Response of maize seedling roots to changing ethylene concentrations

Maize roots (Zea mays, cv. DK 626) growing in aerated solutions showed striking variations in the amount of ethylene produced during different stages of development. As endogenous ethylene increases, root elongation decreases. Exogenous 1-aminocyclopropane-1-carboxylic acid (ACC) supplied to these roots also inhibited their elongation and increased both the fresh weight of the apex and the ethylene produced. The inhibitor of ethylene biosynthesis, 2-aminoethoxyvinyl glycine (AVG), and the inhibitor of ethylene action, silver thiosulfate (STS), also reduced growth and increased swelling. As growth diminishes at reduced ethylene concentrations or with impeded ethylene action, these results support the view that ethylene is necessary for root growth. As ACC treatment also inhibited root elongation, it appears that ethylene was inhibitory at both low and high concentrations. Whereas ACC stimulated ethylene production 4 h after the beginning of treatment, inhibition of root elongation and promotion of fresh weight advanced slowly and needed 24 h to be established. At that time, root elongation reached a maximum response of 60% inhibition and 50% increase in weight. At 48 h, higher doses of ACC were required to provoke the same response as at 24 h. This suggests that the root growth progressively accomodates to higher ethylene concentrations. Published in Russian in Fiziologiya Rastenii, 2009, Vol. 56, No. 4, pp. 539–545. This text was submitted by the authors in English.     

Induction of nitrate transport in maize roots, and kinetics of influx, measured with nitrogen-13

Unlike phosphate or potassium transport, uptake of nitrate by roots is induced, in part, by contact with the substrate ion. Plasmalemma influx of ¹³N-labeled nitrate in maize roots was studied in relation to induction of the uptake system, and the influence of short-term N starvation. Maize (Zea mays) roots not previously exposed to nitrate had a constitutive transport system (state 1), but influx increased 250% during six hours of contact with 100 micromolar nitrate, by which time the transport mechanism appeared to be fully synthesized (state 2). A three-day period of N starvation prior to induction and measurement of nitrate influx resulted in a greater capacity to transport nitrate than in unstarved controls, but this was fully expressed only if roots were kept in contact with nitrate for the six hours needed for full induction (state 2E). A kinetic analysis indicated a 160% increase in maximum influx in N-starved, induced roots with a small decrease in K_m. The inducible component to nitrate influx was induced only by contact with nitrate. Full expression of the nitrate inducible transport system was dependent upon mRNA synthesis. An inhibitor of cytoplasmic protein synthesis (cycloheximide) eliminated the formation of the transport system while inhibition by chloramphenicol of mitochondrial- or plastid-coded protein synthesis had no effect. Poisoning of membrane-bound proteins effectively disabled both the constitutive and induced transport systems.     

不同抗旱性玉米幼苗根系抗氧化系统对水分胁迫的反应

以抗旱性不同的2个玉米品种为材料,研究不同程度水分胁迫下玉米根系活性氧清除系统的变化及膜脂过氧化水平。明确了轻度水分胁迫下玉米根系POD、CAT、APX等保护酶活性明显提高;中、重度胁迫下其活性急剧下降,但几种酶对水分胁迫的敏感程度不同。SOD对水分胁迫表现最不敏感,在中度水分胁迫下仍保持上升趋势;抗氧化剂GSH含量变化趋势与保护酶相似;而AsA含量在不同程度水分胁迫下持续下降;MDA含量随水分胁迫程度加剧而增加。其中抗旱性强的鲁玉14与抗旱性弱的掖单13相比具有较高的保护酶活性和抗氧化剂含量,膜脂过氧化程度较轻,除POD外,品种间抗氧化酶活性（抗氧化剂含量）呈极显著差异,说明抗氧化能力强是抗旱性品种具有较强抗旱性的重要原因之一。     

Excretion products of maize roots from seedling to seed development stage

Summary Excretion products of maize roots (Zea mays cv. Koshu) were estimated. All excreted products were the highest by fresh weight basis at the young seedling stage. In amino acids excreted, glutamic acid accounting for 60% of the total was the highest and followed by alanine. These two amino acids showed the diverse fluctuation according to the growing age, that is, glutamic acid increased while alanine decreased. Stachyose was a main soluble sugar excepting the stage prior to the heading. At this stages, glucose and fructose together with stachyose were observed. Lactic acid was the most dominant organic acid through whole growing stages. These excreted materials could be positive factors for the growth ofSpirillum lipoferum which can fix nitrogen non-symbiotically at the rhizosphere of maize.     

Nitrogen-stimulated potassium influx into maize roots: differential response of components resistant and sensitive to ambient ammonium1

Abstract Potassium (⁸⁶Rb) influx from 200 mmol m ^?3 KCl into dark grown, decapitated maize seedlings 6 d old) was stimulated by nitrate pretreatment. The stimulus was clearly evident by 6h exposure to nitrate and required 12–24 h for maximal expression. Decay of the nitrate-stimulated potassium influx was more than 50% complete within 3 h after transfer to nitrogen-free solutions. The stimulation of potassium influx was entirely accounted for by an increase in the influx component that was resistant to inhibition by presence of 200 mmol m^?3 ambient ammonium. In contrast, the component of potassium influx that was sensitive to inhibition by ambient ammonium was unaffected by nitrate pretreatment. Exposure to the glutamine synthetase inhibitor L-methionine-dl-sulphoximine (MSX) during nitrate pretreatment stimulated the resistant component but the sensitive component was nearly eliminated. Pretreatment with ammonium increased the resistant component of potassium influx within 3 h, i.e. before it was increased by nitrate pretreatment, but the sensitive component was concomitantly restricted. The latter recovered partially during extended pretreatment with ammonium. The data indicate that the resistant component responded positively to increases in tissue ammonium concentrations whereas the sensitive component was unaffected by tissue ammonium except at concentrations in excess of 10μmol g^?1. Ammonium influx was also stimulated by nitrate pretreatment and to a greater extent than potassium influx. Presence of MSX with nitrate during pretreatment resulted in a further stimulation in ammonium influx. The parallel increases in root ammonium concentrations with the two pretreatments imply that part of the increase in ammonium influx was a consequnce of increased counter-transport with endogenous ammonium.     

Cyanidin as endogenous chelator of metal ions in maize seedling roots

The property of cyanidin complexation with different metal ions in maize seedlings roots was established using parameters of reflectance spectroscopy and colorimetry. The spectral criteria of pigment association with metals in vivo were evaluated.     

Properties of potassium uptake by seedling roots of grape cultivars

Uptake rates of (⁸⁶Rb)K⁺ by seedling roots of six cultivars were measured and compared with K⁺ content of the root, K⁺ leakage, H⁺ efflux, and K⁺-ATPase activity of a partially purified plasmalemma fraction.Different cultivars showed significantly different rates of (⁸⁶Rb)K⁺ uptake. The uptake rates of the first (0–5 min) period did not correlate with K⁺ content of the seedling roots.The rates of uptake in the 10 to 30 min period, supposed to be active, were negatively correlated with K⁺ content of the root. Roots consistently leaked K⁺ during the first 5 min. This leakage was positively correlated with the endogenous K⁺ content of the tissue.H⁺ efflux was significantly different among the cultivars and correlated with the K⁺-ATPase activity of a microsomal fraction partially purified on discontinuous (18/34%) sucrose gradient. The relationships among transport parameters are discussed.Part of the present research was carried out at the Department of Agricultural Biotechnology of University of Padova, Italy.Part of the present research was carried out at the Department of Agricultural Biotechnology of University of Padova, Italy.     

Acidic phosphoprotein complex of the 60S ribosomal subunit of maize seedling roots. Components and changes in response to flooding.

We determined that ribosomes of seedling roots of maize (Zea mays L.) contain the acidic phosphoproteins (P-proteins) known to form a flexible lateral stalk structure of the 60S subunit of eukaryotic ribosomes. The P-protein stalk, composed of P0, P1, and P2, interacts with elongation factors, mRNA, and tRNA during translation. Acidic proteins of 13 to 15.5 kD were released as a complex from ribosomes with 0.4 M NH4Cl/50% ethanol. Protein and cDNA sequence analysis confirmed that maize ribosomes contain one type of P1, two types of P2, and a fourth and novel P1/P2-type protein. This novel P-protein, designated P3, has the conserved C terminus of P1 and P2. P1, P2, and P3 are similar in deduced mass (11.4-12.2 kD) and isoelectric point (4.1-4.3). A 35.5- to 36-kD acidic protein was released at low levels from ribosomes with 1.0 M NH4Cl/50% ethanol and identified as P0. Labeling of roots with [32P]inorganic phosphate confirmed the in vivo phosphorylation of the P-proteins. Flooding caused dynamic changes in the P-protein complex, which affected the potential of ribosome-associated kinases and casein kinase II to phosphorylate the P-proteins. We discuss possible alterations of the ribosomal P-protein complex and consider that these changes may be involved in the selective translation of mRNA in flooded roots.     

Tissue zinc distribution in maize seedling roots and its action on growth

Zinc (Zn) distribution over tissues and organs of maize (Zea mays L.) seedlings and its action on root growth, cell division, and cell elongation were studied. Two-day-old seedlings were incubated in the 0.25-strength Hoagland solution containing 2 or 475 μM Zn(NO₃)₂. Zn toxicity was assessed after the inhibition of primary root increment during the first and second days of incubation. The content of Zn was determined by atomic absorption spectrometry in the apical (the first centimeter from the root tip) and basal (the third centimeter from the kernel) root parts. Zn distribution in various tissues was studied by histochemical methods, using a metallochromic indicator zincon and fluorescent indicator Zinpyr-1 and light and confocal scanning fluorescent light microscopy, respectively. To evaluate Zn effects on growth processes, the average length of the meristem; the length of fully elongated cells; the number of meristematic cells in the cortex row; and duration of the cell cycle were measured. When the Zn concentration in the solution was high, the Zn content per weight unit was higher in the basal root part due to its accumulation in lateral root primordial. Zn was also accumulated in both the meristem apoplast and cell protoplasts. In the basal and middle root parts, Zn was detected essentially in all tissues predominantly in the apoplast. Zn inhibited both cell division and elongation. Under Zn influence, the size of the meristem and the number of meristematic cells decreased, which was determined by an increase in the cell cycle duration. The length of the fully elongated cells was also reduced. A comparison of Zn distribution and growth-suppressing activity with other heavy metals studied earlier allows a conclusion that toxic action of heavy metals is mainly determined by physical and chemical properties of their ions and specific patterns of their transport and distribution. As a result, two basic processes determining root growth, e.g., cell division and elongation, could be affected differently.     

Auxin transport in maize roots in response to localized nitrate supply

Background and Aims

Roots typically respond to localized nitrate by enhancing lateral-root growth. Polar auxin transport has important roles in lateral-root formation and growth; however, it is a matter of debate whether or how auxin plays a role in the localized response of lateral roots to nitrate.

Methods

Treating maize (Zea mays) in a split-root system, auxin levels were quantified directly and polar transport was assayed by the movement of [³H]IAA. The effects of exogenous auxin and polar auxin transport inhibitors were also examined.

Key Results

Auxin levels in roots decreased more in the nitrate-fed compartment than in the nitrate-free compartment and nitrate treatment appeared to inhibit shoot-to-root auxin transport. However, exogenous application of IAA only partially reduced the stimulatory effect of localized nitrate, and auxin level in the roots was similarly reduced by local applications of ammonium that did not stimulate lateral-root growth.

Conclusions

It is concluded that local applications of nitrate reduced shoot-to-root auxin transport and decreased auxin concentration in roots to a level more suitable for lateral-root growth. However, alteration of root auxin level alone is not sufficient to stimulate lateral-root growth.     

Suberin lamella development in maize seedling roots grown in aerated and stagnant conditions

Hypoxia can stimulate the development of a suberized exodermis in aquatic plants; however, its influence on this aspect of terrestrial root development is sparsely documented. To determine the effects of hypoxia on maize (Zea mays cv. Seneca Horizon) roots, seedlings were grown in vermiculite (VERM), aerated hydroponics (AER), stagnant hydroponics with agar (STAG), or aerated hydroponics with agar (AERAG). The endo- and exodermis were examined for wall modifications. Lateral root emergence and aerenchyma formation were documented qualitatively. The endodermal Casparian band formation was unaffected by treatment. Endodermal and exodermal suberin lamella formation was earliest and most extensive in VERM. Suberization, especially in the exodermis of aerated treatments, was depressed in all hydroponic media. In comparison with AER, STAG exodermal lamellae were increased, but endodermal lamellae were decreased. Since the suberized exodermis forms a barrier to radial oxygen loss from roots to the medium, its stimulation in STAG roots (which also developed extensive aerenchyma) would help retain oxygen in the root. The reduction of endodermal lamellae should facilitate oxygen diffusion into the stele. Clearly, the response to environmental conditions is variable within individual cortical cell layers. Additionally, the observed patterns of lamellae, aerenchyma and lateral root development indicate a tight radial co-ordination of root development.     

Comparative proteomic analysis of salt response proteins in seedling roots of two wheat varieties

A comparative proteomic analysis was made of salt response in seedling roots of wheat cultivars Jing-411 (salt tolerant) and Chinese Spring (salt sensitive) subjected to a range of salt stress concentrations (0.5%, 1.5% and 2.5%) for 2 days. One hundred and ninety eight differentially expressed protein spots (DEPs) were located with at least two-fold differences in abundance on 2-DE maps, of which 144 were identified by MALDI-TOF-TOF MS. These proteins were involved primarily in carbon metabolism (31.9%), detoxification and defense (12.5%), chaperones (5.6%) and signal transduction (4.9%). Comparative analysis showed that 41 DEPs were salt responsive with significant expression changes in both varieties under salt stress, and 99 (52 in Jing-411 and 47 in Chinese Spring) were variety specific. Only 15 and 9 DEPs in Jing-411 and Chinese Spring, respectively, were up-regulated in abundance under all three salt concentrations. All dynamics of the DEPs were analyzed across all treatments. Some salt responsive DEPs, such as guanine nucleotide-binding protein subunit beta-like protein, RuBisCO large subunit-binding protein subunit alpha and pathogenesis related protein 10, were up-regulated significantly in Jing-411 under all salt concentrations, whereas they were down-regulated in salinity-stressed Chinese Spring.     

Activities of proteinases in maize and soybean roots in response to anoxic stress

This study characterized the changes in proteinase activities in maize inbred line H60 and soybean cultivar Keller roots in response to anoxia. After 24 h of anoxia, crude protein extracts from both maize and soybean root tips (10 cm) were assayed for proteinase activities at pHs ranging from 4.5 to 10.2. In anoxic roots of both maize and soybean, activities of proteinases with alkaline pH optima increased, and activities of proteinases with acidic pH optima declined. Proteinases with neutral pH increased in anoxic maize roots, but declined in anoxic soybean roots. Whether the differences in proteinase activities in anaerobic maize and soybean roots contribute to the differental susceptibility of the two species requires further study.Journal Article No. 265-89.