SUMMARYRoot system architecture responds plastically to some abiotic stresses, including phosphorus (P), iron (Fe) and water deficiency, but its response mechanism is still unclear. We cloned and characterized a vegetative b-expansin gene, GmEXPB2, from a Pi starvation-induced soybean cDNA library. Transient expression of 35S::GmEXPB2-GFP in onion epidermal cells verified that GmEXPB2 is a secretory protein located on the cell wall. GmEXPB2 was found to be primarily expressed in roots, and was highly induced by Pi starvation, and the induction pattern was confirmed by GUS staining in transgenic soybean hairy roots. Results from intact soybean composite plants either over-expressing GmEXPB2 or containing knockdown constructs, showed that GmEXPB2 is involved in hairy root elongation, and subsequently affects plant growth and P uptake, especially at low P levels. The results from a heterogeneous transformation system indicated that overexpressing GmEXPB2 in Arabidopsis increased root cell division and elongation, and enhanced plant growth and P uptake at both low and high P levels. Furthermore, we found that, in addition to Pi starvation, GmEXPB2 was also induced by Fe and mild water deficiencies. Taken together, our results suggest that GmEXPB2 is a critical root b-expansin gene that is intrinsically involved in root system architecture responses to some abiotic stresses, including P, Fe and water deficiency. In the case of Pi starvation responses, GmEXPB2 may enhance both P efficiency and P responsiveness by regulating adaptive changes of the root system architecture. This finding has great agricultural potential for improving crop P uptake on both low-P and P-fertilized soils.

Low soil P availability is a primary constraint to common bean (Phaseolus vulgaris L.) production in Latin America and Africa. Substantial genotypic variation in bean adaptation to low phosphorus (LP) availability has been linked with root traits that enhance the efficiency of soil foraging. The objectives of this study were to identify quantitative trait loci (QTLs) for P accumulation and associated root architectural traits, to facilitate genetic improvement and to reveal physiological relationships. Eighty-six F 5.7 recombinant inbred lines (RILs) were developed from a cross between G19833, an Andean landrace with high total P accumulation, and DOR 364, a Mesoamerican cultivar with low total P accumulation in LP conditions. A genetic map constructed with restriction fragment length polymorphisms (RFLPs), microsatellites, and PCR-based markers covering 1703 centimorgans (cM) total genetic distance and all eleven linkage groups (LGs) was used for QTL analysis. Seventy-one RILs were evaluated in the field at high phosphorus (HP) and LP for P accumulation, total root length (RL), specific RL, and plant dry weight (DW), while all 86 RILs were evaluated in a hydroponic system in the greenhouse for tap, basal, total, and specific RL and plant DW. Phosphorus accumulation in the field correlated with root parameters measured in the greenhouse. A total of 26 individual QTLs were identified for P accumulation and associated root characters using composite interval mapping (CIM) analysis. Phosphorus accumulation QTLs often coincided with those for basal root development, thus, basal roots appear to be important in P acquisition. Independent QTLs were identified for basal and taproot development, and for specific RL. Distinct QTLs for greater specific RL had positive, null and negative effects on P accumulation. Our results confirm the importance of root structure for LP adaptation and highlight the need for a more detailed understanding of root architectural traits for phenotypic as well as marker-aided selection of more P-efficient crops.Abbreviations: AFLP, amplified fragment length polymorphism; CIM, composite interval mapping; cM, centimorgan; DW, dry weight; HP, high phosphorus; LG, linkage group; LOD, base 10 algorithm of the likelihood ratio; LP, low phosphorus; QTL, quantitative trait locus; R 2 , proportion of variance explained by QTL at test site; RL, root length; SCAR, sequence characterized amplified region; RAPD, randomly amplified polymorphic DNA; RFLP, restriction fragment length polymorphism; RIL, recombinant inbred line; TR 2 , proportion of variance explained for the QTL and the background markers; TSP, triple super phosphate.

(J.S., L.V.K.) Aluminum (Al) toxicity and phosphorus (P) deficiency often coexist in acid soils that severely limit crop growth and production, including soybean (Glycine max). Understanding the physiological mechanisms relating to plant Al and P interactions should help facilitate the development of more Al-tolerant and/or P-efficient crops. In this study, both homogeneous and heterogeneous nutrient solution experiments were conducted to study the effects of Al and P interactions on soybean root growth and root organic acid exudation. In the homogenous solution experiments with a uniform Al and P distribution in the bulk solution, P addition significantly increased Al tolerance in four soybean genotypes differing in P efficiency. The two P-efficient genotypes appeared to be more Al tolerant than the two P-inefficient genotypes under these high-P conditions. Analysis of root exudates indicated Al toxicity induced citrate exudation, P deficiency triggered oxalate exudation, and malate release was induced by both treatments. To more closely mimic low-P acid soils where P deficiency and Al toxicity are often much greater in the lower soil horizons, a divided root chamber/nutrient solution approach was employed to impose elevated P conditions in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. Under these conditions, we found that the two P-efficient genotypes were more Al tolerant during the early stages of the experiment than the P-inefficient lines. Although the same three organic acids were exuded by roots in the divided chamber experiments, their exudation patterns were different from those in the homogeneous solution system. The two P-efficient genotypes secreted more malate from the taproot tip, suggesting that improved P nutrition may enhance exudation of organic acids in the root regions dealing with the greatest Al toxicity, thus enhancing Al tolerance. These findings demonstrate that P efficiency may play a role in Al tolerance in soybean. Phosphorus-efficient genotypes may be able to enhance Al tolerance not only through direct Al-P interactions but also through indirect interactions associated with stimulated exudation of different Al-chelating organic acids in specific roots and root regions.