Lateral root elongation in maize is related to auxin synthesis and transportation mediated by N metabolism under a mixed NO_(3)^(–) and NH_(4)^(+) supply

A mixed nitrate (NO_(3)^(–)) and ammonium (NH_(4)^(+)) supply can promote root growth in maize (Zea mays),however,the changes in root morphology and the related physiological mechanism under different N forms are still unclear.Here,maize seedlings were grown hydroponically with three N supplied in three different forms (NO_(3)^(–)only,75/25 NO_(3)^(–)/NH_(4)^(+)and NH_(4)^(+)only).Compared with sole NO_(3)^(–)or NH_(4)^(+),the mixed N supply increased the total root length of maize but did not affect the number of axial roots.The main reason was the increased total lateral root length,while the average lateral root (LR) length in each axle was only slightly increased.In addition,the average LR density of 2nd whorl crown root under mixed N was also increased.Compared with sole nitrate,mixed N could improve the N metabolism of roots (such as the N influx rate,nitrate reductase (NR) and glutamine synthase (GS)enzyme activities and total amino content of the roots).Experiments with exogenously added NR and GS inhibitors suggested that the increase in the average LR length under mixed N was related to the process of N assimilation,and whether the NR mediated NO synthesis participates in this process needs further exploration.Meanwhile,an investigation of the changes in root-shoot ratio and carbon (C) concentration showed that C transportation from shoots to roots may not be the key factor in mediating lateral root elongation,and the changes in the sugar concentration in roots further proved this conclusion.Furthermore,the synthesis and transportation of auxin in axial roots may play a key role in lateral root elongation,in which the expression of ZmPIN1B and ZmPIN9 may be involved in this pathway.This study preliminarily clarified the changes in root morphology and explored the possible physiological mechanism under a mixed N supply in maize,which may provide some theoretical basis for the cultivation of crop varieties with high N efficiency.

Tolerance of Salix matsudana to Heavy Metals Determined by Root Elongation Method

[Objective]The research aimed to discuss the tolerance of Salix matsudana to single or compound heavy metals and provide theoretical basis for renovating polluted soil by heavy metals with woody plants.[Method]Using root elongation method,the effects of heavy metal Cu^2＋,Pb^2＋,Zn^2＋ and their mixed solution on the adventitious roots growth of S.matsudana cuttings were studied.[Result]The adventitious roots growth of S.matsudana cuttings was obviously affected by different concentrations of heavy metals solution.Adventitious roots of S.matsudana cuttings could not grow while the concentration of Cu^2＋ was higher than 15 mg/L,the mixture solution concentration was higher than 20 mg/L and Zn^2＋ concentration was higher than 30 mg/L.When the solution concentration reached 40 mg/L,adventitious roots of S.matsudana cuttings could grow only in Pb^2＋ treatment group.With the increasing of the solution concentration,the number of adventitious roots of S.matsudana cuttings gradually decreased.In 5 mg/L Zn^2＋ treatment group,the number of adventitious roots of S.matsudana cuttings was the most,the longest root length and average root length were the longest and the rooting rate was the highest.[Conclusion]The tolerance of S.matsudana to Pb^2＋ was strongest and its tolerance to Cu^2＋ was the weakest.The tolerance order of S.matsudana to three kinds of heavy metals and their mixed solution was as following：Pb^2＋〉Zn^2＋〉Cu^2＋＋Pb^2＋＋Zn^2＋〉Cu^2＋.

Cell Production and Expansion in the Primary Root of Maize in Response to Low-Nitrogen Stress

Maize plants respond to low-nitrogen stress by enhancing root elongation. The underlying physiological mechanism remains unknown. Seedlings of maize （Zea mays L., cv. Zhengdan 958） were grown in hydroponics with the control （4 mmol L-1） or low-nitrogen （40 μmol L-1） for 12 d, supplied as nitrate. Low nitrogen enhanced root elongation rate by 4.1-fold, accompanied by increases in cell production rate by 2.2-fold, maximal elemental elongation rate （by 2.5-fold）, the length of elongation zone （by 1.5-fold）, and ifnal cell length by 1.8-fold. On low nitrogen, the higher cell production rate resulted from a higher cell division rate and in fact the number of dividing cells was reduced. Consequently, the residence time of a cell in the division zone tended to be shorter under low nitrogen. In addition, low nitrogen increased root diameter, an increase that occurred speciifcally in the cortex and was accompanied by an increase in cell number. It is concluded that roots elongates in response to low-nitrogen stress by accelerating cell production and expansion.

OsABA8ox2, an ABA catabolic gene, suppresses root elongation of rice seedlings and contributes to drought response

In rice, OsABA8ox encodes abscisic acid(ABA) 8′-hydroxylase, which catalyzes the committed step of ABA catabolism. The contribution of ABA catabolism in root development remains unclear. We investigated the role of OsABA8ox2 in root growth and development and drought response. GUS staining results showed that OsABA8ox2 was expressed mainly in roots at seedling stage and was strongly expressed in the meristematic zone of the radicle. OsABA8ox2 expression in roots was markedly decreased after 0.5 h polyethylene glycol(PEG) treatment and increased after 0.5 h rehydration, implying that OsABA8ox2 is a drought-responsive gene.OsABA8ox2 knockout mediated by the CRISPR-Cas9 system increased drought-induced ABA and indole-3-acetic acid accumulation in roots, conferred increased ABA sensitivity, and promoted a more vertically oriented root system architecture(RSA) beneficial to drought tolerance.OsABA8ox2 overexpression suppressed root elongation and increased stomatal conductance and transpiration rate. Consequently, OsABA8ox2 knockout dramatically improved rice drought tolerance, whereas OsABA8ox2 overexpression seedlings were hypersensitive to drought stress,suggesting that OsABA8ox2 contributes to drought response in rice. Compared with wild type,functional leaves of OsABA8ox2 knockout seedlings showed higher ABA levels, whereas overexpression lines showed lower ABA levels, suggesting that OsABA8ox2, as an ABA catabolic gene, modulates ABA concentration through ABA catabolism. OsABA8ox2 and OsABA8ox3 were both localized in the endoplasmic reticulum. Together, these results indicate that OsABA8ox2 suppresses root elongation of rice seedlings, increases water transpiration, and contributes to drought response through ABA catabolism, and that OsABA8ox2 knockout dramatically improves rice drought tolerance. They highlight the key role of ABA catabolism mediated by OsABA8ox2 on root growth and development. OsABA8ox2, as a novel RSA gene, would be a potential genetic target for the improvement of rice drought tolerance.

The inf luence of soil drying- and tillage-induced penetration resistance on maize root growth in a clayey soil

Soil drying may induce a number of stresses on crops. This paper investigated maize（Zea mays L.） root growth as affected by drought and soil penetration resistance（PR）, which was caused by soil drying and tillage in a clayey red soil. Compared with conventional tillage（C） and deep tillage（D）, soil compaction（P） and no-till（N） significantly increased soil PR in the 0-15 cm layer. The PR increased dramatically as the soil drying increased, particularly in soil with a high bulk density. Increased soil PR reduced the maize root mass density distribution not only in the vertical profile（0-20 cm） but also in the horizontal layer at the same distance（0-5, 5-10, 10-15 cm） from the maize plant. With an increase in soil PR in pots, the maize root length, root surface area and root volume significantly decreased. Specifically, the maize root length declined exponentially from 309 to 64 cm per plant with an increase in soil PR from 491 to 3 370 k Pa; the roots almost stopped elongating when the soil PR was larger than 2 200 k Pa. It appeared that fine roots（〈2.5 mm in diameter） thickened when the soil PR increased, resulting in a larger average root diameter. The average root diameter increased linearly with soil PR, regardless of soil irrigation or drought. The results suggest that differences in soil PR caused by soil drying is most likely responsible for inconsistent root responses to water stress in different soils.

The Cotton GhWRKY91 Gene Negatively Regulates Root Elongation in Overexpressed Transgenic Arabidopsis thaliana

WRKY transcription factors play important roles in plant growth,development,and stress responses.Our previous research has shown that the GhWRKY91 gene can delay age-,abscisic acid(ABA)-,and drought-induced leaf senescence when overexpressed in transgenic Arabidopsis plants.To explore in more depth the biological functions of the GhWRKY91 gene,we further observed the root growth of overexpressing transgenic Arabidopsis thaliana under ABA and drought treatment.In this study,we transplanted the germinated seeds of wild-type(WT)and three transgenic lines(OE-12,OE-13 and OE-20)to 1/2 MS solid medium containing ABA and different concentrations of mannitol(simulated drought treatment)for culturing.The results showed that the transgenic plants had dark green leaves and short root lengths when no stress treatment was added.After ABA and mannitol treatment,the root growth of the WT and transgenic Arabidopsis was inhibited to varying degrees,and the root length downregulation of the transgenic plants was higher than that of the WT,indicating that they were more sensitive to ABA and drought.A bimolecular fluorescence complementation(BiFC)assay showed that the GhWRKY91 and GhWRKY3 proteins interact and emit yellow fluorescence in tobacco leaf cells.These results indicate that the GhWRKY91 gene negatively regulates root elongation in transgenic Arabidopsis and provide a basis for further research on the molecular mechanism of its involvement in regulating cotton root development.

Effect of Inoculation with Three Phytohormone Producers Phytobacteria with ACC Deaminase Activity on Root Length of Lens esculenta Seedlings

Plant-associated bacteria that inhabit the rhizosphere may influence the plant growth by their contribution to the endogenous pool of phytohormones and by the activity of ACC deaminase to decrease the ethylene concentration. The aim of this study was to analyse the root length growth by the promoting effect of indole acetic acid producers phytobacteria with ACC deaminase activity, on inoculated seeds of Lens esculenta as synergistic effect on root elongation. In this study, although the roots of L. esculenta seedlings do not show a significant promotion, these phytobacteria could be recommended to treat plants analyzing their added inoculum to increase plant biomass and retard the effect of ethylene on cultures supplied with Tryptophan and ACC.

Responses of Sesbania rostrata and S. cannabina to Pb, Zn, Cu and Cd toxi-cities

Responses of Sesbania rostrata and S. cannabina to Pb, Zn, Cu and Cd toxicities were assessed by a seed-suspending seedbed(SSS) approach. The results showed that the SSS approach was suitable for testing the tolerance of a plant to the stress of toxic metals. The endpoints include seed germination success, straightened radicle and hypocotyl of the seedlings from the seeds. The measurements could be done easily and accurately. It was found that the elongation of radicle was the most sensitive indicator to the stress of heavy metals among the endpoints. When exposure to lower or medium concentrations of Pb, Zn, and Cd, the development of the lateral roots were favorable. Species of S. rostrata was more tolerant than S. cannabina to the heavy metals, especially to Zn and Cd. The ED 50 of Pb, Zn, Cu and Cd were 32.90, 5.32, 4.40 and 12.00 μg/ml for S. rostrata, respectively, and they were 30.11, 2.87, 4.05 and 4.94 μg/ml respectively for S. cannabina.

Soil Temperature Dependent Growth of Cotton Seedlings Before Emergence

Soil temperature is an important variable governing plant growth and development. Studies were conducted under laboratory conditions to determine the effect of soil temperature on root and shoot growth of cotton during emergence. Cotton seedlings were grown for 192 h at 20, 32 and 38℃ in soil packed in 300 mm long and 50 mm diameter cylinders. The data indicated that the longest roots （173 ram） as well as shoots （152 mm） were recorded at 32℃ followed by 20 （130 mm root and 82 mm shoot） and 38℃ （86 mm root and 50 mm shoot）. Roots grown at 20 and 38 ℃ were 20% and 50% shorter, respectively, than those grown at 32℃ after 192 h. Roots and shoots exhibited the lowest length and dry biomass at 38 ℃. Shoot lengths grown at 20 （74 mm） and 38℃（51 mm） were 44% and 61% shorter than those grown at 32℃（131 mm） after 180 h growth period, respectively. Growth at all three temperatures followed a similar pattern. Initially there was a linear growth phase followed by the reduction or cessation of growth. Time to cessation of growth varied with temperature and decreased faster at higher temperatures. Sowing of cotton should be accomplished before seedbed reaches a soil temperature （≥ 38 ℃） detrimental for emergence. Further, the seedbeds should be capable of providing sufficient moisture and essential nutrients for emerging seedling before its seed reserves are exhausted to enhance seedling establishment in soil.

HY5 regulates light-responsive transcription of microRNA163 to promote primary root elongation in Arabidopsis seedlings

MicroRNAs(miRNAs)play key roles in the post-transcriptional regulation of gene expression in plants.Many miRNAs are responsive to environmental signals.Light is the first environmental signal perceived by plants after emergence from the soil.However,less is known about the roles and regulatory mechanism of miRNAs in response to light signal.Here,using small RNA sequencing,we determined that miR163 is significantly rapidly induced by light signaling in Arabidopsis thaliana seedlings.The light-inducible response of miR163 functions genetically downstream of LONG HYPOCOTYL 5(HY5),a central positive regulator of photomorphogenesis.HY5 directly binds to the two G/C-hybrid elements in the miR163 promoter with unequal affinity;one of these elements,which is located next to the transcription start site,plays a major role in light-induced expression of miR163.Overexpression of miR163 rescued the defective primary root elongation of hy5 seedlings without affecting lateral root growth,whereas overexpressing of miR163 target PXMT1 inhibited primary root elongation.These findings provide insight into understanding the post-transcriptional regulation of root photomorphogenesis mediated by the HY5-miR163-PXMT1 network.

PDX1.1-dependent biosynthesis of vitamin B6 protects roots from ammonium-induced oxidative stress

Despite serving as a major inorganic nitrogen source for plants,ammonium causes toxicity at elevated con-centrations,inhibiting root elongation early on.While previous studies have shown that ammonium-inhibited root development relates to ammonium uptake and formation of reactive oxygen species(ROS)in roots,it remains unclear about the mechanisms underlying the repression of root growth and how plants cope with this inhibitory effect of ammonium.In this study,we demonstrate that ammonium-induced apo-plastic acidification co-localizes with Fe precipitation and hydrogen peroxide(H_(2)O_(2))accumulation along the stele of the elongation and differentiation zone in root tips,indicating Fe-dependent ROS formation.By screening ammonium sensitivity in T-DNA insertion lines of ammonium-responsive genes,we identified PDX1.1,which is upregulated by ammonium in the root stele and whose product catalyzes de novo biosyn-thesis of vitamin B6.Root growth of pdx1.1 mutants is hypersensitive to ammonium,while chemical complementation or overexpression of PDX1.1 restores root elongation.This salvage strategy requires non-phosphorylated forms of vitamin B6 that are able to quench ROS and rescue root growth from ammo-nium inhibition.Collectively,these results suggest that PDX1.1-mediated synthesis of non-phosphorylated B6 vitamers acts as a primary strategy to protect roots from ammonium-dependent ROS formation.

Ethylene-induced microtubule reorientation is essential for fast inhibition of root elongation in Arabidopsis

Microtubule reorientation is a long-standing observation that has been implicated in regulating the inhibitory effect of ethylene on axial elongation of plant cells. However, the signaling mechanism underlying ethylene-induced microtubule reorientation has re- mained elusive. Here, we reveal, by live confocal imaging and kinetic root elongation assays, that the time courses of ethylene-induced microtubule reorientation and root elongation inhibition are highly correlated, and that microtubule reorientation is required for the full responsiveness of root elongation to ethylene treatment. Our genetic analysis demonstrated that the effect of ethylene on microtubule orientation and root elongation is mainly transduced through the canonical linear ethylene signaling pathway. By using pharmacological and genetic analyses, we demonstrate further that the TIR1/AFBs-Aux/IAAs-ARFs auxin signaling pathway, but not the ABP1-ROP6-RlC1 auxin signaling branch, is essential for ethylene-induced microtubule reorientation and root elongation inhibition. Together, these findings offer evidence for the functional significance and elucidate the signaling mechanism for ethylene-induced microtubule reorientation in fast root elongation inhibition in Arabidopsis.

Simultaneous mutations in ITPK4 and MRP5 genes result in a low phytic acid level without compromising salt tolerance in Arabidopsis

Generation of crops with low phytic acid(myoinositol-1,2,3,4,5,6-hexakisphosphate(InsP6))is an important breeding direction,but such plants often display less desirable agronomic traits.In this study,through ethyl methanesulfonate-mediated mutagenesis,we found that inositol 1,3,4-trisphosphate5/6-kinase 4(ITPK4),which is essential for producing InsP6,is a critical regulator of salt tolerance in Arabidopsis.Loss of function of ITPK4 gene leads to reduced root elongation under salt stress,which is primarily because of decreased root meristem length and reduced meristematic cell number.The itpk4 mutation also results in increased root hair density and increased accumulation of reactive oxygen species during salt exposure.RNA sequencing assay reveals that several auxin-responsive genes are down-regulated in the itpk4-1 mutant compared to the wild-type.Consistently,the itpk4-1 mutant exhibits a reduced auxin level in the root tip and displays compromised gravity response,indicating that ITPK4 is involved in the regulation of the auxin signaling pathway.Through suppressor screening,it was found that mutation of Multidrug Resistance Protein 5(MRP5)5 gene,which encodes an ATP-binding cassette(ABC)transporter required for transporting InsP6from the cytoplasm into the vacuole,fully rescues the salt hypersensitivity of the itpk4-1 mutant,but in the itpk4-1 mrp5 double mutant,InsP6remains at a very low level.These results imply that InsP6homeostasis rather than its overall amount is beneficial for stress tolerance in plants.Collectively,this study uncovers a pair of gene mutations that confer low InsP6content without impacting stress tolerance,which offers a new strategy for creating“low-phytate”crops.

Wheat morphological and biochemical responses to copper oxide nanoparticles in two soils

The widespread application of copper oxide nanoparticles(CuO NPs)in agricultural production has caused growing concerns about their impact on crops.In this study,wheat root elongation was used to evaluate the toxic effect concentrations of CuO NPs in two soils with differing properties,collected from farmlands in Guangdong(GD)and Shandong(SD)provinces,China.Plant morphological and biochemical properties were also assessed to explore the toxicity mechanism of CuO NPs on wheat seedlings.The root elongation results revealed lower toxic effect concentration values in the plants grown in GD soil than in SD soil.Furthermore,the treatment with CuO NPs at 200 mg Cu kg^(-1) significantly reduced wheat root and shoot biomass by 35.8%and 15.8%,respectively,in GD soil.Electron microscopy showed that CuO NPs deformed wheat roots and entered leaf cells,causing deformation and damaging the cell structure.The CuO NP treatments also decreased chlorophyll content,increased antioxidant enzyme activity,and increased membrane lipid peroxidation in wheat leaves.The addition of CuO NPs significantly reduced the Zn(by 17.3%)and Fe(by 26.9%)contents in the leaves of plants grown in GD and SD soils,respectively.However,the contents of Cu,Mg,and Mn were increased by 27.4%–52.5%in GD soil and by 17.9%–71.6%in SD soil.These results suggested that CuO NPs showed greater toxicity to wheat plants grown in acidic soil than in alkaline soil and that the adverse effects of CuO NP treatments on wheat seedlings were due to a combination of CuO NPs and released Cu^(2+).

Physiological and Antioxidant Responses of Germinating Mung Bean Seedlings to Phthalate Esters in Soil

Single phytotoxicity of two representative phthalate esters(PAEs),di-n-butyl phthalate(DnBP)and bis(2-ethylhexyl)phthalate(DEHP),was tested in mung bean(Vigna radiata)seedlings germinated for 72 h in soils spiked with varying concentrations(0-500 mg kg^(-1)soil)of DnBP or DEHP.PAEs added at up to 500 mg kg^(-1)soil exerted no significant effect on germination but both pollutants significantly inhibited root elongation(P<0.01),DEHP inhibited shoot elongation(P<0.01)and DnBP depressed biomass on a fresh weight basis(P<0.05).Seedling shoot and root malondialdehyde(MDA)contents tended to be stimulated by DnBP but inhibited by DEHP.However,increases in superoxide dismutase,peroxidase,ascorbate peroxidase and polyphenol oxidase activities,as well as glutathione(GSH)content,were induced at higher concentrations(e.g.,20 mg kg^(-1))of both compounds.Accumulation of proline in both roots and shoots and the storage compounds,such as free amino acids and total soluble sugars,in whole plant was induced under the stress exerted by both PAEs.The general responses of mung bean seedlings indicated higher toxicity of DnBP than DEHP on primary growth,during which root elongation was a more responsive index.MDA and GSH were more sensitive parameters in the roots than in the shoots and they might be recommended as physiologically sensitive parameters to assess the toxicity of PAE compounds in soils in future long-term studies.