Role of Root Clusters in Phosphorus Acquisition and Increasing Biological Diversity in Agriculture

Abstract

Abstract. Soils in the south-west of Western Australia and South Africa are among the most phosphorusimpoverished in the world, and at the same time both of these regions are Global Biodiversity Hotspots. This unique combination offers an excellent opportunity to study root adaptations that are significant in phosphorus (P) acquisition. A large proportion of species from these P-poor environments cannot produce an association with mycorrhizal fungi, but, instead, produce 'root clusters'. In Western Australia, rootcluster-bearing Proteaceae occur on the most P-impoverished soils, whereas the mycorrhizal Myrtaceae tend to inhabit the less P-impoverished soils in this region. Root clusters are an adaptation both in structure and in functioning; characterized by high densities of short lateral roots that release large amounts of exudates, in particular carboxylates (anions of di-and tri-carboxylic acids). The functioning of root clusters in Proteaceae ('proteoid' roots) and Fabaceae ('cluster' roots) has received considerable attention, but that of 'dauciform' root clusters developed by species in Cyperaceae has barely been explored. Research on the physiology of 'capillaroid' root clusters formed by species in Restionaceae has yet to be published. Root-cluster initiation and growth in species of the Cyperaceae, Fabaceae and Proteaceae are systemically stimulated when plants are grown at a very low P supply, and are suppressed as leaf P concentrations increase. Root clusters in Proteaceae, Fabaceae and Cyperaceae are short-lived structures, which release large amounts of carboxylates, briefly, at a particular stage of root development. The rates of carboxylate release are considerably faster than reported for non-specialized roots of a wide range of species. Root clusters play a pivotal role in mobilization of P from P-sorbing soil. Because the world P reserves are being depleted whilst vast amounts of P are stored in fertilized soils, there is a growing need for crops with a high efficiency of P acquisition. Some Australian and African native species as well as some existing crops have traits that would be highly desirable for future crops. The possibilities of introducing P-acquisition-efficient species in new cropping and pasture systems are explored. In addition, possible strategies to introduce traits associated with a high P-acquisition efficiency into future crop species are discussed.

Plant adaptations to severely phosphorus-impoverished soils

“…Sandy soils in south-western Australia, for example, contain little total P and plant-available P; a range of carboxylate-releasing P-mining strategies have evolved in different plant families in different parts of the world, including species that are used as crop plants for food or fibre production [64]. The specialised cluster-root structures in south-western Australia are short-lived and release relatively large amounts of carboxylates when compared with mycorrhizal species, but not as much as those in Proteaceae from southern South America which occur on young volcanic soils [13].…”

Phosphorus Nutrition of Proteaceae in Severely Phosphorus-Impoverished Soils: Are There Lessons To Be Learned for Future Crops?

“…Species with root clusters Lambers et al, 2006) are relatively more abundant than mycorrhizal species on the most P-impoverished soils within old landscapes (Lambers et al, 2008bBrundrett, 2009). Even though the majority of crop and forest species are mycorrhizal, some do form cluster roots, such as Macadamia integrifolia (macadamia nut), Aspalathus linearis (rooibos tea), Casuarina cunninghamiana (sheoak), Gevuina avellana, and Lupinus albus (white lupin); most of these species are nonmycorrhizal, but Casuarina species have both cluster roots and mycorrhizas (Halloy et al, 1996;Lambers and Shane, 2007).…”

Plant Phosphate Nutrition and Environmental Challenges

“…Modification of RSA enable plant roots to explore the upper parts of the soil, a strategy described as 'topsoil foraging' [57]. Symbiotic associations with fungi (Vesicular-Arbuscular Mycorrhizae; see below) and formation of cluster roots are adaptive responses to increase Pi uptake in many plants which allow competent exploration of soils for fixed Pi [58][59][60][61][62][63].…”

“…There are over 200 P minerals existing on the earth but only a few can be used for commercial extraction of Pi [61]. Phosphate Rocks (PR) is the commercial term applied to all Pi bearing minerals suitable for Pi production.…”

“…Such examples are very easy to find in the literatures dated early 19 th to 20 th centuries when the application of Pi fertilizers were becoming dominant. The resultant high yield is accompanied by Pi removal of 6 kg/ha for cereals and 20 Kg/ha for other crops from soil [61].…”