Plants without arbuscular mycorrhizae
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Although mycorrhizal symbioses (described elsewhere in this volume) are the most important adaptation for angiosperms to acquire scarce phosphorus (P), many plant families contain species that either do not form or rarely form this pivotal association (Skene 1998; Miller et al. 1999; Cripps and Eddington 2005; Miller 2005). This review will address adaptations and mechanisms for acquisition and use of scarce P in plants lacking effective mycorrhizal symbioses. The primary focus will be on root adaptations in species that develop specialized-complex roots (cluster and dauciform) in response to P deficiency. Although not producing cluster or dauciform roots in response to P deficiency, Arabidopsis will also be considered because it does not form mycorrhizal symbiosis and is a model species for evaluating plant adaptation to P deficiency.
Plants have evolved two broad strategies for improved P acquisition and use in nutrient-limiting environments: (1) those aimed at conservation of use; and (2) those directed toward enhanced acquisition or uptake (Vance et al. 2003; Ticconi and Abel 2004; Misson et al. 2005; Morcuende et al. 2007). Processes that conserve the use of P involve decreased growth rate, increased growth per unit of P uptake, remobilization of internal P, modifications in carbon (C) metabolism that bypass P-requiring steps, alternative respiratory pathways, and alterations in membrane biosynthesis requiring less P (Plaxton and Carswell 1999; Uhde-Stone et al. 2003a,b; Wasaki et al. 2003; Misson et al. 2005; Lambers et al. 2006). In comparison, processes that lead to enhanced P uptake include modified root architecture and greater root growth, prolific development of root hairs leading to expanded root surface area, enhanced expression of Pi transporter genes, and increased production and exudation of phosphatases and organic acids (Marschner et al. 1986; López- Bucio et al. 2002; Shane and Lambers 2005). These numerous adaptive responses to P-deficiency are not mutually exclusive and all may occur within a single species.
KeywordsLateral Root Root Hair Plant Cell Environ Root Architecture White Lupin
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