Number Of Lateral Root Primordia Research Articles

Root ABA Accumulation Delays Lateral Root Emergence in Osmotically Stressed Barley Plants by Decreasing Root Primordial IAA Accumulation

Increased auxin levels in root primordia are important in controlling root branching, while their interaction with abscisic acid (ABA) likely regulates lateral root development in water-deficient plants. The role of ABA accumulation in regulating root branching was investigated using immunolocalization to detect auxin (indoleacetic acid, IAA) and ABA (abscisic acid) in root primordia of the ABA-deficient barley mutant Az34 and its parental genotype (cv. Steptoe) barley plants. Osmotic stress strongly inhibited lateral root branching in Steptoe plants, but hardly affected Az34. Root primordial cells of Steptoe plants had increased immunostaining for ABA but diminished staining for IAA. ABA did not accumulate in root primordia of the Az34, and IAA levels and distribution were unaltered. Treating Az34 plants with exogenous ABA decreased root IAA concentration, while increasing root primordial ABA accumulation and decreasing root primordial IAA concentration. Although ABA treatment of Az34 plants increased the root primordial number, it decreased the number of visible emerged lateral roots. These effects were qualitatively similar to that of osmotic stress on the number of lateral root primordia and emerged lateral roots in Steptoe. Thus ABA accumulation (and its crosstalk with auxin) in root primordia seems important in regulating lateral root branching in response to water stress.

24-epibrassinolide improves cadmium tolerance and lateral root growth associated with regulating endogenous auxin and ethylene in Kentucky bluegrass

The application of phytohormones is a viable technique to increase the efficiency of phytoremediation in heavy metal-contaminated soils. The objective of this study was to determine how the application of 24-epibrassinolide (EBR), a brassinosteroid analog, could regulate root growth and tolerance to cadmium (Cd) stress in Kentucky bluegrass. As a result, the number of lateral root primordia and total root length in the Cd-treated seedlings decreased by 33.1 % and 56.5 %, respectively. After the application of EBR, Cd accumulation in roots and leaves, and the negative effect of Cd on root growth were reduced under Cd stress. Additionally, the expression of the brassinosteroid signaling gene PpBRI1 was significantly upregulated by exogenous EBR. Moreover, exogenous EBR upregulated the expression of genes encoding antioxidant enzymes and improved the activity of antioxidant enzymes, thereby reduced oxidative stress in roots. Finally, targeted hormonomics analysis highlighted the utility of the application of EBR to alleviate the effect of Cd on the reduction in auxin (IAA) content and the increase in ethylene (ACC) content. These were known to be associated with the upregulation in the expression of auxin biosynthesis gene PpYUCCA1 and downregulation in the expression of ethylene biosynthesis gene PpACO1 in the roots treated with Cd stress. Overall, the application of EBR alleviated Cd-induced oxidative stress in addition to improving root elongation and lateral root growth crosstalk with auxin and ethylene in Kentucky bluegrass subjected to Cd stress. This study further highlights the potential role of brassinosteroids in improving the efficiency of phytoremediation for Cd-contaminated soils.

Analysis of unigenes involved in lateral root development in Bupleurum chinense and B. scorzonerifolium.

We identified IAA13 negatively associated with lateral root number by comparing the differential expressed genes between Bupleurum chinense and B. scorzonerifolium. Dried roots of the genus Bupleurum L. are used as a herbal medicine for diseases in Asia. Bupleurum chinense has a greater number of lateral roots than B. scorzonerifolium, but the genetic mechanisms for such differences are largely unknown. We (a) compared the transcriptome profiles of the two species and (b) identified a subset of candidate genes involved in auxin signal transduction and explored their functions in lateral root development. By isoform sequencing (Iso-Seq) analyses of the whole plant, more unigenes were found in B. scorzonerifolium (118,868) than in B. chinense (93,485). Given the overarching role of indole-3-acetic acid (IAA) as one of the major regulators of lateral root development, we identified 539 unigenes associated with auxin signal transduction. Fourteen and 44 unigenes in the pathway were differentially expressed in B. chinense and B. scorzonerifolium, respectively, and 3 unigenes (LAX2, LAX4, and IAA13) were expressed in both species. The number of lateral root primordia increased after exogenous auxin application at 8h and 12h in B. scorzonerifolium and B. chinense, respectively. Since overexpression of IAA13 in Arabidopsis reduced the number of lateral roots, we hypothesized that IAA13 is involved in the reduction of the number of lateral roots in B. scorzonerifolium.

Microcystin-LR, a cyanobacterial toxin affects root development by changing levels of PIN proteins and auxin response in Arabidopsis roots

Microcystin-LR (MCY-LR) is a heptapeptide toxin produced mainly by freshwater cyanobacteria. It strongly inhibits protein phosphatases PP2A and PP1. Functioning of the PIN family of auxin efflux carriers is crucial for plant ontogenesis and their functions depend on their reversible phosphorylation. We aimed to reveal the adverse effects of MCY-LR on PIN and auxin distribution in Arabidopsis roots and its consequences for root development.Relatively short-term (24 h) MCY-LR treatments decreased the levels of PIN1, PIN2 and PIN7, but not of PIN3 in tips of primary roots. In contrast, levels of PIN1 and PIN2 increased in emergent lateral roots and their levels depended on the type of PIN in lateral root primordia. DR5:GFP reporter activity showed that the cyanotoxin-induced decrease of auxin levels/responses in tips of main roots in parallel to PIN levels. Those alterations did not affect gravitropic response of roots. However, MCY-LR complemented the altered gravitropic response of crk5-1 mutants, defective in a protein kinase with essential role in the correct membrane localization of PIN2. For MCY-LR treated Col-0 plants, the number of lateral root primordia but not of emergent laterals increased and lateral root primordia showed early development. In conclusion, inhibition of protein phosphatase activities changed PIN and auxin levels, thus altered root development. Previous data on aquatic plants naturally co-occurring with the cyanotoxin showed similar alterations of root development. Thus, our results on the model plant Arabidopsis give a mechanistic explanation of MCY-LR phytotoxicity in aquatic ecosystems.

The WUSCHEL Related Homeobox Protein WOX7 Regulates the Sugar Response of Lateral Root Development in Arabidopsis thaliana

Sugars promote lateral root formation at low levels but become inhibitory at high C/N or C/P ratios. How sugars suppress lateral root formation is unclear, however. Here we report that WOX7, a member of the WUSCHEL related homeobox (WOX) family transcription factors, inhibits lateral root development in a sugar-dependent manner. The number of lateral root primordia increased in wox7 mutants but decreased in plants over-expressing WOX7. Plants expressing the WOX7-VP16 fusion protein produced even more lateral roots than wox7, suggesting that WOX7 acts as a transcriptional repressor in lateral root development. WOX7 is expressed at all stages of lateral root development, but it is primarily involved in lateral root initiation. Consistent with this, the wox7 mutant had a higher mitotic activity only at early stages of lateral root development. Further studies suggest that WOX7 regulates lateral root development through direct repression of cell cycle genes, particularly CYCD6;1. WOX7 expression was enhanced by sugar, reduced by auxin, but did not respond to salt and mannitol. In the wox7 mutant, the effect of sugar on lateral root formation was mitigated. These results together suggest that WOX7 plays an important role in coupling the lateral root development program and sugar status in plants.

The Emerging Role of Reactive Oxygen Species Signaling during Lateral Root Development

Overall root architecture is the combined result of primary and lateral root growth and is influenced by both intrinsic genetic programs and external signals. One of the main questions for root biologists is how plants control the number of lateral root primordia and their emergence through the main root. We recently identified S-phase kinase-associated protein2 (SKP2B) as a new early marker for lateral root development. Here, we took advantage of its specific expression pattern in Arabidopsis (Arabidopsis thaliana) in a cell-sorting and transcriptomic approach to generate a lateral root-specific cell sorting SKP2B data set that represents the endogenous genetic developmental program. We first validated this data set by showing that many of the identified genes have a function during root growth or lateral root development. Importantly, genes encoding peroxidases were highly represented in our data set. Thus, we next focused on this class of enzymes and showed, using genetic and chemical inhibitor studies, that peroxidase activity and reactive oxygen species signaling are specifically required during lateral root emergence but, intriguingly, not for primordium specification itself.

Histological, cytological and biochemical alterations induced by microcystin-LR and cylindrospermopsin in white mustard (Sinapis albaL.) seedlings

This study compares the histological, cytological and biochemical effects of the cyanobacterial toxins microcystin-LR (MCY-LR) and cylindrospermopsin (CYN) in white mustard (Sinapis alba L.) seedlings, with special regard to the developing root system. Cyanotoxins induced different alterations, indicating their different specific biochemical activities. MCY-LR stimulated mitosis of root tip meristematic cells at lower concentrations (1 μg ml-1) and inhibited it at higher concentrations, while CYN had only inhibitory effects. Low CYN concentrations (0.01 μg ml-1) stimulated lateral root formation, whereas low MCY-LR concentrations increased only the number of lateral root primordia. Both inhibited lateral root development at higher concentrations. They induced lignifications, abnormal cell swelling and inhibited xylem differentiation in roots and shoots. MCY-LR and CYN induced the disruption of metaphase and anaphase spindles, causing altered cell divisions. Similar alterations could be related to decreased protein phosphatase (PP1 and PP2A) activities in shoots and roots. However, in vitro phosphatase assay with purified PP1 catalytic subunit proved that CYN in contrast to MCY-LR, decreased phosphatase activities of mustard in a non-specific way. This study intends to contribute to the understanding of the mechanisms of toxic effects of a protein phosphatase (MCY-LR) and a protein synthesis (CYN) inhibitory cyanotoxin in vascular plants.

The Snf1-related protein kinases SnRK2.4 and SnRK2.10 are involved in maintenance of root system architecture during salt stress.

The sucrose non-fermenting-1-related protein kinase 2 (SnRK2) family represents a unique family of plant-specific protein kinases implicated in cellular signalling in response to osmotic stress. In our studies, we observed that two class 1 SnRK2 kinases, SnRK2.4 and SnRK2.10, are rapidly and transiently activated in Arabidopsis roots after exposure to salt. Under saline conditions, snrk2.4 knockout mutants had a reduced primary root length, while snrk2.10 mutants exhibited a reduction in the number of lateral roots. The reduced lateral root density was found to be a combinatory effect of a decrease in the number of lateral root primordia and an increase in the number of arrested lateral root primordia. The phenotypes were in agreement with the observed expression patterns of genomic yellow fluorescent protein (YFP) fusions of SnRK2.10 and -2.4, under control of their native promoter sequences. SnRK2.10 was found to be expressed in the vascular tissue at the base of a developing lateral root, whereas SnRK2.4 was expressed throughout the root, with higher expression in the vascular system. Salt stress triggered a rapid re-localization of SnRK2.4–YFP from the cytosol to punctate structures in root epidermal cells. Differential centrifugation experiments of isolated Arabidopsis root proteins confirmed recruitment of endogenous SnRK2.4/2.10 to membranes upon exposure to salt, supporting their observed binding affinity for the phospholipid phosphatidic acid. Together, our results reveal a role for SnRK2.4 and -2.10 in root growth and architecture in saline conditions.

Oligosaccharins and Pectimorf® stimulate root elongation and shorten the cell cycle in higher plants

The aim was to test promotive effects of oligosaccharins on root growth and development at the root apical meristem and the cell cycle using the model systems, Arabidopsis thaliana and the tobacco (Nicotiana tabacum) BY-2 cell line. Arabidopsis was grown on medium supplemented with 0.1 mg L−1 oligoxyloglucan (OX), 10 mg L−1 Pectimorf® (P) or 0.5 mg L−1 indole butyric acid (IBA). Primary root length, number of lateral root primordia, root apical meristem (RAM) length and epidermal cell length were recorded. Three genotypes were used: wild type (WT) and transgenic lines expressing either Schizosaccharomyces pombe (Sp) cdc25 or over-expressing(oe) Arath;WEE1. All treatments promoted primary root elongation but repressed lateral root production. Only P had a clear positive effect on meristem length whereas all other genotype × treatment interactions showed shorter RAMs. Whilst IBA, OX and P induced an increase in cell length in Spcdc25, the same treatments caused a significant decrease in WEE1 oe . Mitotic indices were also significantly higher in roots treated with oligosaccharins suggesting a shortening of the cell cycle. This hypothesis was tested in the BY-2 cell line. Both OX and P shortened the cell cycle exclusively through a shortening of G1 whilst mitotic cell size remained constant between treatments. In conclusion, both OX and P do indeed stimulate growth and shorten the cell cycle in higher plants and at the cellular level are able to reverse large and small cell size phenotypes normally exhibited by WEE1 oe and Spcdc25 genotypes, respectively.

Plant Development: Early Events in Lateral Root Initiation

How are the lateral root founder cells specified in the pericycle to initiate lateral root development? An Aux/IAA28 signaling module activates transcription factor GATA23 to control founder cell identity.

SKP2A protein, an F-box that regulates cell division, is degraded via the ubiquitin pathway

The ubiquitin pathway is emerging as a powerful system that controls the stability of key regulatory proteins. In plants, this pathway plays an important role in controlling several developmental processes, responses to environmental changes and also cell division. Arabidopsis SKP2A is an F-box protein that regulates the stability of the E2FC-DPB transcription factor, a repressor of cell proliferation. Although the function of SKP2A is to recruit targets for degradation, we have shown that SKP2A is also degraded through the Ub/26S pathway and, interestingly, auxin stimulates such degradation. Overexpression of SKP2A positively regulates cell division, increasing the number of cells in G2/M, reducing the level of ploidy and developing higher number of lateral root primordia. In addition, we showed in this report that overexpression of SKP2A increased the survival of Arabidopsis plants when they grown on a medium with high levels of sucrose, likely by maintaining cell division active. Thus, it is likely that SKP2A connects cell division with stress responses. Addendum to: Jurado S, Díaz-Triviño S, Abraham Z, Manzano C, Gutierrez C, Del Pozo C. SKP2A, an F-box protein that regulates cell division, is degraded via the ubiquitin pathway. Plant J 2008; 53:828-41.

SKP2A, an F‐box protein that regulates cell division, is degraded via the ubiquitin pathway

Coordination between cell division and cell differentiation is crucial for growth and development of eukaryotic organisms. Progression through the different phases of cell division requires the specific degradation of proteins through the ubiquitin/proteasome 26S (Ub/26S) pathway. In plants, this pathway plays a key role in controlling several developmental processes and responses, including cell proliferation. SKP2A, an F-box protein, regulates the stability of the cell division E2FC-DPB transcription factor. Here, we show that the SKP2A forms a Skp, Cullin containing (SCF) complexin vivo that has E3 ubiquitin ligase activity. Interestingly, SKP2A is degraded through the Ub/26S pathway, and auxin regulates such degradation. SKP2A positively regulates cell division, at least in part by degrading the E2FC/DPB transcription repressor. Plants that overexpress SKP2A increase the number of cells in G2/M, reduce the level of ploidy and develop a higher number of lateral root primordia. Taken together, our results indicate that SKP2A is a positive regulator of cell division, and its stability is controlled by auxin-dependent degradation.

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Abstract: Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition. Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter, compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone‐like when seedlings were grown under conditions of low phosphate availability. Herringbone‐like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattern.(Managing editor: Wei WANG)

Control of root growth: effects of carbohydrates on the extension, branching and rate of respiration of different fractions of wheat roots

Rates of extension, numbers of laterals and rates of respiration were measured in different fractions of wheat (Triticum aestivum L. cv. Alexandria) roots following changes in carbohydrate supply. The supply of carbohydrate was varied by selective pruning and exogenously fed sugars. Pruning shoots to a single leaf (leaf‐pruning) reduced the rate of O2 uptake by intact roots. Rates were not stimulated by shortterm feeding of sucrose (25 mM), but were stimulated by the uncoupler p‐trifluoro‐methoxy(carbonylcyanide)phenylhydrazone (FCCP). Feeding glucose to roots of leaf‐pruned and non‐pruned plants for 16–24 h increased the rate of O2 uptake. It is concluded that respiration is under fine control by adenylates and coarse control by carbohydrate supply, with carbohydrates regulating directly the rate of some energy consuming process(es). These energy consuming processes are located in growing tissue fractions. Feeding glucose to leaf‐pruned and non‐pruned plants increased rates of O2 uptake in seminal root tips, the zone of developing lateral primordia and mature root sections with elongating laterals, but had no effect on mature sections from which the laterals had been excised. Leaf‐pruning reduced the extension rate of seminal axes and first‐order laterals when measured over 24 h. Feeding glucose to roots from the time of pruning increased the rate, but did not fully restore it to control values. Pruning roots to a single seminal axis (root‐pruning) and feeding glucose to non‐pruned plants had no effect on the extension rate of the seminal axis or its laterals over this time period, although rates were increased by root‐pruning when measured over 3 days. The number of lateral root primordia was reduced by leaf‐pruning and increased by root‐pruning and feeding glucose. The results are discussed in terms of the role of carbohydrates in the control of root growth and branching.

The Control of Lateral Root Development in Cultured Pea Seedlings. III. Spacing Intervals

AbstractThe spacing of lateral root primordia in the primary root of Pisum sativum (cv. Alaska) seedlings is influenced by both predetermined lateral root initiation sites in the embryonic radicle and by factors present during seedling growth. When pea seeds were germinated in the presence of the mitotic inhibitor, colchicine, the triarch radicle produced three ranks of primordiomorphs indicating sites of embryonic lateral root primordia. The number of primordiomorphs was not the same along the three xylem strands in the radicle. Normally germinated seedling roots (5 days old) also showed a different number of lateral root primordia associated with the three strands. In both cases, the strand with the greatest number of primordia (or primordiomorphs) was associated with a cotyledonary trace. This indicated a possible role for the cotyledons in setting the pattern of lateral root distribution during radicle development. The spacing of lateral root primordia could be altered by the application of growth regulators. Seedling root tips (2 mm) were removed (− rt) and replaced with indoleacetic acid (+IAA), and in some instances seedlings were also treated with the auxin transport inhibitor, 3,3a‐dihydro‐2‐(p‐methoxyphenyl)‐8H‐pyrazolo[5, 1‐α]isoindol‐8‐one (+DPX). In the growth regulator treatments, primary root elongation was inhibited, a greater number of lateral root primordia were initiated compared to controls, and the spacing intervals between primordia were greatly reduced. The — rt, +IAA, +DPX‐treatment resulted in the closest possible spacing intervals (av. 0.4 − 0.6 mm), but resulted in fused or fasciated laterals. The — rt, + IAA‐treatment produced the shortest spacing intervals which resulted in “normal” lateral roots (0.8 − 1.1 mm).

Rapidity of cellular changes induced by zinc in a zinc tolerant and non‐tolerant cultivar of Festuca rubra L.

summary7 day‐old seedlings of a Zn‐tolerant cultivar (Merlin) and a non‐tolerant cultivar (S59) of Festuca rubra L. were transferred to aerated solution culture containing 3 concentrations of Zn (0, 80 and 120 μM). Roots were sampled at 0, 6, 12, 24, 48, 72 and 96 h following transfer. Zn markedly reduced root extension in S59; this inhibition was evident after only 12 h exposure. In contrast Zn stimulated the root extension of Merlin. The length of the apical meristem of S59 was reduced within 12 h after transfer to the Zn‐containing solutions, followed by a further reduction to 69 % and 61 % of that of control roots (0 μM Zn) after 96 h at 80 μM Zn and 120 μM Zn respectively. The meristem length of Merlin was much less affected by Zn. Treatment with 80 μM Zn resulted in a stimulation in the number of lateral root primordia in both cultivars but their formation was depressed relative to the control roots at 120 μM Zn in S59, though not in Merlin. The area of mitotic cells was reduced by 25 % after only 12 h exposure to 120 μM Zn in S59 and this reduction increased with duration of exposure. However, in Merlin, the area of mitotic cells increased with increasing length of exposure to Zn. The protein content of mitotic cells of S59 was reduced by 28 %, relative to that of roots grown in the control solutions, after 12 h exposure to 120 μM. Zn, culminating in a 50 % reduction after 96 h exposure. However, the cellular protein content of Merlin increased progressively with time of exposure to Zn. The sensitivity and rapidity of response of these cellular changes to Zn in S59 may make them useful early markers of metal toxicity in roots.

Regulation of lateral root formation in lettuce (Lactuca sativa) seedling roots: Interacting effects of α‐naphthaleneacetic acid and kinetin

Growth substances, α‐naphthaleneacetic (NAA) and kinetin, had an important role in the regulation of lateral root (LR) formation in lettuce (Lactuca sativa L. cv. Grand Rapids) seedling roots. NAA (10‐5M) was a potent stimulator of LR initiation and caused a 600% increase in the number of lateral root primordia (LRP) compared to untreated roots. NAA was required for only the first 20 h of the 72 h treatment period for maximum stimulation of LRP initiation. Kinetin (2 × 10‐5M) effectively prevented the spontaneous formation of LRP and inhibited the NAA‐stimulated production of LRP. Kinetin inhibition was maximal during the first 20 h of NAA treatment and this effect was not overcome by subsequent supply of NAA. Also, lettuce roots were most sensitive to kinetin at 20 h of NAA treatment, when the first signs of cell division were observed in the pericycle.

Increase in Size and Cell Number of Lateral Root Primordia in the Primary of Intact Plants and in Excised Roots of Pisum sativum and Vicia faba

Increase in cell number, and in anlage volume and length have been investigated during the development of lateral root primordia in roots of intact plants of Pisum sativum and Vicia faba and in excised roots of both species cultured in White's medium supplemented with 2% sucrose. With the exception of primordia in excised roots of Vicia, the general equation which best described increase in each aspect of primoridium growth measured against time was that for exponential growth. When the times necessary for cell number and primordium volume and length to double were determined at intervals over the period of development studied, however, they were found to vary. Similarly, estimates of the size of the proliferative fraction of cells at different times during anlage development indicated that this index of meristematic activity also fluctuated over the developmental period investigated, i.e., increase in cell number and in primordium volume and length do not occur in a truly exponential fashion as the primordia increase in size and cell number. One difference between anlage development in the roots of intact plants and in those grown in culture was that whereas the former primordia completed their development and emerged as lateral roots over the period of the investigation, the latter did not. Moreover, cell doubling time and anlage volume and length doubling times were longer, and the proliferation fraction of cells lower, over the whole period of, and at intervals during, primordium development in the excised roots compared with the results obtained for the roots of the corresponding intact plants.

The possible rôle of xanthoxin in plant growth and development

In higher plants, abscisic acid and xanthoxin are two potent growth regulators. Although similar properties in both substances have been demonstrated in several biological tests including biochemical interconversion of the substances, evidence is available that in the plant as a whole, xanthoxin has regulatory functions other than those of abscisic acid. Several environmental factors, such as water supply, photoperiod and low temperature, which affect growth and development also greatly change the level of abscisic acid in the plant; however, only small variations in the xanthoxin level have been observed in response to changes in the environmental conditions. On the other hand, a strong enhancement of the xanthoxin level can be induced when dark-grown seedlings are briefly illuminated; this treatment, however, has no influence on the abscisic acid level. This observation supports the hypothesis that light-induced inhibition of growth may be mediated by an increased formation of the growth inhibitor xanthoxin. Light-induced enhancement of the xanthoxin level may also contribute to the phototropic bending in dictyledonous seedlings. Evidence has been obtained from experiments in this laboratory that xanthoxin may be involved in the regulation of root branching. Decapitation of root tips causes a significant increase in the number of lateral root primordia. Chromatographic studies reveal the presence of two substances in the root, which, in a specific bioassay, are active inhibitors of the development of root primordia. The activity of these root inhibitors in the basal part decreases when the root tip is removed. They are probably produced in the root tip and are transported to the base. One of these inhibitors has been identified as xanthoxin, the other is cytokinin. The hormonal regulation of abscission is another process where xanthoxin may have a regulatory function. Senescent, abscinding petioles contain a factor called ‘senescence factor’ which promotes the abscission of leaves. In an attempt to identify its chemical nature, it was found that at least three different abscission accelerating substances, including xanthoxin, participate in the composition of the senescence factor.

Apical dominance in roots of Pisum sativum L.

The influence of several natural and synthetic hormones on apical dominance of pea roots was investigated. Extracts of roots decapitated just before extraction or 12 h earlier were compared in a lateral root formation assay, in the Amaranthus assay for cytokinins and the Avena coleoptile assay. Indole-3-acetic acid shows a strong promotive effect on the initiation of lateral roots. Gibberellic acid has no significant effect. Abscisic acid and its presumable precursor xanthoxin show a strong inhibitory effect, and so do all cytokinins investigated (kinetin, benzyladenine, and zeatin). Decapitation of roots caused a significant increase in the number of lateral root primordia within 12 h. Paper chromatograms of root extracts indicate the presence of at least two inhibitors of lateral root formation in the alkaline fraction, and of two promoters in the acid fraction. The amount of both inhibitors seems to decrease if the tip of the main root is removed 12 h before extraction whereas the promoters of lateral root formation do not change in their concentration. The Amaranthus test shows cytokinin activity in the region of one of the two inhibitors, and this activity decreases after decapitation. The second inhibitor has the Rf of xanthoxin; it inhibits the growth of Avena coleoptiles and its activity also decreases after decapitation.