The actin motor protein OsMYA1 associates with OsExo70H1 and contributes to rice secretory defense by modulating OsSyp121 distribution.

Magnaporthe oryzae (M. oryzae) is a devastating hemibiotrophic pathogen. Its biotrophic invasive hyphae (IH) are enclosed in the extrainvasive hyphal membrane produced by plant cells, thus generating a front line of the battlefield between the pathogen and the host plants. In plants, defense-related complexes such as proteins, callose-rich materials and vesicles, are directionally secreted to this interface to confer defense responses, but the underlying molecular mechanism is poorly understood. In this study, we found that a Myosin gene, Myosin A1 (OsMYA1), contributed to rice defense. The OsMYA1 knockout mutant exhibited decreased resistance to M. oryzae infection. OsMYA1 localizes to the actin cytoskeleton and surrounds the IH of M. oryzae. OsMYA1 interacts with an exocyst subunit, OsExo70H1, and regulates its accumulation at the plasma membrane (PM) and pathogen-plant interface. Furthermore, OsExo70H1 interacted with the rice syntaxin of the plants121 protein (OsSyp121), and the distribution of OsSyp121 to the PM or the pathogen-plant interface was disrupted in both the OsMYA1 and OsExo70H1 mutants. Overall, these results not only reveal a new function of OsMYA1 in rice blast resistance, but also uncover a molecular mechanism by which plants regulate defense against M. oryzae by OsMYA1-initiated vesicle secretory pathway, which originates from the actin cytoskeleton to the PM.

Correction: Magnaporthe oryzae fimbrin organizes actin networks in the hyphal tip during polar growth and pathogenesis.

[This corrects the article DOI: 10.1371/journal.ppat.1008437.].

Fimbrin associated with Pmk1 to regulate the actin assembly during Magnaporthe oryzae hyphal growth and infection.

The dynamic assembly of the actin cytoskeleton is vital for Magnaporthe oryzae development and host infection. The actin-related protein MoFim1 is a key factor for organizing the M. oryzae actin cytoskeleton. Currently, how MoFim1 is regulated in M. oryzae to precisely rearrange the actin cytoskeleton is unclear. In this study, we found that MoFim1 associates with the M. oryzae mitogen-activated protein (MAP) kinase Pmk1 to regulate actin assembly. MoFim1 directly interacted with Pmk1, and the phosphorylation level of MoFim1 was decreased in Δpmk1, which led to a change in the subcellular distribution of MoFim1 in the hyphae of Δpmk1. Moreover, the actin cytoskeleton was aberrantly organized at the hyphal tip in the Δpmk1, which was similar to what was observed in the Δmofim1 during hyphal growth. Furthermore, phosphorylation analysis revealed that Pmk1 could phosphorylate MoFim1 at serine 94. Loss of phosphorylation of MoFim1 at serine 94 decreased actin bundling activity. Additionally, the expression of the site mutant of MoFim1 S94D (in which serine 94 was replaced with aspartate to mimic phosphorylation) in Δpmk1 could reverse the defects in actin organization and hyphal growth in Δpmk1. It also partially rescues the formation of appressorium failure in Δpmk1. Taken together, these findings suggest a regulatory mechanism in which Pmk1 phosphorylates MoFim1 to regulate the assembly of the actin cytoskeleton during hyphal development and pathogenesis.

VILLIN2 regulates cotton defense against Verticillium dahliae by modulating actin cytoskeleton remodeling.

The active structural change of actin cytoskeleton is a general host response upon pathogen attack. This study characterized the function of the cotton (Gossypium hirsutum) actin-binding protein VILLIN2 (GhVLN2) in host defense against the soilborne fungus Verticillium dahliae. Biochemical analysis demonstrated that GhVLN2 possessed actin-binding, -bundling, and -severing activities. A low concentration of GhVLN2 could shift its activity from actin bundling to actin severing in the presence of Ca2+. Knockdown of GhVLN2 expression by virus-induced gene silencing reduced the extent of actin filament bundling and interfered with the growth of cotton plants, resulting in the formation of twisted organs and brittle stems with a decreased cellulose content of the cell wall. Upon V. dahliae infection, the expression of GhVLN2 was downregulated in root cells, and silencing of GhVLN2 enhanced the disease tolerance of cotton plants. The actin bundles were less abundant in root cells of GhVLN2-silenced plants than in control plants. However, upon infection by V. dahliae, the number of actin filaments and bundles in the cells of GhVLN2-silenced plants was raised to a comparable level as those in control plants, with the dynamic remodeling of the actin cytoskeleton appearing several hours in advance. GhVLN2-silenced plants exhibited a higher incidence of actin filament cleavage in the presence of Ca2+, suggesting that pathogen-responsive downregulation of GhVLN2 could activate its actin-severing activity. These data indicate that the regulated expression and functional shift of GhVLN2 contribute to modulating the dynamic remodeling of the actin cytoskeleton in host immune responses against V. dahliae.

Twinfilin regulates actin assembly and Hexagonal peroxisome 1 (Hex1) localization in the pathogenesis of rice blast fungus Magnaporthe oryzae.

Actin assembly at the hyphal tip is key for polar growth and pathogenesis of the rice blast fungus Magnaporthe oryzae. The mechanism of its precise assemblies and biological functions is not understood. Here, we characterized the role of M. oryzae Twinfilin (MoTwf) in M. oryzae infection through organizing the actin cables that connect to Spitzenkörper (Spk) at the hyphal tip. MoTwf could bind and bundle the actin filaments. It formed a complex with Myosin2 (MoMyo2) and the Woronin body protein Hexagonal peroxisome 1 (MoHex1). Enrichment of MoMyo2 and MoHex1 in the hyphal apical region was disrupted in a ΔMotwf loss-of-function mutant, which also showed a decrease in the number and width of actin cables. These findings indicate that MoTwf participates in the virulence of M. oryzae by organizing Spk-connected actin filaments and regulating MoHex1 distribution at the hyphal tip.

Magnaporthe oryzae fimbrin organizes actin networks in the hyphal tip during polar growth and pathogenesis.

Magnaporthe oryzae causes rice blast disease, but little is known about the dynamic restructuring of the actin cytoskeleton during its polarized tip growth and pathogenesis. Here, we used super-resolution live-cell imaging to investigate the dynamic organization of the actin cytoskeleton in M. oryzae during hyphal tip growth and pathogenesis. We observed a dense actin network at the apical region of the hyphae and actin filaments originating from the Spitzenkörper (Spk, the organizing center for hyphal growth and development) that formed branched actin bundles radiating to the cell membrane. The actin cross-linking protein Fimbrin (MoFim1) helps organize this actin distribution. MoFim1 localizes to the actin at the subapical collar, the actin bundles, and actin at the Spk. Knockout of MoFim1 resulted in impaired Spk maintenance and reduced actin bundle formation, preventing polar growth, vesicle transport, and the expansion of hyphae in plant cells. Finally, transgenic rice (Oryza sativa) expressing RNA hairpins targeting MoFim1 exhibited improved resistance to M. oryzae infection, indicating that MoFim1 represents an excellent candidate for M. oryzae control. These results reveal the dynamics of actin assembly in M. oryzae during hyphal tip development and pathogenesis, and they suggest a mechanism in which MoFim1 organizes such actin networks.

The Cotton Apoplastic Protein CRR1 Stabilizes Chitinase 28 to Facilitate Defense against the Fungal Pathogen Verticillium dahliae.

The apoplast serves as the first battlefield between the plant hosts and invading microbes; therefore, work on plant-pathogen interactions has increasingly focused on apoplastic immunity. In this study, we identified three proteins in the apoplast of cotton (Gossypium sp) root cells during interaction of the plant with the fungal pathogen Verticillium dahliae Among these proteins, cotton host cells secrete chitinase 28 (Chi28) and the Cys-rich repeat protein 1 (CRR1), while the pathogen releases the protease VdSSEP1. Biochemical analysis demonstrated that VdSSEP1 hydrolyzed Chi28, but CRR1 protected Chi28 from cleavage by Verticillium dahliae secretory Ser protease 1 (VdSSEP1). In accordance with the in vitro results, CRR1 interacted with Chi28 in yeast and plant cells and attenuated the observed decrease in Chi28 level that occurred in the apoplast of plant cells upon pathogen attack. Knockdown of CRR1 or Chi28 in cotton plants resulted in higher susceptibility to V. dahliae infection, and overexpression of CRR1 increased plant resistance to V dahliae, the fungus Botrytis cinerea, and the oomycete Phytophthora parasitica var nicotianae By contrast, knockout of VdSSEP1 in V. dahliae destroyed the pathogenicity of this fungus. Together, our results provide compelling evidence for a multilayered interplay of factors in cotton apoplastic immunity.

Automatic Radiographic Position Recognition from Image Frequency and Intensity.

Purpose: With the development of digital X-ray imaging and processing methods, the categorization and analysis of massive digital radiographic images need to be automatically finished. What is crucial in this processing is the automatic retrieval and recognition of radiographic position. To address these concerns, we developed an automatic method to identify a patient's position and body region using only frequency curve classification and gray matching. Methods: Our new method is combined with frequency analysis and gray image matching. The radiographic position was determined from frequency similarity and amplitude classification. The body region recognition was performed by image matching in the whole-body phantom image with prior knowledge of templates. The whole-body phantom image was stitched by radiological images of different parts. Results: The proposed method can automatically retrieve and recognize the radiographic position and body region using frequency and intensity information. It replaces 2D image retrieval with 1D frequency curve classification, with higher speed and accuracy up to 93.78%. Conclusion: The proposed method is able to outperform the digital X-ray image's position recognition with a limited time cost and a simple algorithm. The frequency information of radiography can make image classification quicker and more accurate.

Overexpression of GhFIM2 propels cotton fiber development by enhancing actin bundle formation.

Cell elongation and secondary wall deposition are two consecutive stages during cotton fiber development. The mechanisms controlling the progression of these two developmental phases remain largely unknown. Here, we report the functional characterization of the actin-bundling protein GhFIM2 in cotton fiber. Overexpression of GhFIM2 increased the abundance of actin bundles, which was accompanied with accelerated fiber growth at the fast-elongating stage. Meanwhile, overexpression of GhFIM2 could propel the onset of secondary cell wall biogenesis. These results indicate that the dynamic rearrangement of actin higher structures involving GhFIM2 plays an important role in the development of cotton fiber cells.

The Thioredoxin GbNRX1 Plays a Crucial Role in Homeostasis of Apoplastic Reactive Oxygen Species in Response to Verticillium dahliae Infection in Cotton.

Examining the proteins that plants secrete into the apoplast in response to pathogen attack provides crucial information for understanding the molecular mechanisms underlying plant innate immunity. In this study, we analyzed the changes in the root apoplast secretome of the Verticillium wilt-resistant island cotton cv Hai 7124 (Gossypium barbadense) upon infection with Verticillium dahliae Two-dimensional differential gel electrophoresis and matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis identified 68 significantly altered spots, corresponding to 49 different proteins. Gene ontology annotation indicated that most of these proteins function in reactive oxygen species (ROS) metabolism and defense response. Of the ROS-related proteins identified, we further characterized a thioredoxin, GbNRX1, which increased in abundance in response to V. dahliae challenge, finding that GbNRX1 functions in apoplastic ROS scavenging after the ROS burst that occurs upon recognition of V. dahliae Silencing of GbNRX1 resulted in defective dissipation of apoplastic ROS, which led to higher ROS accumulation in protoplasts. As a result, the GbNRX1-silenced plants showed reduced wilt resistance, indicating that the initial defense response in the root apoplast requires the antioxidant activity of GbNRX1. Together, our results demonstrate that apoplastic ROS generation and scavenging occur in tandem in response to pathogen attack; also, the rapid balancing of redox to maintain homeostasis after the ROS burst, which involves GbNRX1, is critical for the apoplastic immune response.

The cotton MYB108 forms a positive feedback regulation loop with CML11 and participates in the defense response against Verticillium dahliae infection.

Accumulating evidence indicates that plant MYB transcription factors participate in defense against pathogen attack, but their regulatory targets and related signaling processes remain largely unknown. Here, we identified a defense-related MYB gene (GhMYB108) from upland cotton (Gossypium hirsutum) and characterized its functional mechanism. Expression of GhMYB108 in cotton plants was induced by Verticillium dahliae infection and responded to the application of defense signaling molecules, including salicylic acid, jasmonic acid, and ethylene. Knockdown of GhMYB108 expression led to increased susceptibility of cotton plants to V. dahliae, while ecotopic overexpression of GhMYB108 in Arabidopsis thaliana conferred enhanced tolerance to the pathogen. Further analysis demonstrated that GhMYB108 interacted with the calmodulin-like protein GhCML11, and the two proteins form a positive feedback loop to enhance the transcription of GhCML11 in a calcium-dependent manner. Verticillium dahliae infection stimulated Ca(2+) influx into the cytosol in cotton root cells, but this response was disrupted in both GhCML11-silenced plants and GhMYB108-silenced plants in which expression of several calcium signaling-related genes was down-regulated. Taken together, these results indicate that GhMYB108 acts as a positive regulator in defense against V. dahliae infection by interacting with GhCML11. Furthermore, the data also revealed the important roles and synergetic regulation of MYB transcription factor, Ca(2+), and calmodulin in plant immune responses.

The RING finger protein NtRCP1 is involved in the floral transition in tobacco (Nicotiana tabacum).

The transition from the vegetative phase to the reproductive phase is a major developmental process in flowering plants. The underlying mechanism controlling this cellular process remains a research focus in the field of plant molecular biology. In the present work, we identified a gene encoding the C3H2C3-type RING finger protein NtRCP1 from tobacco BY-2 cells. Enzymatic analysis demonstrated that NtRCP1 is a functional E3 ubiquitin ligase. In tobacco plants, expression level of NtRCP1 was higher in the reproductive shoot apices than in the vegetative ones. NtRCP1-overexpressing plants underwent a more rapid transition from the vegetative to the reproductive phase and flowered markedly earlier than the wild-type control. Histological analysis revealed that the shoot apical meristem of NtRCP1-overexpressing plants initiated inflorescence primordia precociously compared to the wild-type plant due to accelerated cell division. Overexpression of NtRCP1 in BY-2 suspension cells promoted cell division, which was a consequence of the shortened G2 phase in the cell cycle. Together, our data suggest that NtRCP1 may act as a regulator of the phase transition, possibly through its role in cell cycle regulation, during vegetative/reproductive development in tobacco plant.

Cotton major latex protein 28 functions as a positive regulator of the ethylene responsive factor 6 in defense against Verticillium dahliae.

In this study, we identified a defense-related major latex protein (MLP) from upland cotton (designated GhMLP28) and investigated its functional mechanism. GhMLP28 transcripts were ubiquitously present in cotton plants, with higher accumulation in the root. Expression of the GhMLP28 gene was induced by Verticillium dahliae inoculation and was responsive to defense signaling molecules, including ethylene, jasmonic acid, and salicylic acid. Knockdown of GhMLP28 expression by virus-induced gene silencing resulted in increased susceptibility of cotton plants to V. dahliae infection, while ectopic overexpression of GhMLP28 in tobacco improved the disease tolerance of the transgenic plants. Further analysis revealed that GhMLP28 interacted with cotton ethylene response factor 6 (GhERF6) and facilitated the binding of GhERF6 to GCC-box element. Transient expression assay demonstrated that GhMLP28 enhanced the transcription factor activity of GhERF6, which led to the augmented expression of some GCC-box genes. GhMLP28 proteins were located in both the nucleus and cytoplasm and their nuclear distribution was dependent on the presence of GhERF6. Collectively, these results demonstrate that GhMLP28 acts as a positive regulator of GhERF6, and synergetic actions of the two proteins may contribute substantially to protection against V. dahliae infection in cotton plants.

The dual functions of WLIM1a in cell elongation and secondary wall formation in developing cotton fibers.

LIN-11, Isl1 and MEC-3 (LIM)-domain proteins play pivotal roles in a variety of cellular processes in animals, but plant LIM functions remain largely unexplored. Here, we demonstrate dual roles of the WLIM1a gene in fiber development in upland cotton (Gossypium hirsutum). WLIM1a is preferentially expressed during the elongation and secondary wall synthesis stages in developing fibers. Overexpression of WLIM1a in cotton led to significant changes in fiber length and secondary wall structure. Compared with the wild type, fibers of WLIM1a-overexpressing plants grew longer and formed a thinner and more compact secondary cell wall, which contributed to improved fiber strength and fineness. Functional studies demonstrated that (1) WLIM1a acts as an actin bundler to facilitate elongation of fiber cells and (2) WLIM1a also functions as a transcription factor to activate expression of Phe ammonia lyase-box genes involved in phenylpropanoid biosynthesis to build up the secondary cell wall. WLIM1a localizes in the cytosol and nucleus and moves into the nucleus in response to hydrogen peroxide. Taken together, these results demonstrate that WLIM1a has dual roles in cotton fiber development, elongation, and secondary wall formation. Moreover, our study shows that lignin/lignin-like phenolics may substantially affect cotton fiber quality; this finding may guide cotton breeding for improved fiber traits.

The cotton transcription factor TCP14 functions in auxin-mediated epidermal cell differentiation and elongation.

Plant-specific TEOSINTE-BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors play crucial roles in development, but their functional mechanisms remain largely unknown. Here, we characterized the cellular functions of the class I TCP transcription factor GhTCP14 from upland cotton (Gossypium hirsutum). GhTCP14 is expressed predominantly in fiber cells, especially at the initiation and elongation stages of development, and its expression increased in response to exogenous auxin. Induced heterologous overexpression of GhTCP14 in Arabidopsis (Arabidopsis thaliana) enhanced initiation and elongation of trichomes and root hairs. In addition, root gravitropism was severely affected, similar to mutant of the auxin efflux carrier PIN-FORMED2 (PIN2) gene. Examination of auxin distribution in GhTCP14-expressing Arabidopsis by observation of auxin-responsive reporters revealed substantial alterations in auxin distribution in sepal trichomes and root cortical regions. Consistent with these changes, expression of the auxin uptake carrier AUXIN1 (AUX1) was up-regulated and PIN2 expression was down-regulated in the GhTCP14-expressing plants. The association of GhTCP14 with auxin responses was also evidenced by the enhanced expression of auxin response gene IAA3, a gene in the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) family. Electrophoretic mobility shift assays showed that GhTCP14 bound the promoters of PIN2, IAA3, and AUX1, and transactivation assays indicated that GhTCP14 had transcription activation activity. Taken together, these results demonstrate that GhTCP14 is a dual-function transcription factor able to positively or negatively regulate expression of auxin response and transporter genes, thus potentially acting as a crucial regulator in auxin-mediated differentiation and elongation of cotton fiber cells.

Ectopic expression of a bacterium NhaD-type Na+/H+ antiporter leads to increased tolerance to combined salt/alkali stresses.

AaNhaD, a gene isolated from the soda lake alkaliphile Alkalimonas amylolytica, encodes a Na(+) /H(+) antiporter crucial for the bacterium's resistance to salt/alkali stresses. However, it remains unknown whether this type of bacterial gene may be able to increase the tolerance of flowering plants to salt/alkali stresses. To investigate the use of extremophile genetic resources in higher plants, transgenic tobacco BY-2 cells and plants harboring AaNhaD were generated and their stress tolerance was evaluated. Ectopic expression of AaNhaD enhanced the salt tolerance of the transgenic BY-2 cells in a pH-dependent manner. Compared to wild-type controls, the transgenic cells exhibited increased Na(+) concentrations and pH levels in the vacuoles. Subcellular localization analysis indicated that AaNhaD-GFP fusion proteins were primarily localized in the tonoplasts. Similar to the transgenic BY-2 cells, AaNhaD-overexpressing tobacco plants displayed enhanced stress tolerance when grown in saline-alkali soil. These results indicate that AaNhaD functions as a pH-dependent tonoplast Na(+) /H(+) antiporter in plant cells, thus presenting a new avenue for the genetic improvement of salinity/alkalinity tolerance.

The tobacco BLADE-ON-PETIOLE2 gene mediates differentiation of the corolla abscission zone by controlling longitudinal cell expansion.

The BLADE-ON-PETIOLE (BOP) genes of Arabidopsis (Arabidopsis thaliana) have been shown to play an essential role in floral abscission by specializing the abscission zone (AZ) anatomy. However, the molecular and cellular mechanisms that underlie differentiation of the AZ are largely unknown. In this study, we identified a tobacco (Nicotiana tabacum) homolog of BOP (designated NtBOP2) and characterized its cellular function. In tobacco plants, the NtBOP2 gene is predominantly expressed at the base of the corolla in an ethylene-independent manner. Both antisense suppression of NtBOP genes and overexpression of NtBOP2 in tobacco plants caused a failure in corolla shedding. Histological analysis revealed that the differentiation of the corolla AZ was blocked in the transgenic flowers. This blockage was due to uncontrolled cell elongation at the region corresponding to wild-type AZ. The role of NtBOP2 in regulating cell elongation was further demonstrated in Bright Yellow 2 single cells: perturbation of NtBOP2 function by a dominant negative strategy led to the formation of abnormally elongated cells. Subcellular localization analysis showed that NtBOP2-green fluorescent protein fusion proteins were targeted to both the nucleus and cytoplasm. Yeast two-hybrid, firefly luciferase complementation imaging, and in vitro pull-down assays demonstrated that NtBOP2 proteins interacted with TGA transcription factors. Taken together, these results indicated that NtBOP2 mediated the differentiation of AZ architecture by controlling longitudinal cell growth. Furthermore, NtBOP2 may achieve this outcome through interaction with the TGA transcription factors and via an ethylene-independent signaling pathway.

Heterologous expression of a chloroplast outer envelope protein from Suaeda salsa confers oxidative stress tolerance and induces chloroplast aggregation in transgenic Arabidopsis plants.

Suaeda salsa is a euhalophytic plant that is tolerant to coastal seawater salinity. In this study, we cloned a cDNA encoding an 8.4 kDa chloroplast outer envelope protein (designated as SsOEP8) from S. salsa and characterized its cellular function. Steady-state transcript levels of SsOEP8 in S. salsa were up-regulated in response to oxidative stress. Consistently, ectopic expression of SsOEP8 conferred enhanced oxidative stress tolerance in transgenic Bright Yellow 2 (BY-2) cells and Arabidopsis, in which H(2) O(2) content was reduced significantly in leaf cells. Further studies revealed that chloroplasts aggregated to the sides of mesophyll cells in transgenic Arabidopsis leaves, and this event was accompanied by inhibited expression of genes encoding proteins for chloroplast movements such as AtCHUP1, a protein involved in actin-based chloroplast positioning and movement. Moreover, organization of actin cytoskeleton was found to be altered in transgenic BY-2 cells. Together, these results suggest that SsOEP8 may play a critical role in oxidative stress tolerance by changing actin cytoskeleton-dependent chloroplast distribution, which may consequently lead to the suppressed production of reactive oxygen species (ROS) in chloroplasts. One significantly novel aspect of this study is the finding that the small chloroplast envelope protein is involved in oxidative stress tolerance.

Overexpression of a profilin (GhPFN2) promotes the progression of developmental phases in cotton fibers.

Cotton fiber development at the stages of elongation and secondary wall synthesis determines the traits of fiber length and strength. To date, the mechanisms controlling the progression of these two phases remain elusive. In this work, the function of a fiber-preferential actin-binding protein (GhPFN2) was characterized by cytological and molecular studies on the fibers of transgenic green-colored cotton (Gossypium hirsutum) through three successive generations. Overexpression of GhPFN2 caused pre-terminated cell elongation, resulting in a marked decrease in the length of mature fibers. Cytoskeleton staining and quantitative assay revealed that thicker and more abundant F-actin bundles formed during the elongation stage in GhPFN2-overexpressing fibers. Accompanying this alteration, the developmental reorientation of transverse microtubules to the oblique direction was advanced by 2 d at the period of transition from elongation to secondary wall deposition. Birefringence and reverse transcription-PCR analyses showed that earlier onset of secondary wall synthesis occurred in parallel. These data demonstrate that formation of the higher actin structure plays a determinant role in the progression of developmental phases in cotton fibers, and that GhPFN2 acts as a critical modulator in this process. Such a function of the actin cytoskeleton in cell phase conversion may be common to other secondary wall-containing plant cells.

Proteomic identification of differentially expressed proteins in the Ligon lintless mutant of upland cotton (Gossypium hirsutum L.).

Cotton fiber is an ideal model for studying plant cell elongation. To date, the underlying mechanisms controlling fiber elongation remain unclear due to their high complexity. In this study, a comparative proteomic analysis between a short-lint fiber mutant (Ligon lintless, Li(1)) and its wild-type was performed to identify fiber elongation-related proteins. By 2-DE combined with local EST database-assisted MS/MS analysis, 81 differentially expressed proteins assigned to different functional categories were identified from Li(1) fibers, of which 54 were down-regulated and 27 were up-regulated. Several novel aspects regarding cotton fiber elongation can be illustrated from our data. First, over half of the down-regulated proteins were newly identified at the protein level, which is mainly involved in protein folding and stabilization, nucleocytoplasmic transport, signal transduction, and vesicular-mediated transport. Second, a number of cytoskeleton-related proteins showed a remarkable decrease in protein abundance in the Li(1) fibers. Accordingly, the architecture of actin cytoskeleton was severely deformed and the microtubule organization was moderately altered, accompanied with dramatic disruption of vesicle trafficking. Third, the expression of several proteins involved in unfolded protein response (UPR) was activated in Li(1) fibers, indicating that the deficiency of fiber cell elongation was related to ER stress. Collectively, these findings significantly advanced our understanding of the mechanisms associated with cotton fiber elongation.