Biology subjects

Ljung, K.

Publications and source records attributed to Ljung, K..

29 records · Page 2Linked to original sources

bioRxiv2021

Redefining the roles of UDP-glycosyltransferases in auxin metabolism and homeostasis during plant development

The levels of the important plant growth regulator indole-3-acetic acid (IAA) are tightly controlled within plant tissues to spatiotemporally orchestrate concentration gradients that drive plant growth and development. Metabolic inactivation of bioactive IAA is known to participate in the modulation of IAA maxima and minima. IAA can be irreversibly inactivated by oxidation and conjugation to Aspartate and Glutamate. Usually overlooked because its reversible nature, the most abundant inactive IAA form is the IAA-glucose (IAA-glc) conjugate. Glycosylation of IAA is reported to be carried out by the UDP-glycosyltransferase 84B1 (UGT84B1), while UGT74D1 has been implicated in the glycosylation of the irreversibly formed IAA catabolite oxIAA. Here we demonstrate that both UGT84B1 and UGT74D1 modulate IAA levels throughout plant development by dual IAA and oxIAA glycosylation. Moreover, we identify a novel UGT subfamily whose members modulate IAA homeostasis during skotomorphogenesis by redundantly mediating the glycosylation of oxIAA.

plant biology↗

bioRxiv2020

DNA damage inhibits root growth by enhancing cytokinin biosynthesis in Arabidopsis thaliana.

Plant root growth is influenced by external factors to adapt to changing environmental conditions. However, the mechanisms by which environmental stresses affect root growth remain elusive. Here we found that DNA double-strand breaks (DSBs) induce the expression of genes for the synthesis of cytokinin hormones and enhance the accumulation of cytokinins in the Arabidopsis root tip. This is a programmed response to DSBs through the DNA damage signaling pathway. Our data showed that activation of cytokinin signalling suppresses the expression of PIN-FORMED genes that encode efflux carriers of another plant hormone, auxin, thereby disturbing downward auxin flow and causing cell cycle retardation in the G2 phase. Elevated cytokinin signalling also promotes an early transition from cell division to endoreplication, resulting in a reduction of the root meristem size. We propose that in response to DNA stress, plants inhibit root growth by orchestrating hormone biosynthesis and signalling.

plant biology↗

bioRxiv2020

Shoot and root thermomorphogenesis are linked by a developmental trade-off

Temperature is one of the most impactful environmental factors to which plants adjust their growth and development. While the regulation of temperature signaling has been extensively investigated for the aerial part of plants, much less is known and understood about how roots sense and modulate their growth in response to fluctuating temperatures. Here we found that shoot and root growth responses to high ambient temperature are coordinated during early seedling development. A shoot signaling module that includes HY5, the phytochromes and the PIFs exerts a central function in coupling these growth responses and control auxin levels in the root. In addition to the HY5/PIF-dependent shoot module, a regulatory axis composed of auxin biosynthesis and auxin perception factors controls root responses to high ambient temperature. Together, our findings show that shoot and root developmental responses to temperature are tightly coupled during thermomorphogenesis and suggest that roots integrate energy signals with local hormonal inputs.

plant biology↗

bioRxiv2019

Conifers exhibit a characteristic inactivation of auxin to maintain tissue homeostasis

O_LIDynamic regulation of the levels of the natural auxin, indol-3-acetic acid (IAA), is essential to coordinate most of the physiological and developmental processes and responses to environmental changes. Oxidation of IAA is a major pathway to control auxin concentrations in Arabidopsis and, along with IAA conjugation, to respond to perturbation of IAA homeostasis. However, these regulatory mechanisms are still poorly investigated in conifers. To reduce this gap of knowledge, we investigated the different contribution of the IAA inactivation pathways in conifers.\nC_LIO_LIMass spectrometry-based quantification of IAA metabolites under steady state conditions and after perturbation was investigated to evaluate IAA homeostasis in conifers. Putative Picea abies GH3 genes (PaGH3) were identified by a comprehensive phylogenetic analysis including Arabidopsis and basal land plants. Auxin-inducible PaGH3 genes were identified by expression analysis and their IAA-conjugating activity was explored.\nC_LIO_LICompared to Arabidopsis, oxidative and conjugative pathways differentially contribute to reduce IAA levels in conifers. We demonstrated that the oxidation pathway plays a marginal role in controlling IAA homeostasis in spruce. On the other hand, an excess of IAA rapidly activates GH3-mediated irreversible conjugation pathways.\nC_LIO_LITaken together, these data indicate that a diversification of IAA inactivation mechanisms evolved specifically in conifers.\nC_LI

plant biology↗

bioRxiv2019

Cell-surface receptors enable perception of extracellular cytokinins

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience 1,2. Although cytokinin receptors are substantially localised to the endoplasmic reticulum 3-5, the cellular sites of cytokinin perception continue to be debated 1,6,7. Several cytokinin types display bioactivity 8,9 and recently a cell-specific cytokinin gradient was reported in roots 10. Yet, the importance of spatially heterogeneous cytokinin distribution and the specific cytokinin(s) that account for the different responses remain unclear. Here we show that cytokinin perception by plasma membrane receptors is an effective path for cytokinin response in root cells. Readout from a Two Component Signalling cytokinin-specific reporter (TCSn::GFP;11) is closely matched to intracellular cytokinin content, yet a proportion of bioactive cytokinins are detected in the extracellular fluid. Using cytokinins covalently linked to beads that could not pass the plasma membrane, we demonstrate that strong TCSn activation still occurs and that this response is greatly diminished in cytokinin receptor mutants. Although intracellular receptors play significant roles, we argue for a revision of concepts of cytokinin perception to include the spatial dimensions. In particular, selective ligand-receptor affinities, cellular localisation and tissue distribution of bioactive cytokinins, their receptors, transporters and inactivation enzymes appear all to be components of the signalling regulatory mechanisms.

plant biology↗

bioRxiv2019

A bacterial assay for rapid screening of IAA catabolic enzymes

BackgroundPlants rely on concentration gradients of the native auxin, indole-3-acetic acid (IAA), to modulate plant growth and development. Both metabolic and transport processes participate in the dynamic regulation of IAA homeostasis. Free IAA levels can be reduced by inactivation mechanisms, such as conjugation and degradation. IAA can be conjugated via ester linkage to glucose, or via amide linkage to amino acids, and degraded via oxidation. Members of the UDP glucosyl transferase (UGT) family catalyze the conversion of IAA to indole-3-acetyl-1-glucosyl ester (IAGlc); by contrast, IAA is irreversibly converted to indole-3-acetyl-L-aspartic acid (IAAsp) and indole-3-acetyl glutamic acid (IAGlu) by Group II of the GRETCHEN HAGEN3 (GH3) family of acyl amido synthetases. DIOXYGENASE OF AUXIN OXIDATION (DAO) irreversibly oxidizes IAA to oxindole-3-acetic acid (oxIAA) and, in turn, oxIAA can be further glucosylated to oxindole-3-acetyl-1-glucosyl ester (oxIAGlc) by UGTs. These metabolic pathways have been identified based on mutant analyses, in vitro activity measurements, and in planta feeding assays. In vitro assays for studying protein activity are based on expressing Arabidopsis enzymes in a recombinant form in bacteria or yeast followed by recombinant protein purification. However, the need to extract and purify the recombinant proteins represents a major obstacle when performing in vitro assays.\n\nResultsIn this work we report a rapid, reproducible and cheap method to screen the enzymatic activity of recombinant proteins that are known to inactivate IAA. The enzymatic reactions are carried out directly in bacteria that express the recombinant protein. The enzymatic products can be measured by direct injection of a small supernatant fraction from the bacterial culture on ultrahigh-performance liquid chromatography coupled to electrospray ionization tandem spectrometry (UHPLC-ESI-MS/MS). Experimental procedures were optimized for testing the activity of different classes of IAA-modifying enzymes without the need to purify recombinant protein.\n\nConclusionsThis new method represents an alternative to existing in vitro assays. It can be applied to the analysis of IAA metabolites that are produced upon supplementation of substrate to engineered bacterial cultures and can be used for a rapid screening of orthologous candidate genes from non-model species.

plant biology↗

bioRxiv2019

Flowering behaviour in Arabis alpina ensures the maintenance of a perennating dormant bud bank

Arabis alpina, similar to woody perennials, has a complex architecture with a zone of axillary vegetative branches and a zone of dormant buds that serve as perennating organs. We show that floral development during vernalization is the key for shaping the dormant bud zone by facilitating a synchronized and rapid growth after vernalization and thereby causing an increase in auxin response and transport and endogenous indole-3-acetic acid (IAA) levels in the stem. Floral development during vernalization is associated with the development of axillary buds in subapical nodes. Our transcriptome analysis indicated that these buds are not dormant during vernalization but only attain sustained growth after the return to warm temperatures. Floral and subapical vegetative branches grow after vernalization and inhibit the development of the buds below. Dormancy in these buds is regulated across the A. alpina life cycle by low temperatures and by apical dominance in a BRANCHED 1-dependent mechanism.

plant biology↗

bioRxiv2019

Auxin export from proximal fruits drives arrest in competent inflorescence meristems

A well-defined set of regulatory pathways control entry into the reproductive phase in flowering plants [1]. Conversely, little is known about the mechanisms that control the end of the reproductive phase ( floral arrest), despite this being a critical process for optimising fruit and seed production. Complete fruit removal or lack of fertile fruit-set in male sterile mutants, for example male sterile1 (ms1), prevents timely floral arrest in the model plant Arabidopsis [2]. These observations formed the basis for Hensel and colleagues model in which end-of-flowering was proposed to result from a cumulative fruit/seed-derived signal that caused simultaneous global proliferative arrest (GPA) in all inflorescences [2]. Recent studies have suggested that end-of-flowering involves gene expression changes at the floral meristem which are at least in part controlled by the FRUITFULL-APETELA2 pathway [3,4], however there is limited understanding of how this process is controlled and the communication needed at the whole plant level. Here, we provide new information providing a framework for the fruit-to-meristem (F-M) communication implied by the GPA model [5]. We show that floral arrest in Arabidopsis is not global and does not occur synchronously between branches, but rather that the arrest of each inflorescence is a local process, driven by auxin export from fruit proximal to the inflorescence meristem (IM). Furthermore, we show that inflorescence meristems are only competent for floral arrest once they reach a certain developmental age. Understanding the regulation of floral arrest is of major importance for the future manipulation of flowering to extend and maximise crop yields.

plant biology↗

bioRxiv2019

KAI2 regulates root and root hair development by modulating auxin distribution

Strigolactones (SLs) are endogenous signalling molecules that play important roles in controlling plant development. SL perception is closely related to that of karrikins, smoke-derived compounds presumed to mimic endogenous signalling molecules (KLs). SLs have been suggested to regulate root development. However, perception of both molecules requires the F-box protein MAX2 and the use of max2 mutants has hampered defining the exact role of SLs in roots. Here we dissect the role of SL and KL signalling in Arabidopsis root development using mutants defective in the /{beta} hydrolase receptors D14 and KAI2, which specifically perceive SLs and KLs, respectively. Both pathways together regulate lateral root density (LRD), but contrary to previous reports, KL signalling alone controls root hair density, root hair length and additionally root skewing, straightness and diameter. Members of the SMXL protein family are downstream targets of SL (SMXL6, 7, 8) and KL (SMAX1, SMXL2) signalling. We identified distinct and overlapping roles of these proteins in the regulation of root development. Both SMAX1/SMXL2 and SMXL6/SMXL7/SMXL8 regulate LRD, confirming that SL and KL signalling act together to regulate this trait, while the KL-signalling specific SMAX1 and SMXL2 regulate all other investigated root traits. Finally, we show that KL signalling regulates root hair development by modulating auxin distribution within the root.

plant biology↗

bioRxiv2018

Anthranilic acid regulates subcellular localization of auxin transporters during root gravitropism

O_LIDistribution of auxin within plant tissues is of great importance for developmental plasticity, including root gravitropic growth. Auxin flow is directed by the subcellular polar distribution and dynamic relocalization of auxin transporters such as the PIN-FORMED (PIN) efflux carriers, which can be influenced by the main natural plant auxin indole-3-acetic acid (IAA). Anthranilic acid (AA) is an important early precursor of IAA and previously published studies with AA analogues suggested that AA may also regulate PIN localization.\nC_LIO_LIUsing Arabidopsis thaliana as a model species, we studied an AA-deficient mutant displaying agravitropic root growth, treated seedlings with AA and AA analogues and transformed lines to over-produce AA while inhibiting its conversion to downstream IAA precursors.\nC_LIO_LIWe showed that AA rescues root gravitropic growth in the AA-deficient mutant at concentrations that do not rescue IAA levels. Overproduction of AA affects root gravitropism without affecting IAA levels. Treatments with, or deficiency in, AA result in defects in PIN polarity and gravistimulus-induced PIN relocalization in root cells.\nC_LIO_LIOur results reveal a previously unknown role for AA in the regulation of PIN subcellular localization and dynamics involved in root gravitropism, which is independent of its better-known role in IAA biosynthesis.\nC_LI

plant biology↗

bioRxiv2018

Selective auxin agonists induce specific AUX/IAA protein degradation to modulate plant development

Auxin phytohormones control most aspects of plant development through a complex and interconnected signaling network. In the presence of auxin, AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors are targeted for degradation by the SKP1-CULLIN1-F-BOX (SCF) ubiquitin-protein ligases containing TRANSPORT INHIBITOR RESISTANT 1/AUXIN SIGNALING F-BOX (TIR1/AFB). CULLIN1-neddylation is required for SCFTIR1/AFB functionality as exemplified by mutants deficient in the NEDD8-activating enzyme subunit AUXIN-RESISTANT 1 (AXR1). Here, we report a chemical biology screen that identifies small molecules requiring AXR1 to modulate plant development. We selected four molecules of interest, RubNeddin1 to 4 (RN1 to 4), among which RN3 and RN4 trigger selective auxin responses at transcriptional, biochemical and morphological levels. This selective activity is explained by their ability to promote the interaction between TIR1 and a specific subset of AUX/IAA proteins, stimulating the degradation of particular AUX/IAA combinations. Finally, via a genetic screen using RN4, we revealed that the chromatin remodeling ATPase BRAHMA is implicated in auxin-mediated apical hook development. These results demonstrate the power of selective auxin agonists to dissect auxin perception for plant developmental functions.

plant biology↗