to senescence at day 20 (far right); that is, stages I and II are immature, stage III is mature, stage IV is senescing. (d) Claviform cluster root of the southern South American Gevuina avellana showing club-like tips of mature rootles. (e,f) Simple cluster roots of the southern South American Embothrium coccineum grown in porous pumice (e) or hydroponically (f). The cluster roots of only one Proteaceae species, Gevuina avellana, rootlets develop unusual club-shaped tips (d), referred to as claviform cluster roots (Ramirez et al., 2004). Photographs a–c, courtesy of Michael W. Shane; photographs d–f, courtesy of Michael W. Shane and A Zu´niga-Feest.˜ Metabolic adaptations of the non-mycotrophic Proteaceae 293 500 4000 Total surface area of rootlets Number of rootlets 400 3000 root axis −1 300 2000 root axis) −1 cm 200 2 Surface area (mm 1000 Number of rootlets cm 100 0 0 I II III IV Stage of cluster root development Figure 11.2 Rootlet number and total surface area per cm of main root axis for simple cluster roots of Hakea prostrata. All data are mean ± standard error (n = 5). Each developmental stage is labelled according to the number of days following rootlet emergence observed at day 1 until senescence at day 20, as shown in Figure 11.1c. M.W. Shane, unpublished data. a ‘compound’ cluster root, as in Australian Banksia species (Figure 11.1a). The formation of clusters increases the root mass ratio (i.e. root mass as a fraction of total plant mass), while proliferation at a density of about 300 rootlets per cm main root axis in, for example, Hakea prostrata (harsh hakea) results in an approximately fivefold increase in root surface area (Figure 11.2).