BackgroundRice is the world's most important cereal crop and phosphorus (P) and zinc (Zn) deficiency are major constraints to its production. Where fertilizer is applied to overcome these nutritional constraints it comes at substantial cost to farmers and the efficiency of fertilizer use is low. Breeding crops that are efficient at acquiring P and Zn from native soil reserves or fertilizer sources has been advocated as a cost-effective solution, but would benefit from knowledge of genes and mechanisms that confer enhanced uptake of these nutrients by roots.ScopeThis review discusses root traits that have been linked to P and Zn uptake in rice, including traits that increase mobilization of P/Zn from soils, increase the volume of soil explored by roots or root surface area to recapture solubilized nutrients, enhance the rate of P/Zn uptake across the root membrane, and whole-plant traits that affect root growth and nutrient capture. In particular, this review focuses on the potential for these traits to be exploited through breeding programmes to produce nutrient-efficient crop cultivars.ConclusionsFew root traits have so far been used successfully in plant breeding for enhanced P and Zn uptake in rice or any other crop. Insufficient genotypic variation for traits or the failure to enhance nutrient uptake under realistic field conditions are likely reasons for the limited success. More emphasis is needed on field studies in mapping populations or association panels to identify those traits and underlying genes that are able to enhance nutrient acquisition beyond the level already present in most cultivars.

Keywords 16Organic matter, microbial phosphorus, mineralisation, phosphate, soil phosphorus pools 17 18 19 1 2 to enable accurate predictions of the required external P inputs to achieve optimum growth of 46 subsequent crops. While our understanding of soil inorganic phosphate (Pi) pools is relatively 47 comprehensive, the value of P returned to the soil in crop residues has not been fully resolved. 48 Agronomically significant amounts of P can be present in crop residues and the microbial biomass 49 associated with their decomposition, and the potential contribution of this pool to the P nutrition of 50 cropping systems is significant (eg. Chauhan et al., 1979;Dalal, 1979;White and Ayoub, 1983; Thibaud 51 et al., 1988;Umrit and Friesen, 1994;Kwabiah et al., 2003a;Nachimuthu et al., 2009). The main factors 52 influencing the amount of crop residue P, its rate of mineralisation and subsequent availability to crops 53 have been identified (Stockdale and Brookes, 2006;Guppy and McLaughlin, 2009;; 54 but their interactions remain poorly elucidated and largely unquantified. By reviewing the published 55 literature in which quantitative measurements of P transformations from plant residues applied to soil 56 have been reported, we will evaluate the contribution of crop residue-derived P to the P nutrition of 57 subsequent crops, assess the key factors involved and summarise the knowledge as an empirical model. 58 59The dynamics of organically-derived nitrogen (N) and carbon (C) in agricultural soils has been extensively 60 described, and a wide range of predictive tools have been developed. These have proved a valuable 61 asset for landholders, agronomists and policy makers by providing good estimates of the impacts of 62 agronomic management options on the dynamics of both C (eg. Parton et al., 1988; Coleman and 63 Jenkinson, 1999;Grace et al., 2006) and N (see Herridge et al., 2008) in agricultural soils. Considering 64 our extensive knowledge of the N cycle in agricultural systems, and the benefits (economic, social and 65 environmental) that have been obtained by our ability to predict and manipulate it, similar knowledge 66 of the organic P cycle could also yield significant benefits. Yet, although the principal driving factors of 67 organic P cycling have long been recognised and modelled (Cole et al., 1977), models have not proven 68 to be universally applicable (Gijsman et al., 1996;Schnepf et al., 2011). Several models have 69 demonstrated a capacity to incorporate P release from crop residues and manures into projected crop 70 growth and yield, notably The Agricultural Production Systems Simulation (APSIM) (Keating et al., 2003), 71 3 Century (Parton et al., 1988) and CERES-Wheat (Ritchie et al., 1988;Godwin et al., 1989; Singh et al., 72 1991;Daroub et al., 2003) modelling frameworks. However, these models require detailed climate and 73 site information that may not be available, and are specialised tools that cannot be operated by the 74 layperson. The contribution of crop residue P to the nutriti...

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Root traits such as root angle and hair length influence resource acquisition particularly for immobile nutrients like phosphorus (P). Here, we attempted to modify root angle in rice by disrupting the OsAUX1 auxin influx transporter gene in an effort to improve rice P acquisition efficiency. We show by X-ray microCT imaging that root angle is altered in the osaux1 mutant, causing preferential foraging in the top soil where P normally accumulates, yet surprisingly, P acquisition efficiency does not improve. Through closer investigation, we reveal that OsAUX1 also promotes root hair elongation in response to P limitation. Reporter studies reveal that auxin response increases in the root hair zone in low P environments. We demonstrate that OsAUX1 functions to mobilize auxin from the root apex to the differentiation zone where this signal promotes hair elongation when roots encounter low external P. We conclude that auxin and OsAUX1 play key roles in promoting root foraging for P in rice.

Summary• Zinc (Zn)-deficient soils constrain rice (Oryza sativa) production and cause Zn malnutrition. The identification of Zn-deficiency-tolerant rice lines indicates that breeding might overcome these constraints. Here, we seek to identify processes underlying Zn-deficiency tolerance in rice at the physiological and transcriptional levels.• A Zn-deficiency-tolerant line RIL46 acquires Zn more efficiently and produces more biomass than its nontolerant maternal line (IR74) at low [Zn] ext under field conditions. We tested if this was the result of increased expression of Zn 2+ transporters; increased root exudation of deoxymugineic acid (DMA) or low-molecularweight organic acids (LMWOAs); and ⁄ or increased root production. Experiments were performed in field and controlled environment conditions.• There was little genotypic variation in transcript abundance of Zn-responsive root Zn 2+ -transporters between the RIL46 and IR74. However, root exudation of DMA and LMWOA was greater in RIL46, coinciding with increased root expression of putative ligand-efflux genes. Adventitious root production was maintained in RIL46 at low [Zn] ext , correlating with altered expression of root-specific auxinresponsive genes.• Zinc-deficiency tolerance in RIL46 is most likely the result of maintenance of root growth, increased efflux of Zn ligands, and increased uptake of Zn-ligand complexes at low [Zn] ext ; these traits are potential breeding targets.

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Given the non-renewable nature of global phosphate reserves, there is a push to increase the phosphorus (P) efficiency of agricultural crops. Research has typically focussed on investigating P acquisition efficiency or internal P utilization efficiency to reduce crop fertilizer requirements. A novel option that would reduce the amount of P exported from fields at harvest, and may ultimately reduce P fertilizer requirements, would be to reduce the amount of P translocated to grains to minimize grain P concentrations. While such a trait has been mentioned in a number of studies over the years, there has not been a concerted effort to target this trait in breeding programs. In this perspective piece we explore the reasons why a low grain P trait has not been pursued, and discuss the potential benefits and drawbacks of such a trait in the context of breeding to improve the P efficiency of cropping systems.

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