Possible Involvement of Cytokinin in Nitrate-mediated Root Growth in Maize

Response of root system architecture to nutrient availability in soils is an essential way for plants to adapt to soil environments. Nitrate can affect root development either as a result of changes in the external concentration, or through changes in the internal nutrient status of the plant. Nevertheless, less is known about the physiological mechanisms. In the present study, two maize (Zea mays L.) inbred lines (478 and Wu312) were used to study a possible role of cytokinin in nitrate-mediated root growth in nutrient solutions. Root elongation of 478 was more sensitive to high nitrate supply than that of Wu312. Medium high nitrate (5 mM) inhibited root elongation in 478, while, root elongation in Wu312 was only inhibited at high NO ₃ ⁻ supply (20 mM). Under high nitrate supply, the root elongation zone in 478 became swollen and the site of lateral root elongation was close towards the root tip. Both of the phenomena are typical of root growth induced by exogenous cytokinin treatments. Correspondingly, zeatin and zeatin nucleotide (Z + ZR) concentrations were increased at higher nitrate supply in 478, whereas they were constant in Wu312. Furthermore, exogenous cytokinin 6-benzylaminopurine (6-BA) completely reversed the stimulatory effect of low nitrate on root elongation. Therefore, it is supposed that the inhibitory effect of high concentration of nitrate on root elongation is, at least in part, mediated by increased cytokinin level in roots. High nitrate supply may have negative influences on root apex activity by affecting cytokinin metabolism so that root apical dominance is weakened and, therefore, root elongation is suppressed and lateral roots grow closer to the root apex. Nitrate suppressed lateral root elongation in Wu312 at concentration higher than 5 mM. In 478, however, this phenomenon was not significant even at 20 mM nitrate. Although exogenous 6-BA (20 nM) could suppress lateral root elongation as well, the inhibitory effect of high NO ₃ ⁻ concentration of nitrate on lateral root growth cannot be explained by changes in endogenous cytokinin alone.     

Factors affecting the establishment and maintenance of ‘Titan’ red raspberry root organ cultures

Root organ cultures of red raspberry (Rubus ideaus cv. Titan) were established from two cm long terminal segments of adventitious roots induced in vitro on micropropagated shoot cultures. Growth of root organ cultures was almost entirely by initiation and elongation of lateral roots rather than by terminal elongation from the apical meristem of the original root explants. Indolebutyric acid (IBA) was required for lateral root initiation and elongation. The optimal IBA concentration for lateral root initiation decreased from 0.5 to 0.1 mg/L from the first to the fifth passage in culture. Two cm root explants initiated more lateral roots per unit length than four cm explants. Liquid Anderson's medium was superior to other basal nutrient and vitamin formulations tested. Root growth in liquid culture was stimulated by aeration. Root organ cultures also grew on media solidified with gelrite or agar. A rapid transfer technique was developed for subculturing these which involved cutting and transferring 1 cm discs of roots and underlying medium. Adventitious bud formation occurred spontaneously, but sporadically in liquid cultures, and was not influenced by cytokinin, auxin (spat) light, or chilling.     

Apical diameter and branching density affect lateral root elongation rates in banana

Thick roots elongate faster than thinner ones. However, within one species, the growth achieved by roots of a given diameter can be very variable, and root diameter can only be considered as a determinant of root potential elongation rate. As root elongation is highly correlated to carbon availability, it can be hypothesized that local competition for resources, expressed as the number of lateral roots per unit length (i.e. the branching density), modulates root elongation. Using novel methods in field conditions, we have estimated apical diameters, elongation rates and growth durations of nearly 3500 banana lateral roots, in a field experiment with high radiations and a shaded glasshouse experiment with low radiations. Apical diameters and branching densities were lower in the experiment with low radiation, but elongation rates were higher. In both experiments, mean elongation rates of first-order laterals and thick second-order laterals were negatively correlated with bearing root branching densities. It is hypothesized that, even though apical diameters were lower, low branching densities in the shaded glasshouse allowed enhanced lateral root elongation. In both experiments, second-order laterals elongated more slowly than first-order laterals of similar diameter. A specific effect of root order, independent of branching density and apical diameter, contributed to explain these slow second-order lateral elongation rates. Most lateral roots elongated between 9 and 21 days and growth duration was mainly correlated with root diameter.     

Reactive oxygen species localization in roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> seedlings grown under phosphate deficiency

Arabidopsis plants responding to phosphorus (P) deficiency increase lateral root formation and reduce primary root elongation. In addition the number and length of root hairs increases in response to P deficiency. Here we studied the patterns of radical oxygen species (ROS) in the roots of Arabidopsis seedlings cultured on media supplemented with high or low P concentration. We found that P availability affected ROS distribution in the apical part of roots. If plants were grown on high P medium, ROS were located in the root elongation zone and quiescent centre. At low P ROS were absent in the elongation zone, however, their synthesis was detected in the primary root meristem. The proximal part of roots was characterized by ROS production in the lateral root primordia and in elongation zones of young lateral roots irrespective of P concentration in the medium. On the other hand, plants grown at high or low P differed in the pattern of ROS distribution in older lateral roots. At high P, the elongation zone was the primary site of ROS production. At low P, ROS were not detected in the elongation zone. However, they were present in the proximal part of the lateral root meristem. These results suggest that P deficiency affects ROS distribution in distal parts of Arabidopsis roots. Under P-sufficiency ROS maximum was observed in the elongation zone, under low P, ROS were not synthesized in this segment of the root, however, they were detected in the apical root meristem.     

Relationship between root elongation rate and diameter and duration of growth of lateral roots of maize

The objective of this work was to describe the relationship between elongation rate and diameter of maize roots and to estimate the length and growth duration of lateral roots of maize. Diameters and elongation rates of roots were measuredin situ on plants grown 5 weeks in small rhizotrons under greenhouse conditions. At the end of the experimental period the roots were harvested and diameters of axile and lateral roots were measured. The frequency distribution of diameters of harvested roots was bimodal with a minimum at 0.6 mm; 97% of axile roots were larger than this value and 98% of the lateral roots were smaller. Root elongation per day increased as diameter increased but the slope of the relationship with lateral roots was about 2.5 times that with axile roots when separate linear regressions were fitted to the two populations. The length of lateral roots found on axillary roots between the base and about 30 cm from the apex was approximately 2.2 cm. All of the data was consistent with the hypothesis that the lateral roots grew for about 2.5 days and then ceased growing. The axillary roots continued to grow throughout the experimental period at a rate of about 3 cm day⁻¹. Contribution from the Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Agronomy paper No. 1661. This research is part of the program of the Center for Root-Soil Research.     

Influence of auxin and mycorrhizal fungi on thein vitro formation and growth ofPinus pinaster roots

Summary Experiments, performed withPinus pinaster cloned shoots submitted to an auxin treatment (NAA 10^–6 M, 18 days), demonstrated that rooting abilityin vitro persists over 5 successive induction cycles (through out a 9-month period). Rooting ability needs a permanent synthesis of auxin synergists which activate the metabolism of cell dedifferentiation and root primordium initiation. Agar culture permitted intense meristem initiation, but prevented active root elongation. In the presence of a mycorrhizal fungus,Pisolithus tinctorius orHebeloma cylindrosporum, roots resumed growth and short lateral root formation was stimulated. These two phenomena induced by fungal association improve the quality of the root systems required to facilitate successful transplantation from test-tubes to field conditions.     

Root growth, developmental changes in the apex, and hydraulic conductivity for Opuntia ficus-indica during drought

Developmental changes in the root apex and accompanying changes in lateral root growth and root hydraulic conductivity were examined for Opuntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper soil layers. During 7 d of rapid drying (in containers with a 3-cm depth of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not due to meristem exhaustion but rather to meristem mortality after 3 d of drying. The length of the meristem, the duration of the cell division cycle, and the length of the elongation zone were unchanged during rapid drying. During 14 d of gradual drying (in containers with a 6-cm depth of vermiculite), root mortality was relatively low; the length of the elongation zone decreased by 70%, the number of meristematic cells decreased 30%, and the duration of the cell cycle increased by 36%. Root hydraulic conductivity ( L _P) decreased to one half during both drying treatments; L _P was restored by 2 d of rewetting owing to the emergence of lateral roots following rapid drying and to renewed apical elongation following gradual drying. Thus, in response to drought, the apical meristems of roots of O. ficus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in response to rainfall in the field can be enhanced by lateral root proliferation near the soil surface and additionally by resumption of apical growth for deeper roots.     

Seasonal variation of growth and development of the roots of five second year conifer species in the nursery

Every other week over their second growing season, stem height, collar diameter, shoot and root dry masses, number of lateral roots and length of the tap root were measured on nursery grown seedlings ofAbies balsamea L. Mill.,Pinus banksiana Lamb.,Pinus resinosa Ait.,Picea mariana Mill. BSP andPicea glauca Moench Voss. Root elongation, branching and mycorrhizal development were also recorded.Given species showed distinct seasonal growth patterns. The rate and timing of maximum root growth (mg/dry weight/week) differed markedly between species.Except for the increase in height ofPinus banksiana, root and shoot growth were not negatively correlated.The results are discussed in relation to the performance of tree seedlings in the nursery.     

The effect of phytohormones on growth and artemisinin production in <Emphasis Type="Italic">Artemisia annua</Emphasis> hairy roots

Summary Few studies have focused on the effect of a broad range of phytohormones on growth and secondary metabolism of a single hairy root species. We measured growth, development, and production of the antimalarial drug, artemisinin, in Artemisia annua hairy roots in response to the five main hormones: auxins, cytokinins, ethylene, gibberellins (GA), and abscisic acid (ABA). Single roots grown in six-well plates in medium B5 with 0.01 mgl⁻¹ (0.029 μM) GA₃ produced the highest values overall in terms of the number of lateral roots, length of the primary root, lateral root tip density, total lateral root length, and total root length. When the total root lengths are compared, the best conditions for stimulating elongation appear to be: GA 0.01 mgl⁻¹ (0.029μM)> ABA 1.0 mgl⁻¹ (3.78μM)=GA 0.02 mgl⁻¹ (0.058μM). Bulk yields of biomass were inversely proportional to the concentration of each hormone tested. All cultures provided with ABA yielded the highest amount of biomass. Both 6-benzylaminopurine and 2-isopentenyladenine inhibited root growth, however, only 2-isopentenyladenine stimulated artemisinin production, more than twice that of the B5 controls, and more than any other hormone studied. These results will prove useful in increasing hairy root growth and artemisinin production.     

Influence of wind loading on root system development and architecture in oak (Quercus robur L.) seedlings.

The effect of wind loading on seedlings of English oak (Quercus robur L.) was investigated. Instead of using a traditional wind tunnel, an innovative ventilation system was designed. This device was set up in the field and composed of a rotating arm supporting an electrical fan, which emitted an air current similar to that of wind loading. Oaks were sown from seed in a circle around the device. A block of control plants was situated nearby, and was not subjected to artificial wind loading. After 7 months, 16 plants from each treatment were excavated, and root architecture and morphological characteristics measured using a 3D digitiser. The resulting geometrical and topological data were then analysed using AMAPmod software. Results showed that total lateral root number and length in wind stressed plants were over two times greater than that in control trees. However, total lateral root volume did not differ significantly between treatments. In comparing lateral root characters between the two populations, it was found that mean root length, diameter and volume were similar between the two treatments. In trees subjected to wind loading, an accentuated asymmetry of root distribution and mean root length was found between the windward and leeward sides of the root system, with windward roots being significantly more numerous and longer than leeward roots. However, no differences were found when the two sectors perpendicular to the wind direction were compared. Mean tap root length was significantly higher in control samples compared to wind stressed plants, whilst mean diameter was greater in the latter. Wind loading appears to result in increased growth of lateral roots at the expense of the tap root. Development of the lateral root system may therefore ensure better anchorage of young trees subjected to wind loading under certain conditions.     

Comparing simple root phenotyping methods on a core set of rice genotypes

Interest in belowground plant growth is increasing, especially in relation to arguments that shallow‐rooted cultivars are efficient at exploiting soil phosphorus while deep‐rooted ones will access water at depth. However, methods for assessing roots in large numbers of plants are diverse and direct comparisons of methods are rare. Three methods for measuring root growth traits were evaluated for utility in discriminating rice cultivars: soil‐filled rhizotrons, hydroponics and soil‐filled pots whose bottom was sealed with a non‐woven fabric (a potential method for assessing root penetration ability). A set of 38 rice genotypes including the OryzaSNP set of 20 cultivars, additional parents of mapping populations and products of marker‐assisted selection for root QTLs were assessed. A novel method of image analysis for assessing rooting angles from rhizotron photographs was employed. The non‐woven fabric was the easiest yet least discriminatory method, while the rhizotron was highly discriminatory and allowed the most traits to be measured but required more than three times the labour of the other methods. The hydroponics was both easy and discriminatory, allowed temporal measurements, but is most likely to suffer from artefacts. Image analysis of rhizotrons compared favourably to manual methods for discriminating between cultivars. Previous observations that cultivars from the indica subpopulation have shallower rooting angles than aus or japonica cultivars were confirmed in the rhizotrons, and indica and temperate japonicas had lower maximum root lengths in rhizotrons and hydroponics. It is concluded that rhizotrons are the preferred method for root screening, particularly since root angles can be assessed.     

Evidence for nitrate reductase expression during initiation of lateral roots by NAA in chicory

The supply of -naphthaleneacetic acid (NAA), to excised chicory roots induced the formation of lateral root meristems mainly localized proximal to the pre-existing apical root meristem, in a region which does not initiate any lateral roots in control conditions. Inhibition of root elongation and concomitant enlargement of the apices were also observed. Quantification of NAA and cytokinin levels showed that the most reproducible and significant changes occurring after the NAA treatment consisted of a decrease in the level of zeatin-O-glucoside conjugates. Hydrolysis of these conjugates might deliver free zeatin-type compounds which were consumed during the lateral root growth. After 5 d, control excised roots contained a high level of amino acids, mainly as asparagine and arginine, probably issued from proteolysis associated to a senescent-like process. Conversely, in the presence of NAA, neither accumulation of amino acids nor a decrease of the total protein content of the tissue could be detected. Newly initiated meristems expressed the nia gene which encodes nitrate reductase, the first enzyme of the nitrate assimilatory pathway. Thus the increased expression of nitrate reductase which was observed in excised roots of chicory supplied with NAA (Vuylsteker et al., 1997b) may be ascribed to lateral root formation and development. The reinduction of nitrate reduction activity was driven by the increased demand for reduced nitrogen. Thus, the nia gene is one of the genes expressed during the early stages of root meristem formation.Keywords: Auxins, chicory, in situ hybridization, lateral root, nitrate reductase.      

Evaluation of parameters describing the root system architecture of field grown maize plants (Zea mays L.)

The objective of this work was to study elongation curves of maize axile roots throughout their elongation period under field conditions. Relationships between their elongation rate and the extension rate of their branched region were also studied. Maize, early-maturing cultivar Dea, was grown on a deep, barrier-free clay loam (depth 1.80m). Trenches were dug during four periods until after silking and axile roots were excavated. Parameters measured were total length and the lengths of basal and apical unbranched zones. The rank of the bearing phytomer and general data about the carrying plant were also recorded. Results showed that axile roots from lower phytomers had similar elongation rates irrespective of the rank of the carrying phytomer. This elongation rate declined with root age. A monomolecular elongation model was fitted to the experimental data. Elongation was much slower in roots from upper phytomers. A rough linear relationship was found between the elongation rate of axile roots and the length of the apical unbranched zone. This result suggests that laterals appeared on a root segment a constant time after it was formed. Possible mechanisms with may account for the declining elongation rate with root age (increasing distance from aerial parts or adverse environmental conditions in deep soil layers) and variability between individual roots are also discussed.     

Soil penetration resistance, a suitable soil property to account for variations in root elongation and branching

Background and Aims

Root system development is affected by soil conditions. The effects of bulk density, water content and penetration resistance on root development processes were investigated in peach trees.

Methods

Peach tree rootstocks were grown in various soil conditions, combining two bulk densities (1.2 and 1.5?g soil.cm^-3) and three water contents (0.14, 0.17 and 0.20?g.g^-1soil). Root parameters (tip diameter, length of apical unbranched zone, branching density and diameters of main and lateral roots) and plant growth (leaves, branches, trunk, root dry mass) were measured. Root growth processes (elongation, branching) were studied using relationships between root parameters.

Results

The proportion of biomass allocated to each plant compartment was similar whatever the soil conditions. Variations in root development were best explained by the variation in penetration resistance, rather than other soil properties. Increased soil penetration resistance reduced the root elongation rate, especially for thick roots. In addition, the branching pattern was affected. In soil with a high penetration resistance, the root system shape differs from a typical herringbone pattern.

Conclusions

These results allow quantification of the root system plasticity, and improve our understanding of the interactions between root development and soil properties.     

Interactive effects of mycorrhization and elevated carbon dioxide on growth of young pedunculate oak (Quercus robur L.) trees

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO₂ level and 650 ppmv CO₂ in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 6 month under nutrient non-limiting conditions. Mycorrhization and elevated atmospheric CO₂ each supported the growth of the trees. Stem height, stem diameter, and dry matter accumulation of pedunculate oak were increased by mycorrhization. Elevated atmospheric CO₂ enhanced stem height, stem diameter, fresh weight and dry weight, as well as lateral root formation of the trees. In combination, mycorrhization and elevated atmospheric CO₂ had a more than additive, positive effect on tree height and biomass accumulation, and further improved lateral root formation of the trees. From these findings it is suggested that the efficiency of the roots in supporting the growth of the shoot is increased in mycorrhized oak trees at elevated atmospheric CO₂.Abbreviations DW dry weight - FW fresh weight - RWC relative water content     

Exogenous auxin effects on growth and phenotype of normal and hairy roots of Pueraria lobata (Willd.) Ohwi

Pueraria lobata hairy roots have faster elongationand more branches than normal roots. The responses of hairy roots and normalroots to treatment with three auxins, indole-3-acetic acid (IAA),indole-3-butyric acid (IBA), and naphthalene acetic acid (NAA) were different.In normal roots, all three auxins strongly stimulated lateral root formation atall tested concentrations. Responses to IAA and IBA in primary root growth andlateral root elongation were similar and depended on concentration; promotionat0.1 M, no effect at 1.0 M, and inhibition at2.5 M. In hairy roots, lateral root formation varied inresponseto the different auxins, i.e. depressed by NAA, unaffected by IAA, and promotedby IBA. Primary root growth was slightly inhibited by IBA and was unaffected byIAA. However, mean lateral root length was reduced in response to IAA and IBA.Only NAA exerted strong inhibition on primary and lateral root elongation inboth root types. The similar free IAA and conjugated IAA content but quitedifferent basal ethylene production and biosynthesis in hairy and normal rootssuggested different mechanisms of response to exogenous auxins in the two roottypes.     

Measuring maximum root growth instead of longest root elongation in metal tolerance tests for grasses (Agrostis capillaris,Agrostis delicatula and Agrostis castellana)

Longest root elongation diminished significantly in the three species tested from 6 mm d^-1 to 3 mm d^-1 in 3 weeks. During this period S.D. increased considerably (from 49% to 112%, A. castellana), and accounted on the average for 68% (A. capillaris) till 94% (A. castellana) of the mean. Maximum root growth stabilized at 6 mm d^-1 and showed less variation in the measurements (S.D. 52% of the mean). Growth of the originally longest root approaches zero in all three species, in accordance with the natural cease of growth of roots in grasses fascicular root system. Measuring maximum root growth instead of longest root elongation is proposed for testing metal tolerance of grasses in sequential experiments.     

Lateral root formation in the strawberry Fragaria × ananassa Duch. revealed by histone H4 in situ hybridisation

Abstract

Our study was carried out in bench rhizotrons using the Camarosa variety of strawberry (Fragaria × ananassa Duch.), by exciding the apex of fast-growing primary roots at two distances (1 or 8 cm) from the apex. It was demonstrated that new lateral meristems were rapidly induced by excision of the root apex at either distance; after 24 h, histone H4 in situ hybridisation detected groups of cell-organising root primordia just a few millimetres below the cut. After a further 24 h, new lateral roots were about to protrude from the original root. Results show that lateral roots can be formed anywhere along the primary roots of strawberry plants from a few stem cells distributed along the pericycle close to the protoxylem arches.     

Effect of auxins on <Emphasis Type="Italic">Karwinskia humboldtiana</Emphasis> root cultures

Effects of auxins (IAA, IBA and NAA) on K. humboldtiana root culture cultivated in 16-h photoperiod or in dark have been observed. Light affected positively the production of biomass when cultivated on medium supplemented with NAA in 10 and 25 mol ^–1 concentrations. In the presence of IAA and IBA these values were significantly lower. The growth dynamics of root cultures depended on the auxin used. The best adventitious roots elongation and lateral roots induction on media supplemented with IBA has been ascertained. Morphological and anatomical differences in dependence on auxin used were observed. NAA supported the formation of huge callus-like mass besides mostly very short roots, especially under the light. Similarly IAA induced short roots, and IBA seems to be the most effective substance for the root elongation in this model system. NAA induced roots with larger diameter under the light compared with the other two auxins used. The reason is in the different anatomical structure of roots which was characterized by higher number of cell layers and large intercellulars in the cortex. The shape of cortical cells in the presence of IBA depended on the light conditions. Isodiametric cortical cells were present in roots cultivated in 16-h photoperiod, irregularly-shaped cells in the dark. The effect of light conditions was the smallest in the case of roots grown on IAA enriched media.