ARF5/MONOPTEROS(MP)调控作用研究进展

生长素响应因子(auxin response factors,ARFs)是生长素响应机制中的重要元件,其中,ARF5/MONOPTEROS(MP)参与调控许多生长发育过程。该文对近年来国内外有关ARF5/MP的研究进展,以及由ARF5/MP介导的生长素应答通路在拟南芥的胚根原特化、维管组织发育、茎尖发育等过程中的作用以及鉴于ARFs成员之间在结构和功能上的保守性等研究进展进行综述,为阐明植物体对生长素响应的分子机理提供参考。     

Arabidopsis AXR6 encodes CUL1 implicating SCF E3 ligases in auxin regulation of embryogenesis

The AXR6 gene is required for auxin signaling in the Arabidopsis embryo and during postembryonic development. One of the effects of auxin is to stimulate degradation of the Aux/IAA auxin response proteins through the action of the ubiquitin protein ligase SCF(TIR1). Here we show that AXR6 encodes the SCF subunit CUL1. The axr6 mutations affect the ability of mutant CUL1 to assemble into stable SCF complexes resulting in reduced degradation of the SCF(TIR1) substrate AXR2/IAA7. In addition, we show that CUL1 is required for lateral organ initiation in the shoot apical meristem and the inflorescence meristem. These results indicate that the embryonic axr6 phenotype is related to a defect in SCF function and accumulation of Aux/IAA proteins such as BDL/IAA12. In addition, we show that CUL1 has a role in auxin response throughout the life cycle of the plant.     

植物生长素反应因子研究进展

生长素反应因子（ARFs）是植物生长和发育的重要调节因子,在生长素早期反应蛋白（Aux／IAAs）的参与下,通过和生长素反应基因启动子区AuxRE元件的JTGTCTC序列结合,共同调控这些基因的表达。近年来关于生长素反应因子的分子结构和ARF与Aux／IAA的相互作用及其对植物生长和发育的影响、作用的靶基因以及分子机制受到人们的重视,并在这些方面做了大量的研究。     

Auxin triggers transient local signaling for cell specification in Arabidopsis embryogenesis

The Arabidopsis embryonic root meristem is initiated by the specification of a single cell, the hypophysis. This event critically requires the antagonistic auxin response regulators MONOPTEROS and BODENLOS, but their mechanism of action is unknown. We show that these proteins interact and transiently act in a small subdomain of the proembryo adjacent to the future hypophysis. Here they promote transport of auxin, which then elicits a second response in the hypophysis itself. Our results suggest that hypophysis specification is not the direct result of a primary auxin response but rather depends on cell-to-cell signaling triggered by auxin in adjacent cells.     

Tissue-specific expression of stabilized SOLITARY-ROOT/IAA14 alters lateral root development in Arabidopsis

Auxin is important for lateral root (LR) initiation and subsequent LR primordium development. However, the roles of tissue-specific auxin signaling in these processes are poorly understood. We analyzed transgenic Arabidopsis plants expressing the stabilized mutant INDOLE-3 ACETIC ACID 14 (IAA14)/SOLITARY-ROOT (mIAA14) protein as a repressor of the auxin response factors (ARFs), under the control of tissue-specific promoters. We showed that plants expressing the mIAA14-glucocorticoid receptor (GR) fusion protein under the control of the native IAA14 promoter had the solitary-root/iaa14 mutant phenotypes, including the lack of LR formation under dexamethasone (Dex) treatment, indicating that mIAA14-GR is functional in the presence of Dex. We then demonstrated that expression of mIAA14-GR under the control of the stele-specific SHORT-ROOT promoter suppressed LR formation, and showed that mIAA14-GR expression in the protoxylem-adjacent pericycle also blocked LR formation, indicating that the normal auxin response mediated by auxin/indole-3 acetic acid (Aux/IAA) signaling in the protoxylem pericycle is necessary for LR formation. In addition, we demonstrated that expression of mIAA14-GR under either the ARF7 or the ARF19 promoter also suppressed LR formation as in the arf7 arf19 double mutants, and that IAA14 interacted with ARF7 and ARF19 in yeasts. These results strongly suggest that mIAA14-GR directly inactivates ARF7/ARF19 functions, thereby blocking LR formation. Post-embryonic expression of mIAA14-GR under the SCARECROW promoter, which is expressed in the specific cell lineage during LR primordium formation, caused disorganized LR development. This indicates that normal auxin signaling in LR primordia, which involves the unknown ARFs and Aux/IAAs, is necessary for the establishment of LR primordium organization. Thus, our data show that tissue-specific expression of a stabilized Aux/IAA protein allows analysis of tissue-specific auxin responses in LR development by inactivating ARF functions.     

Multiple AUX/IAA�CARF modules regulate lateral root formation: the role of Arabidopsis SHY2/IAA3-mediated auxin signalling

In Arabidopsis thaliana, lateral root (LR) formation is regulated by multiple auxin/indole-3-acetic acid (Aux/IAA)–AUXIN RESPONSE FACTOR (ARF) modules: (i) the IAA28–ARFs module regulates LR founder cell specification; (ii) the SOLITARY-ROOT (SLR)/IAA14–ARF7–ARF19 module regulates nuclear migration and asymmetric cell divisions of the LR founder cells for LR initiation; and (iii) the BODENLOS/IAA12–MONOPTEROS/ARF5 module also regulates LR initiation and organogenesis. The number of Aux/IAA–ARF modules involved in LR formation remains unknown. In this study, we isolated the shy2-101 mutant, a gain-of-function allele of short hypocotyl2/suppressor of hy2 (shy2)/iaa3 in the Columbia accession. We demonstrated that the shy2-101 mutation not only strongly inhibits LR primordium development and emergence but also significantly increases the number of LR initiation sites with the activation of LATERAL ORGAN BOUNDARIES-DOMAIN16/ASYMMETRIC LEAVES2-LIKE18, a target gene of the SLR/IAA14–ARF7–ARF19 module. Genetic analysis revealed that enhanced LR initiation in shy2-101 depended on the SLR/IAA14–ARF7–ARF19 module. We also showed that the shy2 roots contain higher levels of endogenous IAA. These observations indicate that the SHY2/IAA3–ARF-signalling module regulates not only LR primordium development and emergence after SLR/IAA14–ARF7–ARF19 module-dependent LR initiation but also inhibits LR initiation by affecting auxin homeostasis, suggesting that multiple Aux/IAA–ARF modules cooperatively regulate the developmental steps during LR formation.     

Control of root cap formation by MicroRNA-targeted auxin response factors in Arabidopsis

The plant root cap mediates the direction of root tip growth and protects internal cells. Root cap cells are continuously produced from distal stem cells, and the phytohormone auxin provides position information for root distal organization. Here, we identify the Arabidopsis thaliana auxin response factors ARF10 and ARF16, targeted by microRNA160 (miR160), as the controller of root cap cell formation. The Pro(35S):MIR160 plants, in which the expression of ARF10 and ARF16 is repressed, and the arf10-2 arf16-2 double mutants display the same root tip defect, with uncontrolled cell division and blocked cell differentiation in the root distal region and show a tumor-like root apex and loss of gravity-sensing. ARF10 and ARF16 play a role in restricting stem cell niche and promoting columella cell differentiation; although functionally redundant, the two ARFs are indispensable for root cap development, and the auxin signal cannot bypass them to initiate columella cell production. In root, auxin and miR160 regulate the expression of ARF10 and ARF16 genes independently, generating a pattern consistent with root cap development. We further demonstrate that miR160-uncoupled production of ARF16 exerts pleiotropic effects on plant phenotypes, and miR160 plays an essential role in regulating Arabidopsis development and growth.