非TIR1受体依赖型激活生长素信号的新机制

依赖于受体TIR1以及下游Aux/IAAs-ARFs介导的信号通路是目前研究最为深入的生长素信号转导途径。徐通达课题组最新研究发现, 高浓度生长素能够诱导质膜定位的TMK1激酶发生剪切, 导致其羧基(C-)端部分转入细胞核并磷酸化修饰细胞核内的非经典IAA32/34, 后者通过与生长素响应转录因子ARFs互作, 调控下游基因表达, 从而解析了生长素通过TMK1-IAA32/34-ARFs通路调控植物顶端弯钩内外侧差异性生长的分子机制。该研究发现了一条新的生长素TMK1- IAA32/34-ARFs信号途径, 此信号通路独立于经典生长素受体TIR1介导的生长素信号转导通路。     

非TIR1受体依赖型激活生长素信号的新机制

依赖于受体TIR1以及下游Aux/IAAs-ARFs介导的信号通路是目前研究最为深入的生长素信号转导途径。徐通达课题组最新研究发现, 高浓度生长素能够诱导质膜定位的TMK1激酶发生剪切, 导致其羧基(C-)端部分转入细胞核并磷酸化修饰细胞核内的非经典IAA32/34, 后者通过与生长素响应转录因子ARFs互作, 调控下游基因表达, 从而解析了生长素通过TMK1-IAA32/34-ARFs通路调控植物顶端弯钩内外侧差异性生长的分子机制。该研究发现了一条新的生长素TMK1- IAA32/34-ARFs信号途径, 此信号通路独立于经典生长素受体TIR1介导的生长素信号转导通路。     

SMALL ACIDIC PROTEIN1 Acts with RUB Modification Components, the COP9 Signalosome, and AXR1 to Regulate Growth and Development of Arabidopsis

Previously, a dysfunction of the SMALL ACIDIC PROTEIN1 (SMAP1) gene was identified as the cause of the anti-auxin resistant1 (aar1) mutant of Arabidopsis (Arabidopsis thaliana). SMAP1 is involved in the response pathway of synthetic auxin, 2,4-dichlorophenoxyacetic acid, and functions upstream of the auxin/indole-3-acetic acid protein degradation step in auxin signaling. However, the exact mechanism by which SMAP1 functions in auxin signaling remains unknown. Here, we demonstrate that SMAP1 is required for normal plant growth and development and the root response to indole-3-acetic acid or methyl jasmonate in the auxin resistant1 (axr1) mutation background. Deletion analysis and green fluorescent protein/glutathione S-transferase pull-down assays showed that SMAP1 physically interacts with the CONSTITUTIVE PHOTOMORPHOGENIC9 SIGNALOSOME (CSN) via the SMAP1 F/D region. The extremely dwarf phenotype of the aar1-1 csn5a-1 double mutant confirms the functional role of SMAP1 in plant growth and development under limiting CSN functionality. Our findings suggest that SMAP1 is involved in the auxin response and possibly in other cullin-RING ubiquitin ligase-regulated signaling processes via its interaction with components associated with RELATED TO UBIQUITIN modification.     

The nuclear pore and plant development

All macromolecules that traffic between the nucleus and the cytoplasm traverse the nuclear pore. While yeast and mammalian nuclear pore structure and function have recently been substantially refined, our understanding of the plant nuclear pore is still far from comprehensive. Nevertheless, a number of nuclear pore and nucleocytoplasmic trafficking components have recently been identified as required for diverse developmental and signaling pathways. In addition, some aspects of the nuclear pore composition itself now appear under developmental control and nuclear pore components have recently surfaced as novel players in plant cytokinesis. Here, we review these new findings in context and attempt to correlate molecular functions with developmental processes.     

Mutations in an auxin receptor homolog AFB5 and in SGT1b confer resistance to synthetic picolinate auxins and not to 2,4-dichlorophenoxyacetic acid or indole-3-acetic acid in Arabidopsis

Although a wide range of structurally diverse small molecules can act as auxins, it is unclear whether all of these compounds act via the same mechanisms that have been characterized for 2,4-dichlorophenoxyacetic acid (2,4-D) and indole-3-acetic acid (IAA). To address this question, we used a novel member of the picolinate class of synthetic auxins that is structurally distinct from 2,4-D to screen for Arabidopsis (Arabidopsis thaliana) mutants that show chemically selective auxin resistance. We identified seven alleles at two distinct genetic loci that conferred significant resistance to picolinate auxins such as picloram, yet had minimal cross-resistance to 2,4-D or IAA. Double mutants had the same level and selectivity of resistance as single mutants. The sites of the mutations were identified by positional mapping as At4g11260 and At5g49980. At5g49980 is previously uncharacterized and encodes auxin signaling F-box protein 5, one of five homologs of TIR1 in the Arabidopsis genome. TIR1 is the recognition component of the Skp1-cullin-F-box complex associated with the ubiquitin-proteasome pathway involved in auxin signaling and has recently been shown to be a receptor for IAA and 2,4-D. At4g11260 encodes the tetratricopeptide protein SGT1b that has also been associated with Skp1-cullin-F-box-mediated ubiquitination in auxin signaling and other pathways. Complementation of mutant lines with their corresponding wild-type genes restored picolinate auxin sensitivity. These results show that chemical specificity in auxin signaling can be conferred by upstream components of the auxin response pathway. They also demonstrate the utility of genetic screens using structurally diverse chemistries to uncover novel pathway components.     

Interdependency of Brassinosteroid and Auxin Signaling in Arabidopsis

How growth regulators provoke context-specific signals is a fundamental question in developmental biology. In plants, both auxin and brassinosteroids (BRs) promote cell expansion, and it was thought that they activated this process through independent mechanisms. In this work, we describe a shared auxin:BR pathway required for seedling growth. Genetic, physiological, and genomic analyses demonstrate that response from one pathway requires the function of the other, and that this interdependence does not act at the level of hormone biosynthetic control. Increased auxin levels saturate the BR-stimulated growth response and greatly reduce BR effects on gene expression. Integration of these two pathways is downstream from BES1 and Aux/IAA proteins, the last known regulatory factors acting downstream of each hormone, and is likely to occur directly on the promoters of auxin:BR target genes. We have developed a new approach to identify potential regulatory elements acting in each hormone pathway, as well as in the shared auxin:BR pathway. We show that one element highly overrepresented in the promoters of auxin- and BR-induced genes is responsive to both hormones and requires BR biosynthesis for normal expression. This work fundamentally alters our view of BR and auxin signaling and describes a powerful new approach to identify regulatory elements required for response to specific stimuli.     

Interdependency of brassinosteroid and auxin signaling in Arabidopsis

How growth regulators provoke context-specific signals is a fundamental question in developmental biology. In plants, both auxin and brassinosteroids (BRs) promote cell expansion, and it was thought that they activated this process through independent mechanisms. In this work, we describe a shared auxin:BR pathway required for seedling growth. Genetic, physiological, and genomic analyses demonstrate that response from one pathway requires the function of the other, and that this interdependence does not act at the level of hormone biosynthetic control. Increased auxin levels saturate the BR-stimulated growth response and greatly reduce BR effects on gene expression. Integration of these two pathways is downstream from BES1 and Aux/IAA proteins, the last known regulatory factors acting downstream of each hormone, and is likely to occur directly on the promoters of auxin:BR target genes. We have developed a new approach to identify potential regulatory elements acting in each hormone pathway, as well as in the shared auxin:BR pathway. We show that one element highly overrepresented in the promoters of auxin- and BR-induced genes is responsive to both hormones and requires BR biosynthesis for normal expression. This work fundamentally alters our view of BR and auxin signaling and describes a powerful new approach to identify regulatory elements required for response to specific stimuli.     

GADD45 proteins: central players in tumorigenesis

The Growth Arrest and DNA Damage-inducible 45 (GADD45) proteins have been implicated in regulation of many cellular functions including DNA repair, cell cycle control, senescence and genotoxic stress. However, the pro-apoptotic activities have also positioned GADD45 as an essential player in oncogenesis. Emerging functional evidence implies that GADD45 proteins serve as tumor suppressors in response to diverse stimuli, connecting multiple cell signaling modules. Defects in the GADD45 pathway can be related to the initiation and progression of malignancies. Moreover, induction of GADD45 expression is an essential step for mediating anti-cancer activity of multiple chemotherapeutic drugs and the absence of GADD45 might abrogate their effects in cancer cells. In this review, we present a comprehensive discussion of the functions of GADD45 proteins, linking their regulation to effectors of cell cycle arrest, DNA repair and apoptosis. The ramifications regarding their roles as essential and central players in tumor growth suppression are also examined. We also extensively review recent literature to clarify how different chemotherapeutic drugs induce GADD45 gene expression and how its up-regulation and interaction with different molecular partners may benefit cancer chemotherapy and facilitate novel drug discovery.     

Function of the ubiquitin-proteasome pathway in auxin response

The plant hormone auxin regulates many aspects of growth and development. Despite the importance of this hormone, the molecular basis for auxin action has remained elusive. Recent advances using molecular genetics in Arabidopsis have begun to elucidate the mechanisms involved in auxin signaling. These results suggest that protein degradation by the ubiquitin pathway has a central role in auxin response.     

The novel plant homeodomain protein rhinoceros antagonizes Ras signaling in the Drosophila eye

The sequential specification of cell fates in the Drosophila eye requires repeated activation of the epidermal growth factor receptor (EGFR)/Ras/MAP kinase (MAPK) pathway. Equally important are the multiple layers of inhibitory regulation that prevent excessive or inappropriate signaling. Here we describe the molecular and genetic analysis of a previously uncharacterized gene, rhinoceros (rno), that we propose functions to restrict EGFR signaling in the eye. Loss of rno results in the overproduction of photoreceptors, cone cells, and pigment cells and a corresponding reduction in programmed cell death, all phenotypes characteristic of hyperactivated EGFR signaling. Genetic interactions between rno and multiple EGFR pathway components support this hypothesis. rno encodes a novel but evolutionarily conserved nuclear protein with a PHD zinc-finger domain, a motif commonly found in chromatin-remodeling factors. Future analyses of rno will help to elucidate the regulatory strategies that modulate EGFR signaling in the fly eye.     

Mitogen-activated protein kinase cascades in plant signaling

Mitogen-activated protein kinase (MAPK) cascades are key signaling modules downstream of receptors/sensors that perceive either endogenously produced stimuli such as peptide ligands and damage-associated molecular patterns (DAMPs) or exogenously originated stimuli such as pathogen/microbe-associated molecular patterns (P/MAMPs), pathogen-derived effectors, and environmental factors. In this review, we provide a historic view of plant MAPK research and summarize recent advances in the establishment of MAPK cascades as essential components in plant immunity, response to environmental stresses, and normal growth and development. Each tier of the MAPK cascades is encoded by a small gene family, and multiple members can function redundantly in an MAPK cascade. Yet, they carry out a diverse array of biological functions in plants. How the signaling specificity is achieved has become an interesting topic of MAPK research. Future investigations into the molecular mechanism(s) underlying the regulation of MAPK activation including the activation kinetics and magnitude in response to a stimulus, the spatiotemporal expression patterns of all the components in the signaling pathway, and functional characterization of novel MAPK substrates are central to our understanding of MAPK functions and signaling specificity in plants.     

植物的环境信号分子茉莉酸及其生物学功能

茉莉酸信号分子参与植物生长发育众多生理过程的调控,尤其是作为环境信号分子能有效地介导植物对生物及非生物胁迫的防御反应。迄今已知具有信号分子生理功能的至少包括茉莉酸(jasmonic acid,JA)以及茉莉酸甲酯(methyl jasmonate,Me JA)和茉莉酸-异亮氨酸复合物(jasmonoyl-isoleucine,JA-Ile)等茉莉酸衍生物,统称为茉莉酸类化合物(jasmonates,JAs)。从环境信号分子角度介绍了茉莉酸信号的启动(环境信号感知与转导、茉莉酸类化合物合成)、传递(局部传递、维管束传输、空气传播)和生物学功能(茉莉酸信号受体、调控的转录因子、参与的生物学过程)。     

Auxin transport - shaping the plant

Plant growth is marked by its adaptability to continuous changes in environment. A regulated, differential distribution of auxin underlies many adaptation processes including organogenesis, meristem patterning and tropisms. In executing its multiple roles, auxin displays some characteristics of both a hormone and a morphogen. Studies on auxin transport, as well as tracing the intracellular movement of its molecular components, have suggested a possible scenario to explain how growth plasticity is conferred at the cellular and molecular level. The plant perceives stimuli and changes the subcellular position of auxin-transport components accordingly. These changes modulate auxin fluxes, and the newly established auxin distribution triggers the corresponding developmental response.     

A synthetic approach reveals extensive tunability of auxin signaling

Explaining how the small molecule auxin triggers diverse yet specific responses is a long-standing challenge in plant biology. An essential step in auxin response is the degradation of Auxin/Indole-3-Acetic Acid (Aux/IAA, referred to hereafter as IAA) repressor proteins through interaction with auxin receptors. To systematically characterize diversity in degradation behaviors among IAA|receptor pairs, we engineered auxin-induced degradation of plant IAA proteins in yeast (Saccharomyces cerevisiae). We found that IAA degradation dynamics vary widely, depending on which receptor is present, and are not encoded solely by the degron-containing domain II. To facilitate this and future studies, we identified a mathematical model able to quantitatively describe IAA degradation behavior in a single parameter. Together, our results demonstrate the remarkable tunability conferred by specific configurations of the auxin response pathway.     

ATGs: Scaffolds for MAPK/ERK signaling

Autophagy maintains cellular homeostasis by sequestering unwanted material within autophagosomes and transferring these to lysosomes for degradation. Several signaling cascades activate or suppress autophagy in response to diverse environmental cues. However, whether autophagic structures per se regulate cell signaling was not known. The MAPK/ERK (mitogen-activated protein kinase) pathway controls several functions in the cell, and studies have identified the importance of scaffold proteins in modulating MAPK signaling through the spatial coordination of the RAF1-MAP2K/MEK-MAPK cascade. Growth factors increase the nuclear localization and activity of MAPK, and since the nucleus has been reported to contain LC3, an autophagy-related protein, we asked whether autophagic structures could serve as cytosolic and nuclear scaffolds for growth factor-induced MAPK phosphorylation.     

ATGs

Autophagy maintains cellular homeostasis by sequestering unwanted material within autophagosomes and transferring these to lysosomes for degradation. Several signaling cascades activate or suppress autophagy in response to diverse environmental cues. However, whether autophagic structures per se regulate cell signaling was not known. The MAPK/ERK (mitogen-activated protein kinase) pathway controls several functions in the cell, and studies have identified the importance of scaffold proteins in modulating MAPK signaling through the spatial coordination of the RAF1-MAP2K/MEK-MAPK cascade. Growth factors increase the nuclear localization and activity of MAPK, and since the nucleus has been reported to contain LC3, an autophagy-related protein, we asked whether autophagic structures could serve as cytosolic and nuclear scaffolds for growth factor-induced MAPK phosphorylation.