Update on White Lupin Cluster Root Acclimation to Phosphorus Deficiency Update on Lupin Cluster Roots

Phosphorus (P) is one of 17 essential elements (nutrients) required for plant growth (Tiessen, 2008; Cordell et al., 2009). Although bound P is quite abundant in many soils, it is largely unavailable for uptake. As such, P is frequently the most limiting element for plant growth and development. Crop yield on 40% to 60% of the world’s arable land is limited by P availability. Mined rock phosphate is the primary source of P fertilizer. Approximately 90% of all mined rock phosphate is used for agriculture (Tiessen, 2008; Cordell et al., 2009). However, rock phosphate is a nonrenewable resource (Steen, 1998; Cordell et al., 2009), and easily mined, high-quality rock phosphate sources are projected to be depleted within 30 to 50 years (Steen, 1998; Tiessen, 2008; Cordell et al., 2009). Peak P production is projected to occur in 2035 to 2040 (Cordell et al., 2009). In addition, the world’s major reserves of rock phosphate are located in geographical areas where uncertain political issues could limit access to the world’s P resources. Sustainable management of P in agriculture requires that plant biologists discover mechanisms that enhance P acquisition and exploit these adaptations to make plants more efficient at acquiring P, develop P-efficient germplasm, and advance crop management schemes that increase soil P availability. Cluster roots (Fig. 1), extremely specialized tertiary lateral root structures, are an important adaptive strategy of plants to cope with nutrient-poor, P-depleted soils (Dinkelaker et al., 1995; Neumann and Martinoia, 2002; Vance et al., 2003; Lambers et al., 2006). They are produced on plants from a diverse range of families (Dinkelaker et al., 1995; Watt and Evans, 1999; Shane and Lambers, 2005). White lupin (Lupinus albus) forms cluster roots in response to P starvation. Cluster roots are characterized as concentrated zones of tertiary lateral roots emerging in waves from secondary roots. Root hair density appears to be greater in mature cluster root zones than typical lateral roots. Such an adaptation leads to a striking increase in root surface area available for P uptake from the rhizosphere (Keerthisinghe et al., 1998; Neumann et al., 1999). Cluster root development and function involve a highly synchronous series of molecular and biochemical processes, including highly enhanced lateral root initiation, increased root hair formation, root exudation of organic acid chelators (citrate and malate), modified carbon assimilation, release of enzymes (acid phosphatase, ferric chelate reductases) into the rhizosphere, and more efficient uptake of P from the rhizosphere (Dinkelaker et al., 1989; Neumann et al., 1999; Watt and Evans, 1999; Liu et al., 2001, 2005; Miller et al., 2001; Uhde-Stone et al., 2003a, 2005; Wasaki et al., 2003). Advances have recently been made in understanding the molecular and biochemical events surrounding cluster root formation and function. As a crop, white lupin is a practical alternative to evaluate acclimation to P deficiency, particularly as related to cluster-rooted species (Johnson et al., 1996; Keerthisinghe et al., 1998; Watt and Evans, 1999; Neumann and Martinoia, 2002). Figure 1. White lupin P deficiency cluster roots emerge as waves of tertiary lateral roots along the axis of secondary roots.