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Appropriate root system architecture (RSA) can improve maize yields in densely planted fields, but little is known about its genetic basis in maize. Here we performed root phenotyping of 14,301 field-grown plants from an association mapping panel to study the genetic architecture of maize RSA. A genome-wide association study identified 81 high-confidence RSA-associated candidate genes and revealed that 28 (24.3%) of known root-related genes were selected during maize domestication and improvement. We found that modern maize breeding has selected for a steeply angled root system. Favourable alleles related to steep root system angle have continuously accumulated over the course of modern breeding, and our data pinpoint the root-related genes that have been selected in different breeding eras. We confirm that two auxin-related genes, ZmRSA3.1and ZmRSA3.2, contribute to the regulation of root angle and depth in maize. Our genome-wide identification of RSA-associated genes provides new strategies and genetic resources for breeding maize suitable for high-density planting.

Nitrogen (N) deficiency is one of the main factors limiting maize (Zea maysL.) productivity. Genetic improvement of root traits could improve nitrogen use efficiency. An association panel of 461 maize inbred lines was assayed for root growth at seedling emergence under high-nitrate (HN, 5 mmol L−1) and low-nitrate (LN, 0.05 mmol L−1) conditions. Twenty-one root traits and three shoot traits were measured. Under LN conditions, the root-to-shoot ratio, root dry weight, total root length, axial root length, and lateral root length on the primary root were all increased. Under LN conditions, the heritability of the plant traits ranged from 0.43 to 0.82, a range much wider than that of 0.27 to 0.55 observed under HN conditions. The panel was genotyped with 542,796 high-density single-nucleotide polymorphism (SNP) markers. Totally 328 significant SNP markers were identified using either mixed linear model (MLM) or general linear model analysis, with 34 detected by both methods. In the 100-kb intervals flanking these SNP markers, four candidate genes were identified. Under LN conditions, the protoporphyrinogen IX oxidase 2 gene was associated with total root surface area and the DELLA protein-encoding gene was associated with the length of the visible lateral root zone of the primary root. Under HN conditions, a histone deacetylase gene was associated with plant height. Under both LN and HN conditions, the gene encoding MA3 domain-containing protein was associated with the first whorl crown root number. The phenotypic and genetic information from this study may be exploited for genetic improvement of root traits aimed at increasing NUE in maize.

High yield and efficient use of nitrogen is one of the major target for maize breeding worldwide. Both yield potential and nitrogen use efficiency (NUE) of maize is lower in China compared to that in the US. In this study, we analyzed the change of the plant traits related to high yield and NUE of maize cultivars registered and planted in China and US during the past 40 decades. Compared to the cultivar released in the US, Chinese cultivar are less tolerant to high planting density, higher ratio of ear height to plant height, low grain filling rate, more stay-green and longer growth period. We suggest an ideal plant architecture of maize for high yield and NUE. To achieve high yield and high NUE under intensive production conditions (medium and high nitrogen input, high density planting), an ideotype plant architecture is proposed which includes compact plant architecture, low ratio of ear height to plant height (around 0.36–0.38, close to the golden rate.), medium ear, small tassel, medium root size, one-layer brace root, half stay-green, efficient N remobilization, fast grain filling rate etc. The parameters can be utilized to guide future breeding.

Root system architecture (RSA) plays an important role in phosphorus (P) acquisition, but enhancing P use efficiency (PUE) in maize viagenetic manipulation of RSA has not yet been reported. Here, using a maize recombinant inbred line (RIL) population, we investigated the genetic relationships between PUE and RSA, and developed P-efficient lines by selection of quantitative trait loci (QTLs) that coincide for both traits. In low-P (LP) fields, P uptake efficiency (PupE) was more closely correlated with PUE (r = 0.48–0.54), and RSA in hydroponics was significantly related to PupE (r = 0.25–0.30) but not to P utilization efficiency (PutE). QTL analysis detected a chromosome region where two QTLs for PUE, three for PupE and three for RSA were assigned into two QTL clusters, Cl-bin3.04aand Cl-bin3.04b. These QTLs had favorable effects from alleles derived from the large-rooted and high-PupE parent. Marker-assisted selection (MAS) identified nine advanced backcross-derived lines carrying Cl-bin3.04aor Cl-bin3.04bthat displayed mean increases of 22%–26% in PUE in LP fields. Furthermore, a line L224 pyramiding Cl-bin3.04aand Cl-bin3.04bshowed enhanced PupE, relying mainly on changes in root morphology, rather than root physiology, under both hydroponic and field conditions. These results highlight the physiological and genetic contributions of RSA to maize PupE, and provide a successful study case of developing P-efficient crops through QTL-based selection.

Although the use of heterosis in maize breeding has increased crop productivity, the genetic causes underlying heterosis for nitrogen (N) use efficiency (NUE) have been insufficiently investigated. In this study, five N-response traits and five low-N-tolerance traits were investigated using two inbred line populations (ILs) consisting of recombinant inbred lines (RIL) and advanced backcross (ABL) populations, derived from crossing Ye478 with Wu312. Both populations were crossed with P178 to construct two testcross populations. IL populations, their testcross populations, and the midparent heterosis (MPH) for NUE were investigated. Kernel weight, kernel number, and kernel number per row were sensitive to N level and ILs showed higher N response than did the testcross populations. Based on a high-density linkage map, 138 quantitative trait loci (QTL) were mapped, each explaining 5.6%–38.8% of genetic variation. There were 52, 34 and 52 QTL for IL populations, MPH, and testcross populations, respectively. The finding that 7.6% of QTL were common to the ILs and their testcross populations and that 11.7% were common to the MPH and testcross population indicated that heterosis for NUE traits was regulated by non-additive and non-dominant loci. A QTL on chromosome 5 explained 27% of genetic variation in all of the traits and Gln1-3was identified as a candidate gene for this QTL. Genome-wide prediction of NUE traits in the testcross populations showed 14%–51% accuracy. Our results may be useful for clarifying the genetic basis of heterosis for NUE traits and the candidate gene may be used for genetic improvement of maize NUE.

During the grain‐filling period in maize (Zea maysL.), increasing photosynthesis per unit plant N, namely photosynthetic N‐use efficiency (PNUE) is a potential way to increase N‐utilization efficiency (NUtE). This study aimed to understand whether the spatial distribution of PNUE within the canopy is optimized under low N conditions as a method of maximizing canopy productivity. Maize plants were grown in the field for 2 yr with three N levels (0, 180, 240 kg N ha−1). Changes in specific leaf N (SLN), net photosynthetic rate (PN), PNUE (calculated as PN/SLN), stomatal conductance (gs), green leaf area (GLA), and light interception of leaves at different positions were measured every 10 d after silking (DAS). Significant differences in SLN, PN, gs, and PNUE occurred among leaves at different positions: upper leaf ≥ ear leaf > lower leaf. A positive correlation was found between the spatial difference in gsand that in PNUE and in PN. Compared with the average of N180 and N240 across 2 yr, grain yield (GY) decreased by 56% but physiological NUtE increased by 22% without N (N0). Correspondingly, PNUE increased by 44, 25, and 35% in the lower, ear, and upper leaves, respectively. Light interception by the middle leaves was unchanged with N0. We concluded that the vertical distribution of PNUE was optimized with the light distribution pattern in the canopy. The greater whole‐plant NUtE we found with N0 may be explained by the overall increase in PNUE in the leaves and the continued interception of light by middle leaves.

The size of roots and their physiological activity during the grain‐filling stage affect water and nutrient uptake, and grain yield (GY) in maize (Zea maysL.). The aim of this study was to determine the effects of different N levels on postanthesis root senescence in field‐grown maize. Three N levels (0, 120, 240 kg N ha−1) was applied to field‐grown maize, and the length and weight of roots in the 0‐ to 40‐cm soil layer, nitrogenous compounds in xylem sap, N uptake, dry matter (DM), N accumulation, and GY were analyzed. Shoot N accumulation, but not grain yield, was higher in the N240 treatment than in the N120 treatment. At the silking stage, the root length was longest in the N0 treatment and shortest in the N240 treatment. This was because of the high root/shoot (R/S) and root C/N ratios in N0. Total root length decreased throughout the grain‐filling period, mainly because of the reduced length of early developed nodal roots. The root length decreased by 27% in the N0 treatment, to a smaller extent compared to that in N120 (52%) and N240 (51%). The amounts of NO3−–N, NH4+–N and amino acid‐N in the xylem sap were highest in the N240 treatment during the entire grain‐filling stage, suggesting that a high N supply increased root N‐uptake activity. Based on these results, we conclude that high root N‐uptake activity, not delayed root senescence, resulted in the large accumulation of N in the shoot under excess N.

Breeding high-yielding and nutrient-efficient cultivars is one strategy to simultaneously resolve the problems of food security, resource shortage, and environmental pollution. However, the potential increased yield and reduction in fertilizer input achievable by using high-yielding and nutrient-efficient cultivars is unclear. In the present study, we evaluated the yield and nitrogen use efficiency (NUE) of 40 commercial maize hybrids at five locations in North and Northeast China in 2008 and 2009. The effect of interaction between genotype and nitrogen (N) input on maize yield was significant when the yield reduction under low-N treatment was 25%–60%. Based on the average yields achieved with high or low N application, the tested cultivars were classified into four types based on their NUE: efficient-efficient (EE) were efficient under both low and high N inputs, high-N efficient (HNE) under only high N input, low-N efficient (LNE) under only low N input, and nonefficient-nonefficient under neither low nor high N inputs. Under high N application, EE and HNE cultivars could potentially increase maize yield by 8%-10% and reduce N input by 16%–21%. Under low N application, LNE cultivars could potentially increase maize yield by 12%. We concluded that breeding for N-efficient cultivars is a feasible strategy to increase maize yield and/or reduce N input.

Interspecific root/rhizosphere interactions affect phosphorus (P) uptake and the productivity of maize/faba bean and maize/wheat intercropping systems. The aim of these experiments was to determine whether manipulation of maize root growth could improve the productivity of the two intercropping systems. Two near isogenic maize hybrids (the larger-rooted T149 and smaller-rooted T222) were intercropped with faba bean and wheat, under conditions of high- and low-P availability. The larger-rooted T149 showed greater competitive ability than the smaller-rooted T222 in both maize/faba bean and maize/wheat intercropping systems. The higher competitive ability of T149 improved the productivity of the maize/faba bean intercropping system in P-sufficient conditions. In maize/wheat intercropping systems, root growth, shoot biomass, and P uptake of maize were inhibited by wheat, regardless of the P-supply. Compared with T222, the larger-rooted T149 suffered less in the intercropping systems. The total biomass of the maize/wheat intercropping system was higher for wheat/T149 than for wheat/T222 under low-P conditions. These data suggested that genetic improvement of maize root size could enhance maize growth and its ability to compete for P resources in maize/faba bean and maize/wheat intercropping systems. In addition, depending on the P availability, larger maize roots could increase the productivity of intercropping systems.

Root growth has a fundamental role in nitrogen (N) use efficiency. Nevertheless, little is known about how modern breeding progress has affected root growth and its responses to N supply. The root and shoot growth of a core set of 11 representative Chinese maize (Zea maysL.) hybrids released between 1973 and 2009 were investigated under high N (4 mmol L−1, HN) and low N (0.04 mmol L−1, LN) levels in a solution culture system. Compared with LN, HN treatment decreased root dry weight (RDW), the root: shoot ratio (R/S), and the relative growth rate for root dry weight (RGRroot), but increased the total root length (TRL) and the total lateral root length (LRL). The total axial root length (ARL) per plant was reduced under HN, mostly in hybrids released before the 1990s. The number of seminal roots (SRN) was largely unaffected by different N levels. More recently released hybrids showed higher relative growth rates in the shoot under both HN and LN. However, the roots only showed increased RGR under HN treatment. Correspondingly, there was a positive linear relationship with the year of hybrid release for TRL, LRL and ARL under HN treatment. Together, these results suggest that while shoot growth of maize has improved, its root growth has only improved under high N conditions over the last 36 years of selective breeding in China. Improving root growth under LN conditions may be necessary to increase the N use efficiency of maize.

The use of nitrogen (N) fertilizers has contributed to the production of a food supply sufficient for both animals and humans despite some negative environmental impact. Sustaining food production by increasing N use efficiency in intensive cropping systems has become a major concern for scientists, environmental groups, and agricultural policymakers worldwide. In high-yielding maize systems the major method of N loss is nitrate leaching. In this review paper, the characteristic of nitrate movement in the soil, N uptake by maize as well as the regulation of root growth by soil N availability are discussed. We suggest that an ideotype root architecture for efficient N acquisition in maize should include (i) deeper roots with high activity that are able to uptake nitrate before it moves downward into deep soil; (ii) vigorous lateral root growth under high N input conditions so as to increase spatial N availability in the soil; and (iii) strong response of lateral root growth to localized nitrogen supply so as to utilize unevenly distributed nitrate especially under limited N conditions.

Feline Leukemia Virus, Subgroup C (FLVCR) is a newly found cytoplasmic heme exporter. Retroviral interference with FLVCR display results in a loss of erythroid cells after the CFU-E/proerythroblast stage of differentiation and severe anemia in cats. We studied expression of FLVCR in thalassemia with relatively redundant free cytoplasmic heme, evaluated the correlation of FLVCR expression with clinical parameters, and explore the importance of FLVCR for erythropoiesis in human cells. Samples of bone marrow from 50 children with thalassemia (n=21), other hemolytic anemia(AIHA, G6PD Deficiency, n=17) and non-anemia controls (n=12) were collected. Expression of FLVCR protein and mRNA in mononuclear red cells was quantitated in CFU-E/proerythroblasts (CD71 bright, CD117+) and later erythroid precursors (CD71 bright, CD117 negative cells) by 3 color flow cytometry and fluorescence quantitative Real-time PCR. Expression of FLVCR is higher in early precursors than later erythroid cells: Specifically, the percentage of specimens with high levels of FLVCR protein expression in proerythroblasts and the mean fluorescence intensities(MFI) of thalassemia, other hemolytic anemia and non-anemias were 78.9%, 84.2%,74.85% and 306,347, 291.5 respectively; while these values in later erythroid precursors were 8.8%,10.6%,6.9% and 150,122,119 respectively, P<0.01. Expression of FLVCR tended to be higher in proerythroblasts of thalassemia and other hemolytic anemia:Positive rate and MFI of FLVCR protein expression in thess two groups were 78.9%,306 and 84.2%,347 respectively, as compared with cells of non-anemias (74.85%,291.5), with no statistical differences, P>0.05. Expression of FLVCR is significantly higher in later erythroid precursors of thalassemia:Positive rate and MFI of FLVCR protein expression in later erythroid precursors of thalassemia were 8.8%, and 150 respectively, with significantly higher levels of expression compared with non-anemias (6.9%,119) P<0.05. Expression of FLVCR mRNA in proerythroblasts of thalassemia, other hemolytic anemia and non-anemias were 3.6,4.85,3.7 respectively; while in later erythroblast were 4.13,6.06,3.92 respectively, P<0.01. Hemolytic anemia patients with higher expression of FLVCR had higher reticulocyte count, higher percentage of later erythroblast and lower LDH, P<0.05. Thalassemic patients with higher expression of FLVCR had higher percentage of sideroblasts and serum ferritin, P<0.05. Expression of FLVCR protein was remarkably upregulated in proerythroblasts, combined with previous studies in vitro and identification of FLVCR as a human heme exporter, supporting FLVCR plays an important role in developing erythroid cells to protect them from heme toxicity. There was no significantly up-regulation of FLVCR expression in proerythroblasts of thalassemia, indicating probably other pathways exist in regulating heme metabolic balance combined with FLVCR; Expression of FLVCR in later erythroid precursors of thalassemia was higher than that of non-anemias, which implies FLVCR up-regulates with compensation, this may help counteract the ineffectiveness of late erythropoiesis that results from the poor coordination of heme and globin synthesis. The disparity between FLVCR protein and mRNA may reflect posttranscriptional regulation of protein level. Clinical results also support effect of FLVCR on erythropoiesis. FLVCR expression has close relation with iron metabolic parameters.

Feline Leukemia Virus, Subgroup C (FLVCR) is a newly found cytoplasmic heme exporter. Retroviral interference with FLVCR display results in a loss of erythroid cells after the CFU-E/proerythroblast stage of differentiation and severe anemia in cats. We studied expression of FLVCR in thalassemia with relatively redundant free cytoplasmic heme, evaluated the correlation of FLVCR expression with clinical parameters, and explore the importance of FLVCR for erythropoiesis in human cells. Samples of bone marrow from 50 children with thalassemia (n=21), other hemolytic anemia(AIHA, G6PD Deficiency, n=17) and non-anemia controls (n=12) were collected. Expression of FLVCR protein and mRNA in mononuclear red cells was quantitated in CFU-E/proerythroblasts (CD71 bright, CD117+) and later erythroid precursors (CD71 bright, CD117 negative cells) by 3 color flow cytometry and fluorescence quantitative Real-time PCR. Expression of FLVCR is higher in early precursors than later erythroid cells: Specifically, the percentage of specimens with high levels of FLVCR protein expression in proerythroblasts and the mean fluorescence intensities(MFI) of thalassemia, other hemolytic anemia and non-anemias were 78.9%, 84.2%,74.85% and 306,347, 291.5 respectively; while these values in later erythroid precursors were 8.8%,10.6%,6.9% and 150,122,119 respectively, P<0.01. Expression of FLVCR tended to be higher in proerythroblasts of thalassemia and other hemolytic anemia:Positive rate and MFI of FLVCR protein expression in thess two groups were 78.9%,306 and 84.2%,347 respectively, as compared with cells of non-anemias (74.85%,291.5), with no statistical differences, P>0.05. Expression of FLVCR is significantly higher in later erythroid precursors of thalassemia:Positive rate and MFI of FLVCR protein expression in later erythroid precursors of thalassemia were 8.8%, and 150 respectively, with significantly higher levels of expression compared with non-anemias (6.9%,119) P<0.05. Expression of FLVCR mRNA in proerythroblasts of thalassemia, other hemolytic anemia and non-anemias were 3.6,4.85,3.7 respectively; while in later erythroblast were 4.13,6.06,3.92 respectively, P<0.01. Hemolytic anemia patients with higher expression of FLVCR had higher reticulocyte count, higher percentage of later erythroblast and lower LDH, P<0.05. Thalassemic patients with higher expression of FLVCR had higher percentage of sideroblasts and serum ferritin, P<0.05. Expression of FLVCR protein was remarkably upregulated in proerythroblasts, combined with previous studies in vitro and identification of FLVCR as a human heme exporter, supporting FLVCR plays an important role in developing erythroid cells to protect them from heme toxicity. There was no significantly up-regulation of FLVCR expression in proerythroblasts of thalassemia, indicating probably other pathways exist in regulating heme metabolic balance combined with FLVCR; Expression of FLVCR in later erythroid precursors of thalassemia was higher than that of non-anemias, which implies FLVCR up-regulates with compensation, this may help counteract the ineffectiveness of late erythropoiesis that results from the poor coordination of heme and globin synthesis. The disparity between FLVCR protein and mRNA may reflect posttranscriptional regulation of protein level. Clinical results also support effect of FLVCR on erythropoiesis. FLVCR expression has close relation with iron metabolic parameters.