Auxin Levels In Roots Research Articles

Actin-bundling protein fimbrin serves as a new auxin biosynthesis orchestrator in Arabidopsis root tips.

Plants delicately regulate endogenous auxin levels through the coordination of transport, biosynthesis, and inactivation, which is crucial for growth and development. While it is well-established that the actin cytoskeleton can regulate auxin levels by affecting polar transport, its potential role in auxin biosynthesis has remained largely unexplored. Using LC-MS/MS-based methods combined with fluorescent auxin marker detection, we observed a significant increase in root auxin levels upon deletion of the actin bundling proteins AtFIM4 and AtFIM5. Fluorescent observation, immunoblotting analysis, and biochemical approaches revealed that AtFIM4 and AtFIM5 affect the protein abundance of the key auxin synthesis enzyme YUC8 in roots. AtFIM4 and AtFIM5 regulate the auxin synthesis enzyme YUC8 at the protein level, with its degradation mediated by the 26S proteasome. This regulation modulates auxin synthesis and endogenous auxin levels in roots, consequently impacting root development. Based on these findings, we propose a molecular pathway centered on the 'actin cytoskeleton-26S proteasome-YUC8-auxin' axis that controls auxin levels. Our findings shed light on a new pathway through which plants regulate auxin synthesis. Moreover, this study illuminates a newfound role of the actin cytoskeleton in regulating plant growth and development, particularly through its involvement in maintaining protein homeostasis via the 26S proteasome.

Manganese toxicity elicits the degradation of auxin transport carriers to restrain arabidopsis root growth

Manganese (Mn) is an essential microelement for plant growth but can be toxic when in excessive. Mn toxicity-inhibited root growth possible involves in auxin biosynthesis and transport in plants, but the mechanism remains elusive. Our results have suggested that Mn-inhibited root growth was possible associated with a reduced auxin levels in roots, as shown in experiments with (1) transgenic DII-VENUS and DR5rev::GFP; (2) the application of L-kynurenine (an inhibitor of TAA1 activity), 1-naphthylacetic acid (NAA), 1-naphthylphthalamic acid (NPA) to manipulate auxin levels; (3) using taa1 (defection in auxin biosynthesis) mutants; and (4) determining IAA by enzyme-linked immunosorbent assay. Those results were explained via down-regulated transcriptional levels of auxin biosynthesis-related genes in plants, and reduced abundance of PM-localized auxin transport carriers in roots. Combination of auxin transport-related transgenic pPINs-GFP or pAUX1::AUX1-YFP lines and a real-time in vitro observation of PM-localized PIN2/3/4 carriers using transgenic Dendra2 lines have revealed that excessive Mn induced degradation of these proteins, which might involve in Mn toxicity-elicited up-regulation of partially ubiquitin-, clathrin-meidiated endocytosis (CME)-, and endosomal sorting complex required for transport (ESCRT)-related genes. Overall, our results suggest that Mn toxicity reduces auxin level in the root apex possible via down-regulation of auxin biosynthesis-related genes and post-translational stimulation of the degradation of auxin transport proteins, leading to root growth inhibition caused by Mn toxicity.

Disruption of the amino acid transporter CsAAP2 inhibits auxin-mediated root development in cucumber.

Amino acid transporters are the principal mediators of organic nitrogen distribution within plants and are essential for plant growth and development. Despite this importance, relatively few amino acid transporter genes have been explored and elucidated in cucumber (Cucumis sativus). Here, a total of 86 amino acid transporter genes were identified in the cucumber genome. We further identified Amino Acid Permease (AAP) subfamily members that exhibited distinct expression patterns in different tissues. We found that the CsAAP2 as a candidate gene encoding a functional amino acid transporter is highly expressed in cucumber root vascular cells. CsAAP2 knockout lines exhibited arrested development of root meristem, which then caused the delayed initiation of lateral root and the inhibition of root elongation. What is more, the shoot growth of aap2 mutants was strongly retarded due to defects in cucumber root development. Moreover, aap2 mutants exhibited higher concentrations of amino acids and lignin in roots. We found that the mutant roots had a stronger ability to acidize medium. Furthermore, in the aap2 mutants, polar auxin transport was disrupted in the root tip, leading to high auxin levels in roots. Interestingly, slightly alkaline media rescued their severely reduced root growth by stimulating auxin pathway.

Effects of exogenous melatonin on plant growth, root hormones and photosynthetic characteristics of trifoliate orange subjected to salt stress

Soil salinity has negative effects on crop growth and production, and melatonin (MT) plays an important role in regulating plant salt stress. However, it is not clear whether exogenous melatonin mitigates the negative effect on citrus plants subjected to salt stress. This study aimed to explore the response of exogenous melatonin (0, 50, 100, 150 µmol/L) on plant growth, root hormone levels and the photosynthetic system of trifoliate orange (Citrus (Poncirus) trifoliata L.) seedlings exposed to 0 or 150 mmol/L NaCl for 4 weeks. The 150 mmol/L NaCl treatment significantly increased root zeatin riboside, gibberellin and brassinolide levels, while dramatically reducing plant growth, root auxin levels, leaf photosynthesis and fluorescence indexes of seedlings. However, melatonin treatment partially ameliorated reductions in plant height and dry matter accumulation caused by salt stress. Melatonin (100 µmol/L) appears to interact with IAA but not the other hormones studied. Furthermore, the effects of NaCl stress on the net photosynthetic rate, stomatal conductance, maximum photochemical efficiency, PSII effective photon yield, photochemical quenching and other indicators of seedlings leaves were also partially alleviated and the damage of NaCl stress was also reduced when seedlings were treated with melatonin. This suggests that 100 µmol/L melatonin may be an effective treatment.

Nitrate Modulates Lateral Root Formation by Regulating the Auxin Response and Transport in Rice.

Nitrate () plays a pivotal role in stimulating lateral root (LR) formation and growth in plants. However, the role of in modulating rice LR formation and the signalling pathways involved in this process remain unclear. Phenotypic and genetic analyses of rice were used to explore the role of strigolactones (SLs) and auxin in -modulated LR formation in rice. Compared with ammonium (), stimulated LR initiation due to higher short-term root IAA levels. However, this stimulation vanished after 7 d, and the LR density was reduced, in parallel with the auxin levels. Application of the exogenous auxin α-naphthylacetic acid to -treated rice plants promoted LR initiation to levels similar to those under at 7 d; conversely, the application of the SL analogue GR24 to -treated rice inhibited LR initiation to levels similar to those under supply by reducing the root auxin levels at 10 d. D10 and D14 mutations caused loss of sensitivity of the LR formation response to . The application of and GR24 downregulated the transcription of PIN-FORMED 2 (PIN2), an auxin efflux carrier in roots. LR number and density in pin2 mutant lines were insensitive to treatment. These results indicate that LR formation by affecting the auxin response and transport in rice, with the involvement of SLs.

Potential involvement of root auxins in drought tolerance by modulating nocturnal and daytime water use in wheat.

The ability of wheat genotypes to save water by reducing their transpiration rate (TR) at times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA), has not been explored to explain the shoot-root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day. Eight double-haploid lines were selected from a mapping population descending from two parents with contrasting water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. We examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits. Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits. The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought.

Auxin Acts Downstream of Ethylene and Nitric Oxide to Regulate Magnesium Deficiency-Induced Root Hair Development in Arabidopsis thaliana.

This study examines the association of auxin with ethylene and nitric oxide (NO) in regulating the magnesium (Mg) deficiency-induced root hair development in Arabidopsis thaliana. With Mg deficiency, both ethylene and NO promoted the elevation of root auxin levels in roots by inducing the expression of AUXIN-RESISTANT1 (AUX1), PIN-FORMED 1 (PIN1) and PIN2 transporters. In turn, auxin stimulated ethylene and NO production by activating the activities of 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO), ACC synthase (ACS), nitrate reductase (NR) and NO synthase-like (NOS-L). These processes constituted an NO/ethylene-auxin feedback loop. Interestingly, however, the roles of ethylene and NO in regulating Mg deficiency-induced root hair development required the action of auxin, but not vice versa. In summary, these results suggest that Mg deficiency induces a positive interaction between the accumulation of auxin and ethylene/NO in roots, with auxin acting downstream of ethylene and NO signals to regulate Mg deficiency-induced root hair morphogenesis.

Glucose inhibits root meristem growth via ABA INSENSITIVE 5, which represses PIN1 accumulation and auxin activity in Arabidopsis

Glucose functions as a hormone-like signalling molecule that modulates plant growth and development in Arabidopsis thaliana. However, the role of glucose in root elongation remains elusive. Our study demonstrates that high concentrations of glucose reduce the size of the root meristem zone by repressing PIN1 accumulation and thereby reducing auxin levels. In addition, we verified the involvement of ABA INSENSITIVE 5 (ABI5) in this process by showing that abi5-1 is less sensitive to glucose than the wild type, whereas glucose induces ABI5 expression and the inducible overexpression of ABI5 reduces the size of the root meristem zone. Furthermore, the inducible overexpression of ABI5 in PIN1::PIN1-GFP plants reduces the level of PIN1-GFP, but glucose reduces the level of PIN1-GFP to a lesser extent in abi5-1 PIN1::PIN1-GFP plants than in the PIN1::PIN1-GFP control, suggesting that ABI5 is involved in glucose-regulated PIN1 accumulation. Taken together, our data suggest that ABI5 functions in the glucose-mediated inhibition of the root meristem zone by repressing PIN1 accumulation, thus leading to reduced auxin levels in roots.

Multiple Facets ofArabidopsisSeedling Development Require 
Indole-3-Butyric Acid–Derived Auxin

Levels of auxin, which regulates both cell division and cell elongation in plant development, are controlled by synthesis, inactivation, transport, and the use of storage forms. However, the specific contributions of various inputs to the active auxin pool are not well understood. One auxin precursor is indole-3-butyric acid (IBA), which undergoes peroxisomal β-oxidation to release free indole-3-acetic acid (IAA). We identified ENOYL-COA HYDRATASE2 (ECH2) as an enzyme required for IBA response. Combining the ech2 mutant with previously identified iba response mutants resulted in enhanced IBA resistance, diverse auxin-related developmental defects, decreased auxin-responsive reporter activity in both untreated and auxin-treated seedlings, and decreased free IAA levels. The decreased auxin levels and responsiveness, along with the associated developmental defects, uncover previously unappreciated roles for IBA-derived IAA during seedling development, establish IBA as an important auxin precursor, and suggest that IBA-to-IAA conversion contributes to the positive feedback that maintains root auxin levels.

Further investigation of the roles of auxin and cytokinin in the NH4+-induced stimulation of nodulation using white clover transformed with the auxin-sensitive reporter GH3:gusA

Although mineral N (nitrate and ammonium) is believed to have generally negative effects on nodulation in legume-rhizobia symbioses, previous studies have shown that low, static concentrations of ammonium stimulate nodulation in pea, and that this enhancement may be due to an elevation in cytokinin to auxin levels in roots. Here, the effects of ammonium (0.0, 0.1, 0.5 and 2.5mM) on nodulation and auxin levels were investigated in wild-type (WT) white clover (Trifolium repens cv. Haifa) and its transformants (lines 38 and 41) which contain the auxin-sensitive reporter gene (GH3:gusA). The effects of exogenous application (10 -10 , 10 -9 and 10 -8 M) of the cytokinin 6-benzylaminopurine (BAP) were also assessed. Whole-plant nodulation (nodules plant -1 ) and dry weight (DW)-specific nodulation (nodules g -1 root DW) were stimulated (up to 49%) in all white clover lines by 0.1mM NH 4 +. This represents the first confirmation of an NH 4 +-induced stimulation of DW-specific nodulation in a species other than pea. At 2.5mM NH 4 +, the effect was lost on whole-plant nodulation and was inhibitory on DW-specific nodulation. Rhizobial inoculation resulted in a decline in the expression of GH3:gusA in root tips as expected; however, ammonium treatment did not affect GH3 expression in any root zones. Exogenous application of BAP at 10 -9 and 10 -8 M stimulated whole-plant and DW-specific nodulation in wild-type white clover to a similar degree as treatment with 0.1 mM NH 4 +. These results support our previous hypothesis that the stimulation of nodulation by low concentrations of ammonium involves the alteration of the ratio of cytokinin to auxin, specifically by increasing cytokinin.

AUX1 promotes lateral root formation by facilitating indole-3-acetic acid distribution between sink and source tissues in the Arabidopsis seedling.

Arabidopsis root architecture is regulated by shoot-derived signals such as nitrate and auxin. We report that mutations in the putative auxin influx carrier AUX1 modify root architecture as a result of the disruption in hormone transport between indole-3-acetic acid (IAA) source and sink tissues. Gas chromatography-selected reaction monitoring-mass spectrometry measurements revealed that the aux1 mutant exhibited altered IAA distribution in young leaf and root tissues, the major IAA source and sink organs, respectively, in the developing seedling. Expression studies using the auxin-inducible reporter IAA2::uidA revealed that AUX1 facilitates IAA loading into the leaf vascular transport system. AUX1 also facilitates IAA unloading in the primary root apex and developing lateral root primordium. Exogenous application of the synthetic auxin 1-naphthylacetic acid is able to rescue the aux1 lateral root phenotype, implying that root auxin levels are suboptimal for lateral root primordium initiation in the mutant.