Polar Transport Inhibitor Research Articles

Molecular hydrogen positively influences root gravitropism involving auxin signaling and starch accumulation.

Although geoscience of natural hydrogen (H2), hydrogen-producing soil bacteria, and especially plant-based H2, has been observed, it is not clear whether or how above H2 resources influence root gravitropic responses. Here, pharmacological, genetic, molecular, and cell biological tools were applied to investigate how plant-based H2 coordinates gravity responses in Arabidopsis roots. Since roots show higher H2 production than shoots, exogenous H2 supply was used to mimic this function. After H2 supplementation, the asymmetric expression of the auxin-response reporter DR5 driven by auxin influx and efflux carriers, and thereafter positive root gravitropism were observed. These positive responses in root gravitropism were sensitive to auxin polar transport inhibitors, and importantly, the defective phenotypes observed in aux1-7, pin1, and pin2 mutants were not significantly altered by exogenous H2. The observed starch accumulation was matched with the reprogramming gene expression linked to starch synthesis and degradation. Transgenic plants expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only displayed higher endogenous H2 concentrations, the inducible AUX1 gene expression and starch accumulation, but also showed pronounced root gravitropism. Collectively, above evidence preliminarily provides a framework for understanding the molecular basis of the possible functions of both plant/soil-based and nature H2 in root architecture.

Target of rapamycin coordinates auxin are involved in exogenous melatonin regulated low temperature tolerance in cucumber seedlings

Low temperature (LT) is an important environmental factor affecting the growth and yield of plants. Melatonin (MT) can effectively enhance the LT tolerance of cucumber. This study found that LT stress induced the expression of Comt1 (caffeic acid O-methyltransferase 1), with the highest expression being about 2-times that of the control. Meanwhile, the content of MT was found to be roughly 63.16% of that in the control samples. Compared with LT treatment alone, exogenous MT pretreatment upregulated the expression levels of TOR (Target of rapamycin), PIN1 (Pin-formed 1), and YUC4 (YUCCA 4), with maximum upregulations reaching approximately 66.67%, 79.32%, and 42.86%, respectively. These results suggest that MT may modulate the tolerance of cucumber seedlings to LT stress by regulating the expression of TOR, PIN1, and YUC4. In addition, co-treatment with AZD-8055 (a TOR inhibitor) or NPA (N-1-naphthylphthalamic acid, an auxin polar transport inhibitor) and MT attenuated MT-induced resistance to LT stress, leading to higher levels of reactive oxygen species (ROS), reduced antioxidant defense capacity, and increased damage to the membrane system in cucumber seedlings. Concurrently, the content of osmoregulatory substances and the photosynthesis decreased. These results demonstrate that both TOR and auxin were required for MT to alleviate LT-induced damage in cucumber. In summary, the present study demonstrates that TOR and auxin signaling synergistically contribute to alleviating LT damage in cucumber seedlings by exogenous MT. These findings help us understand the function of MT and provide insights into the regulatory network of MT that regulates the LT tolerance of plants.

Radicle Growth Regulation of Root Parasitic Plants by Auxin-related Compounds.

Root parasitic plants in the Orobanchaceae, such as Striga and Orobanche, cause significant damage to crop production. The germination step of these root parasitic plants is induced by host-root-derived strigolactones. After germination, the radicles elongate toward the host and invade the host root. We have previously discovered that a simple amino acid, tryptophan (Trp), as well as its metabolite, the plant hormone indole-3-acetic acid (IAA), can inhibit radicle elongation of Orobanche minor. These results suggest that auxin plays a crucial role in the radicle elongation step in root parasitic plants. In this report, we used various auxin chemical probes to dissect the auxin function in the radicle growth of O. minor and Striga hermonthica. We found that synthetic auxins inhibited radicle elongation. In addition, auxin receptor antagonist, auxinole, rescued the inhibition of radicle growth by exogenous IAA. Moreover, a polar transport inhibitor of auxin, N-1-naphthylphthalamic acid, affected radicle bending. We also proved that exogenously applied Trp is converted into IAA in O. minor seeds, and auxinole partly rescued this radicle elongation. Taken together, our data demonstrate a pivotal role for auxin in radicle growth. Thus, manipulation of auxin function in root parasitic plants should offer a useful approach to combat these parasites.

Auxin regulates bulbil initiation by mediating sucrose metabolism in Lilium lancifolium.

Lily bulbils, which serve as advantageous axillary organs for vegetative propagation, have not been extensively studied in terms of the mechanism of bulbil initiation. The functions of auxin and sucrose metabolism have been implicated in axillary organ development, but their relationship in regulating bulbil initiation remains unclear. In this study, exogenous indole-3-acetic acid (IAA) treatment increased the endogenous auxin levels at leaf axils and significantly decreased bulbil number, whereas treatment with the auxin polar transport inhibitor N-1-naphthylphthalamic acid (NPA), which resulted in a low auxin concentration at leaf axils, stimulated bulbil initiation and increased bulbil number. A low level of auxin caused by NPA spraying or silencing of auxin biosynthesis genes YUCCA FLAVIN MONOOXYGENASE-LIKE 6 (LlYUC6) and TRYPTOPHAN AMINOTRANSFERASERELATED 1 (LlTAR1) facilitated sucrose metabolism by activating the expression of SUCROSE SYNTHASES 1 (LlSusy1) and CELL WALL INVERTASE 2 (LlCWIN2), resulting in enhanced bulbil initiation. Silencing LlSusy1 or LlCWIN2 hindered bulbil initiation. Moreover, the transcription factor BASIC HELIX-LOOP-HELIX 35 (LlbHLH35) directly bound the promoter of LlSusy1, but not the promoter of LlCWIN2, and activated its transcription in response to the auxin content, bridging the gap between auxin and sucrose metabolism. In conclusion, our results reveal that an LlbHLH35-LlSusy1 module mediates auxin-regulated sucrose metabolism during bulbil initiation.

IAA-miR164a-NAC100L1 module mediates symbiotic incompatibility of cucumber/pumpkin grafted seedlings through regulating callose deposition.

Grafting is one of the key technologies to overcome the obstacles of continuous cropping, and improve crop yield and quality. However, the symbiotic incompatibility between rootstock and scion affects the normal growth and development of grafted seedlings after survival. The specific molecular regulation mechanism of graft incompatibility is still largely unclear. In this study, we found that the IAA-miR164a-NAC100L1 module induced callose deposition to mediate the symbiotic incompatibility of cucumber/pumpkin grafted seedlings. The incompatible combination (IG) grafting interface accumulated more callose, and the activity of callose synthase (CmCalS1) and IAA content were significantly higher than in the compatible combination (CG). Treatment with IAA polar transport inhibitor in the root of the IG plants decreased CmCalS activity and callose content. Furthermore, IAA negatively regulated the expression of Cm-miR164a, which directly targeted cleavage of CmNAC100L1. Interestingly, CmNAC100L1 interacted with CmCalS1 to regulate its activity. Further analysis showed that the interaction between CmNAC100L1 and CmCalS1 increased the activity of CmCalS1 in the IG plants but decreased it in the CG plants. Point mutation analysis revealed that threonine at the 57th position of CmCalS1 protein played a critical role to maintain its enzyme activity in the incompatible rootstock. Thus, IAA inhibited the expression of Cm-miR164a to elevate the expression of CmNAC100L1, which promoted CmNAC100L1 interaction with CmCalS1 to enhance CmCalS1 activity, resulting in callose deposition and symbiotic incompatibility of cucumber/pumpkin grafted seedlings.

Phytochrome B1/B2 and auxin transport are involved in the regulation of shoot: root ratio by far-red radiation in tomato

Plants possess a set of photoreceptors to perceive changes in the light spectrum. Phytochromes (PHY) B1/B2 sense changes in red: far red ratios and are involved in mediating the shade-avoidance responses (SAR) in tomato. Far red (FR) increases the fraction of dry mass partitioning to the shoot at the expense of the root in tomato, but the control of this response has not yet been explained. We studied the role of phytochromes and auxin transport in the regulation of shoot: root ratio. We hypothesized that a loss-of-function mutation in phyB1/B2 leads to a strong increase in shoot: root ratio, similar to the effect of reduced R: FR ratio in wildtype plants. We also hypothesized that the phyB1/B2 double mutation suppresses shoot: root ratio responses when exposed to a reduced R: FR ratio. Furthermore, we hypothesized that the increased shoot: root ratio is linked to the changes in auxin transport between shoot and root. To test these hypotheses, we conducted an experiment in a climate chamber where both wildtype and phyB1/B2 double mutant tomato plants (Solanum lycopersicum cv. Moneymaker) were grown for 21 days with 0, 55 or 85 μmol m−2 s−1 FR in a background of white + red LED light at 150 μmol m−2 s−1. On the 14th day, auxin polar transport inhibitor 1-N-naphthylphthalamic acid (NPA) was applied at the shoot-root junction. PhyB1/B2 mutants showed a higher shoot: root ratio than the wildtype plants. Both wildtype and phyB1/B2 mutants responded to FR with an increase in shoot: root ratio. Blocking auxin transport from shoot to root led to an increase in shoot: root ratio for both genotypes under all light conditions. These results suggest that, similar to other SAR responses like stem elongation, the response of shoot: root ratio to additional FR also involves the regulation by phytochromes, possibly via affecting auxin transport between shoot and root.

SlGH3.15, a member of the GH3 gene family, regulates lateral root development and gravitropism response by modulating auxin homeostasis in tomato

Multiple Gretchen Hagen 3 (GH3) genes have been implicated in a range of processes in plant growth and development through their roles in maintaining hormonal homeostasis. However, there has only been limited study on the functions of GH3 genes in tomato (Solanum lycopersicum). In this work, we investigated the important function of SlGH3.15, a member of the GH3 gene family in tomato. Overexpression of SlGH3.15 led to severe dwarfism in both the above- and below-ground sections of the plant, accompanied by a substantial decrease in free IAA content and reduction in the expression of SlGH3.9, a paralog of SlGH3.15. Exogenous supply of IAA negatively affected the elongation of the primary root and partially restored the gravitropism defects in SlGH3.15-overexpression lines. While no phenotypic change was observed in the SlGH3.15 RNAi lines, double knockout lines of SlGH3.15 and SlGH3.9 were less sensitive to treatments with the auxin polar transport inhibitor. Overall, these findings revealed important roles of SlGH3.15 in IAA homeostasis and as a negative regulator of free IAA accumulation and lateral root formation in tomato.

Integration of root architecture, root nitrogen metabolism, and photosynthesis of ‘Hanfu’ apple trees under the cross-talk between glucose and IAA

Sugars and auxin have important effects on almost all phases of plant life cycle, which are so fundamental to plants and regulate similar processes. However, little is known about the effect of cross-talk between glucose and indole-3-acetic acid (IAA) on growth and development of apple trees. To examine the potential roles of glucose and IAA in root architecture, root nitrogen (N) metabolism and photosynthetic capacity in ‘Hanfu’ (Malus domestica), a total of five treatments was established: single application of glucose, IAA, and auxin polar transport inhibitor (2, 3, 5-triiodobenzoic acid, TIBA), combined application of glucose with TIBA and that of glucose with IAA. The combined application of glucose with IAA improved root topology system and endogenous IAA content by altering the mRNA levels of several genes involved in root growth, auxin transport and biosynthesis. Moreover, the increased N metabolism enzyme activities and levels of genes expression related to N in roots may suggest higher rates of transformation of nitrate (NO3--N) into amino acids application of glucose and IAA. Contrarily, single application of TIBA decreased the expression levels of auxin transport gene, hindered root growth and decreased endogenous IAA content. Glucose combined with TIBA application effectively attenuated TIBA-induced reductions in root topology structure, photosynthesis and N metabolism activity, and mRNA expression levels involved in auxin biosynthesis and transport. Taken together, glucose application probably changes the expression level of auxin synthesis and transport genes, and induce the allocation of endogenous IAA in root, and thus improves root architecture and N metabolism of root in soil with deficit carbon.

Enhancement of shoot regeneration by treatment with inhibitors of auxin biosynthesis and transport during callus induction in tissue culture of Arabidopsis thaliana.

In two-step culture systems for efficient shoot regeneration, explants are first cultured on auxin-rich callus-inducing medium (CIM), where cells are activated to proliferate and form calli containing root-apical meristem (RAM)-type stem cells and stem cell niche, and then cultured on cytokinin-rich shoot-inducing medium (SIM), where stem cells and stem cell niche of the shoot apical meristem (SAM) are established eventually leading to shoot regeneration. In the present study, we examined the effects of inhibitors of auxin biosynthesis and polar transport in the two-step shoot regeneration culture of Arabidopsis and found that, when they were applied during CIM culture, although callus growth was repressed, shoot regeneration in the subsequent SIM culture was significantly increased. The regeneration-stimulating effect of the auxin biosynthesis inhibitor was not linked with the reduction in the endogenous indole-3-acetic acid (IAA) level. Expression of the auxin-responsive reporter indicated that auxin response was more uniform and even stronger in the explants cultured on CIM with the inhibitors than in the control explants. These results suggested that the shoot regeneration competence of calli was enhanced somehow by the perturbation of the endogenous auxin dynamics, which we discuss in terms of the transformability between RAM and SAM stem cell niches.

The Effect of TIBA and NPA on Shoot Regeneration of Cannabis sativa L. Epicotyl Explants

Industrial hemp (Cannabis sativa L., family Cannabaceae) is a multi-purpose crop, used in the production of food, nutraceuticals, cosmetics and medicines. Therefore, development of new varieties with specific chemical profiles is necessary. In vitro culture methods could be complementary to conventional breeding and a useful tool for large-scale propagation. Strong apical dominance is considered as one of the factors contributing to the recalcitrance of industrial hemp in shoot proliferation. In this study, we tested the polar transport inhibitors N-1-naphtylphtalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA) to enhance shoot regeneration as the result of suppression of apical dominance and to develop in vitro protocols for Diana, Finola and Fedora 17 cultivars. Shoot tips derived from epicotyls were cultivated on Murashige and Skoog medium (MS) supplemented with meta-topolin (mT) and NPA, and also thidiazuron (TDZ) with a combination of TIBA and NPA. The results showed that the combination of TDZ with NPA (1–5 mg L−1) and TDZ with TIBA (0.5–2.5 mg L−1) increased the response of explants and the multiplication rate, but the effect was genotype-dependent and malformations were observed. To optimize the developed protocol, a two-step procedure with shortened time of exposure to inhibitors and reduced concentrations of them was applied. Shoots were rooted on media containing indole-3-butyric acid (IBA) and then successfully acclimatized. The obtained results will be useful in micropropagation of recalcitrant industrial hemp varieties.

H2O2 Functions as a Downstream Signal of IAA to Mediate H2S-Induced Chilling Tolerance in Cucumber.

Hydrogen sulfide (H2S) plays a crucial role in regulating chilling tolerance. However, the role of hydrogen peroxide (H2O2) and auxin in H2S-induced signal transduction in the chilling stress response of plants was unclear. In this study, 1.0 mM exogenous H2O2 and 75 μM indole-3-acetic acid (IAA) significantly improved the chilling tolerance of cucumber seedlings, as demonstrated by the mild plant chilling injury symptoms, lower chilling injury index (CI), electrolyte leakage (EL), and malondialdehyde content (MDA) as well as higher levels of photosynthesis and cold-responsive genes under chilling stress. IAA-induced chilling tolerance was weakened by N, N′-dimethylthiourea (DMTU, a scavenger of H2O2), but the polar transport inhibitor of IAA (1-naphthylphthalamic acid, NPA) did not affect H2O2-induced mitigation of chilling stress. IAA significantly enhanced endogenous H2O2 synthesis, but H2O2 had minimal effects on endogenous IAA content in cucumber seedlings. In addition, the H2O2 scavenger DMTU, inhibitor of H2O2 synthesis (diphenyleneiodonium chloride, DPI), and IAA polar transport inhibitor NPA reduced H2S-induced chilling tolerance. Sodium hydrosulfide (NaHS) increased H2O2 and IAA levels, flavin monooxygenase (FMO) activity, and respiratory burst oxidase homolog (RBOH1) and FMO-like protein (YUCCA2) mRNA levels in cucumber seedlings. DMTU, DPI, and NPA diminished NaHS-induced H2O2 production, but DMTU and DPI did not affect IAA levels induced by NaHS during chilling stress. Taken together, the present data indicate that H2O2 as a downstream signal of IAA mediates H2S-induced chilling tolerance in cucumber seedlings.

Melatonin promotes Arabidopsis primary root growth in an IAA-dependent manner

Melatonin has been characterized as a growth regulator in plants. Melatonin shares tryptophan as the precursor with the auxin indole-3-acetic acid (IAA), but the interplay between melatonin and IAA remains controversial. In this study, we aimed to dissect the relationship between melatonin and IAA in regulating Arabidopsis primary root growth. We observed that melatonin concentrations ranging from 10-9 to 10-6 M functioned as IAA mimics to promote primary root growth in Arabidopsis wild type, as well as in pin-formed (pin) single and double mutants. Transcriptome analysis showed that changes in gene expression after melatonin and IAA treatment were moderately correlated. Most of the IAA-regulated genes were co-regulated by melatonin, indicating that melatonin and IAA regulated a similar subset of genes. Melatonin partially rescued primary root growth defects in pin single and double mutant plants. However, melatonin treatment had little effect on primary root growth in the presence of high concentrations of auxin biosynthesis inhibitors, or polar transport inhibitor, and could not rescue the root length defect of the IAA biosynthesis quintuple mutant yucQ. Therefore, we propose that melatonin promotes primary root growth in an IAA-dependent manner.

Auxin is involved in arbuscular mycorrhizal fungi-promoted tomato growth and NADP-malic enzymes expression in continuous cropping substrates

BackgroundDespite significant limitations of growth medium reuse, a large amount of organic substrate is reused in soilless cultivation of horticultural crops in China. Arbuscular mycorrhizal fungi (AMF) can promote nutrient absorption and improve plant tolerance to biotic and abiotic stresses. However, the mechanisms governing the effects of AMF on crop growth in organic continuous cropping substrates have not been elucidated.ResultsIn this study, we showed that the inoculation of AMF in continuous cropping substrates promoted growth and root development, and increased the root and NADP-malic enzyme (NADP-ME) activity of tomato seedlings. Root transcriptome analysis demonstrated that the plant hormone signal transduction pathway was highly enriched, and 109 genes that positively correlated with the AMF-inoculated plant phenotype were obtained by gene set enrichment analysis (GSEA), which identified 9 genes related to indole acetic acid (IAA). Importantly, the levels of endogenous IAA in tomato seedlings significantly increased after AMF inoculation. Furthermore, the application of AMF significantly increased the expression levels of NADP-ME1 and NADP-ME2, as well as the activity of NADP-ME, and enhanced the root activity of tomato seedlings in comparison to that observed without inoculation of AMF. However, these effects were blocked in plants treated with 2,3,5-triiodobenzoic acid (TIBA), a polar transport inhibitor of IAA.ConclusionsThese results suggest that IAA mediates the AMF-promoted tomato growth and expression of NADP-MEs in continuous cropping substrates. The study provides convincing evidence for the reuse of continuous cropping substrates by adding AMF as an amendment.

Cadmium Inhibits Lateral Root Emergence in Rice by Disrupting OsPIN-Mediated Auxin Distribution and the Protective Effect of OsHMA3.

Cadmium (Cd) strongly inhibits root growth, especially the formation of lateral roots (LRs). The mechanism of Cd inhibition on LR formation in rice (Oryza sativa) remains unclear. In this study, we found that LR emergence in rice was inhibited significantly by 1 �M Cd and almost completely arrested by 5 �M Cd. Cd suppressed both the formation and subsequent development of the lateral root primordium (LRP). By using transgenic rice expressing the auxin response reporters DR5::GUS and DR5rev::VENUS, we found that Cd markedly reduced the auxin levels in the stele and LRP. Cd rapidly downregulated the expression of the auxin efflux transporter genes OsPIN1b, OsPIN1c and OsPIN9 in the stele and LRP. The emergence of LRs in a rice cultivar with a null allele of OsHMA3 (Heavy Metal ATPase 3) was more sensitive to Cd than cultivars with functional alleles. Overexpression of functional OsHMA3 in rice greatly alleviated the inhibitory effect of Cd, but the protective effect of OsHMA3 was abolished by the auxin polar transport inhibitor 1-N-naphthylphthalamic acid. The results suggest that Cd inhibits LR development in rice by disrupting OsPIN-mediated auxin distribution to LRP and OsHMA3 protects against Cd toxicity by sequestering Cd into the vacuoles.

Auxin acts as a downstream signaling molecule involved in hydrogen sulfide-induced chilling tolerance in cucumber.

This report proves a cross talk between H2S and IAA in cold stress response, which has presented strong evidence that IAA acts as a downstream signal mediating the H2S-induced stress tolerance in cucumber seedlings. We evaluated changes in endogenous hydrogen sulfide (H2S) and indole-3-acetic acid (IAA) emission systems, and the interactive effect of exogenous H2S and IAA on chilling tolerance in cucumber seedlings. The results showed that chilling stress increased the activity and relative mRNA expression of L-/D-cysteine desulfhydrase (L-/D-CD), which in turn induced the accumulation of endogenous H2S. Similarly, the endogenous IAA system was triggered by chilling stress. We found that 1.0mM sodium hydrosulfide (NaHS, an H2S donor) significantly enhanced the activity of flavin monooxygenase (FMO) and relative expression of FMO-like proteins (YUCCA2), which in turn elevated endogenous IAA levels in cucumber seedlings. However, IAA had little effects on activities of L-/D-CD and endogenous H2S levels. H2S-induced IAA production accompanied by increase in chilling tolerance, as shown by the decrease in stress-induced electrolyte leakage (EL) and reactive oxygen species (ROS) accumulation, and increase in gene expressions and enzyme activities of photosynthesis. 1-naphthylphthalamic acid (NPA, an IAA polar transport inhibitor) declined H2S-induced chilling tolerance and defense genes' expression. However, scavenging of H2S had a little effect on IAA-induced chilling tolerance. These results suggest that IAA acting as a downstream signaling molecule is involved in the H2S-induced chilling tolerance in cucumber seedlings.

Exogenous Glucose Mediates the Regulation of Root Morphology and Carbon–Nitrogen Metabolism by Indole-3-Acetic Acid (IAA) in Malus baccata (L.) Borkh. in Soil with Low Organic Carbon Content

The effects of an exogenous organic carbon source (i.e., glucose) and indole-3-acetic acid (IAA) on root architecture and carbon–nitrogen (C–N) metabolism of apple rootstock were evaluated under soil organic matter (SOM)-restricted conditions. Malus baccata (L.) Borkh. seedlings were exposed to 1.5 g kg−1 glucose (GLC), 0.09 g kg−1 IAA, 0.06 g kg−1 2,3,5-triiodobenzoic acid (TIBA, a widely used auxin polar transport inhibitor), GLC + IAA, or GLC + TIBA for 30 days. Both GLC and IAA promoted root architecture and growth by regulating SHY2, SHR, ALF4, and LBD11. They also enhanced C–N metabolism, and accelerated nitrate transformation to amino acids. In contrast, TIBA reduced endogenous IAA content in root and inhibited plant growth and C–N metabolism by downregulating expression of auxin polar transport genes, although auxin biosynthesis genes were induced. These adverse effects could be alleviated in GLC + TIBA, which exhibited higher endogenous IAA content in root than TIBA-treated seedlings alone, due to the upregulated expression of auxin biosynthesis genes (YUCCA8, TAR2, TAA1, and CYP79B3) and polar transport genes (PIN1, AUX1, and LAX2). In addition, the enhanced transcription and activities of enzymes involved in C metabolism (PEPC, NADP-ME, and NADP-ICDH) could provide more organic acids, adenosine triphosphate (ATP), and energy charge for N metabolism in roots under GLC + TIBA than in roots under TIBA. The induced NR, GS, NADH-GDH, NADH-GOGAT activities and mRNA levels of genes involved in N metabolism indicated the higher N assimilation ability in roots under GLC + TIBA than in roots under TIBA alone. In conclusion, exogenous glucose-mediated IAA biosynthesis and polar transport regulates root architecture and C–N metabolism of M. baccata (L.) Borkh. under low-SOM conditions.

5-Aminolevulinic acid (ALA) promotes primary root elongation through modulation of auxin transport in Arabidopsis

The specific function of 5-aminolevulinic acid (ALA), a new plant growth regulator, in modulating root growth of plants and the mechanisms underlying ALA-regulated root growth are largely unknown. Here, Arabidopsis seedlings were photographed and collected before and after ALA or 2,3,5-triiodobenzoic acid (TIBA) treatment for determination of root growth, fluorescence intensities of PIN1, PIN2, PIN7, and DR5, and gene expression levels of auxin synthesis, signaling, and transport. We first demonstrated that ALA significantly promoted Arabidopsis primary root elongation. We also found that TIBA, an auxin polar transport inhibitor, inhibited ALA-promoted root elongation, indicating that auxin transport is involved in ALA-regulated root growth. Then, the observations of PIN1, PIN2, and PIN7 at protein and transcript levels suggest that ALA improves auxin transport mainly through regulating auxin efflux carriers. Furthermore, the expression patterns of auxin-responsive reporter DR5rev:GFP were not correlated well with the expression of YUC2, a key biosynthetic gene of auxin, but were consistent with changes of PIN1, PIN2, and PIN7. In addition, ALA did not affect the gene expression of auxin receptor, TRANSPORT-INHIBITOR-RESISTANT1 (TIR1). Taken together, we conclude that ALA promotes primary root elongation of young Arabidopsis seedlings mainly through improving auxin transport. Our data suggest the reciprocal interaction between ALA and auxin, providing new insights into the mechanisms underlying ALA-promoted plant root growth.

Influence of auxin and its polar transport inhibitor on the development of somatic embryos in Digitalis trojana.

The present study reports the role of auxin and its transport inhibitor during the establishment of an efficient and optimized protocol for the somatic embryogenesis in Digitalis trojana Ivan. Hypocotyl segments (5mm long) were placed vertically in the Murashige and Skoog medium supplemented with three sets [indole-3-acetic acid (IAA) alone or 2,3,5-triiodobenzoic acid (TIBA) alone or IAA-TIBAcombination] of formulations of plant growth regulators, to assess their differential influence on induction and proliferation of somatic embryos (SEs). IAA alone was found to be the most effective, at a concentration of 0.5mg/l, inducing~10SEs per explant with 52% induction frequency. On the other hand, the combination of 0.5mg/l of IAA and 1mg/l of TIBA produced significantly fewer (~3.6SEs) and abnormal (enlarged, oblong, jar and cup-shaped) SEs per explant with 24% induction frequency in comparison to that in the IAA alone. The explants treated with IAA-TIBA exhibited a delayed response along with the formation of abnormal SEs. Our study revealed that IAA induces high-frequency SE formation when used singly, but the frequency gradually declines when IAA was coupled with increasing levels of TIBA. Eventually, our findings bring new insights into the roles of auxin and its polar transport in somatic embryogenesis of D. trojana.

A Genome-Scale Analysis of the PIN Gene Family Reveals Its Functions in Cotton Fiber Development.

The PIN-FORMED (PIN) protein, the most important polar auxin transporter, plays a critical role in the distribution of auxin and controls multiple biological processes. However, characterizations and functions of this gene family have not been identified in cotton. Here, we identified the PIN family in Gossypium hirsutum, Gossypium arboreum, and Gossypium raimondii. This gene family was divided into seven subgroups. A chromosomal distribution analysis showed that GhPIN genes were evenly distributed in eight chromosomes and that the whole genome and dispersed duplications were the main duplication events for GhPIN expansion. qRT-PCR analysis showed a tissue-specific expression pattern for GhPIN. Likely due to the cis-element variations in their promoters, transcripts of PIN6 and PIN8 genes from the At (tetraploid genome orginated from G. arboreum) subgenome and PIN1a from the Dt (tetraploid genome orginated from G. raimondii) subgenome in G. hirsutum was significantly increased compared to the transcripts in the diploids. The differential regulation of these PIN genes after the polyploidization may be conducive to fiber initiation and elongation. Exogenously applied auxin polar transport inhibitor significantly suppressed fiber growth, which is consistent with the essential function of these PIN genes for regulating cotton fiber development. Furthermore, the overexpression of GhPIN1a_Dt, GhPIN6_At, and GhPIN8_At in Arabidopsis promoted the density and length of trichomes in leaves.

Effect of the auxin polar transport inhibitor on the morphogenesis of leaves and generative structures during fasciation in Arabidopsis thaliana (L.) Heynh.

An increase in the proliferative activity of a shoot apical meristem (SAM) and the further accumulation of a pool of undifferentiated cells (fasciation) results in phyllotaxis changes. In the case of Arabidopsis thaliana, a typical spiral leaf arrangement is replaced by an opposite or verticillate one (depending on the level of a fasciation manifestation). Pistil development in mutant plants is accompanied by the appearance of a group of undifferentiated meristematic cells in its central part. The addition of N-1-naphthylphthalamic acid (NPA) causes an increase in the meristem volume and number of stipules in both mutant and control plants. The NPA effect on the floral morphogenesis results in a significant growth of meristemic cell pool. The interaction of different mechanisms of a meristem volume control is discussed.