AUX/IAA proteins are active repressors, and their stability and activity are modulated by auxin

CLE3 and its homologs share overlapping functions in the modulation of lateral root formation through CLV1 and BAM1 in Arabidopsis thaliana

Lateral roots are important for a wide range of processes, including uptake of water and nutrients. The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-RELATED (CLE) 1 ~ 7 peptide family and their cognate receptor CLV1 have been shown to negatively regulate lateral root formation under low-nitrate conditions. However, little is known about how CLE signaling regulates lateral root formation. A persistent obstacle in CLE peptide research is their functional redundancies, which makes functional analyses difficult. To address this problem, we generate the cle1 ~ 7 septuple mutant (cle1 ~ 7-cr1, cr stands for mutant allele generated with CRISPR/Cas9). cle1 ~ 7-cr1 exhibits longer lateral roots under normal conditions. Specifically, in cle1 ~ 7-cr1, the lateral root density is increased, and lateral root primordia initiation is found to be accelerated. Further analysis shows that cle3 single mutant exhibits slightly longer lateral roots. On the other hand, plants that overexpress CLE2 and CLE3 exhibit decreased lateral root lengths. To explore cognate receptor(s) of CLE2 and CLE3, we analyze lateral root lengths in clv1 barely any meristem 1(bam1) double mutant. Mutating both the CLV1 and BAM1 causes longer lateral roots, but not in each single mutant. In addition, genetic analysis reveals that CLV1 and BAM1 are epistatic to CLE2 and CLE3. Furthermore, gene expression analysis shows that the LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) genes, which promote lateral root formation, are upregulated in cle1 ~ 7-cr1 and clv1 bam1. We therefore propose that CLE2 and CLE3 peptides are perceived by CLV1 and BAM1 to mediate lateral root formation through LBDs regulation.

RAV1 mediates cytokinin signaling for regulating primary root growth in Arabidopsis

Root growth dynamics is an outcome of complex hormonal crosstalk. The primary root meristem size, for example, is determined by antagonizing actions of cytokinin and auxin. Here we show that RAV1, a member of the AP2/ERF family of transcription factors, mediates cytokinin signaling in roots to regulate meristem size. The rav1 mutants have prominently longer primary roots, with a meristem that is significantly enlarged and contains higher cell numbers, compared with wild-type. The mutant phenotype could be restored on exogenous cytokinin application or by inhibiting auxin transport. At the transcript level, primary cytokinin-responsive genes like ARR1, ARR12 were significantly downregulated in the mutant root, indicating impaired cytokinin signaling. In concurrence, cytokinin induced regulation of SHY2, an Aux/IAA gene, and auxin efflux carrier PIN1 was hindered in rav1, leading to altered auxin transport and distribution. This effectively altered root meristem size in the mutant. Notably, CRF1, another member of the AP2/ERF family implicated in cytokinin signaling, is transcriptionally repressed by RAV1 to promote cytokinin response in roots. Further associating RAV1 with cytokinin signaling, our results demonstrate that cytokinin upregulates RAV1 expression through ARR1, during post-embryonic root development. Regulation of RAV1 expression is a part of secondary cytokinin response that eventually represses CRF1 to augment cytokinin signaling. To conclude, RAV1 functions in a branch pathway downstream to ARR1 that regulates CRF1 expression to enhance cytokinin action during primary root development in Arabidopsis.

A HST1-like gene controls tiller angle through regulating endogenous auxin in common wheat

Tiller angle is one of the most important agronomic traits and one key factor for wheat ideal plant architecture, which can both increase photosynthetic efficiency and greatly enhance grain yield. Here, a deacetylase HST1-like (TaHST1L) gene controlling wheat tiller angle was identified by the combination of a genome-wide association study (GWAS) and bulked segregant analysis (BSA). Ethyl methane sulfonate (EMS)-mutagenized tetraploid wheat lines with the premature stop codon of TaHST1L exhibited significantly smaller tiller angles than the wild type. TaHST1L-overexpressing (OE) plants exhibited significantly larger tiller angles and increased tiller numbers in both winter and spring wheat, while TaHST1L-silenced RNAi plants displayed significantly smaller tiller angles and decreased tiller numbers. Moreover, TaHST1L strongly interacted with TaIAA17 and inhibited its expression at the protein level, and thus possibly improved the content of endogenous auxin in the basal tissue of tillers. The transcriptomics and metabolomics results indicated that TaHST1L might change plant architecture by mediating auxin signal transduction and regulating endogenous auxin levels. In addition, a 242-bp insertion/deletion (InDel) in the TaHST1L-A1 promoter altered transcriptional activity and TaHST1L-A1b allele with the 242-bp insertion widened the tiller angle of TaHST1L-OE transgenic rice plants. Wheat varieties with TaHST1L-A1b allele possessed the increased tiller angle and grain yield. Further analysis in wheat and its progenitors indicated that the 242-bp InDel possibly originated from wild emmer and was strongly domesticated in the current varieties. Therefore, TaHST1L involved in the auxin signalling pathway showed the big potential to improve wheat yield by controlling plant architecture.

Genome-Wide Identification and Co-Expression Analysis of ARF and IAA Family Genes in Euscaphis konishii: Potential Regulators of Triterpenoids and Anthocyanin Biosynthesis

Euscaphis konishii is an evergreen plant that is widely planted as an industrial crop in Southern China. It produces red fruits with abundant secondary metabolites, giving E. konishii high medicinal and ornamental value. Auxin signaling mediated by members of the AUXIN RESPONSE FACTOR (ARF) and auxin/indole-3-acetic acid (Aux/IAA) protein families plays important roles during plant growth and development. Aux/IAA and ARF genes have been described in many plants but have not yet been described in E. konishii. In this study, we identified 34 EkIAA and 29 EkARF proteins encoded by the E. konishii genome through database searching using HMMER. We also performed a bioinformatic characterization of EkIAA and EkARF genes, including their phylogenetic relationships, gene structures, chromosomal distribution, and cis-element analysis, as well as conserved motifs in the proteins. Our results suggest that EkIAA and EkARF genes have been relatively conserved over evolutionary history. Furthermore, we conducted expression and co-expression analyses of EkIAA and EkARF genes in leaves, branches, and fruits, which identified a subset of seven EkARF genes as potential regulators of triterpenoids and anthocyanin biosynthesis. RT-qPCR, yeast one-hybrid, and transient expression analyses showed that EkARF5.1 can directly interact with auxin response elements and regulate downstream gene expression. Our results may pave the way to elucidating the function of EkIAA and EkARF gene families in E. konishii, laying a foundation for further research on high-yielding industrial products and E. konishii breeding.

Non-target-site resistance mechanism of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] to florpyrauxifen-benzyl

BACKGROUND

Florpyrauxifen-benzyl (FPB) is an arylpicolinate herbicide (Group IV) for barnyardgrass control in rice. One susceptible (Sus) and three putative FPB-resistant (R1, R2, and R3) barnyardgrass biotypes were selected based on resistant/susceptible (R/S) ratios obtained from dose-response tests and used to investigate the potential resistance mechanisms.

RESULTS

Based on visual control results, the R/S ratios of barnyardgrass biotypes R1, R2, and R3 were 60-, 33-, and 16-fold greater than the Sus standard, respectively. Sequencing results of TIR1 and AFB genes in the tested barnyardgrass revealed no difference between Sus and R barnyardgrass biotypes. Absorption of [¹⁴ C]-FPB in Sus barnyardgrass increased over time and reached 90%, which was >10 percentage points greater than that in R biotypes. The [¹⁴ C]-FPB absorption in all R barnyardgrass equilibrated after 48 h. For both Sus and R barnyardgrass, most [¹⁴ C]-FPB absorbed was present in the treated leaf (79.8-88.8%), followed by untreated aboveground (9.5-18.6%) and belowground tissues (1.3-2.2%). No differences in translocation were observed. Differences between Sus and R barnyardgrass biotypes were found for FPB metabolism. Production of the active metabolite, florpyrauxifen-acid, was greater in Sus barnyardgrass (21.5-52.1%) than in R barnyardgrass (5.5-34.9%).

CONCLUSION

In conclusion, reductions in FPB absorption and florpyrauxifen-acid production may contribute to the inability to control barnyardgrass with FPB. © 2021 Society of Chemical Industry.

Functional analysis of auxin receptor OsTIR1/OsAFB family members in rice grain yield, tillering, plant height, root system, germination, and auxinic herbicide resistance

Auxin regulates almost every aspect of plant growth and development and is perceived by the TIR1/AFB auxin co-receptor proteins differentially acting in concert with specific Aux/IAA transcriptional repressors. Little is known about the diverse functions of TIR1/AFB family members in species other than Arabidopsis. We created targeted OsTIR1 and OsAFB2-5 mutations in rice using CRISPR/Cas9 genome editing, and functionally characterized the roles of these five members in plant growth and development and auxinic herbicide resistance. Our results demonstrated that functions of OsTIR1/AFB family members are partially redundant in grain yield, tillering, plant height, root system and germination. Ostir1, Osafb2 and Osafb4 mutants exhibited more severe phenotypes than Osafb3 and Osafb5. The Ostir1Osafb2 double mutant displays extremely severe defects in plant development. All five OsTIR1/AFB members interacted with OsIAA1 and OsIAA11 proteins in vivo. Root elongation assay showed that each Ostir1/afb2-5 mutant was resistant to 2,4-dichlorophenoxyacetic acid (2,4-D) treatment. Notably, only the Osafb4 mutants were strongly resistant to the herbicide picloram, suggesting that OsAFB4 is a unique auxin receptor in rice. Our findings demonstrate similarities and specificities of auxin receptor TIR1/AFB proteins in rice, and could offer the opportunity to modify effective herbicide-resistant alleles in agronomically important crops.

Molecular pathways regulating elongation of aerial plant organs: a focus on light, the circadian clock, and temperature

Organs such as hypocotyls and petioles rapidly elongate in response to shade and temperature cues, contributing to adaptive responses that improve plant fitness. Growth plasticity in these organs is achieved through a complex network of molecular signals. Besides conveying information from the environment, this signaling network also transduces internal signals, such as those associated with the circadian clock. A number of studies performed in Arabidopsis hypocotyls, and to a lesser degree in petioles, have been informative for understanding the signaling networks that regulate elongation of aerial plant organs. In particular, substantial progress has been made towards understanding the molecular mechanisms that regulate responses to light, the circadian clock, and temperature. Signals derived from these three stimuli converge on the BAP module, a set of three different types of transcription factors that interdependently promote gene transcription and growth. Additional key positive regulators of growth that are also affected by environmental cues include the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) E3 ubiquitin ligase proteins. In this review we summarize the key signaling pathways that regulate the growth of hypocotyls and petioles, focusing specifically on molecular mechanisms important for transducing signals derived from light, the circadian clock, and temperature. While it is clear that similarities abound between the signaling networks at play in these two organs, there are also important differences between the mechanisms regulating growth in hypocotyls and petioles.

Overexpression of MsGH3.5 inhibits shoot and root development through the auxin and cytokinin pathways in apple plants

Phytohormonal interactions are crucial for plant development. Auxin and cytokinin (CK) both play critical roles in regulating plant growth and development; however, the interaction between these two phytohormones is complex and not fully understood. Here, we isolated a wild apple (Malus sieversii Roem) GRETCHEN HAGEN3 (GH3) gene, MsGH3.5, encoding an indole-3-acetic acid (IAA)-amido synthetase. Overexpression of MsGH3.5 significantly reduced the free IAA content and increased the content of some IAA-amino acid conjugates, and MsGH3.5-overexpressing lines were dwarfed and produced fewer adventitious roots (ARs) than the control. This phenotype is consistent with the role of GH3 in conjugating excess free active IAA to amino acids in auxin homeostasis. Surprisingly, overexpression of MsGH3.5 significantly increased CK concentrations in the whole plant, and altered the expression of genes involved in CK biosynthesis, metabolism and signaling. Furthermore, exogenous CK application induced MsGH3.5 expression through the activity of the CK type-B response regulator, MsRR1a, which mediates the CK primary response. MsRR1a activated MsGH3.5 expression by directly binding to its promoter, linking auxin and CK signaling. Plants overexpressing MsRR1a also displayed fewer ARs, in agreement with the regulation of MsGH3.5 expression by MsRR1a. Taken together, we reveal that MsGH3.5 affects apple growth and development by modulating auxin and CK levels and signaling pathways. These findings provide insight into the interaction between the auxin and CK pathways, and might have substantial implications for efforts to improve apple architecture.

Regulation of auxin transcriptional responses

The plant hormone auxin acts as a signaling molecule to regulate a vast number of developmental responses throughout all stages of plant growth. Tight control and coordination of auxin signaling is required for the generation of specific auxin‐response outputs. The nuclear auxin signaling pathway controls auxin‐responsive gene transcription through the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F‐BOX pathway. Recent work has uncovered important details into how regulation of auxin signaling components can generate unique and specific responses to determine auxin outputs. In this review, we discuss what is known about the core auxin signaling components and explore mechanisms important for regulating auxin response specificity.

A review of recent updates to our understanding of auxin signaling.

Transcriptome analysis reveals the different compatibility between LAAA × AA and LAAA × LL in Lilium

To unveil the mechanism of the compatibility of odd-allotetraploid lily (LAAA) as female with diploid male lily, the differences of expressed unigenes in the ovaries and leaves between LAAA × AA and LAAA × LL were investigated using transcriptome analysis. The results showed the fruits of LAAA × AA well developed, while those of LAAA × LL aborted. The number of differentially expressed genes was less in the ovaries of LAAA × AA than those of LAAA × LL, but it showed opposite trend in those of leaves. The unigenes related with auxins, cytokinins, gibberellins, antioxidants, expansins, chlorophylls, carbohydrates, transport proteins were usually up-expressed in the ovaries and leaves of LAAA × AA but not in LAAA × LL; while those of abscisic acid, ethylene, jasmonic acid, and salicylic acid were increased in the ovaries or leaves of LAAA × LL but not in LAAA × AA. The up-expressed unigenes in the ovaries and leaves of LAAA × AA played positive roles in its fruit development because the products of the genes, like phytohormones and antioxidants, had functions protecting leaves from senescence or scavenging ROS, and thus LAAA was compatible with AA, while those of LAAA × LL played negative roles and caused its fruits aborted, and hence LAAA was incompatible with LL.

Genome-wide Identification, Expression Profiling and Evolutionary Analysis of Auxin Response Factor Gene Family in Potato (Solanum tuberosum Group Phureja)

Auxin response factors (ARFs) play central roles in conferring auxin-mediated responses through selection of target genes in plants. Despite their physiological importance, systematic analysis of ARF genes in potato have not been investigated yet. Our genome-wide analysis identified 20 StARF (Solanum tuberosum ARF) genes from potato and found that they are unevenly distributed in all the potato chromosomes except chromosome X. Sequence alignment and conserved motif analysis suggested the presence of all typical domains in all but StARF18c that lacks B3 DNA-binding domain. Phylogenetic analysis indicated that potato ARF could be clustered into 3 distinct subgroups, a result supported by exon-intron structure, consensus motifs, and domain architecture. In silico expression analysis and quantitative real-time PCR experiments revealed that several StARFs were expressed in tissue-specific, biotic/abiotic stress-responsive or hormone-inducible manners, which reflected their potential roles in plant growth, development or under various stress adaptions. Strikingly, most StARFs were identified as highly abiotic stress responsive, indicating that auxin signaling might be implicated in mediating environmental stress-adaptation responses. Taken together, this analysis provides molecular insights into StARF gene family, which paves the way to functional analysis of StARF members and will facilitate potato breeding programs.

Patterning the Axes: A Lesson from the Root

How the body plan is established and maintained in multicellular organisms is a central question in developmental biology. Thanks to its simple and symmetric structure, the root represents a powerful tool to study the molecular mechanisms underlying the establishment and maintenance of developmental axes. Plant roots show two main axes along which cells pass through different developmental stages and acquire different fates: the root proximodistal axis spans longitudinally from the hypocotyl junction (proximal) to the root tip (distal), whereas the radial axis spans transversely from the vasculature tissue (centre) to the epidermis (outer). Both axes are generated by stereotypical divisions occurring during embryogenesis and are maintained post-embryonically. Here, we review the latest scientific advances on how the correct formation of root proximodistal and radial axes is achieved.

Genome-Wide Investigation of the Auxin Response Factor Gene Family in Tartary Buckwheat (Fagopyrum tataricum)

Auxin signaling plays an important role in plant growth and development. It responds to various developmental and environmental events, such as embryogenesis, organogenesis, shoot elongation, tropical growth, lateral root formation, flower and fruit development, tissue and organ architecture, and vascular differentiation. However, there has been little research on the Auxin Response Factor (ARF) genes of tartary buckwheat (Fagopyrum tataricum), an important edible and medicinal crop. The recent publication of the whole-genome sequence of tartary buckwheat enables us to study the tissue and expression profile of the FtARF gene on a genome-wide basis. In this study, 20 ARF (FtARF) genes were identified and renamed according to the chromosomal distribution of the FtARF genes. The results showed that the FtARF genes belonged to the related sister pair, and the chromosomal map showed that the duplication of FtARFs was related to the duplication of the chromosome blocks. The duplication of some FtARF genes shows conserved intron/exon structure, which is different from other genes, suggesting that the function of these genes may be diverse. Real-time quantitative PCR analysis exhibited distinct expression patterns of FtARF genes in various tissues and in response to exogenous auxin during fruit development. In this study, 20 FtARF genes were identified, and the structure, evolution, and expression patterns of the proteins were studied. This systematic analysis laid a foundation for the further study of the functional characteristics of the ARF genes and for the improvement of tartary buckwheat crops.

Role of alkyl silatranes as plant growth regulators: comparative substitution effect on root and shoot development of wheat and maize

BACKGROUND

The present investigation reports the stimulating effects of different substituted alkyl silatranes (3a-3e) on the early seedling growth of wheat (Triticum aestivum) and maize (Zea mays). Seeds of these plants were exposed to six different concentrations (0, 10, 50, 100, 200 and 500 µmol L^-1 ). The results revealed that different substitutions (3a-3e) had different effects on root and shoot elongation. Silatranes (3a-3e) were synthesized employing microwave irradiation by a solvent-mediated transesterification reaction, thereby reducing reaction times from several hours under conventional reflux conditions to 15 min under microwave irradiation.

RESULTS

It was of interest that the effect of these silatranes did not show a dose-dependent relationship but an optimum concentration, which was 100 µmol L^-1 for maize and 200 µmol L^-1 for wheat. γ-Aminopropyl silatranes (3b and 3e) gave the best results in maize, whereas γ-chloropropyl silatrane (3a) was most efficient for wheat at these optimum concentrations.

CONCLUSION

All the synthesized silatranes were effective in promoting root and shoot growth of wheat and maize. Furthermore, an efficient green microwave methodology was successful for the synthesis of silatranes. These observations pave the way for silatranes as efficient plant growth regulators for crops. © 2018 Society of Chemical Industry.

Arabidopsis VQ10 interacts with WRKY8 to modulate basal defense against Botrytis cinerea

Recent studies in Arabidopsis have revealed that some VQ motif-containing proteins physically interact with WRKY transcription factors; however, their specific biological functions are still poorly understood. In this study, we confirmed the interaction between VQ10 and WRKY8, and show that VQ10 and WRKY8 formed a complex in the plant cell nucleus. Yeast two-hybrid analysis showed that the middle region of WRKY8 and the VQ motif of VQ10 are critical for their interaction, and that this interaction promotes the DNA-binding activity of WRKY8. Further investigation revealed that the VQ10 protein was exclusively localized in the nucleus, and VQ10 was predominantly expressed in siliques. VQ10 expression was strongly responsive to the necrotrophic fungal pathogen, Botrytis cinerea and defense-related hormones. Phenotypic analysis showed that disruption of VQ10 increased mutant plants susceptibility to the fungal pathogen B. cinerea, whereas constitutive-expression of VQ10 enhanced resistance to B. cinerea. Consistent with these findings, expression of the defense-related PLANT DEFENSIN1.2 (PDF1.2) gene was decreased in vq10 mutant plants, after B. cinerea infection, but increased in VQ10-overexpressing transgenic plants. Taken together, our findings provide evidence that VQ10 physically interacts with WRKY8 and positively regulates plant basal resistance against the necrotrophic fungal pathogen B. cinerea.

OsMADS25 regulates root system development via auxin signalling in rice

The phytohormone auxin is essential for root development in plants. OsMADS25 is a homologue of the AGL17-clade MADS-box genes in rice. Despite recent progress, the molecular mechanisms underlying the regulation of root development by OsMADS25 are not well known. It is unclear whether OsMADS25 regulates root development via auxin signalling. In this study, we examined the role of OsMADS25 in root development and characterized the signalling pathway through which OsMADS25 regulates root system development in rice. OsMADS25 overexpression significantly increased, but RNAi gene silencing repressed primary root (PR) length and lateral root (LR) density. Moreover, OsMADS25 promoted LR development in response to NO₃^- . Further study showed that OsMADS25 increased auxin accumulation in the root system by enhancing auxin biosynthesis and transport, while also reducing auxin degradation, therefore stimulating root development. More importantly, OsMADS25 was found to regulate OsIAA14 expression directly by binding to the CArG-box in the promoter region of OsIAA14, which encodes an Aux/indole acetic acid (IAA) transcriptional repressor of auxin signalling. Elevated auxin levels and decreased OsIAA14 expression might lead to reduced OsIAA14 protein accumulation, as a mechanism to regulate auxin signalling. Therefore, our findings reveal a molecular mechanism by which OsMADS25 modulates root system growth and development in rice, at least partilly, via Aux/IAA-based auxin signalling.

Genome-wide identification of the auxin/indole-3-acetic acid (Aux/IAA) gene family in pepper, its characterisation, and comprehensive expression profiling under environmental and phytohormones stress

Auxin is an essential phytohormone that plays a crucial role in the growth and development of plants in stressful environments. Here, we analysed the auxin/indole-3-acetic acid (Aux/IAA) gene family, which produces auxin in pepper, and succeeded in identifying 27 putative members containing four conserved domains (I. II. III and IV) in their protein sequences. Sequence analysis, chromosomal mapping and motif prediction of all identified CaAux/IAA genes were performed. It was observed that these genes contained four conserved motifs divided into nine different groups and distributed across nine chromosomes in pepper plants. RNA-seq analysis revealed the organ specific expression of many CaAux/IAA genes. However, the majority of genes were expressed with high expression levels in the early stages of fruit development. However, the maximum expression level of the CA03g34540 gene was observed in the breaker stage. Moreover, thirteen CaAux/IAA genes were labelled as early responsive genes to various phytohormone and abiotic stresses. Furthermore, RNA-seq analysis in response to pathogen inoculation (PepMoV, TMV strains P0/P1, and Phytophthora capsici) showed distinct expression profiles of all identified genes, suggesting the diverse expression nature of genes under these stress conditions. Overall, this study provides insight into the dynamic response of CaAux/IAA genes under environmental and phytohormones stress conditions, providing bases to further explore the importance of these genes through mutant/transgenic analysis in pepper.

Identification, characterization, and expression analysis of auxin response factor (ARF) gene family in Brachypodium distachyon

Auxin response factors (ARFs) are one type of essential family of transcription factors that bind with auxin response elements (AuxRE), and play vital roles in variety of plant development and physiological processes. Brachypodium distachyon, related to the major cereal grain species, were recently developed to be a good model organism for functional genomics research. So far, genome-wide overview of the ARF gene family in B. distachyon was not available. Here, a systemic analysis of ARF gene family members in B. distachyon was performed. A comprehensive overview of the characterization of the BdARFs was obtained by multiple bioinformatics analyses, including the gene and protein structure, chromosome locations, conserved motifs of proteins, phylogenetic analysis, and cis-elements in promoters of BdARF. Results showed that all BdARFs contained conserved DBD, MR, and CTD could be divided into four classes, Ia, IIa, IIb, and III. Expression profiles of BdARF genes indicated that they were expressed across various tissues and organs, which could be clustered into three main expression groups, and most of BdARF genes were involved in phytohormone signal transduction pathways and regulated physiological process in responding to multiple environmental stresses. And predicted regulatory network between B. distachyon ARFs and IAAs was also discussed. Our genomics analysis of BdARFs could yield new insights into the complexity of the control of BdARF genes and lead to potential applications in the investigation of the accurate regulatory mechanisms of ARFs in herbaceous plants.

Class C ARFs evolved before the origin of land plants and antagonize differentiation and developmental transitions in Marchantia polymorpha

A plethora of developmental and physiological processes in land plants is influenced by auxin, to a large extent via alterations in gene expression by AUXIN RESPONSE FACTORs (ARFs). The canonical auxin transcriptional response system is a land plant innovation, however, charophycean algae possess orthologues of at least some classes of ARF and AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, suggesting that elements of the canonical land plant system existed in an ancestral alga. We reconstructed the phylogenetic relationships between streptophyte ARF and AUX/IAA genes and functionally characterized the solitary class C ARF, MpARF3, in Marchantia polymorpha. Phylogenetic analyses indicate that multiple ARF classes, including class C ARFs, existed in an ancestral alga. Loss- and gain-of-function MpARF3 alleles result in pleiotropic effects in the gametophyte, with MpARF3 inhibiting differentiation and developmental transitions in multiple stages of the life cycle. Although loss-of-function Mparf3 and Mpmir160 alleles respond to exogenous auxin treatments, strong miR-resistant MpARF3 alleles are auxin-insensitive, suggesting that class C ARFs act in a context-dependent fashion. We conclude that two modules independently evolved to regulate a pre-existing ARF transcriptional network. Whereas the auxin-TIR1-AUX/IAA pathway evolved to repress class A/B ARF activity, miR160 evolved to repress class C ARFs in a dynamic fashion.

Identification and expression profiling of the Aux/IAA gene family in Chinese hickory (Carya cathayensis Sarg.) during the grafting process

Auxin is an essential regulator in various aspects of organism growth and development. Members of the Aux/IAA family of genes encode short-lived nuclear proteins and mediate the responses of auxin-regulated gene expression. Here, the first identification and characterization of 22 cDNAs encoding the open reading frame of the Aux/IAA family in Chinese hickory (named as CcIAA) has been performed. The proteins encoded by these genes contain four whole or partially conserved domains of the Aux/IAA family. Phylogenetic analysis indicated that CcIAAs were unevenly distributed among eight different subgroups. The spatio-specific expression profiles showed that most of the CcIAAs preferentially expressed in specific tissues. Three CcIAA genes, including CcIAA11, CcIAA27a2 and CcIAAx, were predominantly expressed in stem. The predominant expression of CcIAA genes in stems might play important roles in vascular reconnection during the graft process. Furthermore, expression profiles of Aux/IAA genes during the grafting process of Chinese hickory have been analysed. Our data suggested that 19 CcIAAs were down-regulated and 3 CcIAAs (including CcIAA28, CcIAA8a and CcIAA27b) were induced, indicating their specializations during the grafting process. The involvement of CcIAA genes at the early stage after grafting gives us an opportunity to understand the role of auxin signalling in the grafting process.

Genetic evidence suggests that GIS functions downstream of TCL1 to regulate trichome formation in Arabidopsis

BACKGROUND

Trichome formation in Arabidopsis is regulated by a MBW complex formed by MYB, bHLH and WD40 transcriptional factors, which can activate GLABRA2 (GL2) and the R3 MYB transcription factor genes. GL2 promotes trichome formation, whereas R3 MYBs are able to block the formation of the MBW complex. It has been reported that the C2H2 transcription factor GIS (GLABROUS INFLORESCENCE STEMS) functions upstream of the MBW activator complex to regulate trichome formation, and that the expression of TCL1 is not regulated by the MBW complex. However, gis and the R3 MYB gene mutant tcl1 (trichomeless 1) have opposite inflorescence trichome phenotypes, but their relationship in regulating trichome formation remained unknown.

RESULTS

By generating and characterization of the gis tcl1 double mutant, we found that trichome formation in the gis tcl1double and the tcl1 single mutants were largely indistinguishable, but the trichome formation in the 35S:TCL1/gis transgenic plant was similar to that in the gis mutant. By using quantitative RT-PCR analysis, we showed that expression level of GIS was increased in the triple mutant tcl1 try cpc, but the expression level of TCL1 was not affected in the gis mutant. On the other hand, trichome morphology in both gis tcl1 and 35S:TCL1/gis plants was similar to that in the gis mutant.

CONCLUSIONS

In summary, our results indicate that GIS may work downstream of TCL1 to regulate trichome formation, and GIS has a dominant role in controlling trichome morphology.

Rice OsMYB5P improves plant phosphate acquisition by regulation of phosphate transporter

Myeloblastosis (MYB) transcription factors play central roles in plant developmental processes and in responses to nutrient deficiency. In this study, OsMYB5P, an R2R3-MYB transcription factor, was isolated and identified from rice (Oryza sativa L. 'Dongjin') under inorganic phosphate (Pi)-deficient conditions. OsMYB5P protein is localized to the nucleus and functions as a transcription activator in plant development. Overexpression of OsMYB5P in rice and Arabidopsis (Arabidopsis thaliana Col-0) increases tolerance to phosphate starvation, whereas OsMYB5P knock-out through RNA interference increases sensitivity to Pi depletion in rice. Furthermore, shoots and roots of transgenic rice plants overexpressing OsMYB5P were longer than those of wild plants under both normal and Pi-deficient conditions. These results indicate that OsMYB5P is associated with the regulation of shoot development and root- system architecture. Overexpression of OsMYB5P led to increased Pi accumulation in shoots and roots. Interestingly, OsMYB5P directly bound to MBS (MYB binding site) motifs on the OsPT5 promoter and induced transcription of OsPT5 in rice. In addition, overexpression of OsMYB5P in Arabidopsis triggered increased expression of AtPht1;3, an Arabidopsis Pi transporter, in shoots and roots under normal and Pi-deficient conditions. Together, these results demonstrate that overexpression of OsMYB5P increases tolerance to Pi deficiency in plants by modulating Pi transporters at the transcriptional level in monocots and dicots.

The novel transcription factor TRP interacts with ZFP5, a trichome initiation-related transcription factor, and negatively regulates trichome initiation through gibberellic acid signaling

The trichome-related protein (TRP) is a novel transcription factor (TF) that negatively regulates trichome initiation-related TFs through gibberellin (GA) signaling. Trichomes, which are outgrowths of leaf epidermal cells, provide the plant with a first line of defense against damage from herbivores and reduce transpiration. The initiation and development of trichomes are regulated by a network of positively or negatively regulating transcription factors (TFs). However, little information is currently available on transcriptional regulation related to trichome formation. Here, we report a novel TF Trichome-Related Protein (TRP) that was observed to negatively regulate the trichome initiation-related TFs through gibberellic acid (GA) signaling. ProTRP:GUS revealed that TRP was only expressed in the trichome. The TRP loss-of-function mutant (trp) had an increased number of trichomes on the flower, cauline leaves, and main inflorescence stems compared to the wild-type. In contrast, TRP overexpression lines (TRP-Ox) exhibited a decreased number of trichomes on cauline leaves and main inflorescence stem following treatment with exogenous GA. Moreover, the expressions of trichome initiation regulators (GIS, GIS2, ZFP8, GL1, and GL3) increased in trp plants but decreased in TRP-Ox lines after GA treatment. TRP was observed to physically interact with ZFP5, a C2H2 TF that controls trichome cell development through GA signaling, both in vivo and in vitro. Based on these results, we suggest that TRP functions upstream of the trichome initiation regulators and represses the binding of ZFP5 to the ZFP8 promoter.

Drought stress responses in maize are diminished by Piriformospora indica

As an endophytic fungus of Sebacinales, Piriformospora indica promotes plant growth and resistance to abiotic stress, including drought. Colonization of maize roots promoted the leaf size, root length and number of tap roots. Under drought stress, the maize seedlings profited from the presence of the fungus and performed visibly better than the uncolonized controls. To identify genes and biological processes involved in growth promotion and drought tolerance conferred by P. indica, the root transcriptome of colonized and uncolonized seedlings was analyzed 0, 6 and 12 h after drought stress (20% polyethylene glycol 6000). The number of P. indica-responsive genes increased from 464 (no stress at 0 h) to 1337 (6 h drought) and 2037 (12 h drought). Gene Ontology analyses showed that the carbon and sulfur metabolisms are major targets of the fungus. Furthermore, the growth promoting effect of P. indica is reflected by higher transcript levels for microtubule associated processes. Under drought stress, the fungus improved the oxidative potential of the roots, and stimulated genes for hormone functions, including those which respond to abscisic acid, auxin, salicylic acid and cytokinins. The comparative analyses of our study provides systematic insight into the molecular mechanism how P. indica promotes plant performance under drought stress, and presents a collection of genes which are specifically targeted by the fungus under drought stress in maize roots.

Alternative polyadenylation is involved in auxin-based plant growth and development

Auxin is widely involved in plant growth and development. However, the molecular mechanism on how auxin carries out this work is unclear. In particular, the effect of auxin on pre-mRNA post-transcriptional regulation is mostly unknown. By using a poly(A) tag (PAT) sequencing approach, mRNA alternative polyadenylation (APA) profiles after auxin treatment were revealed. We showed that hundreds of poly(A) site clusters (PACs) are affected by auxin at the transcriptome level, where auxin reduces PAC distribution in 5'-untranslated region (UTR), but increases in the 3'UTR. APA site usage frequencies of 42 genes were switched by auxin, suggesting that auxin affects the choice of poly(A) sites. Furthermore, poly(A) signal selection was altered after auxin treatment. For example, a mutant of poly(A) signal binding protein CPSF30 showed altered sensitivity to auxin treatment, indicating interactions between auxin and the poly(A) signal recognition machinery. We also found that auxin activity on lateral root development is likely mediated by altered expression of ARF7, ARF19 and IAA14 through poly(A) site switches. Our results shed light on the molecular mechanisms of auxin responses relative to its interactions with mRNA polyadenylation.

Auxin regulates anthocyanin biosynthesis through the Aux/IAA-ARF signaling pathway in apple

Auxin signaling, which is crucial for normal plant growth and development, mainly depends on ARF-Aux/IAA interactions. However, little is known regarding the regulatory effects of auxin signaling on anthocyanin metabolism in apple (Malus domestica). We investigated the functions of MdARF13, which contains a repression domain and is localized to the nucleus. This protein was observed to interact with the Aux/IAA repressor, MdIAA121, through its C-terminal dimerization domain. Protein degradation experiments proved that MdIAA121 is an unstable protein that is degraded by the 26S proteasome. Additionally, MdIAA121 stability is affected by the application of exogenous auxin. Furthermore, the overexpression of MdIAA121 and MdARF13 in transgenic red-fleshed apple calli weakened the inhibitory effect of MdARF13 on anthocyanin biosynthesis. These results indicate that the degradation of MdIAA121 induced by auxin treatment can release MdARF13, which acts as a negative regulator of the anthocyanin metabolic pathway. Additionally, yeast two-hybrid, bimolecular fluorescence complementation, and pull-down assays confirmed that MdMYB10 interacts with MdARF13. A subsequent electrophoretic mobility shift assay and yeast one-hybrid assay demonstrated that MdARF13 directly binds to the promoter of MdDFR, which is an anthocyanin pathway structural gene. Interestingly, chromatin immunoprecipitation-quantitative real-time PCR results indicated that the overexpression of MdIAA121 clearly inhibits the recruitment of MdARF13 to the MdDFR promoter. Our findings further characterized the mechanism underlying the regulation of anthocyanin biosynthesis via Aux/IAA-ARF signaling.

Metabolome and transcriptome profiling reveal new insights into somatic embryo germination in Norway spruce (Picea abies)

Transcriptome, metabolome and histological profiling were performed on normal and aberrant somatic embryo germinants of Norway spruce (Picea abies L. Karst) providing a simplistic systems biology description of conifer germination. Aberrant germinants (AGs) formed periderm-like tissue at the apical pole and lacked shoot growth above the cotyledons. Transcriptome profiling (RNA-Sequencing) revealed a total of 370 differentially expressed genes at ≥1 or ≤-1 log2-fold change, where 92% were down-regulated in AGs compared with normal germinants (NGs). Genes associated with shoot apical meristem formation were down-regulated in AGs, or not differentially expressed between AGs and NGs. Genes involved in hormone signaling and transport were also down-regulated. Metabolite profiling by gas chromatography-mass spectrometry (MS) and liquid chromatography-MS revealed biochemical difference between AGs and NGs, notably increased levels of sugars including glucose in AGs. Genes involved in glucose signaling were down-regulated and genes involved in starch biosynthesis were up-regulated, suggesting involvement of sugar signaling during late embryo development and germination. The overall results provide new data enabling further studies to confirm potential markers for a normal germination process in conifers.

A novel family of transcription factors conserved in angiosperms is required for ABA signalling

The plant hormone abscisic acid (ABA) plays a crucial role in regulating plant responses to environmental stresses. Interplay of several different proteins including the PYR/PYL/RCAR receptors, A-group PP2C protein phosphatases, SnRK2 protein kinases, and downstream transcription factors regulates ABA signalling. We report here the identification of a family of ABA-induced transcription repressors (AITRs) that act as feedback regulators in ABA signalling. We found that the expression of all the 6 Arabidopsis AITR genes was induced by exogenously ABA, and their expression levels were decreased in ABA biosynthesis mutant aba1-5. BLAST searches showed that AITRs are exclusively present in angiosperms. When recruited to the promoter region of a reporter gene by a fused DNA binding domain, all AITRs inhibited reporter gene expression in transfected protoplasts. In Arabidopsis, aitr mutants showed reduced sensitivity to ABA and to stresses such as salt and drought. Quantitative RT-PCR analysis demonstrated that the ABA-induced response of PP2C and some PYR/PYL/RCAR genes was reduced in AITR5 transgenic plants but increased in an aitr2 aitr5 aitr6 triple mutant. These results provide important new insights into the regulation of ABA signalling in plants, and such information may lead to the production of plants with enhanced resistance to environmental stresses.

Evolution Analysis of the Aux/IAA Gene Family in Plants Shows Dual Origins and Variable Nuclear Localization Signals

The plant hormone auxin plays pivotal roles in many aspects of plant growth and development. The auxin/indole-3-acetic acid (Aux/IAA) gene family encodes short-lived nuclear proteins acting on auxin perception and signaling, but the evolutionary history of this gene family remains to be elucidated. In this study, the Aux/IAA gene family in 17 plant species covering all major lineages of plants is identified and analyzed by using multiple bioinformatics methods. A total of 434 Aux/IAA genes was found among these plant species, and the gene copy number ranges from three (Physcomitrella patens) to 63 (Glycine max). The phylogenetic analysis shows that the canonical Aux/IAA proteins can be generally divided into five major clades, and the origin of Aux/IAA proteins could be traced back to the common ancestor of land plants and green algae. Many truncated Aux/IAA proteins were found, and some of these truncated Aux/IAA proteins may be generated from the C-terminal truncation of auxin response factor (ARF) proteins. Our results indicate that tandem and segmental duplications play dominant roles for the expansion of the Aux/IAA gene family mainly under purifying selection. The putative nuclear localization signals (NLSs) in Aux/IAA proteins are conservative, and two kinds of new primordial bipartite NLSs in P. patens and Selaginella moellendorffii were discovered. Our findings not only give insights into the origin and expansion of the Aux/IAA gene family, but also provide a basis for understanding their functions during the course of evolution.

De Novo Transcriptome Sequencing and the Hypothetical Cold Response Mode of Saussurea involucrata in Extreme Cold Environments

Saussurea involucrata grows in high mountain areas covered by snow throughout the year. The temperature of this habitat can change drastically in one day. To gain a better understanding of the cold response signaling pathways and molecular metabolic reactions involved in cold stress tolerance, genome-wide transcriptional analyses were performed using RNA-Seq technologies. A total of 199,758 transcripts were assembled, producing 138,540 unigenes with 46.8 Gb clean data. Overall, 184,416 (92.32%) transcripts were successfully annotated. The 365 transcription factors identified (292 unigenes) belonged to 49 transcription factor families associated with cold stress responses. A total of 343 transcripts on the signal transduction (132 upregulated and 212 downregulated in at least any one of the conditions) were strongly affected by cold temperature, such as the CBL-interacting serine/threonine-protein kinase (CIPKs), receptor-like protein kinases, and protein kinases. The circadian rhythm pathway was activated by cold adaptation, which was necessary to endure the severe temperature changes within a day. There were 346 differentially expressed genes (DEGs) related to transport, of which 138 were upregulated and 22 were downregulated in at least any one of the conditions. Under cold stress conditions, transcriptional regulation, molecular transport, and signal transduction were involved in the adaptation to low temperature in S. involucrata. These findings contribute to our understanding of the adaptation of plants to harsh environments and the survival traits of S. involucrata. In addition, the present study provides insight into the molecular mechanisms of chilling and freezing tolerance.

Expression profiling of AUXIN RESPONSE FACTOR genes during somatic embryogenesis induction in Arabidopsis

Extensive modulation of numerous ARF transcripts in the embryogenic culture of Arabidopsis indicates a substantial role of auxin signaling in the mechanism of somatic embryogenesis induction. Somatic embryogenesis (SE) is induced by auxin in plants and auxin signaling is considered to play a key role in the molecular mechanism that controls the embryogenic transition of plant somatic cells. Accordingly, the expression of AUXIN RESPONSE FACTOR (ARF) genes in embryogenic culture of Arabidopsis was analyzed. The study revealed that 14 of the 22 ARFs were transcribed during SE in Arabidopsis. RT-qPCR analysis indicated that the expression of six ARFs (ARF5, ARF6, ARF8, ARF10, ARF16, and ARF17) was significantly up-regulated, whereas five other genes (ARF1, ARF2, ARF3, ARF11, and ARF18) were substantially down-regulated in the SE-induced explants. The activity of ARFs during SE was also monitored with GFP reporter lines and the ARFs that were expressed in areas of the explants engaged in SE induction were detected. A functional test of ARFs transcribed during SE was performed and the embryogenic potential of the arf mutants and overexpressor lines was evaluated. ARFs with a significantly modulated expression during SE coupled with an impaired embryogenic response of the relevant mutant and/or overexpressor line, including ARF1, ARF2, ARF3, ARF5, ARF6, ARF8, and ARF11 were indicated as possibly being involved in SE induction. The study provides evidence that embryogenic induction strongly depends on ARFs, which are key regulators of the auxin signaling. Some clues on the possible functions of the candidate ARFs, especially ARF5, in the mechanism of embryogenic transition are discussed. The results provide guidelines for further research on the auxin-related functional genomics of SE and the developmental plasticity of somatic cells.

Auxin homeostasis: the DAO of catabolism

Nearly all programmed and plastic plant growth responses are at least partially regulated by auxins, such as indole-3-acetic acid (IAA). Although vectorial, long distance auxin transport is essential to its regulatory function, all auxin responses are ultimately localized in individual target cells. As a consequence, cellular auxin concentrations are tightly regulated via coordinated biosynthesis, transport, conjugation, and oxidation. The primary auxin oxidative product across species is 2-oxindole-3-acetic acid (oxIAA), followed by glucose and amino acid conjugation to oxIAA. Recently, the enzymes catalyzing the oxidative reaction were characterized in Arabidopsis thaliana. DIOXYGENASE OF AUXIN OXIDATION (DAO) comprises a small subfamily of the 2-oxoglutarate and Fe(II) [2-OG Fe(II)] dependent dioxygenase superfamily. Biochemical and genetic studies have revealed critical physiological functions of DAO during plant growth and development. Thus far, DAO has been identified in three species by homology. Here, we review historical and recent studies and discuss future perspectives regarding DAO and IAA oxidation.

Genome-wide analysis and characterization of Aux/IAA family genes related to fruit ripening in papaya (Carica papaya L.)

Background

Auxin/indole-3-acetic acid (Aux/IAA) family genes encode short-lived nuclear proteins that mediate the responses of auxin-related genes and are involved in several plant developmental and growth processes. However, how Aux/IAA genes function in the fruit development and ripening of papaya (Carica papaya L.) is largely unknown.

Results

In this study, a comprehensive identification and a distinctive expression analysis of 18 C. papaya Aux/IAA (CpIAA) genes were performed using newly updated papaya reference genome data. The Aux/IAA gene family in papaya is slightly smaller than that in Arabidopsis, but all of the phylogenetic subfamilies are represented. Most of the CpIAA genes are responsive to various phytohormones and expressed in a tissues-specific manner. To understand the putative biological functions of the CpIAA genes involved in fruit development and ripening, quantitative real-time PCR was used to test the expression profiling of CpIAA genes at different stages. Furthermore, an IAA treatment significantly delayed the ripening process in papaya fruit at the early stages. The expression changes of CpIAA genes in ACC and 1-MCP treatments suggested a crosstalk between auxin and ethylene during the fruit ripening process of papaya.

Conclusions

Our study provided comprehensive information on the Aux/IAA family in papaya, including gene structures, phylogenetic relationships and expression profiles. The involvement of CpIAA gene expression changes in fruit development and ripening gives us an opportunity to understand the roles of auxin signaling in the maturation of papaya reproductive organs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3722-6) contains supplementary material, which is available to authorized users.

The molecular mechanism underlying anthocyanin metabolism in apple using the MdMYB16 and MdbHLH33 genes

MdMYB16 forms homodimers and directly inhibits anthocyanin synthesis via its C-terminal EAR repressor. It weakened the inhibitory effect of MdMYB16 on anthocyanin synthesis when overexpressing MdbHLH33 in callus overexpressing MdMYB16. MdMYB16 could interact with MdbHLH33. Anthocyanins are strong antioxidants that play a key role in the prevention of cardiovascular disease, cancer, and diabetes. The germplasm of Malus sieversii f. neidzwetzkyana is important for the study of anthocyanin metabolism. To date, only limited studies have examined the negative regulatory mechanisms underlying anthocyanin synthesis in apple. Here, we analyzed the relationship between anthocyanin levels and MdMYB16 expression in mature Red Crisp 1-5 apple (M. domestica) fruit, generated an evolutionary tree, and identified an EAR suppression sequence and a bHLH binding motif of the MdMYB16 protein using protein sequence analyses. Overexpression of MdMYB16 or MdMYB16 without bHLH binding sequence (LBSMdMYB16) in red-fleshed callus inhibited MdUFGT and MdANS expression and anthocyanin synthesis. However, overexpression of MdMYB16 without the EAR sequence (LESMdMYB16) in red-fleshed callus had no inhibitory effect on anthocyanin. The yeast one-hybrid assay showed that MdMYB16 and LESMdMYB16 interacted the promoters of MdANS and MdUFGT, respectively. Yeast two-hybrid, pull-down, and bimolecular fluorescence complementation assays showed that MdMYB16 formed homodimers and interacted with MdbHLH33, however, the LBSMdMYB16 could not interact with MdbHLH33. We overexpressed MdbHLH33 in callus overexpressing MdMYB16 and found that it weakened the inhibitory effect of MdMYB16 on anthocyanin synthesis. Together, these results suggested that MdMYB16 and MdbHLH33 may be important part of the regulatory network controlling the anthocyanin biosynthetic pathway.

The Solanum lycopersicum auxin response factor SlARF2 participates in regulating lateral root formation and flower organ senescence

ARF2 as apleiotropic developmental regulator has been reported in Arabidopsis thaliana and tomato (Solanum lycopersicum). The present study showed SlARF2 transcripts in all tomato plant tissues but with higher accumulation in flowers. During bud-anthesis stages, SlARF2 transcripts showed a dynamic expression pattern in sepal, stamen, ovary and petal. Hormone treatment analysis suggested that SlARF2 transcript accumulation was positively regulated by auxin and gibberellic acid, and negatively regulated by ethylene in tomato seedlings. Phenotypes and molecular analyses of SlARF2-upregulated transgenic tomato indicated that SlARF2 regulated tomato lateral root formation and flower organ senescence may be partially mediated by regulating the gene expression of auxin and ethylene response factors. The data enlarges the functional characterization of SlARF2 in tomato, and broadens our understanding of auxin signaling in regulating plant growth and development.

Cytological behaviour of floral organs and in silico characterization of differentially expressed transcript-derived fragments associated with 'floral bud distortion' in soybean

An attempt was made to understand the 'floral bud distortion' (FBD), an unexplored disorder prevailing in soybean. Cytological behaviour of floral reproductive organs and in silico characterization of differentially expressed transcript-derived fragments (TDFs) in symptomatic and asymptomatic soybean plants were carried out. Pollens in asymptomatic plants do not have defects in number, size, shape and function. However, in symptomatic plant, pollens were found nonviable, abnormal in shape and with reduced germination ability. Here, we employed a computational approach, exploring invaluable resources. The tissue-specific transcript profile of symptomatic and asymptomatic sources was compared to determine differentially expressed TDFs associated with FBD to improve its basic understanding. A total of 60 decamer primers produced 197 scorable amplicons, ranged 162-1130 bp, of which 171 were monomorphic and 26 were differentially regulated. Reproducible TDFs were sequenced and characterized for their homology analysis, annotation, protein-protein interaction, subcellular localization and their physical mapping. Homology-based annotation of TDFs in soybean revealed presence of two characterized and seven uncharacterized hits. Annotation of characterized sequences showed presence of genes, namely auxin response factor 9 (ARF9) and forkhead-associated (FHA) domain, which are directly involved in plant development through various pathways, such as hormonal regulation, plant morphology, embryogenesis and DNA repair.

Genome-Wide Identification of miRNAs and Their Targets Involved in the Developing Internodes under Maize Ears by Responding to Hormone Signaling

Internode length is one of the decisive factors affecting plant height (PH) and ear height (EH), which are closely associated with the lodging resistance, biomass and grain yield of maize. miRNAs, currently recognized as important transcriptional/ post-transcriptional regulators, play an essential role in plant growth and development. However, their roles in developing internodes under maize ears remain unclear. To identify the roles of miRNAs and their targets in the development of internodes under maize ears, six miRNA and two degradome libraries were constructed using the 7th, 8th and 9th internodes of two inbred lines, 'Xun928' and 'Xun9058', which had significantly different internode lengths. A total of 45 and 54 miRNAs showed significant changes for each pairwise comparison among the 7th, 8th and 9th internodes of 'Xun9058' and 'Xun928', respectively. The expression of 31 miRNAs showed significant changes were common to the corresponding comparison groups of the 7th, 8th and 9th internodes of 'Xun9058' and 'Xun928'. For the corresponding internodes of 'Xun9058' and 'Xun928', compared with the expression of miRNAs in the 7th, 8th and 9th internodes of 'Xun928', the numbers of up-regulated and down-regulated miRNAs were 11 and 36 in the 7th internode, 9 and 45 in the 8th internode, and 9 and 25 in the 9th internode of 'Xun9058', respectively. Moreover, 10 miRNA families containing 45 members showed significant changes at least in two internodes of 'Xun928' by comparing with the corresponding internodes of 'Xun9058'. Based on the sequencing data, 20 miRNAs related to hormone signaling among the candidates, belonging to five conserved miRNA families, were selected for expression profiling using quantitative reverse-transcription polymerase chain reaction (qRT-PCR). The five miRNA families, zma-miR160, zma-miR167, zma-miR164, zma-miR169 and zma-miR393, targeted the genes encoding auxin response factor, N-acetylcysteine domain containing protein, nuclear transcription factor Y and auxin signaling F-BOX 2 through degradome sequencing. The miRNAs might regulate their targets to respond to hormone signaling, thereby regulating the internode elongation and development under maize ear. These results provide valuable reference for understanding the possible regulation mechanism of the ILs under the ear.

Apoplastic H2 O2 plays a critical role in axillary bud outgrowth by altering auxin and cytokinin homeostasis in tomato plants

Although phytohormones such as indole-3-acetic acid (IAA), cytokinin (CK) and strigolactone are important modulators of plant architecture, it remains unclear whether reactive oxygen species are involved in the regulation of phytohormone-dependent axillary bud outgrowth in plants. We used diverse techniques, including transcriptional suppression, HPLC-MS, biochemical methodologies and gene transcript analysis to investigate the signaling pathway for apoplastic hydrogen peroxide (H2 O2 )-induced axillary bud outgrowth. Silencing of tomato RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1) and WHITEFLY INDUCED 1 (WFI1), two important genes involved in H2 O2 production in the apoplast, enhanced bud outgrowth, decreased transcript of FZY - a rate-limiting gene in IAA biosynthesis and IAA accumulation in the apex - and increased the transcript of IPT2 involved in CK biosynthesis and CK accumulation in the stem node. These effects were fully abolished by the application of exogenous H2 O2 . Both decapitation and the silencing of FZY promoted bud outgrowth, and downregulated and upregulated the transcripts for IAA3 and IAA15, and IPT2, respectively. However, these effects were not blocked by treatment with exogenous H2 O2 but by napthaleneacetic acid (NAA) treatment. These results suggest that RBOHs-dependent apoplastic H2 O2 promotes IAA biosynthesis in the apex, which, in turn, inhibits CK biosynthesis and subsequent bud outgrowth in tomato plants.

A common miRNA160-based mechanism regulates ovary patterning, floral organ abscission and lamina outgrowth in tomato

Plant microRNAs play vital roles in auxin signaling via the negative regulation of auxin response factors (ARFs). Studies have shown that targeting of ARF10/16/17 by miR160 is indispensable for various aspects of development, but its functions in the model crop tomato (Solanum lycopersicum) are unknown. Here we knocked down miR160 (sly-miR160) using a short tandem target mimic (STTM160), and investigated its roles in tomato development. Northern blot analysis showed that miR160 is abundant in developing ovaries. In line with this, its down-regulation perturbed ovary patterning as indicated by the excessive elongation of the proximal ends of mutant ovaries and thinning of the placenta. Following fertilization, these morphological changes led to formation of elongated, pear-shaped fruits reminiscent of those of the tomato ovate mutant. In addition, STTM160-expressing plants displayed abnormal floral organ abscission, and produced leaves, sepals and petals with diminished blades, indicating a requirement for sly-miR160 for these auxin-mediated processes. We found that sly-miR160 depletion was always associated with the up-regulation of SlARF10A, SlARF10B and SlARF17, of which the expression of SlARF10A increased the most. Despite the sly-miR160 legitimate site of SlARF16A, its mRNA levels did not change in response to sly-miR160 down-regulation, suggesting that it may be regulated by a mechanism other than mRNA cleavage. SlARF10A and SlARF17 were previously suggested to function as inhibiting ARFs. We propose that by adjusting the expression of a group of ARF repressors, of which SlARF10A is a primary target, sly-miR160 regulates auxin-mediated ovary patterning as well as floral organ abscission and lateral organ lamina outgrowth.

Transcriptional Responses to the Auxin Hormone

Auxin is arguably the most important signaling molecule in plants, and the last few decades have seen remarkable breakthroughs in understanding its production, transport, and perception. Recent investigations have focused on transcriptional responses to auxin, providing novel insight into the functions of the domains of key transcription regulators in responses to the hormonal cue and prominently implicating chromatin regulation in these responses. In addition, studies are beginning to identify direct targets of the auxin-responsive transcription factors that underlie auxin modulation of development. Mechanisms to tune the response to different auxin levels are emerging, as are first insights into how this single hormone can trigger diverse responses. Key unanswered questions center on the mechanism for auxin-directed transcriptional repression and the identity of additional determinants of auxin response specificity. Much of what has been learned in model plants holds true in other species, including the earliest land plants.

Enhancement of hypocotyl elongation by LOV KELCH PROTEIN2 production is mediated by auxin and phytochrome-interacting factors in Arabidopsis thaliana

Auxin and two phytochrome-interacting factors, PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5, play crucial roles in the enhancement of hypocotyl elongation in transgenic Arabidopsis thaliana plants that overproduce LOV KELCH PROTEIN2 (LKP2). LOV KELCH PROTEIN2 (LKP2) is a positive regulator of hypocotyl elongation under white light in Arabidopsis thaliana. In this study, using microarray analysis, we compared the gene expression profiles of hypocotyls of wild-type Arabidopsis (Columbia accession), a transgenic line that produces green fluorescent protein (GFP), and two lines that produce GFP-tagged LKP2 (GFP-LKP2). We found that, in GFP-LKP2 hypocotyls, 775 genes were up-regulated, including 36 auxin-responsive genes, such as 27 SMALL AUXIN UP RNA (SAUR) and 6 AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, and 21 genes involved in responses to red or far-red light, including PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5; and 725 genes were down-regulated, including 15 flavonoid biosynthesis genes. Hypocotyls of GFP-LKP2 seedlings, but not cotyledons or roots, contained a higher level of indole-3-acetic acid (IAA) than those of control seedlings. Auxin inhibitors reduced the enhancement of hypocotyl elongation in GFP-LKP2 seedlings by inhibiting the increase in cortical cell number and elongation of the epidermal and cortical cells. The enhancement of hypocotyl elongation was completely suppressed in progeny of the crosses between GFP-LKP2 lines and dominant gain-of-function auxin-resistant mutants (axr2-1 and axr3-1) or loss-of-function mutants pif4, pif5, and pif4 pif5. Our results suggest that the enhancement of hypocotyl elongation in GFP-LKP2 seedlings is due to the elevated level of IAA and to the up-regulated expression of PIF4 and PIF5 in hypocotyls.

Auxin responsiveness of the MONOPTEROS-BODENLOS module in primary root initiation critically depends on the nuclear import kinetics of the Aux/IAA inhibitor BODENLOS

Primary root formation in early embryogenesis of Arabidopsis thaliana is initiated with the specification of a single cell called hypophysis. This initial step requires the auxin-dependent release of the transcription factor MONOPTEROS (MP, also known as ARF5) from its inhibition by the Aux/IAA protein BODENLOS (BDL, also known as IAA12). Auxin-insensitive bdl mutant embryos and mp loss-of-function embryos fail to specify the hypophysis, giving rise to rootless seedlings. A suppressor screen of rootless bdl mutant seedlings yielded a mutation in the nuclear import receptor IMPORTIN-ALPHA 6 (IMPα6) that promoted primary root formation through rescue of the embryonic hypophysis defects, without causing additional phenotypic changes. Aux/IAA proteins are continually synthesized and degraded, which is essential for rapid transcriptional responses to changing auxin concentrations. Nuclear translocation of bdl:3×GFP was slowed down in impα6 mutants as measured by fluorescence recovery after photobleaching (FRAP) analysis, which correlated with the reduced inhibition of MP by bdl in transient expression assays in impα6 knock-down protoplasts. The MP-BDL module acts like an auxin-triggered genetic switch because MP activates its own expression as well as the expression of its inhibitor BDL. Using an established simulation model, we determined that the reduced nuclear translocation rate of BDL in impα6 mutant embryos rendered the auxin-triggered switch unstable, impairing the fast response to changes in auxin concentration. Our results suggest that the instability of the inhibitor BDL necessitates a fast nuclear uptake in order to reach the critical threshold level required for auxin responsiveness of the MP-BDL module in primary root initiation.

Inhibition of root meristem growth by cadmium involves nitric oxide-mediated repression of auxin accumulation and signalling in Arabidopsis

The root is the first plant organ to get in contact with the toxin cadmium (Cd), which is a widespread soil contaminant. Cd inhibits the growth of the primary root, but the mechanisms underlying this inhibition remain elusive. In this study, we used physiological, pharmacological and genetic approaches to investigate the roles of nitric oxide (NO) and auxin in Cd-mediated inhibition of Arabidopsis thaliana root meristem growth. Our study demonstrated that in the first 12 h of exposure, Cd inhibits primary root elongation through a decrease in the sizes of both the elongation and meristematic zones. Following Cd exposure, a decrease in auxin levels is associated with reduced PIN1/3/7 protein accumulation, but not with reduced PIN1/3/7 transcript levels. Additionally, Cd stabilized AXR3/IAA17 protein to repress auxin signalling in this Cd-mediated process. Furthermore, decreasing Cd-induced NO accumulation with either NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or NO synthase inhibitor N(ω) -nitro-l-Arg-methylester (l-NAME) compromised the Cd-mediated inhibition of root meristem development, reduction in auxin and PIN1/3/7 accumulation, as well as stabilization of AXR3/IAA17, indicating that NO participates in Cd-mediated inhibition of root meristem growth. Taken together, our data suggest that Cd inhibits root meristem growth by NO-mediated repression of auxin accumulation and signalling in Arabidopsis.

The Control of Auxin Transport in Parasitic and Symbiotic Root-Microbe Interactions

Most field-grown plants are surrounded by microbes, especially from the soil. Some of these, including bacteria, fungi and nematodes, specifically manipulate the growth and development of their plant hosts, primarily for the formation of structures housing the microbes in roots. These developmental processes require the correct localization of the phytohormone auxin, which is involved in the control of cell division, cell enlargement, organ development and defense, and is thus a likely target for microbes that infect and invade plants. Some microbes have the ability to directly synthesize auxin. Others produce specific signals that indirectly alter the accumulation of auxin in the plant by altering auxin transport. This review highlights root-microbe interactions in which auxin transport is known to be targeted by symbionts and parasites to manipulate the development of their host root system. We include case studies for parasitic root-nematode interactions, mycorrhizal symbioses as well as nitrogen fixing symbioses in actinorhizal and legume hosts. The mechanisms to achieve auxin transport control that have been studied in model organisms include the induction of plant flavonoids that indirectly alter auxin transport and the direct targeting of auxin transporters by nematode effectors. In most cases, detailed mechanisms of auxin transport control remain unknown.

SAUR Proteins as Effectors of Hormonal and Environmental Signals in Plant Growth

The plant hormone auxin regulates numerous aspects of plant growth and development. Early auxin response genes mediate its genomic effects on plant growth and development. Discovered in 1987, small auxin up RNAs (SAURs) are the largest family of early auxin response genes. SAUR functions have remained elusive, however, presumably due to extensive genetic redundancy. However, recent molecular, genetic, biochemical, and genomic studies have implicated SAURs in the regulation of a wide range of cellular, physiological, and developmental processes. Recently, crucial mechanistic insight into SAUR function was provided by the demonstration that SAURs inhibit PP2C.D phosphatases to activate plasma membrane (PM) H(+)-ATPases and promote cell expansion. In addition to auxin, several other hormones and environmental factors also regulate SAUR gene expression. We propose that SAURs are key effector outputs of hormonal and environmental signals that regulate plant growth and development.

SmARF8, a transcription factor involved in parthenocarpy in eggplant

Parthenocarpic fruit is a very attractive trait for consumers and especially in eggplants where seeds can lead to browning of the flesh and bitterness. However, the molecular mechanisms underlying parthenocarpy in eggplant still remain unknown. Some auxin response factors have been previously shown in model species, such as Arabidopsis and tomato, to play an important role in such a process. Here, we have identified a natural parthenocarpic mutant and showed that ARF8 from eggplant (SmARF8), is down-regulated in buds compared to wild-type plants. Further characterization of SmARF8 showed that it is a nuclear protein and an active transcriptional regulator. We determined that amino acids 629-773 of SmARF8 act as the transcriptional activation domain, the C terminus of SmARF8 is the protein-binding domain, and that SmARF8 might form homodimers. Expression analysis in eggplant showed that SmARF8 is expressed ubiquitously in all tissues and organs and is responsive to auxin. Eggplant transgenic lines harboring RNA interference of SmARF8 exhibited parthenocarpy in unfertilized flowers, suggesting that SmARF8 negatively regulates fruit initiation. Interestingly, SmARF8-overexpressing Arabidopsis lines also induced parthenocarpy. These results indicate that SmARF8 could affect the dimerization of auxin/indole acetic acid repressors with SmARF8 via domains III and IV and thus induce fruit development. Furthermore, the introduction of SmARF8 full-length cDNA could partially complement the parthenocarpic phenotypes in Arabidopsis arf8-1 and arf8-4 mutants. Collectively, our results demonstrate that SmARF8 may act as a key negative regulator involved in parthenocarpic fruit development of eggplant. These findings give more insights into the conserved mechanisms leading to parthenocarpy in which auxin signaling plays a pivotal role, and provide potential target for eggplant breeding.

Auxin transporters and binding proteins at a glance

The plant hormone auxin is a key regulator of plant growth and development. Differences in auxin distribution within tissues are mediated by the polar auxin transport machinery, and cellular auxin responses occur depending on changes in cellular auxin levels. Multiple receptor systems at the cell surface and in the interior operate to sense and interpret fluctuations in auxin distribution that occur during plant development. Until now, three proteins or protein complexes that can bind auxin have been identified. SCF(TIR1) [a SKP1-cullin-1-F-box complex that contains transport inhibitor response 1 (TIR1) as the F-box protein] and S-phase-kinase-associated protein 2 (SKP2) localize to the nucleus, whereas auxin-binding protein 1 (ABP1), predominantly associates with the endoplasmic reticulum and cell surface. In this Cell Science at a Glance article, we summarize recent discoveries in the field of auxin transport and signaling that have led to the identification of new components of these pathways, as well as their mutual interaction.

Characterization of an activation-tagged mutant uncovers a role of GLABRA2 in anthocyanin biosynthesis in Arabidopsis

In Arabidopsis, anthocyanin biosynthesis is controlled by a MYB-bHLH-WD40 (MBW) transcriptional activator complex. The MBW complex activates the transcription of late biosynthesis genes in the flavonoid pathway, leading to the production of anthocyanins. A similar MBW complex regulates epidermal cell fate by activating the transcription of GLABRA2 (GL2), a homeodomain transcription factor required for trichome formation in shoots and non-hair cell formation in roots. Here we provide experimental evidence to show that GL2 also plays a role in regulating anthocyanin biosynthesis in Arabidopsis. From an activation-tagged mutagenized population of Arabidopsis plants, we isolated a dominant, gain-of-function mutant with reduced anthocyanins. Molecular cloning revealed that this phenotype is caused by an elevated expression of GL2, thus the mutant was named gl2-1D. Consistent with the view that GL2 acts as a negative regulator of anthocyanin biosynthesis, gl2-1D seedlings accumulated less whereas gl2-3 seedlings accumulated more anthocyanins in response to sucrose. Gene expression analysis indicated that expression of late, but not early, biosynthesis genes in the flavonoid pathway was dramatically reduced in gl2-1D but elevated in gl2-3 mutants. Further analysis showed that expression of some MBW component genes involved in the regulation of late biosynthesis genes was reduced in gl2-1D but elevated in gl2-3 mutants, and chromatin immunoprecipitation results indicated that some MBW component genes are targets of GL2. We also showed that GL2 functions as a transcriptional repressor. Taken together, these results indicate that GL2 negatively regulates anthocyanin biosynthesis in Arabidopsis by directly repressing the expression of some MBW component genes.

Peptidyl-prolyl isomerization targets rice Aux/IAAs for proteasomal degradation during auxin signalling

In plants, auxin signalling is initiated by the auxin-promoted interaction between the auxin receptor TIR1, an E3 ubiquitin ligase, and the Aux/IAA transcriptional repressors, which are subsequently degraded by the proteasome. Gain-of-function mutations in the highly conserved domain II of Aux/IAAs abolish the TIR1-Aux/IAA interaction and thus cause an auxin-resistant phenotype. Here we show that peptidyl-prolyl isomerization of rice OsIAA11 catalysed by LATERAL ROOTLESS2 (LRT2), a cyclophilin-type peptidyl-prolyl cis/trans isomerase, directly regulates the stability of OsIAA11. NMR spectroscopy reveals that LRT2 efficiently catalyses the cis/trans isomerization of OsIAA11. The lrt2 mutation reduces OsTIR1-OsIAA11 interaction and consequently causes the accumulation of a higher level of OsIAA11 protein. Moreover, knockdown of the OsIAA11 expression partially rescues the lrt2 mutant phenotype in lateral root development. Together, these results illustrate cyclophilin-catalysed peptidyl-prolyl isomerization promotes Aux/IAA degradation, as a mechanism regulating auxin signalling.

A Simple Auxin Transcriptional Response System Regulates Multiple Morphogenetic Processes in the Liverwort Marchantia polymorpha

In land plants comparative genomics has revealed that members of basal lineages share a common set of transcription factors with the derived flowering plants, despite sharing few homologous structures. The plant hormone auxin has been implicated in many facets of development in both basal and derived lineages of land plants. We functionally characterized the auxin transcriptional response machinery in the liverwort Marchantia polymorpha, a member of the basal lineage of extant land plants. All components known from flowering plant systems are present in M. polymorpha, but they exist as single orthologs: a single MpTOPLESS (TPL) corepressor, a single MpTRANSPORT inhibitor response 1 auxin receptor, single orthologs of each class of auxin response factor (ARF; MpARF1, MpARF2, MpARF3), and a single negative regulator auxin/indole-3-acetic acid (MpIAA). Phylogenetic analyses suggest this simple system is the ancestral condition for land plants. We experimentally demonstrate that these genes act in an auxin response pathway--chimeric fusions of the MpTPL corepressor with heterodimerization domains of MpARF1, MpARF2, or their negative regulator, MpIAA, generate auxin insensitive plants that lack the capacity to pattern and transition into mature stages of development. Our results indicate auxin mediated transcriptional regulation acts as a facilitator of branching, differentiation and growth, rather than acting to determine or specify tissues during the haploid stage of the M. polymorpha life cycle. We hypothesize that the ancestral role of auxin is to modulate a balance of differentiated and pluri- or totipotent cell states, whose fates are determined by interactions with combinations of unrelated transcription factors.