Loss of Responsiveness to Osmotic Stress in Maize Root:the Effect of Water Channel Blocker HgCl_2

In order to investigate the effect of water channel blocker HgCl2 on the hydraulic resistance in roots of maize seedlings, a xylem pressure probe was used to monitor the changes in root xylem pressure in response to NaCl- or mannitol-induced osmotic stresses before and after the application of HgCl2. When the maize roots were subjected to 500 umol L-1 HgCl2 in root bathing solution, not only a considerable decline in xylem pressure （increase in xylem tension） was observed, but the loss of responsiveness of the plant to both salt- and mannitol-induced osmotic stresses in terms of xylem pressure change was seen as well when the transpiration rate of the plant was not significantly changed. The results are similar but different from the reversed osmosis by the Fenton reaction in the internodes of Chara coralline, showing that the mechanisms of water transport across cell membrane in plant roots are far more complicated than expected.

Effects of NaCl and Ca^2+on Membrane Potential of Epidermal Cells of Maize Roots

The effects of salt-stress on plants involve not only the water stress caused by low osmotic pressure, but also the toxicity of excess Na^＋. A large amount of Na^＋ entering cells would reduce K^＋ uptake, which leads to an imbalance of K：Na ratio in cells. One of the reasons for the reduced K^＋-uptake is the closure of K^＋-channel which is controlled by membrane potential. Calcium is usually applied to improve the growth of plants on saline soils and shows positive influence in the integrality of cell membrane. This study applied glass microelectrode technique to monitoring the NaCl-induced changes of membrane potential of root epidermal cells of maize （Zea mays L., Denghai 11） seedlings at NaCl concentrations of 0, 8, 20, 50, 100, 200 mmol L^-1, respectively. The effect of Ca^2＋ on the changes of membrane potential caused by NaCl was also studied. The results showed that： NaCl caused cell membrane depolarization. The depolarization became greater and faster with increasing of NaCl concentration. Moreover, the extent of depolarization was positively correlated with NaCl concentration. The addition of calcium postponed the depolarization, and decreased the degree of depolarization caused by NaCl. High NaCl concentration leads to depolarization of maize root cell membrane, which can partly be counteracted by calcium.

Characterization and expression analysis of a novel RING-HC gene,ZmRHCP1,involved in brace root development and abiotic stress responses in maize

RING is a really interesting new gene which plays important regulatory roles in many developmental processes as well as in plant-environment interactions. In the present report, the Zm RHCP1 gene encoding a putative RING-HC protein was isolated from maize and characterized. The Zm RHCP1 protein contained 310 amino acid residues with a conserved RINGHC zinc-finger motif and two transmembrane（TM） domains. Zm RHCP1 was expressed ubiquitously in various organs（root, stem, leaf, seedling, immature ear, and tassel）, but its transcript levels were higher in vegetative organs than in reproductive organs. Moreover, the expression pattern of Zm RHCP1 in brace roots indicated that Zm RHCP1 functions in brace root initiation. In addition, Zm RHCP1 expression was regulated by abiotic stresses. The expression results suggested that Zm RHCP1 plays important roles in brace root development and abiotic stress responses. The findings of the present study provide important information to help us understand the function of Zm RHCP1 in maize.

Ammonium-dependent regulation of ammonium transporter ZmAMT1s expression conferred by glutamine levels in roots of maize

In maize,two root epidermis-expressed ammonium transporters ZmAMT1;1a and ZmAMT1;3 play major roles in highaffinity ammonium uptake.However,the transcriptional regulation of ZmAMT1s in roots for ensuring optimal ammonium acquisition remains largely unknown.Here,using a split root system we showed that ZmAMT1;1a and ZmAMT1;3transcript levels were induced by localized ammonium supply to nitrogen-deficient roots.This enhanced expression of Zm AMT1s correlated with increases in ^(15)NH_(4)^(+)influx rates and tissue glutamine concentrations in roots.When ammonium was supplied together with methionine sulfoximine,an inhibitor of glutamine synthase,ammonium-induced expression of ZmAMT1s disappeared,suggesting that glutamine rather than ammonium itself regulated ZmAMT1s expression.When glutamine was supplied to nitrogen-deficient roots,expression levels of ZmAMT1s were enhanced,and negative feedback regulation could subsequently occur by supply of glutamine at a high level.Thus,our results indicated an ammonium-dependent regulation of ZmAMT1s at transcript levels,and a dual role of glutamine was suggested in the regulation of ammonium uptake in maize roots.

Genetic dissection of maize seedling root system architecture traits using an ultra-high density bin-map and a recombinant inbred line population

Maize（Zea mays） root system architecture（RSA）mediates the key functions of plant anchorage and acquisition of nutrients and water. In this study,a set of 204 recombinant inbred lines（RILs） was derived from the widely adapted Chinese hybrid ZD958（Zheng58 Chang7-2）,genotyped by sequencing（GBS） and evaluated as seedlings for 24 RSA related traits divided into primary,seminal and total root classes. Signi ficant differences between the means of the parental phenotypes were detected for 18 traits,and extensive transgressive segregation in the RIL population was observed for all traits. Moderate to strong relationships among the traits were discovered. A total of 62 quantitative trait loci（QTL） were identi fied that individually explained from1.6% to 11.6%（total root dry weight/total seedling shoot dry weight） of the phenotypic variation. Eighteen,24 and 20 QTL were identi fied for primary,seminal and total root classes of traits,respectively. We found hotspots of 5,3,4 and 12 QTL in maize chromosome bins 2.06,3.02-03,9.02-04,and 9.05-06,respectively,implicating the presence of root gene clusters or pleiotropic effects. These results characterized the phenotypic variation and genetic architecture of seedling RSA in a population derived from a successful maize hybrid.

Mapping the vertical distribution of maize roots in China in relation to climate and soil texture

Aims Optimizing water and fertilizer management for crops requires an understanding of root distribution.Maize(Zea mays L.)is currently the most widely planted cereal crop in China,yet the vertical dis-tribution of maize roots across different regions remains unknown.The aims of this work were(i)to quantify the effects of climate and soil texture on the vertical distribution of maize roots,and(ii)to show the depth distribution of root biomass in China.Methods We used data of maize root biomass from 11 Chinese ecological stations with discontinuous observations from 2004 to 2014 to fit the regression coefficientβfor an asymptotic equation Y=1-βd,where d is the soil depth and Y is the proportion of root biomass from the surface to depth d.A statistical model was then developed to quantify the effects of climate and soil texture on the fittedβval-ues.Using the statistical model,we map the depth distribution of maize root biomass in China.Important Findings Maize root biomass in the 0-100 cm soil depth varied by an order of magnitude at different stations,from 64 to 268 g m−2.Maize planted in sandy soils and/or maize with high accumu-lated temperature for development had higher root biomass and deeper rooting systems.The fittedβvalues ranged from 0.785 to 0.977,which can be modeled by an integration of the accu-mulated temperature during the maize growing period and the soil clay and sand fractions(R2=0.66,n=50,P<0.001).Up to 82%of maize planting regions in China showed shallower rooting systems where more than 90%and 95%of the root bio-mass occurred in the top 20 and 30 cm soil layers,respectively.Deeper rooting systems occurred in some temperate arid and temperate semi-arid regions,with less than 80%of the root bio-mass in the top 20 cm soil.Our findings highlighted the vertical distribution of maize roots,and underlined the spatial variability in the vertical distribution of roots across China’s planting areas of maize.

Root architecture

Numerous research publications over the past 20 years have made it quite clear that a better understanding of the molecular and genetic basis for variation in root system architecture（RSA）will greatly aid the development of crop varieties with improved and more ef ficient nutrient and water acquisition under limiting conditions.

Early Transcriptomic Adaptation to Na_2CO_3 Stress Altered the Expression of a Quarter of the Total Genes in the Maize Genome and Exhibited Shared and Distinctive Profles with NaCl and High pH Stresses

Sodium carbonate （Na2CO3） presents a huge challenge to plants by the combined damaging effects of Na＋, high pH, and CO32. Little is known about the cellular responses to Na2CO3 stress. In this study, the transcriptome of maize （Zea mays L. cv. B73） roots exposed to Na2CO3 stress for 5 h was compared with those of NaCI and NaOH stresses. The expression of 8,319 genes, representing over a quarter of the total number of genes in the maize genome, was altered by Na2CO3 stress, and the downregulated genes （5,232） outnumbered the upregulated genes （3,087）. The effects of Na2CO3 differed from those of NaCI and NaOH, primarily by downregulating different categories of genes. Pathways commonly altered by Na2CO3, NaCI, and NaOH were enriched in phenylpropanoid biosynthesis, oxidation of unsaturated fatty acids, ATP- binding cassette （ABC） transporters, as well as the metabolism of secondary metabolites. Genes for brassinosteroid biosynthesis were specifically upregulated by Na2CO3, while genes involved in ascorbate and aldarate metabolism, protein processing in the endoplasmic reticulum and by N-glycosylation, fatty acid biosynthesis, and the circadian rhythm were downregulated. This work provides the first holistic picture of early transcriptomic adaptation to Na2CO3 stress, and highlights potential molecular pathways that could be manipulated to improve tolerance in maize.