赤霉素作用机理的分子基础与调控模式研究进展

赤霉素(gibberellins或gibberellic acid, GA)作为植物生长的必需激素之一, 调控植物生长发育的各个方面, 如: 种子萌发, 下胚轴的伸长, 叶片的生长和植物开花时间等。近年来随着植物功能基因组学的进一步发展, 有关赤霉素生物合成及其调控, 赤霉素信号转导途径, 以及赤霉素与其他激素和环境因子的互作等领域的研究取得了较大的进展。本文综述了赤霉素生物合成的生物学途径及其调控研究; GA信号转导通道的研究进展, 特别是DELLA蛋白阻遏植物生长发育的分子机理和GA解除阻遏作用(derepress)的分子模型; GA受体研究的新进展; 探讨GA与其它激素之间的相互作用, 以及植物在应答环境过程中的作用。     

A gibberellin methyltransferase modulates the timing of floral transition at the Arabidopsis shoot meristem

The transition from vegetative to reproductive growth is a key event in the plant life cycle. Plants therefore use a variety of environmental and endogenous signals to determine the optimal time for flowering to ensure reproductive success. These signals are integrated at the shoot apical meristem (SAM), which subsequently undergoes a shift in identity and begins producing flowers rather than leaves, while still maintaining pluripotency and meristematic function. Gibberellic acid (GA), an important hormone associated with cell growth and differentiation, has been shown to promote flowering in many plant species including Arabidopsis thaliana, but the details of how spatial and temporal regulation of GAs in the SAM contribute to floral transition are poorly understood. In this study, we show that the gene GIBBERELLIC ACID METHYLTRANSFERASE 2 (GAMT2), which encodes a GA-inactivating enzyme, is significantly upregulated at the SAM during floral transition and contributes to the regulation of flowering time. Loss of GAMT2 function leads to early flowering, whereas transgenic misexpression of GAMT2 in specific regions around the SAM delays flowering. We also found that GAMT2 expression is independent of the key floral regulator LEAFY but is strongly increased by the application of exogenous GA. Our results indicate that GAMT2 is a repressor of flowering that may act as a buffer of GA levels at the SAM to help prevent premature flowering.     

Understanding gibberellic acid signaling--are we there yet?

The phytohormone gibberellic acid (GA) controls important aspects of plant growth such as seed germination, elongation growth, and flowering. The key components of the GA signaling pathway have been identified over the past 10 years. The current view is that GA binds to a soluble GID1 receptor, which interacts with the DELLA repressor proteins in a GA-dependent manner and thereby induces DELLA protein degradation via the E3 ubiquitin ligase SCF(GID2/SLY1). GA-dependent growth responses can generally be correlated with and be explained by changes in DELLA repressor abundance, where the DELLA repressor exerts a growth restraint that is relieved upon its degradation. However, it is obvious that other mechanisms must exist that control the activity of this pathway. This review discusses recent advances in the understanding of GA signaling, of its homeostasis, and of its cross-talk with other signaling pathways.     

Molecular basis and evolutionary pattern of GA–GID1–DELLA regulatory module

The tetracyclic diterpenoid carboxylic acids, gibberellins (GAs), orchestrate a broad spectrum of biological programs. In nature, GAs or GA-like substance is produced in bacteria, fungi, and plants. The function of GAs in microorganisms remains largely unknown. Phytohormones GAs mediate diverse growth and developmental processes through the life cycle of plants. The GA biosynthetic and metabolic pathways in bacteria, fungi, and plants are remarkably divergent. In vascular plants, phytohormone GA, receptor GID1, and repressor DELLA shape the GA–GID1–DELLA module in GA signaling cascade. Sequence reshuffling, functional divergence, and adaptive selection are main driving forces during the evolution of GA pathway components. The GA–GID1–DELLA complex interacts with second messengers and other plant hormones to integrate environmental and endogenous cues, which is beneficial to phytohormones homeostasis and other biological events. In this review, we first briefly describe GA metabolism pathway, signaling perception, and its second messengers. Then, we examine the evolution of GA pathway genes. Finally, we focus on reviewing the crosstalk between GA–GID1–DELLA module and phytohormones. Deciphering mechanisms underlying plant hormonal interactions are not only beneficial to addressing basic biological questions, but also have practical implications for developing crops with ideotypes to meet the future demand.     

Developmental transcriptome analysis of floral transition in <Emphasis Type="Italic">Rosa odorata</Emphasis> var. <Emphasis Type="Italic">gigantea</Emphasis>

Key message

Expression analyses revealed that floral transition of Rosa odorata var. gigantea is mainly regulated by VRN1, COLs, DELLA and KSN, with contributions by the effects of phytohormone and starch metabolism.

Abstract

Seasonal plants utilize changing environmental and developmental cues to control the transition from vegetative growth to flowering at the correct time of year. This study investigated global gene expression profiles at different developmental stages of Rosa odorata var. gigantea by RNA-sequencing, combined with phenotypic characterization and physiological changes. Gene ontology enrichment analysis of the differentially expressed genes (DEGs) between four different developmental stages (vegetative meristem, pre-floral meristem, floral meristem and secondary axillary buds) indicated that DNA methylation and the light reaction played a large role in inducing the rose floral transition. The expression of SUF and FLC, which are known to play a role in delaying flowering until vernalization, was down-regulated from the vegetative to the pre-floral meristem stage. In contrast, the expression of VRN1, which promotes flowering by repressing FLC expression, increased. The expression of DELLA proteins, which function as central nodes in hormone signaling pathways, and probably involve interactions between GA, auxin, and ABA to promote the floral transition, was well correlated with the expression of floral integrators, such as AGL24, COL4. We also identified DEGs associated with starch metabolism correlated with SOC1, AGL15, SPL3, AGL24, respectively. Taken together, our results suggest that vernalization and photoperiod are prominent cues to induce the rose floral transition, and that DELLA proteins also act as key regulators. The results summarized in the study on the floral transition of the seasonal rose lay a foundation for further functional demonstration, and have profound economic and ornamental values.

     

The effects of DELLAs on growth change with developmental stage and brassinosteroid levels

There are two stages in photomorphogenesis. First, seedlings detect light and open their cotyledons. Second, seedlings optimize their light environment by controlled elongation of the seedling stem or hypocotyl. In this study, we used time‐lapse imaging to investigate the relationship between the brassinosteroid (BR) and gibberellin (GA) hormones across both stages of photomorphogenesis. During the transition between one stage and the other, growth promotion by BRs and GAs switched from an additive to a synergistic relationship. Molecular genetic analysis revealed unexpected roles for known participants in the GA pathway during this period. Members of the DELLA family could either repress or enhance BR growth responses, depending on developmental stage. At the transition point for seedling growth dynamics, the BR and GA pathways had opposite effects on DELLA protein levels. In contrast to GA‐induced DELLA degradation, BR treatments increased the levels of REPRESSOR of ga1‐3 (RGA) and mimicked the molecular effects of stabilizing DELLAs. In addition, DELLAs showed complex regulation of genes involved in BR biosynthesis, implicating them in BR homeostasis. Growth promotion by GA alone depended on the PHYTOCHROME INTERACTING FACTOR (PIF) family of master growth regulators. The effects of BR, including the synergistic effects with GA, were largely independent of PIFs. These results point to a multi‐level, dynamic relationship between the BR and GA pathways.     

植物开花调控途径

开花是植物从营养生长转换为生殖生长的生理发育过程,受光周期、温度、激素、年龄等多个因素诱导,在植物生长和物种进化中处于核心地位。综合不断更新的开花分子遗传结果,将植物响应各种内源和外源信号启动开花的途径归纳为:经典的光周期途径、春化途径、自主途径、赤霉素途径和较新的年龄途径共5条。旨在描绘出这些不同途径间既独立又相互影响的复杂网络关系,为进一步探索和阐述更多植物的开花分子机理提供借鉴与参考。     

高等植物开花诱导途径信号整合的分子机制

开花是高等植物从营养生长到生殖生长的重要转折点。花分生组织的形成是开花植物对内外环境信号的响应。近年来在开花诱导方面已获得许多研究成果,我们介绍了高等植物开花诱导的4条主要途径（光周期途径、春化途径、自主途径和赤霉素途径）和复杂的信号整合机制。     

高等植物开花诱导研究进展

高等植物由营养生长向生殖生长转换的过程称为开花诱导。开花诱导过程由遗传和外界环境两个因素决定, 受错综复杂的网络信号传导途径调控。近年来, 在双子叶模式植物拟南芥中, 开花诱导研究取得了很大进展, 探明了控制开花诱导的4条主要途径(光周期途径、春化途径、自主途径和GA途径)及调控机制。研究也表明, 开花基因在拟南芥、水稻以及其他高等植物之间具有很高的保守性。文章对相关研究的最新进展作一综述, 并指出了目前研究中存在的问题及相应的研究对策。     

Interplay between cytochrome c and gibberellins during Arabidopsis vegetative development

We studied the effect of reducing the levels of the mitochondrial electron carrier cytochrome c (CYTc) in Arabidopsis thaliana. Plants with CYTc deficiency have delayed growth and development, and reach flowering several days later than the wild‐type but with the same number of leaves. CYTc‐deficient plants accumulate starch and glucose during the day, and contain lower levels of active gibberellins (GA) and higher levels of DELLA proteins, involved in GA signaling. GA treatment abolishes the developmental delay and reduces glucose accumulation in CYTc‐deficient plants, which also show a lower raise in ATP levels in response to glucose. Treatment of wild‐type plants with inhibitors of mitochondrial energy production limits plant growth and increases the levels of DELLA proteins, thus mimicking the effects of CYTc deficiency. In addition, an increase in the amount of CYTc decreases DELLA protein levels and expedites growth, and this depends on active GA synthesis. We conclude that CYTc levels impinge on the activity of the GA pathway, most likely through changes in mitochondrial energy production. In this way, hormone‐dependent growth would be coupled to the activity of components of the mitochondrial respiratory chain.     

Biochemical Insights on Degradation of Arabidopsis DELLA Proteins Gained From a Cell-Free Assay System

The phytohormone gibberellic acid (GA) regulates diverse aspects of plant growth and development. GA responses are triggered by the degradation of DELLA proteins, which function as repressors in GA signaling pathways. Recent studies in Arabidopsis thaliana and rice (Oryza sativa) have implied that the degradation of DELLA proteins occurred via the ubiquitin-proteasome system. Here, we developed an Arabidopsis cell-free system to recapitulate DELLA protein degradation in vitro. Using this cell-free system, we documented that Lys-29 of ubiquitin is the major site for ubiquitin chain formation to mediate DELLA protein degradation. We also confirmed the specific roles of GA receptors and multisubunit E3 ligase components in regulating DELLA protein degradation. In addition, blocking DELLA degradation with a PP1/PP2A phosphatase inhibitor in our cell-free assay suggested that degradation of DELLA proteins required protein Ser/Thr dephosphorylation activity. Furthermore, our data revealed that the LZ domain of Arabidopsis DELLA proteins is essential for both their stability and activity. Thus, our in vitro degradation system provides biochemical insights into the regulation of DELLA protein degradation. This in vitro assay system could be widely adapted for dissecting cellular signaling pathways in which regulated proteolysis is a key recurrent theme.     

Reduction of the geomagnetic field delays Arabidopsis thaliana flowering time through downregulation of flowering‐related genes

Variations in magnetic field (MF) intensity are known to induce plant morphological and gene expression changes. In Arabidopsis thaliana Col‐0, near‐null magnetic field (NNMF, i.e., <100 nT MF) causes a delay in the transition to flowering, but the expression of genes involved in this response has been poorly studied. Here, we showed a time‐course quantitative analysis of the expression of both leaf (including clock genes, photoperiod pathway, GA20ox, SVP, and vernalization pathway) and floral meristem (including GA2ox, SOC1, AGL24, LFY, AP1, FD, and FLC) genes involved in the transition to flowering in A. thaliana under NNMF. NNMF induced a delayed flowering time and a significant reduction of leaf area index and flowering stem length, with respect to controls under geomagnetic field. Generation experiments (F₁‐ and F₂‐NNMF) showed retention of flowering delay. The quantitative expression (qPCR) of some A. thaliana genes expressed in leaves and floral meristem was studied during transition to flowering. In leaves and flowering meristem, NNMF caused an early downregulation of clock, photoperiod, gibberellin, and vernalization pathways and a later downregulation of TSF, AP1, and FLC. In the floral meristem, the downregulation of AP1, AGL24, FT, and FLC in early phases of floral development was accompanied by a downregulation of the gibberellin pathway. The progressive upregulation of AGL24 and AP1 was also correlated to the delayed flowering by NNMF. The flowering delay is associated with the strong downregulation of FT, FLC, and GA20ox in the floral meristem and FT, TSF, FLC, and GA20ox in leaves. Bioelectromagnetics. 39:361–374, 2018. © 2018 The Authors. Bioelectromagnetics Published by Wiley Periodicals, Inc.     

Overexpression of RoDELLA impacts the height, branching, and flowering behaviour of Pelargonium?×?domesticum transgenic plants

Key message

We reported the cloning of a rose DELLA gene. We obtained transgenic Pelargonium lines overexpressing this gene which presented several phenotypes in plant growth, root growth, flowering time and number of inflorescences.

Abstract

Control of development is an important issue for production of ornamental plant. The plant growth regulator, gibberellins (GAs), plays a pivotal role in regulating plant growth and development. DELLA proteins are nuclear negative regulator of GA signalling. Our objective was to study the role of GA in the plant architecture and in the blooming of ornamentals. We cloned a rose DELLA homologous gene, RoDELLA, and studied its function by genetic transformation of pelargonium. Several transgenic pelargonium (Pelargonium?×?domesticum ‘Autum Haze’) lines were produced that ectopically expressed RoDELLA under the control of the 35S promoter. These transgenic plants exhibited a range of phenotypes which could be related to the reduction in GA response. Most of transgenic plants showed reduced growth associated to an increase of the node and branch number. Moreover, overexpression of RoDELLA blocked or delayed flowering in transgenic pelargonium and exhibited defects in the root formation. We demonstrated that pelargonium could be used to validate ornamental gene as the rose DELLA gene. RoDELLA overexpression modified many aspects of plant developmental pathways, as the plant growth, the transition of vegetative to floral stage and the ability of rooting.     

What is going on with the hormonal control of flowering in plants?

Molecular genetic studies using Arabidopsis thaliana as a model system have overwhelmingly revealed many important molecular mechanisms underlying the control of various biological events, including floral induction in plants. The major genetic pathways of flowering have been characterized in-depth, and include the photoperiod, vernalization, autonomous and gibberellin pathways. In recent years, novel flowering pathways are increasingly being identified. These include age, thermosensory, sugar, stress and hormonal signals to control floral transition. Among them, hormonal control of flowering except the gibberellin pathway is not formally considered a major flowering pathway per se, due to relatively weak and often pleiotropic genetic effects, complex phenotypic variations, including some controversial ones. However, a number of recent studies have suggested that various stress signals may be mediated by hormonal regulation of flowering. In view of molecular diversity in plant kingdoms, this review begins with an assessment of photoperiodic flowering, not in A. thaliana, but in rice (Oryza sativa); rice is a staple crop for human consumption worldwide, and is a model system of short-day plants, cereals and breeding crops. The rice flowering pathway is then compared with that of A. thaliana. This review then aims to update our knowledge on hormonal control of flowering, and integrate it into the entire flowering gene network.