Molecular genetics of auxin signaling

The Arabidopsis thaliana Double-Stranded RNA Binding Proteins DRB1 and DRB2 Are Required for miR160-Mediated Responses to Exogenous Auxin

DOUBLE-STRANDED RNA BINDING (DRB) proteins DRB1, DRB2, and DRB4 are essential for microRNA (miRNA) production in Arabidopsis thaliana (Arabidopsis) with miR160, and its target genes, AUXIN RESPONSE FACTOR10 (ARF10), ARF16, and ARF17, forming an auxin responsive miRNA expression module crucial for root development. Methods: Wild-type Arabidopsis plants (Columbia-0 (Col-0)) and the drb1, drb2, and drb12 mutants were treated with the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), and the miR160-mediated response of these four Arabidopsis lines was phenotypically and molecularly characterized. Results: In 2,4-D-treated Col-0, drb1 and drb2 plants, altered miR160 abundance and ARF10, ARF16, and ARF17 gene expression were associated with altered root system development. However, miR160-directed molecular responses to treatment with 2,4-D was largely defective in the drb12 double mutant. In addition, via profiling of molecular components of the miR160 expression module in the roots of the drb4, drb14, and drb24 mutants, we uncovered a previously unknown role for DRB4 in regulating miR160 production. Conclusions: The miR160 expression module forms a central component of the molecular and phenotypic response of Arabidopsis plants to exogenous auxin treatment. Furthermore, DRB1, DRB2, and DRB4 are all required in Arabidopsis roots to control miR160 production, and subsequently, to appropriately regulate ARF10, ARF16, and ARF17 target gene expression.

A telomere-to-telomere gap-free reference genome of Chionanthus retusus provides insights into the molecular mechanism underlying petal shape changes

Chionanthus retusus, an arbor tree of the Oleaceae family, is an ecologically and economically valuable ornamental plant for its remarkable adaptability in landscaping. During C. retusus breeding, we observed diverse floral shapes; however, no available genome for C. retusus has hindered the widespread identification of genes related to flower morphology. Thus, a de novo telomere-to-telomere (T2T) gap-free genome was generated. The assembly, incorporating high-coverage and long-read sequencing data, successfully yielded two complete haplotypes (687 and 683 Mb). The genome encompasses 42 864 predicted protein-coding genes, with all 46 telomeres and 23 centromeres in one haplotype. Whole-genome duplication analysis revealed that C. retusus underwent one fewer event of whole-genome duplication after differentiation compared to other species in the Oleaceae family. Furthermore, flower vein diversity was the main reason for the differences in floral shapes. Auxin-related genes were responsible for petal shape formation on genome-based transcriptome analysis. Specifically, the removal and retention of the first intron in CrAUX/IAA20 resulted in the production of two transcripts, and the differences in the expression levels of CrAUX/IAA20 resulted in the variations of flower veins. Compared to transcripts lacking the first intron, transcripts with intron retention caused more severe decreases in the number and length of flower veins in transgenic Arabidopsis thaliana. Our findings will deepen our understanding of flower morphology development and provide important theoretical support for the cultivation of Oleaceae.

Exogenous application of the apocarotenoid retinaldehyde negatively regulates auxin-mediated root growth

Root development is essential for plant survival. The lack of carotenoid biosynthesis in the phytoene desaturase 3 (pds3) mutant results in short primary roots (PRs) and reduced lateral root formation. In this study, we showed that short-term inhibition of PDS by fluridone suppresses PR growth in wild type, but to a lesser extent in auxin mutants of Arabidopsis (Arabidopsis thaliana). Such an inhibition of PDS activity increased endogenous indole-3-acetic acid levels, promoted auxin signaling, and partially complemented the PR growth of an auxin-deficient mutant, the YUCCA 3 5 7 8 9 quadruple mutant (yucQ). The exogenous application of retinaldehyde (retinal), an apocarotenoid derived from β-carotene, complemented the fluridone-induced suppression of root growth, as well as the short roots of the pds3 mutant. Retinal also partially complemented the auxin-induced suppression of root growth. These results suggest that retinal may play a role in regulating root growth by modulating endogenous auxin levels.

Control of grain size in rice by TGW3 phosphorylation of OsIAA10 through potentiation of OsIAA10-OsARF4-mediated auxin signaling

Grain size is a key component of grain yield and quality in crops. Several core players of auxin signaling have been revealed to modulate grain size; however, to date, few genetically defined pathways have been reported, and whether phosphorylation could boost degradation of Aux/IAA proteins is uncertain. Here, we show that TGW3 (also called OsGSK5) interacts with and phosphorylates OsIAA10. Phosphorylation of OsIAA10 facilitates its interaction with OsTIR1 and subsequent destabilization, but this modification hinders its interaction with OsARF4. Our genetic and molecular evidence identifies an OsTIR1-OsIAA10-OsARF4 axis as key for grain size control. In addition, physiological and molecular studies suggest that TGW3 mediates the brassinosteroid response, the effect of which can be relayed through the regulatory axis. Collectively, these findings define a auxin signaling pathway to regulate grain size, in which phosphorylation of OsIAA10 enhances its proteolysis and potentiates OsIAA10-OsARF4-mediated auxin signaling.

Auxin participates in regulating the leaf curl development of Wucai (Brassica campestris L.)

Wucai (Brassica campestris L. ssp. chinensis var. rosularis Tsen) belongs to the Brassica genus of the Cruciferae family, and its leaf curl is a typical feature that distinguishes Wucai from other nonheading cabbage subspecies. Our previous research found that plant hormones were involved in the development of the leaf curl in Wucai. However, the molecular mechanisms and the hormones regulating the formation of leaf curl in Wucai have not yet been reported. This study aimed to understand the molecular functions related to hormone metabolism during the formation of leaf curl in Wucai. A total of 386 differentially expressed genes (DEGs) were identified by transcriptome sequencing of two different morphological parts of the same leaf of Wucai germplasm W7-2, and 50 DEGs were found to be related to plant hormones, which were mainly involved in the auxin signal transduction pathway. Then, we measured the content of endogenous hormones in two different forms of the same leaf of Wucai germplasm W7-2. A total of 17 hormones with differential content were identified, including auxin, cytokinins, jasmonic acids, salicylic acids, and abscisic acid. And we found that treatment with auxin transport inhibitor N-1-naphthylphthalamic acid can affect the leaf curl phenotype of Wucai and pak choi (Brassica rapa L. subsp. Chinensis). These results indicated that plant hormones, especially auxin, are involved in developing the leaf curl of Wucai. Our findings provide a potentially valuable reference for future research on the development of leaf curls.

Auxin regulation on crop: from mechanisms to opportunities in soybean breeding

Breeding crop varieties with high yield and ideal plant architecture is a desirable goal of agricultural science. The success of "Green Revolution" in cereal crops provides opportunities to incorporate phytohormones in crop breeding. Auxin is a critical phytohormone to determine nearly all the aspects of plant development. Despite the current knowledge regarding auxin biosynthesis, auxin transport and auxin signaling have been well characterized in model Arabidopsis (Arabidopsis thaliana) plants, how auxin regulates crop architecture is far from being understood, and the introduction of auxin biology in crop breeding stays in the theoretical stage. Here, we give an overview on molecular mechanisms of auxin biology in Arabidopsis, and mainly summarize auxin contributions for crop plant development. Furthermore, we propose potential opportunities to integrate auxin biology in soybean (Glycine max) breeding.

Design, synthesis, and herbicidal activity of indole-3-carboxylic acid derivatives as potential transport inhibitor response 1 antagonists

Auxins as an important class of phytohormones play essential roles in plant life cycle; therefore, developing compounds with auxin-like properties for plant growth regulation and weed control applications is of great significance. Herein, we reported the design, synthesis, and herbicidal activity evaluation of a series of novel indole-3-carboxylic acid derivatives as auxin receptor protein TIR1 antagonists. Petri dish herbicidal activity assay demonstrated that most of the as-synthesized target compounds exhibited good-to-excellent inhibition effects (60–97% inhibitory rates) on roots and shoots of both dicotyledonous rape (B. napus) and monocotyledonous barnyard grass (E. crus-galli). The inhibition rates of compounds 10d and 10h reached up to 96% and 95% for the root of rape (B. napus) at 100 mg/L, and they also maintained 92% and 93% inhibition rates even if at 10 mg/L, respectively. Molecular docking revealed that the interactions between these synthesized target compounds and TIR1 protein include tight π–π stacking, hydrogen bond, and hydrophobic interactions. This work expands the range of auxin chemistry for the development of new auxin mimic herbicides.

Transcriptome Analysis Reveals Key Pathways and Candidate Genes Controlling Seed Development and Size in Ricebean (Vigna umbellata)

Ricebean (Vigna umbellata) is a lesser known pulse with well-recognized potential. Recently, it has emerged as a legume with endowed nutritional potential because of high concentration of quality protein and other vital nutrients in its seeds. However, the genes and pathways involved in regulating seed development and size are not understood in this crop. In our study, we analyzed the transcriptome of two genotypes with contrasting grain size (IC426787: large seeded and IC552985: small seeded) at two different time points, namely, 5 and 10 days post-anthesis (DPA). The bold seeded genotype across the time points (B5_B10) revealed 6,928 differentially expressed genes (DEGs), whereas the small seeded genotype across the time point (S5_S10) contributed to 14,544 DEGs. We have also identified several candidate genes for seed development-related traits like seed size and 100-seed weight. On the basis of similarity search and domain analysis, some candidate genes (PHO1, cytokinin dehydrogenase, A-type cytokinin, and ARR response negative regulator) related to 100-seed weight and seed size showed downregulation in the small seeded genotype. The MapMan and KEGG analysis confirmed that auxin and cytokinin pathways varied in both the contrasting genotypes and can therefore be the regulators of the seed size and other seed development-related traits in ricebeans. A total of 51 genes encoding SCF TIR1/AFB , Aux/IAA, ARFs, E3 ubiquitin transferase enzyme, and 26S proteasome showing distinct expression dynamics in bold and small genotypes were also identified. We have also validated randomly selected SSR markers in eight accessions of the Vigna species (V. umbellata: 6; Vigna radiata: 1; and Vigna mungo: 1). Cross-species transferability pattern of ricebean-derived SSR markers was higher in V. radiata (73.08%) than V. mungo (50%). To the best of our knowledge, this is the first transcriptomic study conducted in this crop to understand the molecular basis of any trait. It would provide us a comprehensive understanding of the complex transcriptome dynamics during the seed development and gene regulatory mechanism of the seed size determination in ricebeans.

Mdm-MIR393b-mediated adventitious root formation by targeted regulation of MdTIR1A expression and weakened sensitivity to auxin in apple rootstock

Adventitious root (AR) formation is of great significance for apple rootstock breeding. It is widely accepted that miR393 influences AR formation in many plant species; however, the molecular mechanism by which factors regulate AR formation remains insufficient. In this study, the evolutionary relationship of mdm-miR393 and candidate target genes MdTIR1/AFB was systematically identified, and the expression patterns were analysed. Multisequence alignment analysis of miR393 family members suggests that miR393 conservatively evolved between different species. The evolutionary relationship of the TIR1/AFBs can be divided into G1, G2 and G3 subgroups. During AR formation, the expression level of mdm-miR393a/b/c was significantly upregulated at 1 d and 7 d by exogenous auxin treatment. Furthermore, the expression levels of MdTIR1A, MdTIR1D, MdAFB1, MdAFB2, MdAFB3, MdAFB4 and MdAFB8 also appeared to be significantly changed by exogenous auxin induction. Subsequently, tissue-specific expression analysis showed that the expression levels of mdm-miR393 and MdTIR1/AFBs in different tissues exhibited significant differences. The promoter of mdm-miR393 contains multiple elements that respond to ABA, adversity and light signals; auxin treatment can activate the mdm-MIR393b promoter but is obviously inhibited by NPA treatment. The targeting relationship between mdm-MIR393b and MdTIR1A was verified by expression patterns, degradation group data, transient tobacco conversion results, and genes functions experiments. Heterologous overexpression of mdm-MIR393b (35S::mdm-MIR393b) decreased the number of ARs in the phenotype and reduced the expression level of the target gene NtTIR1 in tobacco. Compared to the wild type, the 35S::mdm-MIR393b transgenic plants demonstrated insensitivity to auxin. Furthermore, tir1 mutant exhibited reduced root system structure relative to the control. The above results illustrated that mdm-MIR393b is involved in mediating AR formation by targeted regulation of MdTIR1A expression in apple rootstock.

A copy number variant at the HPDA-D12 locus confers compact plant architecture in cotton

Improving yield is a primary mission for cotton (Gossypium hirsutum) breeders; development of cultivars with suitable architecture for high planting density (HPDA) can increase yield per unit area. We characterized a natural cotton mutant, AiSheng98 (AS98), which exhibits shorter height, shorter branch length, and more acute branch angle than wild-type. A copy number variant at the HPDA locus on Chromosome D12 (HPDA-D12), encoding a dehydration-responsive element-binding (DREB) transcription factor, GhDREB1B, strongly affects plant architecture in the AS98 mutant. We found an association between a tandem duplication of a c. 13.5 kb segment in HPDA-D12 and elevated GhDREB1B expression resulting in the AS98 mutant phenotype. GhDREB1B overexpression confers a significant decrease in plant height and branch length, and reduced branch angle. Our results suggest that fine-tuning GhDREB1B expression may be a viable engineering strategy for modification of plant architecture favorable to high planting density in cotton.

Root Silicon Addition Induces Fe Deficiency in Cucumber Plants, but Facilitates Their Recovery After Fe Resupply. A Comparison With Si Foliar Sprays

Silicon has not been cataloged as an essential element for higher plants. However, it has shown beneficial effects on many crops, especially under abiotic and biotic stresses. Silicon fertilization was evaluated for the first time on plants exposed to fluctuations in an Fe regime (Fe sufficiency followed by Fe deficiency and, in turn, by Fe resupply). Root and foliar Si applications were compared using cucumber plants that were hydroponically grown in a growth chamber under different Fe nutritional statuses and Si applied either to the roots or to the shoots. The SPAD index, Fe, and Mn concentration, ROS, total phenolic compounds, MDA concentration, phytohormone balance, and cell cycle were determined. The results obtained showed that the addition of Si to the roots induced an Fe shortage in plants grown under optimal or deficient Fe nutritional conditions, but this was not observed when Si was applied to the leaves. Plant recovery following Fe resupply was more effective in the Si-treated plants than in the untreated plants. A relationship between the ROS concentration, hormonal balance, and cell cycle under different Fe regimes and in the presence or absence of Si was also studied. The contribution of Si to this signaling pathway appears to be related more to the induction of Fe deficiency, than to any direct biochemical or metabolic processes. However, these roles could not be completely ruled out because several hormone differences could only be explained by the addition of Si.

Auxin-mediated responses under salt stress: from developmental regulation to biotechnological applications

As sessile organisms, plants are exposed to multiple abiotic stresses commonly found in nature. To survive, plants have developed complex responses that involve genetic, epigenetic, cellular, and morphological modifications. Among different environmental cues, salt stress has emerged as a critical problem contributing to yield losses and marked reductions in crop production. Moreover, as the climate changes, it is expected that salt stress will have a significant impact on crop production in the agroindustry. On a mechanistic level, salt stress is known to be regulated by the crosstalk of many signaling molecules such as phytohormones, with auxin having been described as a key mediator of the process. Auxin plays an important role in plant developmental responses and stress, modulating a complex balance of biosynthesis, transport, and signaling that among other things, finely tune physiological changes in plant architecture and Na+ accumulation. In this review, we describe current knowledge on auxin's role in modulating the salt stress response. We also discuss recent and potential biotechnological approaches to tackling salt stress.

PINOID is required for lateral organ morphogenesis and ovule development in cucumber

Lateral organ development is essential for cucumber production. The protein kinase PINOID (PID) participates in distinct aspects of plant development by mediating polar auxin transport in different species. Here, we obtained a round leaf (rl) mutant that displayed extensive phenotypes including round leaf shape, inhibited tendril outgrowth, abnormal floral organs, and disrupted ovule genesis. MutMap+ analysis revealed that rl encodes a cucumber ortholog of PID (CsPID). A non-synonymous single nucleotide polymorphism in the second exon of CsPID resulted in an amino acid substitution from arginine to lysine in the rl mutant. Allelic testing using the mutant allele C356 with similar phenotypes verified that CsPID was the causal gene. CsPID was preferentially expressed in young leaf and flower buds and down-regulated in the rl mutant. Subcellular localization showed that the mutant form, Cspid, showed a dotted pattern of localization, in contrast to the continuous pattern of CsPID in the periphery of the cell and nucleus. Complementation analysis in Arabidopsis showed that CsPID, but not Cspid, can partially rescue the pid-14 mutant phenotype. Moreover, indole-3-acetic acid content was greatly reduced in the rl mutant. Transcriptome profiling revealed that transcription factors, ovule morphogenesis, and auxin transport-related genes were significantly down-regulated in the rl mutant. Biochemical analysis showed that CsPID physically interacted with a key polarity protein, CsREV (REVOLUTA). We developed a model in which CsPID regulates lateral organ morphogenesis and ovule development by stimulating genes related to auxin transport and ovule development.

The heterotrimeric G protein β subunit RGB1 is required for seedling formation in rice

BACKGROUND

The heterotrimeric G protein β subunit RGB1 plays an important role in plant growth and development. However, the molecular mechanisms underlying the regulation of rice growth by RGB1 remain elusive.

RESULTS

Here, the rgb1 mutants rgb1-1 (+ 1 bp), rgb1-2 (- 1 bp), and rgb1-3 (- 11 bp) were isolated using the CRISPR/Cas9 system, and they were arrested at 1 day after germination and ultimately exhibited seedling lethality. The dynamic anatomical characteristics of the embryos of the rgb1 seedlings and WT during early postgermination and according to TUNEL assays showed that the suppressed growth of the rgb1 mutants was caused by cell death. In addition to the limited shoot and root development, the development of the embryo shoot-root axis was suppressed in the rgb1 mutants. RGB1 was expressed mainly in the root epidermal and vascular tissues of the embryo. Moreover, transcript profiling analysis revealed that the expression of a large number of auxin-, cytokinin-, and brassinosteroid-inducible genes was upregulated or downregulated in the rgb1 mutant compared to the wild type during seedling development.

CONCLUSIONS

Overall, the rgb1 mutants provide an ideal material for exploring the molecular mechanism underlying rice seedling formation during early postgermination development by G proteins.

SIGNIFICANCE STATEMENT

The heterotrimeric G protein β subunit RGB1 acts as a crucial factor in promoting early postgermination seedling development in rice.

Comparative transcriptome analysis of two reproductive modes in Adiantum reniforme var. sinense targeted to explore possible mechanism of apogamy

Background

Apogamy is a unique asexual reproduction in the ferns, in which somatic cells of gametophytes go through dedifferentiation and then differentiate into haploid sporophytes bypassing fertilization. Restricted to the lack of genomic information, molecular mechanisms of apogamy have remained unclear. Comparative transcriptome analysis was conducted at six stages between sexual reproduction and apogamy in the fern Adiantum reniforme var. sinense, in an effort to identify genes and pathways that might initiate the asexual reproduction.

Results

Approximately 928 million high-quality clean reads were assembled into 264,791 unigenes with an average length of 615 bp. A total of 147,865 (55.84%) unigenes were successfully annotated. Differential genes expression analysis indicated that transcriptional regulation was more active in the early stage of apogamy compared to sexual reproduction. Further comparative analysis of the enriched pathways between the early stages of the two reproductive modes demonstrated that starch and sucrose metabolism pathway responsible for cell wall was only significantly enriched in asexual embryonic cell initiation. Furthermore, regulation of plant hormone related genes was more vigorous in apogamy initiation.

Conclusion

These findings would be useful for revealing the initiation of apogamy and further understanding of the mechanisms related to asexual reproduction.

Electronic supplementary material

The online version of this article (10.1186/s12863-019-0762-8) contains supplementary material, which is available to authorized users.

Exogenous Application of Phytohormones Promotes Growth and Regulates Expression of Wood Formation-Related Genes in Populus simonii × P. nigra

Although phytohormones are known to be important signal molecules involved in wood formation, their roles are still largely unclear. Here, Populus simonii × P. nigra seedlings were treated with different concentrations of exogenous phytohormones, indole-3-acetic acid (IAA), gibberellin (GA₃), and brassinosteroid (BR), and the effects of phytohormones on growth were investigated. Next, 27 genes with known roles in wood formation were selected for qPCR analysis to determine tissue-specificity and timing of responses to phytohormone treatments. Compared to the control, most IAA, GA₃, and BR concentrations significantly increased seedling height. Meanwhile, IAA induced significant seedling stem diameter and cellulose content increases that peaked at 3 and 30 mg·L⁻¹, respectively. Significant increase in cellulose content was also observed in seedlings treated with 100 mg·L⁻¹ GA₃. Neither stem diameter nor cellulose content of seedlings were affected by BR treatment significantly, although slight effects were observed. Anatomical measurements demonstrated improved xylem, but not phloem, development in IAA- and BR-treated seedlings. Most gene expression patterns induced by IAA, GA₃, and BR differed among tissues. Many IAA response genes were also regulated by GA₃, while BR-induced transcription was weaker and slower in Populus than for IAA and GA₃. These results reveal the roles played by phytohormones in plant growth and lay the foundation for exploring molecular regulatory mechanisms of wood formation in Populus.

The WUSCHEL-related homeobox1 gene of cucumber regulates reproductive organ development

The WUSCHEL-related homeobox1 (WOX1) transcription factor plays an important role in lateral growth of plant organs; however, the underlying mechanisms in the regulation of reproductive development are largely unknown. Cucumber (Cucumis sativus) has separate male and female flowers, facilitating the study of the role of WOX1 in stamen and carpel development. Here, we identified a mango fruit (mf) mutant in cucumber, which displayed multiple defects in flower growth as well as male and female sterility. Map-based cloning showed that Mf encodes a WOX1-type transcriptional regulator (CsWOX1), and that the mf mutant encodes a truncated protein lacking the conserved WUS box. Further analysis showed that elevated expression of CsWOX1 was responsible for the mutant phenotype in cucumber and Arabidopsis. Comparative transcriptome profiling revealed certain key players and CsWOX1-associated networks that regulate reproductive development. CsWOX1 directly interacts with cucumber SPOROCYTELESS (CsSPL), and many genes in the CsSPL-mediated pathway were down-regulated in plants with the mutant allele at the Mf locus. In addition, auxin distribution was affected in both male and female flowers of the mutant. Taking together, these data suggest that CsWOX1 may regulate early reproductive organ development and be involved in sporogenesis via the CsSPL-mediated pathway and/or modulate auxin signaling in cucumber.

Overexpression of an auxin receptor OsAFB6 significantly enhanced grain yield by increasing cytokinin and decreasing auxin concentrations in rice panicle

Auxin plays critical roles in many developmental processes of plants. The auxin signaling pathway is a series of plant responses to auxin stimuli. However, the functions of many genes in this pathway are still obscure. As auxin receptors, TIR/AFB family genes encode F-Box proteins that directly bind auxin and then transduce the stimulus through the signaling pathway. In this paper, we generated an overexpression line of Auxin-signaling F-Box 6 (OsAFB6) in rice, which largely delayed heading, greatly increased spikelets per panicle and primary branch number and ultimately enhanced grain yield by 50%. OsAFB6 is preferentially expressed in young tissues with stronger meristem activities and suppresses flowering by upregulating OsRR1 and downregulating Ehd1 expression levels. Overexpression of OsAFB6 delayed heading, increased cytokinin (CK) by suppressing the expression level of Gn1a and simultaneously decreased the IAA concentration in the young panicle, which promoted inflorescence meristem development and resulted in large panicles with more spikelets per panicle, primary branches and increased grain yield. It would be a beneficial strategy to generate lines with varied expression levels of OsAFB6 to breed high-yielding cultivars for specific regions that can fully utilize the local sunlight and temperature resources.

Suppression of microRNA159 impacts multiple agronomic traits in rice (Oryza sativa L.)

BACKGROUND

microRNAs (miRNAs) are important regulators in plant growth and development. miR159 is a conserved miRNA among different plant species and has various functions in plants. Studies on miR159 are mostly done on model plant, Arabidopsis thaliana. In rice, studies on miR159 were either based upon genome-wide expression analyses focused upon responses to different nitrogen forms and abiotic stress or upon phenotypic studies of transgenic plants overexpressing its precursor. STTM (Short Tandem Target Mimic) is an effective tool to block the activity of endogenous mature miRNA activity in plant. Therefore, specific roles of miR159 in rice could be explored by down regulating miR159 through STTM.

RESULTS

In this study, expression of mature miR159 was successfully suppressed by STTM which resulted in the increased expressions of its two targets genes, OsGAMYB and OsGAMYBL1 (GAMYB-LIKE 1). Overall, STTM159 plants exhibited short stature along with smaller organ size and reduction in stem diameter, length of flag leaf, main panicle, spikelet hulls and grain size. Histological analysis of stem, leaf and mature spikelet hull showed the reduced number of small vascular bundles (SVB), less number of small veins (SV) between two big veins (LV) and less cell number in outer parenchyma. Gene Ontology (GO) enrichment analysis of differentially expressed genes between wild type plants and STTM159 transgenic plants showed that genes involved in cell division, auxin, cytokinin (CK) and brassinosteroids (BRs) biosynthesis and signaling are significantly down-regulated in STTM159 plants.

CONCLUSION

Our data suggests that in rice, miR159 positively regulates organ size, including stem, leaf, and grain size due to the promotion of cell division. Further analysis from the RNA-seq data showed that the decreased cell divisions in STTM159 transgenic plants may result, at least partly from the lower expression of the genes involved in cell cycle and hormone homeostasis, which provides new insights of rice miR159-specific functions.

Leaf expansion in Phaseolus: transient auxin-induced growth increase

Control of leaf expansion by auxin is not well understood. Evidence from short term exogenous applications and from treatment of excised tissues suggests auxin positively influences growth. Manipulations of endogenous leaf auxin content, however, suggests that, long-term, auxin suppresses leaf expansion. This study attempts to clarify the growth effects of auxin on unifoliate (primary) leaves of the common bean (Phaseolus vulgaris) by reexamining the response to auxin treatment of both excised leaf strips and attached leaves. Leaf strips, incubated in culture conditions that promoted steady elongation for up to 48 h, treated with 10 μM NAA responded with an initial surge of elongation growth complete within 10 hours followed by insensitivity. A range of NAA concentrations from 0.1 μM to 300 μM induced increased strip elongation after 24 hours and 48 hours. Increased elongation and epinastic curvature of leaf strips was found specific to active auxins. Expanding attached unifoliates treated once with aqueous auxin α-naphthalene acetic acid (NAA) at 1.0 mM showed both an initial surge in growth lasting 4-6 hours followed by growth inhibition sustained at least as long as 24 hours post treatment. Auxin-induced inhibition of leaf expansion was associated with smaller epidermal cell area. Together the results suggest increasing leaf auxin first increases growth then slows growth through inhibition of cell expansion. Excised leaf strips, retain only the initial increased growth response to auxin and not the subsequent growth inhibition, either as a consequence of wounding or of isolation from the plant.

Striking the Right Chord: Signaling Enigma during Root Gravitropism

Plants being sessile can often be judged as passive acceptors of their environment. However, plants are actually even more active in responding to the factors from their surroundings. Plants do not have eyes, ears or vestibular system like animals, still they "know" which way is up and which way is down? This is facilitated by receptor molecules within plant which perceive changes in internal and external conditions such as light, touch, obstacles; and initiate signaling pathways that enable the plant to react. Plant responses that involve a definite and specific movement are called "tropic" responses. Perhaps the best known and studied tropisms are phototropism, i.e., response to light, and geotropism, i.e., response to gravity. A robust root system is vital for plant growth as it can provide physical anchorage to soil as well as absorb water, nutrients and essential minerals from soil efficiently. Gravitropic responses of both primary as well as lateral root thus become critical for plant growth and development. The molecular mechanisms of root gravitropism has been delved intensively, however, the mechanism behind how the potential energy of gravity stimulus converts into a biochemical signal in vascular plants is still unknown, due to which gravity sensing in plants still remains one of the most fascinating questions in molecular biology. Communications within plants occur through phytohormones and other chemical substances produced in plants which have a developmental or physiological effect on growth. Here, we review current knowledge of various intrinsic signaling mechanisms that modulate root gravitropism in order to point out the questions and emerging developments in plant directional growth responses. We are also discussing the roles of sugar signals and their interaction with phytohormone machinery, specifically in context of root directional responses.

Small acidic protein 1 and SCFTIR1 ubiquitin proteasome pathway act in concert to induce 2,4-dichlorophenoxyacetic acid-mediated alteration of actin in Arabidopsis roots

2,4-Dichlorophenoxyacetic acid (2,4-D), a functional analogue of auxin, is used as an exogenous source of auxin as it evokes physiological responses like the endogenous auxin, indole-3-acetic acid (IAA). Previous molecular analyses of the auxin response pathway revealed that IAA and 2,4-D share a common mode of action to elicit downstream physiological responses. However, recent findings with 2,4-D-specific mutants suggested that 2,4-D and IAA might also use distinct pathways to modulate root growth in Arabidopsis. Using genetic and cellular approaches, we demonstrate that the distinct effects of 2,4-D and IAA on actin filament organization partly dictate the differential responses of roots to these two auxin analogues. 2,4-D but not IAA altered the actin structure in long-term and short-term assays. Analysis of the 2,4-D-specific mutant aar1-1 revealed that small acidic protein 1 (SMAP1) functions positively to facilitate the 2,4-D-induced depolymerization of actin. The ubiquitin proteasome mutants tir1-1 and axr1-12, which show enhanced resistance to 2,4-D compared with IAA for inhibition of root growth, were also found to have less disrupted actin filament networks after 2,4-D exposure. Consistently, a chemical inhibitor of the ubiquitin proteasome pathway mitigated the disrupting effects of 2,4-D on the organization of actin filaments. Roots of the double mutant aar1-1 tir1-1 also showed enhanced resistance to 2,4-D-induced inhibition of root growth and actin degradation compared with their respective parental lines. Collectively, these results suggest that the effects of 2,4-D on actin filament organization and root growth are mediated through synergistic interactions between SMAP1 and SCF^TIR1 ubiquitin proteasome components.

Robust root growth in altered hydrotropic response1 (ahr1) mutant of Arabidopsis is maintained by high rate of cell production at low water potential gradient

Hydrotropism is the directional root growth response determined by water stimulus. In a water potential gradient system (WPGS) the roots of the Arabidopsis wild type have a diminished root growth compared to normal medium (NM). In contrast, the altered hydrotropic response1 (ahr1) mutant roots maintain their robust growth in the same WPGS. The aims of this work were to ascertain how ahr1 roots could sustain growth in the WPGS, with a special focus on the integration of cellular processes involved in the signaling that determines root growth during abiotic stress and their relation to hydrotropism. Cellular analysis of the root apical meristem of ahr1 mutant contrary to the wild type showed an absence of changes in the meristem length, the elongation zone length, the length of fully elongated cells, and the cell cycle duration. The robust and steady root growth of ahr1 seedlings in the WPGS is explained by the mutant capacity to maintain cell production and cell elongation at the same level as in the NM. Analysis of auxin response at a transcriptional level showed that roots of the ahr1 mutant had a lower auxin response when grown in the WPGS, compared to wild type, indicating that auxin signaling participates in attenuation of root growth under water stress conditions. Also, wild type plants exhibited a high increase in proline content while ahr1 mutants showed minimum changes in the Normal Medium→Water Stress Medium (NM→WSM), a lower water potential gradient system than the WPGS. Accordingly, in this condition, gene expression of Δ1-6 Pyrroline-5-Carboxylate Synthetase1 (P5CS1) involved in proline synthesis strongly increased in wild type but not in ahr1 seedlings. The ahr1 phenotype shows unique features since the mutant root cells continue to proliferate and grow in the presence of a progressively negative water potential gradient at a level comparable to wild type growing in the NM. As such, it represents an exceptional resource for understanding hydrotropism.

Cloning and expression of SgCYP450-4 from Siraitia grosvenorii

CYP450 plays an essential role in the development and growth of the fruits of Siraitia grosvenorii. However, little is known about the SgCYP450-4 gene in S. grosvenorii. Here, based on transcriptome data, a full-length cDNA sequence of SgCYP450-4 was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR) and rapid-amplification of cDNA ends (RACE) strategies. SgCYP450-4 is 1677 bp in length (GenBank accession No. AEM42985.1) and contains a complete open reading frame (ORF) of 1422 bp. The deduced protein was composed of 473 amino acids, the molecular weight is 54.01 kDa, the theoretical isoelectric point (PI) is 8.8, and the protein was predicted to possess cytochrome P450 domains. SgCYP450-4 gene was highly expressed in root, diploid fruit and fruit treated with hormone and pollination. At 10 days after treatment with pollination and hormones, the expression of SgCYP450-4 had the highest level and then decreased over time, which was consistent with the development of fruits of S. Grosvenorii. Hormonal treatment could significantly induce the expression of SgCYP450-4. These results provide a reference for regulation of fruit development and the use of parthenocarpy to generate seedless fruit, and provide a scientific basis for the production of growth regulator application agents.

Transcriptome Profiling of Petal Abscission Zone and Functional Analysis of an Aux/IAA Family Gene RhIAA16 Involved in Petal Shedding in Rose

Roses are one of the most important cut flowers among ornamental plants. Rose flower longevity is largely dependent on the timing of petal shedding occurrence. To understand the molecular mechanism underlying petal abscission in rose, we performed transcriptome profiling of the petal abscission zone during petal shedding using Illumina technology. We identified a total of 2592 differentially transcribed genes (DTGs) during rose petal shedding. Gene ontology term enrichment and pathway analysis revealed that major biochemical pathways the DTGs were involved in included ethylene biosynthesis, starch degradation, superpathway of cytosolic glycolysis, pyruvate dehydrogenase and TCA cycle, photorespiration and the lactose degradation III pathway. This suggests that alterations in carbon metabolism are an important part of rose petal abscission. Among these DTGs, approximately 150 genes putatively encoding transcription factors were identified in rose abscission zone. These included zinc finger, WRKY, ERF, and Aux/IAA gene families, suggesting that petal abscission involves complex transcriptional reprogramming. Approximately 108 DTGs were related to hormone pathways, of which auxin and ethylene related DTGs were the largest groups including 52 and 41 genes, respectively. These also included 12 DTGs related to gibberellin and 6 DTGs in jasmonic acid pathway. Surprisingly, no DTGs involved in the biosynthesis/signaling of abscisic acid, cytokinin, brassinosteroid, and salicylic acid pathways were detected. Moreover, among DTGs related to auxin, we identified an Aux/IAA gene RhIAA16 that was up-regulated in response to petal shedding. Down-regulation of RhIAA16 by virus-induced gene silencing in rose promoted petal abscission, suggesting that RhIAA16 plays an important role in rose petal abscission.

LEAFY COTYLEDON1-CASEIN KINASE I-TCP15-PHYTOCHROME INTERACTING FACTOR4 Network Regulates Somatic Embryogenesis by Regulating Auxin Homeostasis

Somatic embryogenesis (SE) is an efficient tool for the propagation of plant species and also, a useful model for studying the regulatory networks in embryo development. However, the regulatory networks underlying the transition from nonembryogenic callus to somatic embryos during SE remain poorly understood. Here, we describe an upland cotton (Gossypium hirsutum) CASEIN KINASE I gene, GhCKI, which is a unique key regulatory factor that strongly affects SE. Overexpressing GhCKI halted the formation of embryoids and plant regeneration because of a block in the transition from nonembryogenic callus to somatic embryos. In contrast, defective GhCKI in plants facilitated SE. To better understand the mechanism by which GhCKI regulates SE, the regulatory network was analyzed. A direct upstream negative regulator protein, cotton LEAFY COTYLEDON1, was identified to be targeted to a cis-element, CTTTTC, in the promoter of GhCKI. Moreover, GhCKI interacted with and phosphorylated cotton CINCINNATA-like TEOSINTE BRANCHED1-CYCLOIDEA-PCF transcription factor15 by coordinately regulating the expression of cotton PHYTOCHROME INTERACTING FACTOR4, finally disrupting auxin homeostasis, which led to increased cell proliferation and aborted somatic embryo formation in GhCKI-overexpressing somatic cells. Our results show a complex process of SE that is negatively regulated by GhCKI through a complex regulatory network.

Expression of Brassica napus TTG2, a regulator of trichome development, increases plant sensitivity to salt stress by suppressing the expression of auxin biosynthesis genes

WRKY transcription factors (TFs) are plant specific and play important roles in regulating diverse biological processes. To identify TFs with broad-spectrum effects on various stress responses in Brassica napus, an important oil crop grown across diverse ecological regions worldwide, we functionally characterized Bna.TTG2 genes, which are homologous to the Arabidopsis AtTTG2 (WRKY44) gene. Four Bna.TTG2 genes were capable of rescuing the trichome phenotypes of Arabidopsis ttg2 mutants. Overexpressing one Bna.TTG2 family member, BnaA.TTG2.a.1, remarkably increased trichome numbers in Arabidopsis and B. napus plants. Interestingly, the BnaA.TTG2.a.1-overexpressing plants of both species exhibited increased sensitivity to salt stress. In BnaA.TTG2.a.1-overexpressing Arabidopsis under salt stress, the endogenous indole-3-acetic acid (IAA) content was reduced, and the expression of two auxin biosynthesis genes, TRYPTOPHAN BIOSYNTHESIS 5 (TRP5) and YUCCA2 (YUC2), was downregulated. The results from yeast one-hybrid, electrophoretic mobility shift, and dual-luciferase reporter assays revealed that BnaA.TTG2.a.1 is able to bind to the promoters of TRP5 and YUC2. These data indicated that BnaA.TTG2.a.1 confers salt sensitivity to overexpressing plants by suppressing the expression of IAA synthesis genes and thus lowering IAA levels. Transgenic Arabidopsis plants with an N-terminus-deleted BnaA.TTG2.a.1 no longer showed hypersensitivity to salt stress, suggesting that the N terminus of BnaA.TTG2.a.1 plays a critical role in salt stress responses. Therefore, in addition to its classical function in trichome development, our study reveals a novel role for Bna.TTG2 genes in salt stress responses.

Phytohormonal networks promote differentiation of fiber initials on pre-anthesis cotton ovules grown in vitro and in planta

The number of cotton (Gossypium sp.) ovule epidermal cells differentiating into fiber initials is an important factor affecting cotton yield and fiber quality. Despite extensive efforts in determining the molecular mechanisms regulating fiber initial differentiation, only a few genes responsible for fiber initial differentiation have been discovered. To identify putative genes directly involved in the fiber initiation process, we used a cotton ovule culture technique that controls the timing of fiber initial differentiation by exogenous phytohormone application in combination with comparative expression analyses between wild type and three fiberless mutants. The addition of exogenous auxin and gibberellins to pre-anthesis wild type ovules that did not have visible fiber initials increased the expression of genes affecting auxin, ethylene, ABA and jasmonic acid signaling pathways within 1 h after treatment. Most transcripts expressed differentially by the phytohormone treatment in vitro were also differentially expressed in the ovules of wild type and fiberless mutants that were grown in planta. In addition to MYB25-like, a gene that was previously shown to be associated with the differentiation of fiber initials, several other differentially expressed genes, including auxin/indole-3-acetic acid (AUX/IAA) involved in auxin signaling, ACC oxidase involved in ethylene biosynthesis, and abscisic acid (ABA) 8'-hydroxylase an enzyme that controls the rate of ABA catabolism, were co-regulated in the pre-anthesis ovules of both wild type and fiberless mutants. These results support the hypothesis that phytohormonal signaling networks regulate the temporal expression of genes responsible for differentiation of cotton fiber initials in vitro and in planta.

Molecular functions of genes related to grain shape in rice

Because grain shape is an important component of rice grain yield, the discovery of genes related to rice grain shape has attracted much attention of rice breeding programs. In recent years, some of these genes have been cloned and studied. They have been found not only regulate grain shape by changing the shape of the spikelet hull, but also regulate endosperm development through control of cell division using different molecular mechanisms. In this paper, we review the recent research on genes related to rice grain shape and their possible regulatory mechanisms.

CLUMSY VEIN, the Arabidopsis DEAH-box Prp16 ortholog, is required for auxin-mediated development

Pre-messenger RNA (pre-mRNA) splicing is essential in eukaryotic cells. In animals and yeasts, the DEAH-box RNA-dependent ATPase Prp16 mediates conformational change of the spliceosome, thereby facilitating pre-mRNA splicing. In yeasts, Prp16 also plays an important role in splicing fidelity. Conversely, PRP16 orthologs in Chlamydomonas reinhardtii and nematode do not have an important role in general pre-mRNA splicing, but are required for gene silencing and sex determination, respectively. Functions of PRP16 orthologs in higher plants have not been described until now. Here we show that the CLUMSY VEIN (CUV) gene encoding the unique Prp16 ortholog in Arabidopsis thaliana facilitates auxin-mediated development including male-gametophyte transmission, apical-basal patterning of embryonic and gynoecium development, stamen development, phyllotactic flower positioning, and vascular development. cuv-1 mutation differentially affects splicing and expression of genes involved in auxin biosynthesis, polar auxin transport, auxin perception and auxin signaling. The cuv-1 mutation does not have an equal influence on pre-mRNA substrates. We propose that Arabidopsis PRP16/CUV differentially facilitates expression of genes, which include genes involved in auxin biosynthesis, transport, perception and signaling, thereby collectively influencing auxin-mediated development.

The resemblance and disparity of gene expression in dormant and non-dormant seeds and crown buds of leafy spurge (Euphorbia esula)

BACKGROUND

Leafy spurge (Euphorbia esula L.) is a herbaceous perennial weed and dormancy in both buds and seeds is an important survival mechanism. Bud dormancy in leafy spurge exhibits three well-defined phases of para-, endo- and ecodormancy; however, seed dormancy for leafy spurge is classified as physiological dormancy that requires after-ripening and alternating temperature for maximal germination. Overlaps in transcriptome profiles between different phases of bud and seed dormancy have not been determined. Thus, we compared various phases of dormancy between seeds and buds to identify common genes and molecular processes, which should provide new insights about common regulators of dormancy.

RESULTS

Cluster analysis of expression profiles for 201 selected genes indicated bud and seed samples clustered separately. Direct comparisons between buds and seeds are additionally complicated since seeds incubated at a constant temperature of 20°C for 21 days (21d C) could be considered paradormant (Para) because seeds may be inhibited by endosperm-generated signals, or ecodormant (Eco) because seeds germinate after being subjected to alternating temperature of 20:30°C. Since direct comparisons in gene expression between buds and seeds were problematic, we instead examined commonalities in differentially-expressed genes associated with different phases of dormancy. Comparison between buds and seeds ('Para to Endo buds' and '21d C to 1d C seeds'), using endodormant buds (Endo) and dormant seeds (1d C) as common baselines, identified transcripts associated with cell cycle (HisH4), stress response/transcription factors (ICE2, ERFB4/ABR1), ABA and auxin response (ABA1, ARF1, IAA7, TFL1), carbohydrate/protein degradation (GAPDH_1), and transport (ABCB2). Comparison of transcript abundance for the 'Eco to Endo buds' and '21d C to 1d C seeds' identified transcripts associated with ABA response (ATEM6), auxin response (ARF1), and cell cycle (HisH4). These results indicate that the physiological state of 21d C seeds is more analogous to paradormant buds than that of ecodormant buds.

CONCLUSION

Combined results indicate that common molecular mechanisms associated with dormancy transitions of buds and seeds involve processes associated with ABA and auxin signaling and transport, cell cycle, and AP2/ERF transcription factors or their up-stream regulators.

Rapid Response ofArabidopsisT87 Cultured Cells to Cytokinin through His-to-Asp Phosphorelay Signal Transduction

According to the current consistent model for the higher plant Arabidopsis thaliana, the scheme for an immediate early response to the plant hormone cytokinin can be formulated as Arabidopsis histidine kinase (AHK) cytokinin receptor-mediated His --> Asp phosphorelay signal transduction. Nonetheless, clarification of the comprehensive picture of cytokinin-mediated signal transduction in this higher plant is at a very early stage. As a new approach to this end, we studied whether or not a certain Arabidopsis cell line (named T87) would be versatile for such work on cytokinin signal transduction. We show that T87 cells had the ability to respond to cytokinin, displaying the immediate early induction of type-A Arabidopsis response regulator (ARR) family genes (e.g., ARR6) at the transcriptional level. This event was further confirmed by employing the stable transgenic lines of T87 cells with a set of ARR::LUC reporter transgenes. We also show that T87 cells had the ability to respond to auxin when the expression of a set of AUX/IAA genes (e.g., IAA5) was examined. As postulated for intact plants, in T87 cells too, the induction of IAA5 by auxin was selectively inhibited in the presence of a proteasome inhibitor, while the induction of ARR6 by cytokinin was not significantly affected under the same conditions. Through transient expression assays with T87 protoplasts, it is shown that the intracellular localization profiles of the phosphorelay intermediate Arabidopsis histidine-containing phosphotransfer factor (AHPs; e.g., AHP1 and AHP4) were markedly affected in response to cytokinin, but those of type-A ARRs were not (e.g., ARR15 and ARR16). Taken together, we conclude that, in T87 cells, the AHK-dependent His --> Asp phosphorelay circuitry appears to be propagated in response to cytokinin, as in the case of plants, as far as the immediate early responses were concerned. This cultured cell system might therefore provide us with an alternative means to further characterize the mechanisms underlying cytokinin (and also auxin) responses at the molecular level.

Genome-wide identification and transcriptional profiling analysis of auxin response-related gene families in cucumber

BACKGROUND

Auxin signaling has a vital function in the regulation of plant growth and development, both which are known to be mediated by auxin-responsive genes. So far, significant progress has been made toward the identification and characterization of auxin-response genes in several model plants, while no systematic analysis for these families was reported in cucumber (Cucumis sativus L.), a reference species for Cucurbitaceae crops. The comprehensive analyses will help design experiments for functional validation of their precise roles in plant development and stress responses.

RESULTS

A genome-wide search for auxin-response gene homologues identified 16 auxin-response factors (ARFs), 27 auxin/indole acetic acids (Aux/IAAs), 10 Gretchen Hagen 3 (GH3s), 61 small auxin-up mRNAs (SAURs), and 39 lateral organ boundaries (LBDs) in cucumber. Sequence analysis together with the organization of putative motifs indicated the potential diverse functions of these five auxin-related family members. The distribution and density of auxin response-related genes on chromosomes were not uniform. Evolutionary analysis showed that the chromosomal segment duplications mainly contributed to the expansion of the CsARF, CsIAA, CsGH3, and CsLBD gene families. Quantitative real-time RT-PCR analysis demonstrated that many ARFs, AUX/IAAs, GH3s, SAURs, and LBD genes were expressed in diverse patterns within different organs/tissues and during different development stages. They were also implicated in IAA, methyl jasmonic acid, or salicylic acid response, which is consistent with the finding that a great number of diverse cis-elements are present in their promoter regions involving a variety of signaling transduction pathways.

CONCLUSION

Genome-wide comparative analysis of auxin response-related family genes and their expression analysis provide new evidence for the potential role of auxin in development and hormone response of plants. Our data imply that the auxin response genes may be involved in various vegetative and reproductive developmental processes. Furthermore, they will be involved in different signal pathways and may mediate the crosstalk between various hormone responses.

Identification and Analyses of AUX-IAA target genes controlling multiple pathways in developing fiber cells of Gossypium hirsutum L

Technological development led to an increased interest in systems biological approaches in plants to characterize developmental mechanism and candidate genes relevant to specific tissue or cell morphology. AUX-IAA proteins are important plant-specific putative transcription factors. There are several reports on physiological response of this family in Arabidopsis but in cotton fiber the transcriptional network through which AUX-IAA regulated its target genes is still unknown. in-silico modelling of cotton fiber development specific gene expression data (108 microarrays and 22,737 genes) using Algorithm for the Reconstruction of Accurate Cellular Networks (ARACNe) reveals 3690 putative AUX-IAA target genes of which 139 genes were known to be AUX-IAA co-regulated within Arabidopsis. Further AUX-IAA targeted gene regulatory network (GRN) had substantial impact on the transcriptional dynamics of cotton fiber, as showed by, altered TF networks, and Gene Ontology (GO) biological processes and metabolic pathway associated with its target genes. Analysis of the AUX-IAA-correlated gene network reveals multiple functions for AUX-IAA target genes such as unidimensional cell growth, cellular nitrogen compound metabolic process, nucleosome organization, DNA-protein complex and process related to cell wall. These candidate networks/pathways have a variety of profound impacts on such cellular functions as stress response, cell proliferation, and cell differentiation. While these functions are fairly broad, their underlying TF networks may provide a global view of AUX-IAA regulated gene expression and a GRN that guides future studies in understanding role of AUX-IAA box protein and its targets regulating fiber development.

Mapping quantitative trait loci for tissue culture response in VCS3M-DH population of Brassica rapa

Quantitative trait loci (QTL) controlling callus induction and plant regeneration were identified in the VCS3M-DH population of Brassica rapa. Quantitative trait loci (QTL) controlling callus induction and plant regeneration were identified in the VCS3M-DH population of Brassica rapa. The VCS3M-DH population showed wide and continuous variation in callus induction and shoot regeneration. Significant coefficient correlations were detected between these two parameters. Broad-sense heritability (h (2)) for the two traits was around 0.7, indicating genetic regulation of regeneration ability in this population. In the composite interval mapping analysis, two QTLs for callus induction ability, qCi2 and qCi7, were mapped on chromosome A02 and A07, explaining 28.6 % of phenotypic variation. For plant regeneration, four QTLs, qPr6-1 qPr6-2, qPr7, and qPr9 were identified on chromosome A06, A07, and A09, which in total explained 50.1 % of phenotypic variation. Furthermore, 15 putative candidate genes were found on the interval of the six QTLs, which were related to various plant hormones, MADS-box genes, and serine/threonine related genes. These results provide important information to identify genes related to tissue culture ability in B. rapa.

The manipulation of auxin in the abscission zone cells of Arabidopsis flowers reveals that indoleacetic acid signaling is a prerequisite for organ shedding

A number of novel strategies were employed to examine the role of indoleacetic acid (IAA) in regulating floral organ abscission in Arabidopsis (Arabidopsis thaliana). Analysis of auxin influx facilitator expression in β-glucuronidase reporter plants revealed that AUXIN RESISTANT1, LIKE AUX1, and LAX3 were specifically up-regulated at the site of floral organ shedding. Flowers from mutants where individual family members were down-regulated exhibited a reduction in the force necessary to bring about petal separation; however, the effect was not additive in double or quadruple mutants. Using the promoter of a polygalacturonase (At2g41850), active primarily in cells undergoing separation, to drive expression of the bacterial genes iaaL and iaaM, we have shown that it is possible to manipulate auxin activity specifically within the floral organ abscission zone (AZ). Analysis of petal breakstrength reveals that if IAA AZ levels are reduced, shedding takes place prematurely, while if they are enhanced, organ loss is delayed. The At2g41850 promoter was also used to transactivate the gain-of-function AXR3-1 gene in order to disrupt auxin signaling specifically within the floral organ AZ cells. Flowers from transactivated lines failed to shed their sepals, petals, and anthers during pod expansion and maturity, and these organs frequently remained attached to the plant even after silique desiccation and dehiscence had taken place. These observations support a key role for IAA in the regulation of abscission in planta and reveal, to our knowledge for the first time, a requirement for a functional IAA signaling pathway in AZ cells for organ shedding to take place.

Galactoglucomannan oligosaccharides are assumed to affect tracheary element formation via interaction with auxin in Zinnia xylogenic cell culture

Galactoglucomannan oligosaccharides seem to interact with auxin in xylogenic cell culture, thus influencing mainly metaxylem-like tracheary element differentiation depending on timing with hormones and the process kinetics. Complex mapping of Zinnia mesophyll cell transdifferentiation into tracheary elements with or without prior cell division was documented after palisade and spongy parenchyma cell immobilization during the first 4 days of culture. Here, we report a positive effect of galactoglucomannan oligosaccharides on cell viability and density and higher metaxylem-like tracheary element formation in xylogenic cell culture. The maximal positive effect was achieved by the simultaneous addition of the oligosaccharides and growth hormones (auxin, cytokinin) to the cell culture medium. Moreover, a large number of metaxylem-like tracheary elements were observed in a low-auxin medium supplemented with oligosaccharides, but not in a low-cytokinin medium, suggesting a close relationship between auxin and the oligosaccharides during tracheary element formation.

The BIG gene is required for auxin-mediated organ growth in Arabidopsis

Control of organ size by cell expansion and cell proliferation is a fundamental process during development, but the importance of BIG in this process is still poorly understood. Here, we report the isolation and characterization of a new allele mutant of BIG in Arabidopsis: big-j588. The mutant displayed small aerial organs that were characterized by reduced cell size in the epidermis and short roots with decreased cell numbers. The big-j588 axr1 double and big-j588 arf7 arf19 triple mutants displayed more severe defects in leaf expansion and root elongation than their parents, implying BIG is involved in auxin-dependent organ growth. Genetic analysis suggests that BIG may act synergistically with PIN1 to affect leaf growth. The PIN1 protein level decreased in both the root cells and the tips of leaf pavement cell lobes of big-j588. Further analysis showed that the auxin maxima in the roots and the leaves of big-j588 decreased. Therefore, we concluded that the small leaves and the short roots of big-j588 were associated with reduction of auxin maxima. Overall, our study suggested that BIG is required for Arabidopsis organ growth via auxin action.

Cloning and expression analysis of novel Aux/IAA family genes in Gossypium hirsutum

Members of the Aux/IAA gene family encode proteins that mediate the responses of auxin-regulated gene expression and regulate various aspects of plant morphological development. Here, we provide the first identification and characterization of nine cDNAs encoding the complete open reading frame (ORF) of the Aux/IAA family in cotton. These were designated GhAux1 to GhAux9 (Gossypiumhirsutum Aux/IAA). The proteins encoded by these nine genes had either whole or partially conserved domains of the Aux/IAA superfamily, with sequence identity ranging from 14% to 69%. A pair of homeologs exists for each Aux/IAA in G. hirsutum acc. TM-1 with high identity both in ORF sequences and amino acid level. Tissue- and organ-specific analysis showed that transcripts of GhAux1, GhAux2, and GhAux3 were abundant in vegetative organs, whereas GhAux4, GhAux5, GhAux6, and GhAux7 were preferentially expressed in ovules on the day of anthesis. GhAux8 and GhIAA16 (previously reported) were also preferentially expressed during fiber developmental stages, especially GhAux8 in fiber early elongation stages, and GhIAA16 in fiber initiation and secondary cell wall thickening stage. GhAux9 was specifically expressed in developing fibers. During the fiber initiation stage, except for GhAux3 and GhAux6, the expression of the other eight GhAuxs in various lintless-fuzzless and linted-fuzzless mutants demonstrated that they were significantly up-regulated compared with linted-fuzzy TM-1.

OsTIR1 and OsAFB2 downregulation via OsmiR393 overexpression leads to more tillers, early flowering and less tolerance to salt and drought in rice

The microRNA miR393 has been shown to play a role in plant development and in the stress response by targeting mRNAs that code for the auxin receptors in Arabidopsis. In this study, we verified that two rice auxin receptor gene homologs (OsTIR1 and OsAFB2) could be targeted by OsmiR393 (Os for Oryza sativa). Two new phenotypes (increased tillers and early flowering) and two previously observed phenotypes (reduced tolerance to salt and drought and hyposensitivity to auxin) were observed in the OsmiR393-overexpressing rice plants. The OsmiR393-overexpressing rice demonstrated hyposensitivity to synthetic auxin-analog treatments. These data indicated that the phenotypes of OsmiR393-overexpressing rice may be caused through hyposensitivity to the auxin signal by reduced expression of two auxin receptor genes (OsTIR1 and OsAFB2). The expression of an auxin transporter (OsAUX1) and a tillering inhibitor (OsTB1) were downregulated by overexpression of OsmiR393, which suggested that a gene chain from OsmiR393 to rice tillering may be from OsTIR1 and OsAFB2 to OsAUX1, which affected the transportation of auxin, then to OsTB1, which finally controlled tillering. The positive phenotypes (increased tillers and early flowering) and negative phenotypes (reduced tolerance to salt and hyposensitivity to auxin) of OsmiR393-overexpressing rice present a dilemma for molecular breeding.

The auxin receptor homologue in Solanum lycopersicum stimulates tomato fruit set and leaf morphogenesis

TIR1 and its homologues act as auxin receptors and play a crucial role in auxin-mediated plant development. While the functions of auxin receptor genes have been widely studied in Arabidopsis thaliana, there has been no report on the consequences of TIR1 overexpression in plants that regulate fruit development. Here a putative tomato auxin receptor gene, homologous to Arabidopsis AtTIR1, is reported. This gene, designated as Solanum lycopersicum TIR1 (SlTIR1), was found to be expressed in all the parts of floral buds and flowers at anthesis stages. From bud to anthesis, SlTIR1 expression increases slightly in sepal tissue and decreases dramatically in stamen. From anthesis to post-anthesis when fruit set is expected to occur, the expression of SlTIR1 declines in the ovary and sepal. Overexpression of SlTIR1 results in a pleiotropic phenotype including parthenocarpic fruit formation and leaf morphology. Furthermore, SlTIR1 overexpression altered transcript levels of a number of auxin-responsive genes. The present data demonstrate that the tomato SlTIR1 gene plays an important role at the stages of flower-to-fruit transition and leaf formation.

Repression by an auxin/indole acetic acid protein connects auxin signaling with heat shock factor-mediated seed longevity

The plant hormone auxin regulates growth and development by modulating the stability of auxin/indole acetic acid (Aux/IAA) proteins, which in turn repress auxin response factors (ARFs) transcriptional regulators. In transient assays performed in immature sunflower embryos, we observed that the Aux/IAA protein HaIAA27 represses transcriptional activation by HaHSFA9, a heat shock transcription factor (HSF). We also found that HaIAA27 is stabilized in immature sunflower embryos, where we could show bimolecular fluorescence complementation interaction between native forms of HaIAA27 and HaHSFA9. An auxin-resistant form of HaIAA27 was overexpressed in transgenic tobacco seeds, leading to effects consistent with down-regulation of the ortholog HSFA9 gene, effects not seen with the native HaIAA27 form. Repression of HSFs by HaIAA27 is thus likely alleviated by auxin in maturing seeds. We show that HSFs such as HaHSFA9 are targets of Aux/IAA protein repression. Because HaHSFA9 controls a genetic program involved in seed longevity and embryonic desiccation tolerance, our findings would suggest a mechanism by which these processes can be auxin regulated. Aux/IAA-mediated repression involves transcription factors distinct from ARFs. This finding widens interpretation of auxin responses.

Microarray analysis of the abscission-related transcriptome in the tomato flower abscission zone in response to auxin depletion

The abscission process is initiated by changes in the auxin gradient across the abscission zone (AZ) and is triggered by ethylene. Although changes in gene expression have been correlated with the ethylene-mediated execution of abscission, there is almost no information on the molecular and biochemical basis of the increased AZ sensitivity to ethylene. We examined transcriptome changes in the tomato (Solanum lycopersicum 'Shiran 1335') flower AZ during the rapid acquisition of ethylene sensitivity following flower removal, which depletes the AZ from auxin, with or without preexposure to 1-methylcyclopropene or application of indole-3-acetic acid after flower removal. Microarray analysis using the Affymetrix Tomato GeneChip revealed changes in expression, occurring prior to and during pedicel abscission, of many genes with possible regulatory functions. They included a range of auxin- and ethylene-related transcription factors, other transcription factors and regulatory genes that are transiently induced early, 2 h after flower removal, and a set of novel AZ-specific genes. All gene expressions initiated by flower removal and leading to pedicel abscission were inhibited by indole-3-acetic acid application, while 1-methylcyclopropene pretreatment inhibited only the ethylene-induced expressions, including those induced by wound-associated ethylene signals. These results confirm our hypothesis that acquisition of ethylene sensitivity in the AZ is associated with altered expression of auxin-regulated genes resulting from auxin depletion. Our results shed light on the regulatory control of abscission at the molecular level and further expand our knowledge of auxin-ethylene cross talk during the initial controlling stages of the process.

The Arabidopsis cell cycle F-box protein SKP2A binds to auxin

Arabidopsis thaliana S-Phase Kinase-Associated Protein 2A (SKP2A) is an F-box protein that regulates the proteolysis of cell cycle transcription factors. The plant hormone auxin regulates multiple aspects of plant growth and development, including cell division. We found that auxin induces the ubiquitin-dependent degradation of SKP2A both in vivo and in vitro, suggesting that this hormone acts as a signal to trigger SKP2A proteolysis. In this article, we show that auxin binds directly and specifically to SKP2A. By TIR1-based superposition and docking analyzes, we identified an auxin binding site in SKP2A. Mutations in this binding site reduce the ability of SKP2A to bind to auxin and generate nondegradable SKP2A forms. In addition, these non-auxin binding proteins are unable to promote E2FC/DPB degradation in vivo or to induce cell division in the root meristem. Auxin binds to TIR1 to promote its interaction with the auxin/indole-3-acetic acid target proteins. Here, we show that auxin also enhanced the interaction between SKP2A and DPB. Finally, a mutation in SKP2A leads to auxin-resistant root growth, an effect that is additive with the tir1-1 phenotype. Thus, our data indicate that SKP2A is an auxin binding protein that connects auxin signaling with cell division.

Members of the LATERAL ORGAN BOUNDARIES DOMAIN transcription factor family are involved in the regulation of secondary growth in Populus

Regulation of secondary (woody) growth is of substantial economic and environmental interest but is poorly understood. We identified and subsequently characterized an activation-tagged poplar (Populus tremula × Populus alba) mutant with enhanced woody growth and changes in bark texture caused primarily by increased secondary phloem production. Molecular characterization of the mutation through positioning of the tag and retransformation experiments shows that the phenotype is conditioned by activation of an uncharacterized gene that encodes a novel member of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors. Homology analysis showed highest similarity to an uncharacterized LBD1 gene from Arabidopsis thaliana, and we consequently named it Populus tremula × Populus alba (Pta) LBD1. Dominant-negative suppression of Pta LBD1 via translational fusion with the repressor SRDX domain caused decreased diameter growth and suppressed and highly irregular phloem development. In wild-type plants, LBD1 was most highly expressed in the phloem and cambial zone. Two key Class I KNOTTED1-like homeobox genes that promote meristem identity in the cambium were downregulated, while an Altered Phloem Development gene that is known to promote phloem differentiation was upregulated in the mutant. A set of four LBD genes, including the LBD1 gene, was predominantly expressed in wood-forming tissues, suggesting a broader regulatory role of these transcription factors during secondary woody growth in poplar.

Auxin regulates distal stem cell differentiation in Arabidopsis roots

The stem cell niche in the root meristem is critical for the development of the plant root system. The plant hormone auxin acts as a versatile trigger in many developmental processes, including the regulation of root growth, but its role in the control of the stem cell activity remains largely unclear. Here we show that local auxin levels, determined by biosynthesis and intercellular transport, mediate maintenance or differentiation of distal stem cells in the Arabidopsis thaliana roots. Genetic analysis shows that auxin acts upstream of the major regulators of the stem cell activity, the homeodomain transcription factor WOX5, and the AP-2 transcription factor PLETHORA. Auxin signaling for differentiation of distal stem cells requires the transcriptional repressor IAA17/AXR3 as well as the ARF10 and ARF16 auxin response factors. ARF10 and ARF16 activities repress the WOX5 transcription and restrict it to the quiescent center, where WOX5, in turn, is needed for the activity of PLETHORA. Our investigations reveal that long-distance auxin signals act upstream of the short-range network of transcriptional factors to mediate the differentiation of distal stem cells in roots.