Exogenous Auxin Research Articles

Transcriptome profiling reveals the regulatory mechanism underlying pollination dependent and parthenocarpic fruit set mainly mediated by auxin and gibberellin.

BackgroundFruit set is a key process for crop production in tomato which occurs after successful pollination and fertilization naturally. However, parthenocarpic fruit development can be uncoupled from fertilization triggered by exogenous auxin or gibberellins (GAs). Global transcriptome knowledge during fruit initiation would help to characterize the molecular mechanisms by which these two hormones regulate pollination-dependent and -independent fruit set.Principal FindingsIn this work, digital gene expression tag profiling (DGE) technology was applied to compare the transcriptomes from pollinated and 2, 4-D/GA3-treated ovaries. Activation of carbohydrate metabolism, cell division and expansion as well as the down-regulation of MADS-box is a comprehensive regulatory pathway during pollination-dependent and parthenocarpic fruit set. The signaling cascades of auxin and GA are significantly modulated. The feedback regulations of Aux/IAAs and DELLA genes which functioned to fine-tune auxin and GA response respectively play fundamental roles in triggering fruit initiation. In addition, auxin regulates GA synthesis via up-regulation of GA20ox1 and down-regulation of KNOX. Accordingly, the effect of auxin on fruit set is mediated by GA via ARF2 and IAA9 down-regulation, suggesting that both pollination-dependent and parthenocarpic fruit set depend on the crosstalk between auxin and GA.SignificanceThis study characterizes the transcriptomic features of ovary development and more importantly unravels the integral roles of auxin and GA on pollination-dependent and parthenocarpic fruit set.

Light Quality Regulates Lateral Root Development in Tobacco Seedlings by Shifting Auxin Distributions

Light is an important environmental regulator of diverse growth and developmental processes in plants. However, the mechanisms by which light quality regulates root growth are poorly understood. We analyzed lateral root (LR) growth of tobacco seedlings in response to three kinds of light qualities (red, white, and blue). Primary (1°) LR number and secondary (2°) LR density were elevated under red light (on days 9 and 12 of treatment) in comparison with white and blue lights. Higher IAA concentrations measured in roots and lower in leaves of plants treated with red light suggest that red light accelerated auxin transport from the leaves to roots (in comparison with other light qualities). Corroborative evidence for this suggestion was provided by elevated DR5::GUS expression levels at the shoot/root junction and in the 2° LR region. Applications of N-1-naphthylphthalamic acid (NPA) to red light-treated seedlings reduced both 1° LR number and 2° LR density to levels similar to those measured under white light; DR5::GUS expression levels were also similar between these light qualities after NPA application. Results were similar following exogenous auxin (NAA) application to blue light-treated seedlings. Direct [3H]IAA transport measurement indicated that the polar auxin transport from shoot to root was increased by red light. Red light promoted PIN3 expression levels and blue light reduced PIN1, 3–4 expression levels in the shoot/root junction and in the root, indicating that these genes play key roles in auxin transport regulation by red and blue lights. Overall, our findings suggest that three kinds of light qualities regulate LR formation in tobacco seedlings through modification of auxin polar transport.

A strigolactone signal is required for adventitious root formation in rice.

Strigolactones (SLs) and their derivatives are plant hormones that have recently been identified as regulating root development. This study examines whether SLs play a role in mediating production of adventious roots (ARs) in rice (Oryza sativa), and also investigates possible interactions between SLs and auxin. Wild-type (WT), SL-deficient (d10) and SL-insensitive (d3) rice mutants were used to investigate AR development in an auxin-distribution experiment that considered DR5::GUS activity, [(3)H] indole-3-acetic acid (IAA) transport, and associated expression of auxin transporter genes. The effects of exogenous application of GR24 (a synthetic SL analogue), NAA (α-naphthylacetic acid, exogenous auxin) and NPA (N-1-naphthylphalamic acid, a polar auxin transport inhibitor) on rice AR development in seedlings were investigated. The rice d mutants with impaired SL biosynthesis and signalling exhibited reduced AR production compared with the WT. Application of GR24 increased the number of ARs and average AR number per tiller in d10, but not in d3. These results indicate that rice AR production is positively regulated by SLs. Higher endogenous IAA concentration, stronger expression of DR5::GUS and higher [(3)H] IAA activity were found in the d mutants. Exogenous GR24 application decreased the expression of DR5::GUS, probably indicating that SLs modulate AR formation by inhibiting polar auxin transport. The WT and the d10 and d3 mutants had similar expression of DR5::GUS regardless of exogenous application of NAA or NPA; however, AR number was greater in the WT than in the d mutants. The results suggest that AR formation is positively regulated by SLs via the D3 response pathway. The positive effect of NAA application and the opposite effect of NPA application on AR number of WT plants also suggests the importance of auxin for AR formation, but the interaction between auxin and SLs is complex.

Genome-wide identification and expression profiling analysis of ZmPIN, ZmPILS, ZmLAX and ZmABCB auxin transporter gene families in maize (Zea mays L.) under various abiotic stresses.

The auxin influx carriers auxin resistant 1/like aux 1 (AUX/LAX), efflux carriers pin-formed (PIN) (together with PIN-like proteins) and efflux/conditional P-glycoprotein (ABCB) are major protein families involved in auxin polar transport. However, how they function in responses to exogenous auxin and abiotic stresses in maize is largely unknown. In this work, the latest updated maize (Zea mays L.) reference genome sequence was used to characterize and analyze the ZmLAX, ZmPIN, ZmPILS and ZmABCB family genes from maize. The results showed that five ZmLAXs, fifteen ZmPINs, nine ZmPILSs and thirty-five ZmABCBs were mapped on all ten maize chromosomes. Highly diversified gene structures, nonconservative transmembrane helices and tissue-specific expression patterns suggested the possibility of function diversification for these genes. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to analyze the expression patterns of ZmLAX, ZmPIN, ZmPILS and ZmABCB genes under exogenous auxin and different environmental stresses. The expression levels of most ZmPIN, ZmPILS, ZmLAX and ZmABCB genes were induced in shoots and were reduced in roots by various abiotic stresses (drought, salt and cold stresses). The opposite expression response patterns indicated the dynamic auxin transport between shoots and roots under abiotic stresses. Analysis of the expression patterns of ZmPIN, ZmPILS, ZmLAX and ZmABCB genes under drought, salt and cold treatment may help us to understand the possible roles of maize auxin transporter genes in responses and tolerance to environmental stresses.

A proposed role for selective autophagy in regulating auxin-dependent lateral root development under phosphate starvation in Arabidopsis

Plants respond to limited soil nutrient availability by inducing more lateral roots (LR) to increase the root surface area. At the cellular level, nutrient starvation triggers the process of autophagy through which bulk degradation of cellular materials is achieved to facilitate nutrient mobilization. Whether there is any link between the cellular autophagy and induction of LR had remained unknown. We recently showed that the S-Domain receptor Kinase (ARK2) and U Box/Armadillo Repeat-Containing E3 ligase (PUB9) module is required for lateral root formation under phosphate starvation in Arabidopsis thaliana.1 We also showed that PUB9 localized to autophagic bodies following either activation by ARK2 or under phosphate starvation and ark2–1/pub9–1 plants displayed lateral root defects with inability to accumulate auxin in the root tips under phosphate starvation.1 Supplementing exogenous auxin was sufficient to rescue the LR defects in ark2–1/pub9–1 mutant. Blocking of autophagic responses in wild-type Arabidopsis also resulted in inhibition of both lateral roots and auxin accumulation in the root tips indicating the importance of autophagy in mediating auxin accumulation under phosphate starved conditions.1 Here, we propose a model for ARK2/AtPUB9 module in regulation of lateral root development via selective autophagy.

Adventitious Rooting and Browning are Differentially Controlled by Auxin in Rooting-Recalcitrant Elegia capensis (Burm. f.) Schelpe

Industrial propagation of horticultural species involves adventitious root (AR) induction, which is difficult to establish in rooting-recalcitrant species. One of these recalcitrant species is Elegia capensis (Burm. f.) Schelpe. This monocotyledonous species was used to investigate the correlation between tissue browning and AR formation. To evaluate the root-inducing potential of various auxin treatments, a test system using stem segments of in vitro grown Elegia shoots was developed. In addition to exogenous auxin treatments, the effect of more sucrose in the medium and the effect of different light quality were evaluated. The auxins IAA, IBA, NAA, and 2,4-D did not induce roots under standard light conditions, which confirmed the strong rooting-recalcitrance of E. capensis. However, in the dark, IBA successfully induced roots in 20–40 % of the segments, depending on the auxin concentration applied. Excised stem segments showed browning upon incubation with auxins. Browning was dependent on auxin type and concentration, yet it did not correlate with the capacity to form roots because AR formation was observed on brown stem segments. We therefore conclude that browning does not cause rooting-recalcitrance in E. capensis.

Auxin binding protein 1 (ABP1) is not required for either auxin signaling or Arabidopsis development

Auxin binding protein 1 (ABP1) has been studied for decades. It has been suggested that ABP1 functions as an auxin receptor and has an essential role in many developmental processes. Here we present our unexpected findings that ABP1 is neither required for auxin signaling nor necessary for plant development under normal growth conditions. We used our ribozyme-based CRISPR technology to generate an Arabidopsis abp1 mutant that contains a 5-bp deletion in the first exon of ABP1, which resulted in a frameshift and introduction of early stop codons. We also identified a T-DNA insertion abp1 allele that harbors a T-DNA insertion located 27 bp downstream of the ATG start codon in the first exon. We show that the two new abp1 mutants are null alleles. Surprisingly, our new abp1 mutant plants do not display any obvious developmental defects. In fact, the mutant plants are indistinguishable from wild-type plants at every developmental stage analyzed. Furthermore, the abp1 plants are not resistant to exogenous auxin. At the molecular level, we find that the induction of known auxin-regulated genes is similar in both wild-type and abp1 plants in response to auxin treatments. We conclude that ABP1 is not a key component in auxin signaling or Arabidopsis development.

Expression of auxin synthesis gene tms1 under control of tuber-specific promoter enhances potato tuberization in vitro.

Phytohormones, auxins in particular, play an important role in plant development and productivity. Earlier data showed positive impact of exogenous auxin on potato (Solanum tuberosum L.) tuberization. The aim of this study was to generate potato plants with increased auxin level predominantly in tubers. To this end, a pBinB33-tms1 vector was constructed harboring the Agrobacterium auxin biosynthesis gene tms1 fused to tuber-specific promoter of the class I patatin gene (B33-promoter) of potato. Among numerous independently generated B33:tms1 lines, those without visible differences from control were selected for detailed studies. In the majority of transgenic lines, tms1 gene transcription was detected, mostly in tubers rather than in shoots. Indoleacetic acid (IAA) content in tubers and the auxin tuber-to-shoot ratio were increased in tms1-expressing transformants. The organ-specific increase in auxin synthesis in B33:tms1-transformants accelerated and intensified the process of tuber formation, reduced the dose of carbohydrate supply required for in vitro tuberization, and decreased the photoperiodic dependence of tuber initiation. Overall, a positive correlation was observed between tms1 expression, IAA content in tubers, and stimulation of tuber formation. The revealed properties of B33:tms1 transformants imply an important role for auxin in potato tuberization and offer prospects to magnify potato productivity by a moderate organ-specific enhancement of auxin content.

Effects of combined or single exogenous auxin and/or cytokinin applications on growth and leaf area development in Epipremnum aureum

SUMMARYWe have analysed the effects of combined auxin and cytokinin treatments on whole plant growth and leaf development in Epipremnum aureum and compared both morphological and physiological variables to those obtained with a single hormone application. Rooted cuttings of E. aureum were sprayed with 0, 5, 50, or 100 mg I−1 indole-3- acetic acid (IAA) 7 d after transplanting. One week later, they were then sprayed with 0, 5, 50, or 100 mg I−1 benzylaminopurine (BAP). Whole plant growth, leaf development, carbon fixation, and leaf anatomy were recorded for 6 months after these sequential treatments. Following a single application of IAA or BAP, we observed an increase in the accumulation of whole-plant biomass, which reached a plateau at the highest concentration of either plant hormone. The promotion of growth was associated with increased rates of net C-assimilation and net photosynthesis, as well as with increased leaf thickness and the relative proportion of intracellular spaces in the mesophyll layer. The effect on leaves of applying both hormones, in different combinations and concentrations, resembled the results on whole plants obtained by spraying either the auxin or the cytokinin (at 50 or 100 mg I−1 BAP). Similarities in plant and leaf responses to the auxin and/or the cytokinin suggest that both hormones may act via the same pathway, which agreed with the well-known promotional effect of auxins on the development of new lateral roots, as root apices are the main site of cytokinin biosynthesis. Conversely, our results did not support the occurrence of a significant auxin-driven inhibition of cytokinin synthesis in root apices, at least for E. aureum, as reported in other species.

Low temperature inhibits root growth by reducing auxin accumulation via ARR1/12.

Plants exhibit reduced root growth when exposed to low temperature; however, how low temperature modulates root growth remains to be understood. Our study demonstrated that low temperature reduces both meristem size and cell number, repressing the division potential of meristematic cells by reducing auxin accumulation, possibly through the repressed expression of PIN1/3/7 and auxin biosynthesis-related genes, although the experiments with exogenous auxin application also suggest the involvement of other factor(s). In addition, we verified that ARABIDOPSIS RESPONSE REGULATOR 1 (ARR1) and ARR12 are involved in low temperature-mediated inhibition of root growth by showing that the roots of arr1-3 arr12-1 seedlings were less sensitive than wild-type roots to low temperature, in terms of changes in root length and meristem cell number. Furthermore, low temperature reduced the levels of PIN1/3 transcripts and the auxin level to a lesser extent in arr1-3 arr12-1 roots than in wild-type roots, suggesting that cytokinin signaling is involved in the low-temperature-mediated reduction of auxin accumulation. Taken together, our data suggest that low temperature inhibits root growth by reducing auxin accumulation via ARR1/12.

Cutting Type and IBA Treatment Duration Affect <i>Teucrium fruticans</i> Adventitious Root Quality

Root development of stem cuttings of Silver Germander (Teucrium fruticans) was investigated in relation to cutting type and indole-3-butyric acid (IBA) treatment. Terminal cuttings of a clone grown in Sicily were trimmed to three types: terminal cuttings with apex (TWA), terminal cuttings without apex (TWOA) or sub-terminal cuttings (ST). To verify the cutting response to exogenous auxin, cuttings were dipped to a 2.0 cm depth in a 0.5% indole-3-butyric acid solution for 0, 5 or 7 minutes. Overall percent survival was 97 to 98%. Rooting percent, root number and root length were affected by cutting type and indole-3-butyric acid treatment. In general, TWA cuttings demonstrated a higher capacity to form roots than cuttings without apex (TWOA and ST cuttings). In absence of indole-3-butyric acid treatment, TWA and ST cuttings gave higher rooting percentages than TWOA cuttings. Exposing cuttings to indole-3-butyric acid improved percent rooting, number of roots per cutting and root length. The best results in terms of rooting percentage and root number per cuttings were obtained with TWA cuttings in combination with 7 min indole-3-butyric acid basal dip. However, cuttings taken farther down the stem, such as sub-terminal cuttings gave satisfactory rooting performance as well. We suggest that the use of all cutting types tested associated to indole-3-butyric acid basal dip for 5 or 7 min may be beneficial to propagators wishing to produce T. fruticans rooted cuttings with well-developed root system.

AtrbohD and AtrbohF negatively regulate lateral root development by changing the localized accumulation of superoxide in primary roots of Arabidopsis.

NADPH oxidase AtrbohD an d AtrbohF negatively modulate lateral root development by changing the peroxidase activity and increasing the local generation of superoxide in primary roots of Arabidopsis in an auxin-independent manner. NADPH oxidase subunits AtrbohD and AtrbohF play pivotal roles in regulating growth, development and stress responses in Arabidopsis. However, whether they modulate lateral root (LR) formation has not yet been addressed, and the detailed mechanisms underlying the process remain unanswered. Here, we show that two null double mutants atrbohD1/F1 and atrbohD2/F2, in which both AtrbohD and AtrbohF genes are disrupted, had remarkably higher LR density than wild-type (WT), or the single mutant atrbohD1 and atrbohF1. Compared to WT, the double mutants exhibited early emerged LRs and enhanced density of lateral root primordia (LRP). Unexpectedly, the production of superoxide (O2 (-)), but not hydrogen peroxide, in the mature area of the primary root containing LRs significantly increased in the double mutants relative to that in WT. Further experiments revealed that the local accumulation of O2 (-) led to the enhancement of LR density in the double mutants. Moreover, the deficiency of AtrbohD and AtrbohF caused a marked increase in peroxidase activity in the mature root zone, which contributed to the localized accumulation of O2 (-) and the elevated LR density in the double mutants. Furthermore, the double mutants were not sensitive to exogenous auxin naphthalene acetic acid or auxin transport inhibitor 1-N-naphthylphthalamic acid in terms of LR formation. The auxin response of LRP in vivo in atrbohD1/F1 was also similar to that in WT. Taken together, these results suggest that AtrbohD and AtrbohF negatively modulate LR development by controlling the local generation of superoxide in an auxin-independent manner. These findings provide new insights into the mechanisms of NADPH oxidase-mediated regulation of LR branching in Arabidopsis.

Plasma Membrane-Targeted PIN Proteins Drive Shoot Development in a Moss

SummaryBackgroundPlant body plans arise by the activity of meristematic growing tips during development and radiated independently in the gametophyte (n) and sporophyte (2n) stages of the life cycle during evolution. Although auxin and its intercellular transport by PIN family efflux carriers are primary regulators of sporophytic shoot development in flowering plants, the extent of conservation in PIN function within the land plants and the mechanisms regulating bryophyte gametophytic shoot development are largely unknown.ResultsWe have found that treating gametophytic shoots of the moss Physcomitrella patens with exogenous auxins and auxin transport inhibitors disrupts apical function and leaf development. Two plasma membrane-targeted PIN proteins are expressed in leafy shoots, and pin mutants resemble plants treated with auxins or auxin transport inhibitors. PIN-mediated auxin transport regulates apical cell function, leaf initiation, leaf shape, and shoot tropisms in moss gametophytes. pin mutant sporophytes are sometimes branched, reproducing a phenotype only previously seen in the fossil record and in rare natural moss variants.ConclusionsOur results show that PIN-mediated auxin transport is an ancient, conserved regulator of shoot development.

Modelling of Arabidopsis LAX3 expression suggests auxin homeostasis.

Emergence of new lateral roots from within the primary root in Arabidopsis has been shown to be regulated by the phytohormone auxin, via the expression of the auxin influx carrier LAX3, mediated by the ARF7/19 IAA14 signalling module (Swarup et al., 2008). A single cell model of the LAX3 and IAA14 auxin response was formulated and used to demonstrate that hysteresis and bistability may explain the experimentally observed 'all-or-nothing' LAX3 spatial expression pattern in cortical cells containing a gradient of auxin concentrations. The model was tested further by using a parameter fitting algorithm to match model output with qRT-PCR mRNA expression data following exogenous auxin treatment. It was found that the model is able to show good agreement with the data, but only when the exogenous auxin signal is degraded over time, at a rate higher than that measured in the experimental medium, suggesting the triggering of an endogenous auxin homeostasis mechanism. Testing the model over a more physiologically relevant range of extracellular auxin shows bistability and hysteresis still occur when using the optimised parameters, providing the rate of LAX3 active auxin transport is sufficiently high relative to passive diffusion.

An epiallele at cly1 affects the expression of floret closing (cleistogamy) in barley.

The swelling of the lodicule is responsible for floret opening in many grass species, allowing for pollen dispersal and cross-pollination. In barley, the closed floret habit (cleistogamy) is under the control of cly1, a gene that operates by inhibiting the development of the lodicule. In non-cleistogamous cultivars, cly1 mRNA is degraded by miR172-directed cleavage, allowing the lodicules to swell; however, in cultivars carrying the recessive allele cly1.b, a single-nucleotide substitution destroys the miR172 target site preventing mRNA cleavage. Barley cv. SV235 is cleistogamous; its cly1 coding sequence is identical to that of cly1.b, but its lodicules do develop, although insufficiently to produce a non-cleistogamous flower. In this cultivar, the downregulation of cly1 is unrelated to miR172-directed mRNA degradation, but rather is caused by an epiallele that represses transcription. Allelic relationships between known cly1 alleles were explored by the quantification of lodicule vascularization and an assessment of the response of the spike to the supply of exogenous auxin. The SV235 phenotype can be manipulated by a pre-anthesis application of 2,4-d, a feature that could be of interest in the context of hybrid barley grain production based on cleistogamy.

UNRAVELLING THE REGULATION OF SOMATIC EMBRYOGENESIS BY EXTRACELLULAR CALCIUM AND AUXIN EFFLUX BLOCKERS IN HYPOCOTYL EXPLANTS OF ALBIZZIA LEBBECK L.: SIMILARITY IN ACTION

Somatic embryogenesis involves developmental reprogramming and de-differentiation of somatic cells towards embryo development that is crucial for cellular totipotency in higher plants. It is a prototype for comprehension of the physiological, biochemical and molecular biological events transpiring during plant embryo development. In this study, we analyzed the role of exogenous calcium and auxin during somatic embryogenesis in hypocotyl explants of Albizzia lebbeck L. Light microscopic studies revealed that anterior and posterior ends of hypocotyl explants had entirely different patterns of differentiation . Scanning electron microscopic analysis of cultured hypocotyl explants exhibited globular-, heart- and torpedo-shaped somatic embryos developing from the anterior cut surface and gradually proceeding towards the other end. Surface texture of these somatic embryos showed uni-or bi-celled trichomes. Presence of exogenous calcium (1-4mM) in hormone-free B5 medium is prerequisite for induction of somatic embryos in hypocotyl explants derived from 7-d-old, light grown seedlings of Albizzia lebbeck L. A two-fold increase in calcium concentration from 2mM (B 5 medium) to 4mM led to doubling of somatic embryogenic response whereas, embryogenic callus was induced in explants raised on medium containing 10 or 20mM calcium. Depletion or high dosage in the medium (20 mM) induce non-embryogenic callus in explants. A similar response induced on hypocotyl explants upon treatments with NPA and PCIB, (auxin transport blockers). Results indicate that polar auxin transport or basipetal movement of IAA triggers acropetal transport of calcium. This combination of inverse fluxes acts as the primary signal for embryogenic response at the cut ends proximal to shoot apex and callusing at the cut ends distal to shoot apex of explants. Moreover, a correlation exists between NPA and PCIB-induced disruption of basipetal IAA transport and the inhibitory effect of NPA and PCIB on calcium translocation in the induction of similar morphogenetic responses.

Auxin-modulated root growth inhibition in Arabidopsis thaliana seedlings with ammonium as the sole nitrogen source

Ammonium is the major nitrogen source in some plant ecosystems but is toxic at high concentrations, especially when available as the exclusive nitrogen source. Ammonium stress rapidly leads to various metabolic and hormonal imbalances that ultimately inhibit root and shoot growth in many plant species, including Arabidopsis thaliana (L.) Heynh. To identify molecular and genetic factors involved in seedling survival with prolonged exclusive NH4+ nutrition, a transcriptomic analysis with microarrays was used. Substantial transcriptional differences were most pronounced in (NH4)2SO4-grown seedlings, compared with plants grown on KNO3 or NH4NO3. Consistent with previous physiological analyses, major differences in the expression modules of photosynthesis-related genes, an altered mitochondrial metabolism, differential expression of the primary NH4+ assimilation, alteration of transporter gene expression and crucial changes in cell wall biosynthesis were found. A major difference in plant hormone responses, particularly of auxin but not cytokinin, was striking. The activity of the DR5::GUS reporter revealed a dramatically decreased auxin response in (NH4)2SO4-grown primary roots. The impaired root growth on (NH4)2SO4 was partially rescued by exogenous auxin or in specific mutants in the auxin pathway. The data suggest that NH4+-induced nutritional and metabolic imbalances can be partially overcome by elevated auxin levels.

A genetic screen for mutants defective in IAA1-LUC degradation in Arabidopsis thaliana reveals an important requirement for TOPOISOMERASE6B in auxin physiology

Many plant growth and developmental processes are modulated by the hormone auxin. Auxin-modulated proteolysis of Aux/IAAs, a family of transcriptional repressors, represents a major mode of auxin action. Auxin facilitates the interaction of Aux/IAAs with TIR1/AFB F-box proteins, promoting their ubiquitination by the SCFTIR1/AFB ubiquitin E3 ligase leading to subsequent degradation by the 26S proteasome. To identify new genes regulating Aux/IAA proteolysis in Arabidopsis thaliana, we took a genetic approach, identifying individuals with altered degradation of an IAA1-luciferase fusion protein (IAA1-LUC). A mutant with 2-fold slower IAA1-LUC degradation rate compared with wild-type was isolated. Positional cloning identified the mutant as an allele of TOPOISOMERASE6B, named top6b-7. TOP6B encodes a subunit of a plant and archea-specific enzyme regulating endoreduplication, DNA damage repair and transcription in plants. T-DNA insertion alleles (top6b-8 and top6b-9) were also analyzed. top6b-7 seedlings are less sensitive to exogenous auxin than wild-type siblings in primary root growth assays, and experiments with DR5:GUS. Additionally, top6b-7 seedlings have a 40% reduction in the amount of endogenous IAA. These data suggest that increased IAA1-LUC half-life in top6b-7 probably results from a combination of both lower endogenous IAA levels and reduced sensitivity to auxin.

CUTTING PROPAGATION OF LITTLE-LEAF MOUNTAIN MAHOGANY©

Superior selections of little-leaf mountain mahogany (Cercocarpus ledifolius var. intricatus) (CLI) have potential for use in water-conserving landscapes in the Intermountain region of the USA. The potential for propagating CLI by cuttings was determined by selecting specimens from a range of sources and growing conditions at different seasons and with different rooting conditions in a series of individual experiments. Exogenous auxin treatments at high (4000/2000 ppm indolebutryic acid/naphthaleneacetic acid as Dip-N-Grow®) and low (2000/1000 ppm of the same material) were also tested. Selection CLI-1 (wild source, Ephraim, Utah, Jan. 2010) rooted at 0, 33, and 42% with zero, low, and high auxin treatments. Selections CLI-2 and CLI-3 (wild source, Mt. Charleston, Nevada, May, 2010) had results of 0, 0, and 8% and 15, 28, and 52% rooting for the respective auxin treatments and selections. Selection CLI-4 (landscape source, Logan, Utah, July, 2010) had 8, 96, and 100% rooting for similar auxin treatments. Finally selections CLI-5 and CLI-6 (landscape source, Logan, Utah) and CLI-3 (container-grown stock plants) stuck in July, 2012 had 6, 2, and 85% rooting with the low rate of auxin, respectively. Overall, two selections had greater than 90% rooting while one had approximately 40% and three others less than 10%. For the three selections that did root, supplemental auxin increased the percentage of rooted cuttings from 0-15% for untreated controls to 42-100% for treated. In contrast, the three selections with poor rooting had little response to exogenous auxin at the levels used. Propagation of CLI by cuttings is feasible, though variable. The variability may be due to accession genetic differences, stock plant growing conditions, or rooting conditions.

A Molecular framework for the differential responses of primary and lateral roots to auxin in Arabidopsis thaliana

The Arabidopsis thaliana root system, which comprises primary, lateral, and adventitious roots, responds differentially to environmental and hormonal signals. For example, exposure to high concentrations of the plant hormone auxin promotes lateral root (LR) initiation and emergence; however, high levels of auxin inhibit primary root (PR) growth. Here, we show that transacting small interfering RNA-dependent post-transcriptional gene silencing of AUXIN RESPONSE FACTOR 3 (tasiRNA-ARF PTGS) likely plays a role in promoting the differential responses of PR and LR to auxin. In the presence of 1-naphthaleneacetic acid (NAA), a synthetic auxin, the expression domain of ARF3 (a negative regulator in auxin signaling) expanded into the meristem zone (MZ) in PR. Moreover, we found a remarkable increase in the expression of a cleavage-resistant version of ARF3 (pARF3::ARF3(m)-GUS) in PR even without application of exogenous auxin, suggesting the involvement of posttranscriptional regulation. In addition, the expression levels of microRNA390 (miR390), which triggers biogenesis of tasiRNA-ARF, were substantially reduced in PR, but increased in LR. Taken together, our findings suggest that high auxin levels differentially regulate the growth and development of PR and LR via the tasiRNA-ARF pathway.