Update on Phosphate Stress in Legumes Genomic and Genetic Control of Phosphate Stress in Legumes 1

Phosphorus (P), an essential element for growth and development, is taken up by plants as phosphate (Pi), but Pi is unevenly distributed and relatively immobile in soils. As a result, more than 30% of the world’s arable land requires the application of P fertilizers for cropping (Vance et al., 2003). Unfortunately, P fertilizers are manufactured from nonrenewable resources that are increasingly becoming more costly and less available. Current estimates indicate that easily mined rock Pi reserves could easily be depleted by 2060 (Steen, 1997; Vance, 2001; Vance et al., 2003). Paradoxically, part of the applied P in intensive cropping systems can enter the waterways through runoff and erosion, contributing to pollution of surrounding lakes and marine environments. Improving P acquisition and use by crops is critical to economical and environmentally friendly crop agriculture. Plants have evolved a variety of adaptive strategies to improve their acquisition, use, and remobilization of P (Vance et al., 2003; Hammond et al., 2004; Lambers et al., 2006). Plant responses to P stress conditions involve changes in root morphology and architecture (Lynch, 1995; Liao et al., 2001; Lynch and Brown, 2001; Yan et al., 2004; Beebe et al., 2006; Hill et al., 2006; Ochoa et al., 2006), as well as changes in shoot and flower development (Bucciarelli et al., 2006). Among the legumes, white lupine (Lupinus albus), common bean (Phaseolus vulgaris), and to a lesser extent barrel medic (Medicago truncatula) and soybean (Glycine max) have been the focus of P stress research. White lupine, a nonmycorrhizal species, is adaptable to scarce P and displays a highly synchronous suite of molecular and biochemical adaptations to P stress by developing proteoid (cluster) roots, increasing organic acid exudation, and enhancing the expression of many genes, such as secreted acid phosphatase (LaSAP1) and Pi transporters (LaPT1; Vance, 2001; Uhde-Stone et al., 2003a, 2003b; Vance et al., 2003, and refs. therein). Common bean is the most important food legume worldwide, and genetic variability for the capacity to produce grain in low soil P conditions has been documented (Broughton et al., 2003; Ochoa et al., 2006). Moreover, several thousand ESTs derived from P-stressed common bean roots have been characterized (Ramı́rez et al., 2005). Noteworthy in an accompanying article in this legume focus issue, Hernández et al. (2007) have completed a P stress root transcriptome survey in common bean, identifying some 125 genes responsive to P stress. Recent studies of barrel medic, a model legume for plant biology research, showed that P stress delayed: (1) leaf development and leaf expansion along the main and axillary shoots; (2) axillary shoot emergence and elongation, resulting in stunted plants; and (3) timing and frequency of flower emergence (Bucciarelli et al., 2006). P-stressed barrel medic also formed shorter petioles and shorter blade lengths relative to plants in P-sufficient conditions. Whether or not morphological changes seen in P-starved barrel medic plants are attributable to an overall delay in whole plant development or as a P stress response remains to be seen. However, the lack of a standardized approach to describe plant growth and phenotypic responses to P stress (Bucciarelli et al., 2006), together with the plastic nature of plant morphological traits (Beebe et al., 2006; Ochoa et al., 2006), makes result comparisons from different laboratories difficult. Because of the subterranean nature of growth, plant roots have been recalcitrant to phenotypic study. Root adaptations to P limitations include reduced extension of primary roots, highly branched roots with increased lateral roots, and an increased density of root hair formation. Consistent with a general stress response by plants, however, P-stressed plants tend to allocate a greater proportion of biomass to root dry matter compared to P-sufficient plants (López-Bucio et al., 2002, 2003; Hammond et al., 2004; Hill et al., 2006). Most pasture species studied showed reduced total root mass as a response to P stress conditions. On average, a 32% to 86% reduction in root mass was observed in 1 This work was supported by the U.S. Department of Agriculture Agricultural Research Service (CRIS no. 3640–21000–024–00D, ‘‘Functional Genomics for Improving Nutrient Acquisition and Use in Legumes’’) and by a U.S. Department of Agriculture Agricultural Research Service/University of Minnesota specific cooperative agreement (no. 58–3640–4–107, ‘‘Lupinus, Phaseolus and Medicago Genomics of Plant Nutrition’’). * Corresponding author; e-mail vance004@umn.edu; fax 651–649– 5058. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Carroll P. Vance (vance004@umn.edu). www.plantphysiol. org/cgi/doi/10.1104/pp.107. 097386