Regulating Root Elongation Research Articles

Light modulates the root tip excision induced lateral root formation in tomato

During plant growth and development, root tip performs multifarious functions integrating diverse external and internal stimuli to regulate root elongation and architecture. It is believed that a signal originating from root tip inhibits lateral root formation (LRF). The excision of root tip induced LRF in tomato seedlings associated with accumulation of auxin in pericycle founder cells. The excision of cotyledons slightly reduced LRF, whereas severing shoot from root completely abolished LRF. Exogenous ethylene application did not alter LRF. The response was modulated by light with higher LRF in seedlings exposed to light. Our results indicate that light plays a role in LRF in seedlings by likely modulating shoot derived auxin.

Sulfur nutrient availability regulates root elongation by affecting root indole-3-acetic acid levels and the stem cell niche.

Sulfur is an essential macronutrient for plants with numerous biological functions. However, the influence of sulfur nutrient availability on the regulation of root development remains largely unknown. Here, we report the response of Arabidopsis thaliana L. root development and growth to different levels of sulfate, demonstrating that low sulfate levels promote the primary root elongation. By using various reporter lines, we examined in vivo IAA level and distribution, cell division, and root meristem in response to different sulfate levels. Meanwhile the dynamic changes of in vivo cysteine, glutathione, and IAA levels were measured. Root cysteine, glutathione, and IAA levels are positively correlated with external sulfate levels in the physiological range, which eventually affect root system architecture. Low sulfate levels also downregulate the genes involved in auxin biosynthesis and transport, and elevate the accumulation of PLT1 and PLT2. This study suggests that sulfate level affects the primary root elongation by regulating the endogenous auxin level and root stem cell niche maintenance.

Ethylene-induced changes in lignification and cell wall-degrading enzymes in the roots of mungbean (Vigna radiata) sprouts

As an important regulator, ethylene inhibits root growth and development in plants. To determine the mechanism of ethylene on root elongation growth and lateral root formation, ethylene-induced lignification and cell wall-degrading enzymes in the roots of mungbean sprouts were tested. We initially observed that primary root elongation and lateral root numbers were inhibited, while lignin content was enhanced by ethephon (ETH). Cell wall remolding proteins, polygalacturonase (PG) and carboxymethyl cellulose (Cx) activities were reduced, but α-expansins and xyloglucan endotransglucosylases/hydrolases (XTH) were enhanced by ETH. The promotion in lignin production was correlated with changes in activities of key lignin biosynthesis enzymes and hydrogen peroxide (H2O2) content. These actions induced by ETH were altered via treatment with an ethylene perception antagonist (Ag+). We subsequently demonstrated that the role of endogenous ethylene in regulating root elongation growth and lateral root formation were correlated with lignification and cell wall-degrading enzymes, respectively. These results suggested that the ethylene-regulated inhibition of primary root elongation growth was caused by an increase in lignification that reinforced the cell wall and shortened root length, and the suppression of lateral root formation was linked to activities of PG, Cx, α-expansins and XTH.

Auxin–ethylene interaction in transversal and longitudinal growth in maize primary root

The auxin—ethylene interaction in regulating root elongation and radial expansion was analysed through the application of the ethylene precursor 1-aminocyclopropane-1- carboxylic acid (ACC) and the synthetic auxin 1-naphthaleneacetic acid (NAA). NAA and ACC, in the concentration range of 0.01–0.1 μmol/L and 1–5 μmol/L, respectively, inhibited root elongation and increased root diameter in a concentration-dependent manner. The inhibitory effect of auxin on root elongation was increased by ethylene applied at low as well as high concentrations. Auxin-induced radial expansion was also enhanced by ethylene, although this effect depended on the level of auxin applied. In the presence of very low exogenous auxin (0.01 μmol/L NAA), ethylene stimulated radial growth at both low and high levels (1–5 μmol/L ACC), whereas the effect of higher auxin concentrations (>0.025 μmol/L NAA) was only enhanced by high ethylene production (5 μmol/L ACC). Interaction was analysed by two-way ANOVA; the results showed that both the effect of each hormone and the interactions responsible for controlling elongation and radial expansion were concentration-dependent. The relative effects of auxin and ethylene on the two growth parameters were also analysed; they showed different types of behaviour. In summary, the results obtained suggest that auxin is the main hormone involved in regulating root growth but that ethylene may modulate its effects. Our research also suggests that root elongation and radial expansion may be independently regulated despite being closely related.

Auxin increases the hydrogen peroxide (H2O2) concentration in tomato (Solanum lycopersicum) root tips while inhibiting root growth

The hormone auxin and reactive oxygen species (ROS) regulate root elongation, but the interactions between the two pathways are not well understood. The aim of this study was to investigate how auxin interacts with ROS in regulating root elongation in tomato, Solanum lycopersicum. Wild-type and auxin-resistant mutant, diageotropica (dgt), of tomato (S. lycopersicum 'Ailsa Craig') were characterized in terms of root apical meristem and elongation zone histology, expression of the cell-cycle marker gene Sl-CycB1;1, accumulation of ROS, response to auxin and hydrogen peroxide (H2O2), and expression of ROS-related mRNAs. The dgt mutant exhibited histological defects in the root apical meristem and elongation zone and displayed a constitutively increased level of hydrogen peroxide (H2O2) in the root tip, part of which was detected in the apoplast. Treatments of wild-type with auxin increased the H2O2 concentration in the root tip in a dose-dependent manner. Auxin and H2O2 elicited similar inhibition of cell elongation while bringing forth differential responses in terms of meristem length and number of cells in the elongation zone. Auxin treatments affected the expression of mRNAs of ROS-scavenging enzymes and less significantly mRNAs related to antioxidant level. The dgt mutation resulted in resistance to both auxin and H2O2 and affected profoundly the expression of mRNAs related to antioxidant level. The results indicate that auxin regulates the level of H2O2 in the root tip, so increasing the auxin level triggers accumulation of H2O2 leading to inhibition of root cell elongation and root growth. The dgt mutation affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.

The Arabidopsis B‐type response regulator 18 homomerizes and positively regulates cytokinin responses

In higher plants, the two-component system (TCS) is a signaling mechanism based on a His-to-Asp phosphorelay. The Arabidopsis TCS involves three different types of proteins, namely the histidine kinases (AHKs), the histidine phosphotransfer proteins (AHPs) and the response regulators (ARRs). The ARRs comprise three different families, namely A, B and C types, according to their protein structure. While some members of the B-type family of ARRs have been studied extensively and reported to act as DNA-binding transcriptional regulators, very limited information is available for other B-type ARRs such as ARR18. In this study, we characterize in detail the molecular and functional properties of ARR18. ARR18 acts as a transcriptional regulator in plant cells and forms homodimers in planta as shown by FRET-FLIM studies. As demonstrated by mutational analysis, the aspartate at position 70 (D70) in the receiver domain of ARR18 acts as crucial phosphorylation site. The modification of D70 affects the response regulator's ability to homodimerize and to activate its target genes. Furthermore, physiological investigations of Arabidopsis lines ectopically expressing ARR18 introduce ARR18 as a new member within the cytokinin-regulated response pathway regulating root elongation.

Soil physical strength rather than excess ethylene reduces root elongation of Eucalyptus seedlings in mechanically impeded sandy soils

Seedling establishment in heavily compact soils is hampered by poor root growth caused by soil chemical or physical factors. This study aims to determine the role of ethylene in regulating root elongation through mechanically impeded sandy soils using Eucalyptus todtiana F. Muell seedlings. Concentrations of ethephon (1, 10, and 100 μM) were added to non-compact soils, and endogenous ethylene production from seedling roots was compared to ethylene production of roots grown in physically compacted field soils (98.6 % sand). The ethylene-inhibitor 3,5-diiodo-4-hydroxybenzoic acid (DIHB) (0.1 μM) was included for each treatment to counteract the negative effects of excess ethylene or compact soils on root elongation. Root elongation was reduced in high ethylene soils by 49 % and high bulk density soils by 44 %. Root ethylene production increased ninefold in roots grown in the high ethylene environment (100 μM), but decreased 80 % in compact soils. The use of DIHB did not alter root length and produced varying results with respect to ethylene production, suggesting an interaction effect involving high amounts of soil ethylene. While ethylene regulates root growth, the physical strength of sandy soils is the major factor limiting root elongation in mechanically impeded soils.

A novel histidine kinase gene, ZmHK9, mediate drought tolerance through the regulation of stomatal development in Arabidopsis

Plants have developed complex signaling networks to regulate biochemical and physiological acclimation, environmental signals were perceived and transmitted to cellular machinery to activate adaptive responses. Here, a novel drought responsive histidine kinase gene was identified and designated as ZmHK9. Under normal conditions, ZmHK9 was predominantly expressed in roots, and the roots of ZmHK9-OX transgenic lines are markedly hypersensitive to ABA and ethylene, as compare to wild type. Consistent with its expression induced by PEG and exogenous ABA treatment, promoter sequence of this gene possessed drought and ABA responsive element. Moreover, the transgenic plants were much less affected by drought stress and recovered quickly after rewatering, stomatal complex size and stomatal density in the transgenic plants are significantly smaller and lower than those of the wild-type plants. In addition, ABA induced stomatal closure and the stomatal aperture of ZmHK9-OX lines was smaller than that of wild type. Collectively, it can be concluded that ZmHK9 regulates root elongation, stomatal development and drought tolerance through ABA dependent signaling pathway in Arabidopsis.

OsARF12, a transcription activator on auxin response gene, regulates root elongation and affects iron accumulation in rice (Oryza sativa)

• Auxin has an important role in maintaining optimal root system architecture (RSA) that can cope with growth reductions of crops caused by water or nutrient shortages. However, the mechanism of controlling RSA remains largely unclear. Here, we found a limiting factor of RSA--OsARF12--an auxin response factor whose knockout led to decreased primary root length in rice (Oryza sativa). • OsARF12 as a transcription activator can facilitate the expression of the auxin response element DR5::GFP, and OsARF12 was inhibited by osa-miRNA167d by transient expression in tobacco and rice callus. • The root elongation zones of osarf12 and osarf12/25, which had lower auxin concentrations, were distinctly shorter than for the wild-type, possibly as a result of decreased expression of auxin synthesis genes OsYUCCAs and auxin efflux carriers OsPINs and OsPGPs. The knockout of OsARF12 also altered the abundance of mitochondrial iron-regulated (OsMIR), iron (Fe)-regulated transporter1 (OsIRT1) and short postembryonic root1 (OsSPR1) in roots of rice, and resulted in lower Fe content. • The data provide evidence for the biological function of OsARF12, which is implicated in regulating root elongation. Our investigation contributes a novel insight for uncovering regulation of RSA and the relationship between auxin response and Fe acquisition.

Occlusion regulates tooth-root elongation during root development in rat molars

Occlusion is commenced by contact of a tooth with an opposing tooth and is the mechanical force working against the periodontal ligament (PDL). However, the influences of occlusion during root development remain uncertain. By extracting the unerupted counterpart molars of rats, we established a non-occlusal model that directly examined the effects of the absence of occlusion in developing molars using micro-computed tomography (μ-CT) and histological procedures. The μ-CT data for experimental molars confirmed no attrition and hypogenesis of the alveolar bone. Root lengths in experimental groups increased more than in control groups. Histological findings of experimental molars showed a wide crown pulp, a long and narrow root, immature Sharpey's fibers, and hypogenesis of cementum. Proliferating cells localized in Hertwig's epithelial root sheath (HERS), the apical pulp, and the PDL of experimental teeth. Furthermore, cell-proliferative activity in experimental roots exceeded that in normal roots. These data indicate that cell proliferation is decreased by occlusion during root formation. Thus, occlusion is one factor that regulates root elongation.

The short–root mutation srt5 defines a sugar-mediated root growth in rice ( Oryza sativa L.)

We previously reported a mutant, srt5, in which the short–root phenotype at the seedling stage could be partially rescued by exogenous abscisic acid (ABA) [Plant Sci. 163 (2002) 217]. In this paper, we describe that exogenously applied sucrose (Suc), glucose (Glu) or fructose (Fru), but not mannitol (Mtl) or glucose analogs, can also rescue root growth of the mutant to extents greater than ABA. Combined treatment reveals that root growth of the mutant under 1 μM ABA treatment can be further promoted by the addition of sugars. In contrast, application of ABA shows no promotive effects on 100 mM sugar-treated roots of srt5 seedlings. We also observed that endogenous sucrose contents in seeds and seedlings of srt5 are significantly lower, and starch breakdown in endosperm of srt5 is normal compared to wild-type. Furthermore, when treated with ABA, sucrose content in roots increases greatly in srt5, but decreases significantly in the wild-type. Collectively, these results suggest that energy deficiency is the cause of the srt5 phenotype, and ABA regulates root elongation of the mutant in a sugar-mediated way. The possible mechanism for ABA to promote root growth by enhancing the level of sugar in roots of srt5 is discussed.

RRL1, RRL2 and CRL2 loci regulating root elongation in rice.

To analyze the genetic mechanism regulating root elongation, we characterized five recessive mutants in rice (BRX65, BRX117, BRX430, BRX448 and crl2 mutant). In fifteen-day-old seedlings, the root length of BRX65 was 36% of that of the wild type root, while the length of the roots of BRX117, BRX430 and BRX448 was about 45% of that of the wild type root. These types of mutation were designated as reduced root length with the gene symbol rrl. In contrast, the length of the crl2 mutant root was 114% of that of the wild type root. Allelism test indicated that rrl mutations occurred in two complementation groups and that these mutant genes were not allelic to crl2. The first RRL locus was designated as RRL1 (BRX65) and the other locus as RRL2, and the alleles were designated as rrl2-1 (BRX117), rrl2-2 (BRX430) and rrl2-3 (BRX448). In the rrl1 mutant, only the mature cortical cell length was significantly shorter than that of the wild type, whereas, the values of the cell length, apical meristem size and cell flux of the rrl2-1 mutant root were all clearly lower than those of the wild type root. On the other hand, the values of the mature cortical cell length, apical meristem size and cell flux of the crl2 mutant root were all clearly higher than those of the wild type. Mature cortical cell length of the rrl1 crl2 double mutant was intermediate between that of the parental single mutants, while the cell length, meristem size and cell flux of the rrl2-1 crl2 double mutant were all intermediate between those of the parental single mutants, respectively. These results suggest that there are genes that inhibit or promote the maintenance of root apical meristem and cell elongation, and the opposite effects between these individual genes determine the extent of root elongation in rice.

Effect of zeatin on the growth and indolyl‐3‐acetic acid and abscisic acid levels in maize roots

Elongation, indolyl‐3‐acetic acid (IAA) and abscisic acid (ABA) levels, – gas chromatography‐mass spectrometry quantification –, in the elongating zone were analysed for maize (Zea mays L., Cv. LG11) roots immersed in buffer solution with or without zeatin (Z). The effect of Z depends on the initial extension rate of roots. The slower growing roots are more strongly inhibited by Z (10−7−10−5M) and they show a greater increase in IAA and ABA content. When compared to the rapidly growing roots, the larger reactivity of the 'slow’ones cannot be attributed to a higher Z uptake as shown when using [14C]‐Z. It is suggested that Z could regulate root elongation by acting on the IAA and/or ABA level. The comparative action of these two hormones is discussed.