Genetic dissection of N use efficiency using maize inbred lines and testcrosses

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 highdensity 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 Gln1-3 was 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.

Evaluation of maize root growth and genome-wide association studies of root traits in response to low nitrogen supply at seedling emergence

Nitrogen(N) deficiency is one of the main factors limiting maize(Zea mays L.) 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.

A Critical Role of AMT2;1 in Root-To-Shoot Translocation of Ammonium in Arabidopsis

Ammonium uptake in plant roots is mediated by AMT/MEP/Rh-type ammonium transporters. Out of five AMTs being expressed in Arabidopsis roots, four AMT1-type transporters contribute to ammonium uptake, whereas no physiological function has so far been assigned to the only homolog belonging to the MEP subfamily, AMT2;1. Based on the observation that under ammonium supply, the transcript levels of AMT2;1 increased and its promoter activity shifted preferentially to the pericycle, we assessed the contribution of AMT2;1 to xylem loading. When exposed to ^15N-labeled ammonium, amt2;1 mutant lines translocated less tracer to the shoots and contained less ammonium in the xylem sap. Moreover, in an amtl;1 amtl;2 amtl ;3 amt2;1 quadruple mutant （qko）, co-expression of AMT2;1 with either AMT1;2 or AMT1;3 significantly enhanced ^15N translocation to shoots, indicating a cooperative action between AMT2;1 and AMT1 transporters. Under N deficiency, proAMT2;1-GFP lines showed enhanced promoter activity predominantly in cortical root cells, which coincided with elevated ammonium influx conferred by AMT2;1 at millimolar sub- strate concentrations. Our results indicate that in addition to contributing moderately to root uptake in the low-affinity range, AMT2;1 functions mainly in root-to-shoot translocation of ammonium, depending on its Cell-type-specific expression in response to the plant nutritional status and to local ammonium gradients.

Transporter-Mediated Nuclear Entry of Jasmonoyl-lsoleucine Is Essential for Jasmonate Signaling

To control gene expression by directly responding to hormone concentrations, both animal and plant cells have exploited comparable mechanisms to sense small-molecule hormones in nucleus. Whether nuclear entry of these hormones is actively transported or passively diffused, as conventionally postulated, through the nuclear pore complex, remains enigmatic. Here, we identified and characterized a jasmonate transporter in Arabidopsis thaliana, AtJAT1/AtABCG16, which exhibits an unexpected dual localization at the nuclear envelope and plasma membrane. We show that AtJAT1/AtABCG16 controls the cytoplasmic and nuclear partition of jasmonate phytohormones by mediating both cellular efflux of jasmonic acid （JA） and nuclear influx of jasmonoyl-isoleucine （JA-Ile）, and is essential for maintaining a critical nuclear JA-Ile concentration to activate JA signaling. These results illustrate that transporter-mediated nuclear entry of small hormone molecules is a new mechanism to regulate nuclear hormone signaling. Our findings provide an avenue to develop pharmaceutical agents targeting the nuclear entry of small molecules.

Importers Drive Leaf-to-Leaf Jasmonic Acid Transmission in Wound-Induced Systemic Immunity

The transmission of mobile wound signals along the phloem pathway is essential to the activation of wound-induced systemic response/resistance,which requires an upsurge of jasmonic acid(JA)in the distal undamaged leaves.Among these mobile signals,the electrical signal mediated by the glutamate-dependent activation of several clade three GLUTAMATE RECEPTOR-LIKE(GLR3)proteins is involved in the stimulation of JA production in distal leaves.However,whether JA acts as a mobile wound signal and,if so,how it is transmitted and interacts with the electrical signal remain unclear.Here,we show that JA was translocated from the local to distal leaves in Arabidopsis,and this process was predominantly regulated by two phloem-expressed and plasma membrane-localized jasmonate transporters,AtJAT3 and AtJAT4.In addition to the cooperation between AtJAT3/4 and GLR3.3 in the regulation of long-distance JA translocation,our findings indicate that importer-mediated cell-cell JA transport is important for driving the loading and translocation of JA in the phloem pathway in a self-propagating manner.

Enhancing phosphorus uptake efficiency through QTL-based selection for root system architecture in maize

Root system architecture （RSA） plays an important role in phosphorus （P） acquisition, but enhancing P use efficiency （PUE） in maize via genetic 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-bin3.04a and 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-bin3.04a or Cl-bin3.04b that displayed mean increases of 22%-26% in PUE in LP fields. Furthermore, a line L224 pyramiding Cl- binB.04a and Cl-bin3.04b showed 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.

A NEW APPROACH TO HOLISTIC NITROGEN MANAGEMENT IN CHINA

Since the 1980s,the widespread use of N fertilizer has not only resulted in a strong increase in agricultural productivity but also caused a number of environmental problems,induced by excess reactive N emissions.A range of approaches to improve N management for increased agricultural production together with reduced environmental impacts has been proposed.The 4R principles(right product,right amount,right time and right place)for N fertilizer application have been essential for improving crop productivity and N use efficiency while reducing N losses.For example,site-specific N management(as part of 4R practice)reduced N fertilizer use by 32%and increased yield by 5%in China.However,it has not been enough to overcome the challenge of producing more food with reduced impact on the environment and health.This paper proposes a new framework of food-chainnitrogen-management(FCNM).This involves good N management including the recycling of organic manures,optimized crop and animal production and improved human diets,with the aim of maximizing resource use efficiency and minimizing environmental emissions.FCNM could meet future challenges for food demand,resource sustainability and environmental safety,key issues for green agricultural transformation in China and other countries.

Innovations of phosphorus sustainability:implications for the whole chain

Phosphorus(P)is a non-renewable resource,therefore ensuring global food and environmental security depends upon sustainable P management.To achieve this goal,sustainable P management in the upstream and downstream sectors of agriculture from mineral extraction to food consumption must be addressed systematically.The innovation and feasibility of P sustainability are highlighted from the perspective of the whole P-based chain,including the mining and processing of P rock,production of P fertilizers,soil and rhizosphere processes involving P,absorption and utilization of P by plants,P in livestock production,as well as flow and management of P at the catchment scale.The paper also emphasizes the importance of recycling P and the current challenges of P recovery.Finally,sustainable solutions of holistic P management are proposed from the perspective of technology improvement with policy support.

Use of genotype-environment interactions to elucidate the pattern of maize root plasticity to nitrogen deficiency

Maize（Zea mays L.） root morphology exhibits a high degree of phenotypic plasticity to nitrogen（N） de ficiency,but the underlying genetic architecture remains to be investigated Using an advanced BC_4F_3 population,we investigated the root growth plasticity under two contrasted N levels and identi fied the quantitative trait loci（QTLs） with QTL-environment（Q×E）interaction effects. Principal components analysis（PCA） on changes of root traits to N de ficiency（D LN-HN） showed that root length and biomass contributed for 45.8% in the same magnitude and direction on the first PC,while root traits scattered highly on PC_2 and PC_3. Hierarchical cluster analysis on traits for D LN-HN further assigned the BC_4F_3 lines into six groups,in which the special phenotypic responses to N de ficiency was presented These results revealed the complicated root plasticity of maize in response to N de ficiency that can be caused by genotype environment（G×E） interactions. Furthermore,QTL mapping using a multi-environment analysis identi fied 35 QTLs for root traits. Nine of these QTLs exhibited signi ficant Q×E interaction effects. Taken together,our findings contribute to understanding the phenotypic and genotypic pattern of root plasticity to N de ficiency,which will be useful for developing maize tolerance cultivars to N de ficiency.

Improving the efficiency and effectiveness of global phosphorus use: focus on root and rhizosphere levels in the agronomic system

Phosphorus(P)is essential for life and for efficient crop production,but global P use with limited recycling is inefficient in several sectors,including agronomy.Unfortunately,plant physiologists,agronomists,farmers and end users employ different measures for P use efficiency(PUE),which often masks their values at different scales.The term P use effectiveness,which also considers energetic and sustainability measures in addition to P balances,is also a valuable concept.Major physiological and genetic factors for plant P uptake and utilization have been identified,but there has been limited success in genetically improving PUE of modern crop cultivars.In maize,studies on root architectural and morphological traits appear promising.Rhizosphere processes assist in mobilizing and capturing sparingly soluble phosphate from rock phosphate.Combinations of phosphate-solubilizing microorganisms with ammonium-based nitrogen fertilizer,as well as strategies of fertilizer placement near the roots of target crops,can moderately enhance PUE.The desired concentration of P in the products differs,depending on the final use of the crop products as feed,food or for energy conversion,which should be considered during crop production.

Highlights of special issue on “Sustainable Phosphorus Use in Agri-Food System”

Phosphorus(P)is an essential element for plants,and all other life on Earth including humans.However,P is a nonrenewable resource with a very uneven distribution in the world.In the agri-food system,P is supplied to agricultural land as chemical P fertilizer and/or animal manure for producing food,feed and fiber,followed by their use or consumption by animals and humans.Despite some returns of P in crop production as organic manure,large amounts of P are released into the environment,severely contributing to pollution or eutrophication(mainly surface waters like rivers,lakes and oceans).Therefore,how to close the P cycle in the agri-food system has become a major challenge for ensuring food and environmental security globally.This special issue focuses on the sustainable use of P in agriculture,and comprises 12 review and research articles covering a wide range of topics on P resources,P use in crop and animal systems,P recycling and eco-environment.

Evolving technologies for growing,imaging and analyzing 3D root system architecture of crop plants

A plant＇s ability to maintain or improve its yield under limiting conditions,such as nutrient de ficiency or drought,can be strongly in fluenced by root system architecture（RSA）,the three-dimensional distribution of the different root types in the soil. The ability to image,track and quantify these root system attributes in a dynamic fashion is a useful tool in assessing desirable genetic and physiological root traits. Recent advances in imaging technology and phenotyping software have resulted in substantive progress in describing and quantifying RSA. We have designed a hydroponic growth system which retains the three-dimensional RSA of the plant root system,while allowing for aeration,solution replenishment and the imposition of nutrient treatments,as well as high-quality imaging of the root system. The simplicity and flexibility of the system allows for modi fications tailored to the RSA of different crop species and improved throughput. This paper details the recent improvements and innovations in our root growth and imaging system which allows for greater image sensitivity（detection of fine roots and other root details）,higher ef ficiency,and a broad array of growing conditions for plants that more closely mimic those found under field conditions.

HIGHLIGHTS OF THE SPECIAL ISSUE “PROGRESS ON NITROGEN RESEARCH FROM SOIL TO PLANT AND TO THE ENVIRONMENT”

Nitrogen(N)is an essential nutrient for plants,animals and humans,being a key element in proteins.Rapid increases of global population thus require more N inputs to soil to improve food production.However,N losses induced environmental problems,due to inefficient N use by crops and animals,are threatening our environment via soil acidification,eutrophication and water pollution,air pollution and biodiversity loss/health impacts and by greenhouse gas emissions and climate change.Therefore,humans are facing a huge challenge on how to balance the N requirement(to meet the food demand)and the N restriction(to avoid environmental damage).This includes the need for improving the N use efficiency(NUE),as one pillar to realize green development.This special issue summarizes recent progress on N research at different scales:from soil to plant and to the environment,and includes 15 review and research articles,distinguishing four topics,related to(1)N use and turnover(3 articles),(2)N management(4 articles),(3)N impacts on environment and climate(5 articles),and(4)mitigation approaches to improve NUE and reduce N losses(3 articles).

Highlights of special issue on“Sustainable Phosphorus Use in Agri-Food System”

The original version of this article unfortunately contained a project duration mistake.The correction is given below:The duration“2009-2011”(Page 312,line 4)of Sino-German research collaboration project should be“2008-2012”。