Exogenous Auxin Research Articles

Auxin- and pH-induced guttation in Phycomyces sporangiophores: relation between guttation and diminished elongation growth.

Guttation, the formation of exudation water, is widespread among plants and fungi, yet the underlying mechanisms remain largely unknown. We describe the conditions for inducing guttation in sporangiophores of the mucoracean fungus, Phycomyces blakesleeanus. Cultivation on peptone-enriched potato dextrose agar elicits vigorous guttation mainly below the apical growing zone, while sporangiophores raised on a glucose-mineral medium manifest only moderate guttation. Mycelia do not guttate irrespective of the employed media. The topology of guttation droplets allows identifying the non-growing part of the sporangiophore as a guttation zone, which responds to humidity and medium composition in ways that become relevant for turgor homeostasis and thus the sensor physiology of the growing zone. Apparently, the entire sporangiophore, rather than exclusively the growing zone, participates in signal reception and integration to generate a common growth output. Exogenous auxin applied to the growing zones elicits two correlated responses: (i) formation of guttation droplets in the growing and transition zones below the sporangium and (ii) a diminution of the growth rate. In sporangiophore populations, guttation-induction by exogenous control buffer occurs at low frequencies; the bias for guttation increases with increasing auxin concentration. Synthetic auxins and the transport inhibitor NPA suppress guttation completely, but leave growth rates largely unaffected. Mutants C2 carA and C148 carA madC display higher sensitivities for auxin-induced guttation compared to wild type. A working model for guttation includes aquaporins and mechanosensitive ion channels that we identified in Phycomyces by sequence domain searches.

Effects of exogenous auxin on yield in foxtail millet (Setaria italica L.) when applied at the grain-filling stage.

Foxtail millet (Setaria italica L.) is of high nutritious value, which is an important crop in arid and semi-arid regions. The objective of this experiment was to explore the effects of the synthetic auxin naphthalene acetic acid (NAA) on the physiological processes of foxtail millet, and to provide a theoretical basis and technical approaches for its efficient use in millet cultivation. Two foxtail millet varieties ('Jingu 21' and 'Zhangzagu 5') were treated with six concentrations of NAA from 0-144 mg L-1 at the grain-filling stage in field experiments. The photosynthetic pigment contents, gas exchange parameters, chlorophyll fluorescence parameters, and grain yield were measured in foxtail millet. The results showed that low concentrations of NAA (18-36 mg L-1) increased the contents of photosynthetic pigments, and increased the activities of antioxidant enzymes, the photosynthetic rate, and the activity of photosystem system II (PS II). At higher NAA concentrations, the facilitation effect of the treatments diminished, showing a clear concentration effect. In this study, yield was significantly and positively correlated with PS II effective quantum yield (Y(II)) and the PSII electron transport rate (ETR), and the net photosynthetic rate (Pn) was significantly and positively correlated with chlorophyll content, stomatal conductance (Gs), Y(II), and ETR. These results also indicated that exogenous NAA application promotes the production of ATP and NADPH by increasing the efficiency of electron transfer within the photosystems and also improved photochemical utilization, which facilitates the fixation and reduction of carbon, ultimately leading to an increase in Pn and increasing grain yield in foxtail millet.

ПУТИ МОРФОГЕНЕЗА В КУЛЬТУРЕ IN VITRO ИЗОЛИРОВАННЫХ ПЫЛЬНИКОВ ПШЕНИЦЫ: РОЛЬ БАЛАНСА ФИТОГОРМОНОВ

The study is aimed to the study of the peculiarities of the induction of morphogenesis pathways in vitro of morphogenetically competent anther cells - microspores in the isolated anther culture of a number of spring soft wheat genotypes, recommended for cultivation in the climatic conditions of the Republic of Bashkortostan. The content of IAA in seedlings of the studied genotypes of spring sof wheat was analyzed to determine the class of the genotype – characterized by high or low auxin content. Such morphogenesis pathways in vitro as embryoidogenesis, polyembryoidogenesis and callusogenesis have been identified and studied. Morpho-histological assessment of androclinic structures is given. It was established that the induction of a certain morphogenesis pathway in vitro is determined mainly by the balance between the content of endogenous auxin IAA in anthers before inoculation on a nutrient medium and the concentration of exogenous synthetic auxin 2,4-D in the composition of the nutrient medium. The fundamental possibility of regulation of morphogenesis pathways in vitro in isolated anther culture of wheat by selecting for each genotype an adequate balance of endogenous and exogenous auxins for the induction of a specific pathway has been shown.

ВЛИЯНИЕ ОБРАБОТКИ СЕМЯН РАЗНЫМИ ДОЗАМИ ГЕТЕРОАУКСИНА НА РОСТОВЫЕ ПАРАМЕТРЫ ДВУХ СОРТОВ ФАСОЛИ

Priming of seeds in auxin solution is a model of the action of auxin-producing bacteria on plant growth. The aim of the study was to identify auxin concentrations that stimulate and inhibit the growth of seedlings of two varieties of beans. Seeds were soaked for 3 hours in tenfold dilutions of heteroauxin 0,00001–100 mg/l, germinated on wet filters in the dark, growth parameters (germination energy, germination, length of axial organs of the seedling, number and length of lateral and accessory roots, root and shoot weight) were measured on the 7th day. For the Ufa variety, an increase in all growth indicators by 25-36% relative to the control occurred only at a dose of 0,001 mg/l, while the shoot/root ratio of 1.8 was as in the control. Changing this dose by an order of magnitude led to a neutral or negative effect on all growth indicators. For the Elsa variety, the growth of the length of the main root on the gradient of hormone concentrations was characterized by a two-vertex graph with a maximum value of an increase in root length by 23% to the control at a dose of 0,1 mg/l hormone. At the same time, the shoot/root ratio is 1,0. According to other indicators, no stimulating effect was found. The presence of one or more pessimums of the values of the growth indicators of seedlings on the concentration curve indicates that in both varieties the range of stimulating concentration of exogenous auxin is very narrow. Therefore, to stimulate growth, it is necessary to use precision and individual concentrations of exogenous hormone for each variety.

Auxin contributes to jasmonate-mediated regulation of abscisic acid signaling during seed germination in Arabidopsis.

Abscisic acid (ABA) represses seed germination and postgerminative growth in Arabidopsis thaliana. Auxin and jasmonic acid (JA) stimulate ABA function; however, the possible synergistic effects of auxin and JA on ABA signaling and the underlying molecular mechanisms remain elusive. Here, we show that exogenous auxin works synergistically with JA to enhance the ABA-induced delay of seed germination. Auxin biosynthesis, perception, and signaling are crucial for JA-promoted ABA responses. The auxin-dependent transcription factors AUXIN RESPONSE FACTOR10 (ARF10) and ARF16 interact with JASMONATE ZIM-DOMAIN (JAZ) repressors of JA signaling. ARF10 and ARF16 positively mediate JA-increased ABA responses, and overaccumulation of ARF16 partially restores the hyposensitive phenotype of JAZ-accumulating plants defective in JA signaling in response to combined ABA and JA treatment. Furthermore, ARF10 and ARF16 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a critical regulator of ABA signaling, and the ability of ARF16 to stimulate JA-mediated ABA responses is mainly dependent on ABI5. ARF10 and ARF16 activate the transcriptional function of ABI5, whereas JAZ repressors antagonize their effects. Collectively, our results demonstrate that auxin contributes to the synergetic modulation of JA on ABA signaling, and explain the mechanism by which ARF10/16 coordinate with JAZ and ABI5 to integrate the auxin, JA, and ABA signaling pathways.

Transcriptomics Analysis Reveals a Putative Role for Hormone Signaling and MADS-Box Genes in Mature Chestnut Shoots Rooting Recalcitrance.

Maturation imposes several changes in plants, which are particularly drastic in the case of trees. In recalcitrant woody species, such as chestnut (Castanea sativa Mill.), one of the major maturation-related shifts is the loss of the ability to form adventitious roots in response to auxin treatment as the plant ages. To analyze the molecular mechanisms underlying this phenomenon, an in vitro model system of two different lines of microshoots derived from the same field-grown tree was established. While juvenile-like shoots root readily when treated with exogenous auxin, microshoots established from the crown of the tree rarely form roots. In the present study, a transcriptomic analysis was developed to compare the gene expression patterns in both types of shoots 24 h after hormone and wounding treatment, matching the induction phase of the process. Our results support the hypothesis that the inability of adult chestnut tissues to respond to the inductive treatment relies in a deep change of gene expression imposed by maturation that results in a significant transcriptome modification. Differences in phytohormone signaling seem to be the main cause for the recalcitrant behavior of mature shoots, with abscisic acid and ethylene negatively influencing the rooting ability of the chestnut plants. We have identified a set of related MADS-box genes whose expression is modified but not suppressed by the inductive treatment in mature shoots, suggesting a putative link of their activity with the rooting-recalcitrant behavior of this material. Overall, distinct maturation-derived auxin sensibility and homeostasis, and the related modifications in the balance with other phytohormones, seem to govern the outcome of the process in each type of shoots.

Transcriptional Regulation of zma-MIR528a by Action of Nitrate and Auxin in Maize.

In recent years, miR528, a monocot-specific miRNA, has been assigned multifaceted roles during development and stress response in several plant species. However, the transcription regulation and the molecular mechanisms controlling MIR528 expression in maize are still poorly explored. Here we analyzed the zma-MIR528a promoter region and found conserved transcription factor binding sites related to diverse signaling pathways, including the nitrate (TGA1/4) and auxin (AuxRE) response networks. Accumulation of both pre-miR528a and mature miR528 was up-regulated by exogenous nitrate and auxin treatments during imbibition, germination, and maize seedling establishment. Functional promoter analyses demonstrated that TGA1/4 and AuxRE sites are required for transcriptional induction by both stimuli. Overall, our findings of the nitrogen- and auxin-induced zma-MIR528a expression through cis-regulatory elements in its promoter contribute to the knowledge of miR528 regulome.

A quantitative assessment of factors affecting the rooting of grapevine rootstocks (Vitis vinifera L.)

Grapevine is a fruit species of great agronomic interest since both fresh fruit and wine are highly marketable products. Therefore, there is growing interest in this crop among researchers in the pursuit of increased yields and better cultivation conditions. Asexual propagation is the most commonly used method for propagating grapevine seedlings, with cuttings used for rootstock formation and subsequent grafting of materials to form the canopy. The rootstock is responsible for support, water, and nutrient uptake, thus determining the vigor of the plant; therefore, it is essential to understand the suitable conditions in which roots can thrive and thereby enhance plant growth. The type of cutting, type of substrate, and application of exogenous auxins are among the factors that interfere with the rooting of grapevine rootstocks, and determining and implementing the best combination of these factors could provide for more vigorous plants. Reviewing high-quality published scientific research from diverse countries is a method of identifying data, grouping information, and obtaining reliable answers. Furthermore, it can allow for the detection and grouping of the main factors affecting the rooting of grapevine cuttings, thus enabling a clearer recommendation and better understanding of the issue.

Widely targeted metabolomics analysis reveals the effect of exogenous auxin on postharvest resistance to Botrytis cinerea in kiwifruit (Actinidia chinensis L.)

Kiwifruit has high economic value but is susceptible to fungal infection after harvest. Botrytis cinerea (B. cinerea) is one of these fungal pathogens that cause huge economic losses and potential food safety problems. Auxin is a vital plant growth regulator that has been reported to play a role in plant defense. In this study, postharvest kiwifruit was treated with different concentrations of indole-3-acetic acid (IAA), after which the average incidence rate and lesion area were analyzed to determine the appropriate concentration of IAA for disease resistance. Defense enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were detected. Ultra-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS)-based widely targeted metabolomic analysis was used to investigate the mechanism of kiwifruit active compounds during IAA treatment and B. cinerea infection. Exogenous IAA treatment (especially at 50 μg/ mL) enhanced the resistance of kiwifruit to B. cinerea, resulting in reduced disease incidence and lesion area, and showed higher pathogen resistance-related defense enzyme activity in response to B. cinerea infection. Further metabolomic analysis revealed 776 metabolites in kiwifruit after IAA treatment, among which 51 metabolites in IAA_24h (24 h after IAA treatment) vs. CK (sterilized ddH2O treatment) fruit, 55 metabolites in IAA_72h (72 h after IAA treatment) vs. CK, and 67 metabolites in IAA_24h vs. IAA_72h were found to be significantly different. KEGG analysis showed that the resistance induced in kiwifruit by IAA treatment was closely related to the activation of phenylpropanoids, including flavonoids and phenols, terpenoids, carbohydrate metabolism, and hormone signaling pathways. These results might reveal the mechanism of IAA-induced resistance in kiwifruit and provide a new theoretical basis for the safe and efficient control of postharvest diseases in kiwifruit.

The thiol-reductase activity of YUCCA6 enhances nickel heavy metal stress tolerance in Arabidopsis.

Anthropogenic activities cause the leaching of heavy metals into groundwater and their accumulation in soil. Excess levels of heavy metals cause toxicity in plants, inducing the production of reactive oxygen species (ROS) and possible death caused by the resulting oxidative stress. Heavy metal stresses repress auxin biosynthesis and transport, inhibiting plant growth. Here, we investigated whether nickel (Ni) heavy metal toxicity is reduced by exogenous auxin application and whether Ni stress tolerance in Arabidopsis thaliana is mediated by the bifunctional enzyme YUCCA6 (YUC6), which functions as an auxin biosynthetic enzyme and a thiol-reductase (TR). We found that an application of up to 1 µM exogenous indole-3-acetic acid (IAA) reduces Ni stress toxicity. yuc6-1D, a dominant mutant of YUC6 with high auxin levels, was more tolerant of Ni stress than wild-type (WT) plants, despite absorbing significantly more Ni. Treatments of WT plants with YUCASIN, a specific inhibitor of YUC-mediated auxin biosynthesis, increased Ni toxicity; however yuc6-1D was not affected by YUCASIN and remained tolerant of Ni stress. This suggests that rather than the elevated IAA levels in yuc6-1D, the TR activity of YUC6 might be critical for Ni stress tolerance. The loss of TR activity in YUC6 caused by the point-mutation of Cys85 abolished the YUC6-mediated Ni stress tolerance. We also found that the Ni stress–induced ROS accumulation was inhibited in yuc6-1D plants, which consequently also showed reduced oxidative damage. An enzymatic assay and transcriptional analysis revealed that the peroxidase activity and transcription of PEROXIREDOXIN Q were enhanced by Ni stress to a greater level in yuc6-1D than in the WT. These findings imply that despite the need to maintain endogenous IAA levels for basal Ni stress tolerance, the TR activity of YUC6, not the elevated IAA levels, plays the predominant role inNi stress tolerance by lowering Ni-induced oxidative stress.

The SlTPL3-SlWUS module regulates multi-locule formation in tomato by modulating auxin and gibberellin levels in the shoot apical meristem.

Malformed fruits depreciate a plant's market value. In tomato (Solanum lycopersicum), fruit malformation is associated with the multi-locule trait, which involves genes regulating shoot apical meristem (SAM) development. The expression pattern of TOPLESS3 (SlTPL3) throughout SAM development prompted us to investigate its functional significance via RNA interference (RNAi) and clustered regularly interspaced short palindromic repeats/CRISPR-associated nuclease 9 (Cas9)-mediated gene editing. Lower SlTPL3 transcript levels resulted in larger fruits with more locules and larger SAMs at the 5 d after germination (DAG5) stage. Differentially expressed genes in the SAM of wild-type (WT) and SlTPL3-RNAi plants, identified by transcriptome deep sequencing (RNA-seq), were enriched in the gibberellin (GA) biosynthesis and plant hormone signaling pathways. Moreover, exogenous auxin and paclobutrazol treatments rescued the multi-locule phenotype, indicating that SlTPL3 affects SAM size by mediating auxin and GA levels in the SAM. Furthermore, SlTPL3 interacted with WUSCHEL (SlWUS), which plays an important role in SAM size maintenance. We conducted RNA-seq and DNA affinity purification followed by sequencing (DAP-seq) analyses to identify the genes regulated by SlTPL3 and SlWUS in the SAM and to determine how they regulate SAM size. We detected 24 overlapping genes regulated by SlTPL3 and SlWUS and harboring an SlWUS-binding motif in their promoters. Furthermore, functional annotation revealed a notable enrichment for functions in auxin transport, auxin signal transduction, and GA biosynthesis. Dual-luciferase assays also revealed that SlTPL3 enhances SlWUS-mediated regulation (repression and activation) of SlPIN3 and SlGA2ox4 transcription, indicating that the SlTPL3-SlWUS module regulates SAM size by mediating auxin distribution and GA levels, and perturbations of this module result in enlarged SAM. These results provide novel insights into the molecular mechanism of SAM maintenance and locule formation in tomato and highlight the SlTPL3-SlWUS module as a key regulator.

Ethylene response factor ERF.D7 activates auxin response factor 2 paralogs to regulate tomato fruit ripening.

Despite the obligatory role of ethylene in climacteric fruit ripening and the identification of 77 ethylene response factors (ERFs) in the tomato (Solanum lycopersicum) genome, the role of few ERFs has been validated in the ripening process. Here, using a comprehensive morpho-physiological, molecular, and biochemical approach, we demonstrate the regulatory role of ERF D7 (SlERF.D7) in tomato fruit ripening. SlERF.D7 expression positively responded to exogenous ethylene and auxin treatments, most likely in a ripening inhibitor-independent manner. SlERF.D7 overexpression (OE) promoted ripening, and its silencing had the opposite effect. Alterations in its expression modulated ethylene production, pigment accumulation, and fruit firmness. Consistently, genes involved in ethylene biosynthesis and signaling, lycopene biosynthesis, and cell wall loosening were upregulated in the OE lines and downregulated in RNAi lines. These transgenic lines also accumulated altered levels of indole-3-acetic acid at late-breaker stages. A positive association between auxin response factor 2 (ARF2) paralog's transcripts and SlERF.D7 mRNA levels and that SlARF2A and SlARF2B are direct targets of SlERF.D7 underpinned the perturbed auxin-ethylene crosstalk for the altered ripening program observed in the transgenic fruits. Overall, this study uncovers that SlERF.D7 positively regulates SlARF2A/B abundance to amalgamate auxin and ethylene signaling pathways for controlling tomato fruit ripening.

Effects of α-Naphthylacetic Acid on Cadmium Stress and Related Factors of Tomato by Regulation of Gene Expression

Cadmium (Cd) is absorbed and accumulated by crops, and it adversely affects plant growth and development. To explore the effect of exogenous auxin on Cd stress, we applied different concentrations of α-naphthaleneacetic acid (NAA) and the auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) to tomato plants exposed to Cd stress in a hydroponic system. NAA and TIBA at different concentrations were used under Cd stress. Plant growth, root morphology, and auxin distribution were observed. Lipid peroxidation and antioxidant enzyme activities in leaves, cadmiumcontent, and migration coefficient of plants were determined. Transcriptome sequencing and qRT-PCR were used to analyze the differentially expressed genes. Results showed that auxin was concentrated in the leaf veins, stem base, and roots in P5::GUS “Chico III” transgenic tomato, indicating NAA polar transport to the roots and promotion of root growth under Cd stress. Cd was absorbed by the roots and transported to the shoots. It then inhibited plant growth and promoted antioxidant enzyme activities, O2− production, H2O2 accumulation, and membrane lipid peroxidation. Treatment with 0.5 μM NAA improved antioxidant enzyme activities, reduced reactive oxygen, maintained membrane permeability, and decreased malondialdehyde and proline contents. Transcriptome analysis revealed that NAA activated a large number of genes in the roots: 1998 genes were differentially expressed in response to Cd or NAA treatment, and 1736 genes were specifically expressed in response to NAA treatment under Cd stress. Among the differentially expressed genes, tomato metallocarboxypeptidase inhibitor TCMP-2 (2A11) and Solanum lycopersicum heavy metal-associated isoprenylated plant protein (HIPP) 7-like (LOC101264884), which are closely related to plant response to heavy metal stress, may be the key sites of NAA. In conclusion, the NAA-mediated response to Cd stress was closely associated with “defense response” genes in shoots and “oxidoreductase activity, oxidizing metal ions” and “response to auxin” genes in roots.

The cytokinin type-B response regulator PeRR12 is a negative regulator of adventitious rooting and salt tolerance in poplar

Adventitious root (AR) development is an ecologically and economically important biological process that maintains ecological balance, improves plant survivability, and allows for massive vegetative propagation, but its genetic mechanisms are not well understood. Here, eight Arabidopsis response regulator (ARR) genes were cloned and identified in poplar, most of which were detected in the AR, phloem, and xylem and showed remarkable induction at different time points during AR development. Subcellular localization indicated that most of these PeRR genes are in the nucleus. Based on qRT-PCR expression analysis of some genes related to AR development, we inferred that overexpression of PeRR12 (OE_PeRR12) may inhibited AR formation by suppressing the transcription of PeWOX11, PeWOX5, PePIN1 and PePIN3 in poplar while promoting type-A RR transcripts. Correspondingly, exogenous auxin partially restored the rooting of OE_PeRR12 poplar by inhibiting PeRR12 expression. Moreover, the activities of the antioxidant systems of OE_PeRR12 poplars were lower than those of wild-type poplars under salt stress conditions, indicating that PeRR12 may acts as a repressor that mediates salt tolerance by suppressing the expression of PeHKT1;1. Altogether, these results suggest that PeRR12 plays essential roles in mediating AR formation and salinity tolerance in poplar.

Tomato UVI4 homologue modulates cell expansion to participate heat-stimulated hypocotyl elongation

Raising incubation temperature during seed germination is a conventional strategy to promote the efficiency of seed emergence from soil, which requires robust hypocotyl elongation. Tomato hypocotyl elongation is a consequence of active cell division and expansion, both of which require precisely regulated cell cycle progression. Anaphase-promoting complex/cyclosome (APC/C) is a multiple subunit E3 ubiquitin ligase which degrades certain mitotic cyclins and inhibit M-specific CDK/cyclin complex activity during cell cycle from G2 to M phase. Little is known about the function of APC/C regulators during tomato hypocotyl elongation. Here, we isolated tomato homologue SlUVI4 based on the homology with the negative regulator of APC/C, UVI4 in Arabidopsis. SlUVI4 had high expression in hypocotyls after the seed germination and gradually reduced its transcription during the hypocotyl elongation. We probed SlUVI4 function in heat-stimulated hypocotyl elongation through genetic, physiological and cytological analyses. Both overexpression and loss-of-function of SlUVI4 resulted in defective hypocotyl growth with opposite cell sizes. Microscopic analyses of hypocotyl sections revealed that SlUVI4 plays important roles in repressing cell expansion. Heat repressed SlUVI4 transcription to stimulate cell expansion and hypocotyl elongation. Inhibition of auxin polar transportation activated SlUVI4 transcription and abolished heat-induced cell expansion and hypocotyl elongation. Exogenous auxin repressed SlUVI4 transcription and enhanced hypocotyl elongation under heat condition. Partial loss of function of SlUVI4 largely abrogated heat-stimulated hypocotyl cell expansion. Taken together, our findings illustrate that SlUVI4 is involved in auxin-mediated hypocotyl elongation stimulated by heat. • SlUVI4 negatively modulates cell expansion in tomato hypocotyls. • Auxin represses SlUVI4 transcription in tomato hypocotyls. • Heat-stimulated hypocotyl elongation requires auxin-mediated repression of SlUVI4 transcription.

Key regulatory pathways, microRNAs, and target genes participate in adventitious root formation of Acer rubrum L

Red maple (Acer rubrum L.) is a type of colorful ornamental tree with great economic value. Because this tree is difficult to root under natural conditions and the seedling survival rate is low, vegetative propagation methods are often used. Because the formation of adventitious roots (ARs) is essential for the asexual propagation of A. rubrum, it is necessary to investigate the molecular regulatory mechanisms of AR formation in A. rubrum. To address this knowledge gap, we sequenced the transcriptome and small RNAs (sRNAs) of the A. rubrum variety ‘Autumn Fantasy’ using high-throughput sequencing and explored changes in gene and microRNA (miRNA) expression in response to exogenous auxin treatment. We identified 82,468 differentially expressed genes (DEGs) between the treated and untreated ARs, as well as 48 known and 95 novel miRNAs. We also identified 172 target genes of the known miRNAs using degradome sequencing. Two key regulatory pathways (ubiquitin mediated proteolysis and plant hormone signal transduction), Ar-miR160a and the target gene auxin response factor 10 (ArARF10) were selected based on KEGG pathway and cluster analyses. We further investigated the expression patterns and regulatory roles of ArARF10 through subcellular localization, transcriptional activation, plant transformation, qRT-PCR analysis, and GUS staining. Experiments overexpressing ArARF10 and Ar-miR160a, indicated that ArARF10 promoted AR formation, while Ar-miR160a inhibited AR formation. Transcription factors (TFs) and miRNAs related to auxin regulation that promote AR formation in A. rubrum were identified. Differential expression patterns indicated the Ar-miR160a-ArARF10 interaction might play a significant role in the regulation of AR formation in A. rubrum. Our study provided new insights into mechanisms underlying the regulation of AR formation in A. rubrum.

Transporter NRT1.5/NPF7.3 suppresses primary root growth under low K+ stress by regulating the degradation of PIN-FORMED2

BackgroundThe availability of potassium is one of the main environmental factors for modifying the plasticity of root architecture. Many potassium channels and transporters are involved in regulating primary root growth in response to low potassium stress. NRT1.5/NPF7.3 transporter is a NO3−/H+ and K+/H+ cotransporter, and participates in NO3− and K+ translocation from the roots to the shoots. However, the underlying mechanism of NRT1.5-regulated primary root growth under low potassium stress is unclear.ResultsWe show that NRT1.5/NPF7.3 inhibited primary root growth under low potassium conditions by regulating the accumulation of PIN2 protein and auxin levels. Under low potassium conditions, the mutants nrt1.5 and lks2 exhibited longer primary roots, longer meristem regions and elongation zones of primary roots, and more cell activity in the meristem region compared to WT plants, revealing the involvement of NRT1.5 in LK (low potassium)-inhibition primary root growth. In addition, exogenous auxin (IAA), auxin analogue (NAA, 2.4-D) or auxin precursor (IBA) promoted the primary root growth of WT and the complementation line NRT1.5 COM plants. In addition, the application of NPA inhibited the primary root growth of the nrt1.5 and lks2 mutants. Auxin accumulation was higher in the root tip of nrt1.5 plants than in WT plants, indicating that NRT1.5 regulates root growth inhibition by regulating auxin distribution. Furthermore, PIN2 was degraded more quickly in nrt1.5 plants under LK stress.ConclusionsOur findings reveal that NRT1.5 inhibits primary root growth by modulating the auxin level in the root tip via the degradation of PIN2.

Auxin facilitates cell wall phosphorus reutilization in a nitric oxide-ethylene dependent manner in phosphorus deficient rice (Oryza sativa L.)

Auxin is involved in stress responses of plants, such as phosphorus (P) deficiency in rice. Studies on whether auxin participates in cell-wall inorganic phosphorous (Pi) reutilization in Pi-starved rice are scarce. This study explored the mechanisms underlying auxin-facilitated cell-wall Pi-reutilization in rice roots. Pi deficiency rapidly induced auxin accumulation in roots; exogenous auxin [α-naphthaleneacetic acid (NAA), a permeable analog of auxin] elevated soluble Pi content in roots and shoots by increasing pectin content by enhancing activity of pectin methylesterase, and upregulating the transcript level of PHOSPHORUS-TRANSPORTER-2, such that more Pi was translocated to the shoot. Irrespective of the Pi status, exogenous auxin induced nitric oxide (NO) and ethylene production, while exogenous sodium nitroprusside (an NO donor) and 1-aminocyclopropane-1-carboxylic acid (a precursor of ethylene) had no effect on auxin content, suggesting that auxin may act upstream of NO and ethylene. The beneficial effect of NAA in increasing soluble Pi content in roots and shoots disappeared when 2-(4-carboxyphenyl)− 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (a scavenger of NO) or aminoethoxyvinylglycine (an inhibitor of ethylene) were applied, suggesting that auxin facilitates cell-wall Pi-reutilization in a NO-ethylene-dependent manner in Pi-deficient rice. Our study results suggest auxin application as an effective agronomic practice for improving plant Pi nutrition in P-deficient conditions.

Auxin Coordinates Achene and Receptacle Development During Fruit Initiation in Fragaria vesca.

In strawberries, fruit set is considered as the transition from the quiescent ovary to a rapidly growing fruit. Auxin, which is produced from the fertilized ovule in the achenes, plays a key role in promoting the enlargement of receptacles. However, detailed regulatory mechanisms for fruit set and the mutual regulation between achenes and receptacles are largely unknown. In this study, we found that pollination promoted fruit development (both achene and receptacle), which could be stimulated by exogenous auxin treatment. Interestingly, auxin was highly accumulated in achenes, but not in receptacles, after pollination. Further transcriptome analysis showed that only a small portion of the differentially expressed genes induced by pollination overlapped with those by exogenous auxin treatment. Auxin, but not pollination, was able to activate the expression of growth-related genes, especially in receptacles, which resulted in fast growth. Meanwhile, those genes involved in the pathways of other hormones, such as GA and cytokinin, were also regulated by exogenous auxin treatment, but not pollination. This suggested that pollination was not able to activate auxin responses in receptacles but produced auxin in fertilized achenes, and then auxin might be able to transport or transduce from achenes to receptacles and promote fast fruit growth at the early stage of fruit initiation. Our work revealed a potential coordination between achenes and receptacles during fruit set, and auxin might be a key coordinator.

Excessive nitrogen in field-grown rice suppresses grain filling of inferior spikelets by reducing the accumulation of cytokinin and auxin

Excessive nitrogen fertilizer is often applied to achieve higher rice yield in many regions of the world. However, the effect of excessive nitrogen on grain filling of inferior spikelets and its underlying mechanisms are still poorly understood. A three-year field experiment was conducted to reveal the mechanisms by which excessive nitrogen application regulated grain filling. We found that excessive application of nitrogen reduced the starch biosynthesis and grain weight of inferior spikelets but not superior spikelets. Plant hormone detection revealed that both cytokinin (CTK) and 3-Indoleacetic acid (IAA) were reduced by excessive nitrogen at the early stage of grain filling in inferior spiekelts. Quantitative real-time PCR results showed that inhibition of biosynthesis genes led to a decrease in IAA content, while elevation of CTK oxidase genes resulted in a lower level of CTK under excessive nitrogen treatment. Application of exogenous IAA or CTK significantly impaired the inhibitory effect of excessive nitrogen on starch accumulation and grain weight of inferior spikelets. Further enzyme assays revealed that excessive nitrogen application significantly suppressed the enzyme activities of sucrose synthase (SuS), adenosine diphosphate glucose pyrophosphorylase (AGP) and starch synthase (StS) in inferior spikelets. Application of exogenous IAA or CTK to the panicle under excessive nitrogen treatment significantly increased the enzyme activities of SuS, AGP and StS. Taken together, our results revealed that excessive nitrogen application in field-grown rice suppressed grain filling of inferior spikelets by reducing the accumulation of cytokinin and auxin. • Grain filling of inferior spikelets was reduced by excessive nitrogen application. • HN application reduced the contents of CTK and auxin in inferior spikelets at 6 DAA. • Exogenous CTK and auxin both increased grain filling under HN conditions. • HN downregulated auxin biosynthesis genes while upregulated CTK catabolism genes. • Activities of key enzymes for starch biosynthesis were regulated by CTK and auxin.