Save your TIRs - more to auxin than meets the eye

Regulation of PILS genes by bZIP transcription factor TGA7 in tomato plant growth

Auxin plays a pivotal role in plant growth regulation. The PIN-FORMED (PIN) proteins facilitate long-distance polar auxin transport, whereas the recently identified PIN-LIKES (PILS) proteins regulate intracellular auxin homeostasis. However, the auxin transport mechanisms in horticultural crops remain largely unexplored. Here, we identified and characterized PILS genes in tomato (Solanum lycopersicum). Promoter analysis revealed enrichment in TGA[C/T]G motifs, suggesting transcriptional regulation by TGA factors in the bZIP family. Subcellular localization studies confirmed that all tomato PILS proteins localize in the endoplasmic reticulum. PILS2 exhibited the highest expression across examined tissues, and its close homologue PILS6 showed a similar but less pronounced expression pattern. Silencing PILS2 significantly inhibited shoot and root growth. Phylogenetic and expression analyses identified the homologs of Arabidopsis TGA1, TGA3, TGA4, and TGA7 in tomato genome, with tomato TGA7 showing higher expression in roots. Notably, silencing tomato TGA7, but not TGA1, TGA3, or TGA4, strongly impaired shoot and root growth. Molecular assays demonstrated that TGA7 directly binds to the PILS2 promoter to activate its transcription. These findings uncover a TGA7-PILS2 regulatory module that governs plant growth and offer new insights into the function and regulation of PILS genes in tomato.

Plasma membrane H+-ATPase activation increases global transcript levels and promotes the shoot growth of light-grown Arabidopsis seedlings

Plant cell growth requires the elongation of cells mediated by cell wall remodelling and turgor pressure changes. The plasma membrane (PM) H+-ATPase facilitates both cell wall loosening and turgor pressure changes by acidifying the apoplast of cells, referred to as acid growth. The acid growth theory is mostly established on the auxin-induced activation of PM H+-ATPase in non-photosynthetic tissues. However, how PM H+-ATPase affects the growth in photosynthetic tissues of Arabidopsis remains unclear. Here, a combination of transcriptomics and cis-regulatory element analysis was conducted to identify the impact of PM H+-ATPase on global transcript levels and the molecular mechanism downstream of the PM H+-ATPase. The PM H+-ATPase activation increased transcript levels globally, especially cell wall modification-related genes. The transcript level changes were in PM H+-ATPase-dependent manner. Involvement of Ca2+ was suggested as CAMTA motif was enriched in the promoter of PM H+-ATPase-induced genes and cytosolic Ca2+ elevated upon PM H+-ATPase activation. PM H+-ATPase activation in photosynthetic tissues promotes the expression of cell wall modification enzymes and shoot growth, adding a novel perspective of photosynthesis-dependent PM H+-ATPase activation in photosynthetic tissues to the acid growth theory that has primarily based on findings from non-photosynthetic tissues.

AuxSynBio: synthetic biology tools to understand and engineer auxin

The plant hormone auxin is a crucial coordinator of nearly all plant growth and development processes. Because of its centrality to plant physiology and the modular nature of the signaling pathway, auxin has played a critical role at the forefront of plant synthetic biology. This review will highlight how auxin is both a subject and an object of synthetic biology. Engineering biology approaches are deepening our understanding of how auxin pathways are wired and tuned, particularly through the creative use of signaling pathway recapitulation in yeast and engineered orthogonal auxin-receptor pairs. Auxin biology has also been mined for parts by synthetic biologists, with components being used for inducible protein degradation systems (auxin-inducible degron), auxin biosensors, synthetic cell-cell communication, and plant engineering.

Role of TIR1/AFB family genes during grafting in Carya cathayensis

Auxins play significant roles in plant growth and development. The transporter inhibitor response1/auxin signaling F-box (TIR1/AFB) gene family encodes the auxin receptor proteins and plays an essential role in the auxin signaling pathway. Here we identified and characterized the TIR1/AFB family in Carya cathayensis (Cc) plants (named as CcTIR1/AFB). Seven CcTIR1/AFBs were identified and further confirmed by cloning. All proteins encoded by these genes conservatively contained two domains, the F-box and leucine-rich repeat (LRR) domains. The CcTIR1/AFBs were located in the nucleus. Phylogenetic analysis suggested that CcTIR1/AFBs were evenly scattered in four different subgroups. The cis-acting element analysis indicates that CcTIR1/AFBs might be activated by auxin. The spatial and temporal expression of CcTIR1/AFBs during grafting suggested that both CcAFB1 and CcAFB2 in scions and CcAFB4 in the rootstocks were significantly upregulated at 3 days after grafting, which indicated the specialization of three CcAFBs during grafting. The Y2H assay indicated that three CcAFBs were capable of interacting with CcIAA16, CcIAA27b, and CcIAA29a, among which CcAFB4 interacted strongly with CcIAA1 and CcIAA16. Our study provides the opportunity to understand the potential role of not only CcTIR1/AFBs but also special CcAFBs (CcAFB1, CcAFB2, and CcAFB4), which is a great aspect to further explore the molecular mechanism during the grafting process.

Protein degradation in auxin response

The signaling molecule auxin sits at the nexus of plant biology where it coordinates essentially all growth and developmental processes. Auxin molecules are transported throughout plant tissues and are capable of evoking highly specific physiological responses by inducing various molecular pathways. In many of these pathways, proteolysis plays a crucial role for correct physiological responses. This review provides a chronology of the discovery and characterization of the auxin receptor, which is a fascinating example of separate research trajectories ultimately converging on the discovery of a core auxin signaling hub that relies on degradation of a family of transcriptional inhibitor proteins-the Aux/IAAs. Beyond describing the "classical" proteolysis-driven auxin response system, we explore more recent examples of the interconnection of proteolytic systems, which target a range of other auxin signaling proteins, and auxin response. By highlighting these emerging concepts, we provide potential future directions to further investigate the role of protein degradation within the framework of auxin response.

Modulation of early gene expression responses to water deprivation stress by the E3 ubiquitin ligase ATL80: implications for retrograde signaling interplay

BACKGROUND

Primary response genes play a pivotal role in translating short-lived stress signals into sustained adaptive responses. In this study, we investigated the involvement of ATL80, an E3 ubiquitin ligase, in the dynamics of gene expression following water deprivation stress. We observed that ATL80 is rapidly activated within minutes of water deprivation stress perception, reaching peak expression around 60 min before gradually declining. ATL80, despite its post-translational regulation role, emerged as a key player in modulating early gene expression responses to water deprivation stress.

RESULTS

The impact of ATL80 on gene expression was assessed using a time-course microarray analysis (0, 15, 30, 60, and 120 min), revealing a burst of differentially expressed genes, many of which were associated with various stress responses. In addition, the diversity of early modulation of gene expression in response to water deprivation stress was significantly abolished in the atl80 mutant compared to wild-type plants. A subset of 73 genes that exhibited a similar expression pattern to ATL80 was identified. Among them, several are linked to stress responses, including ERF/AP2 and WRKY transcription factors, calcium signaling genes, MAP kinases, and signaling peptides. Promoter analysis predicts enrichment of binding sites for CAMTA1 and CAMTA5, which are known regulators of rapid stress responses. Furthermore, we have identified a group of differentially expressed ERF/AP2 transcription factors, proteins associated with folding and refolding, as well as pinpointed core module genes which are known to play roles in retrograde signaling pathways that cross-referenced with the early ATL80 transcriptome.

CONCLUSIONS

Based on these findings, we propose that ATL80 may target one or more components within the retrograde signaling pathways for degradation. In essence, ATL80 serves as a bridge connecting these signaling pathways and effectively functions as an alarm signal.

Manipulation of Auxin Signaling by Smut Fungi during Plant Colonization

A common feature of many plant-colonizing organisms is the exploitation of plant signaling and developmental pathways to successfully establish and proliferate in their hosts. Auxins are central plant growth hormones, and their signaling is heavily interlinked with plant development and immunity responses. Smuts, as one of the largest groups in basidiomycetes, are biotrophic specialists that successfully manipulate their host plants and cause fascinating phenotypes in so far largely enigmatic ways. This review gives an overview of the growing understanding of how and why smut fungi target the central and conserved auxin growth signaling pathways in plants.

What a tangled web it weaves: auxin coordination of stem cell maintenance and flower production

Robust agricultural yields require consistent flower production throughout fluctuating environmental conditions. Floral primordia are produced in the inflorescence meristem, which contains a pool of continuously dividing stem cells. Daughter cells of these divisions either retain stem cell identity or are pushed to the SAM periphery, where they become competent to develop into floral primordia after receiving the appropriate signal. Thus, flower production is inherently linked to regulation of the stem cell pool. The plant hormone auxin promotes flower development throughout its early phases and has been shown to interact with the molecular pathways regulating stem cell maintenance. Here, we will summarize how auxin signaling contributes to stem cell maintenance and promotes flower development through the early phases of initiation, outgrowth, and floral fate establishment. Recent advances in this area suggest that auxin may serve as a signal that integrates stem cell maintenance and new flower production.

Auxins and grass shoot architecture: how the most important hormone makes the most important plants

Cereals are a group of grasses cultivated by humans for their grain. It is from these cereal grains that the majority of all calories consumed by humans are derived. The production of these grains is the result of the development of a series of hierarchical reproductive structures that form the distinct shoot architecture of the grasses. Being spatiotemporally complex, the coordination of grass shoot development is tightly controlled by a network of genes and signals, including the key phytohormone auxin. Hormonal manipulation has therefore been identified as a promising potential approach to increasing cereal crop yields and therefore ultimately global food security. Recent work translating the substantial body of auxin research from model plants into cereal crop species is revealing the contribution of auxin biosynthesis, transport, and signalling to the development of grass shoot architecture. This review discusses this still-maturing knowledge base and examines the possibility that changes in auxin biology could have been a causative agent in the evolution of differences in shoot architecture between key grass species, or could underpin the future selective breeding of cereal crops.

Roles of auxin pathways in maize biology

Phytohormones play a central role in plant development and environmental responses. Auxin is a classical hormone that is required for organ formation, tissue patterning, and defense responses. Auxin pathways have been extensively studied across numerous land plant lineages, including bryophytes and eudicots. In contrast, our understanding of the roles of auxin in maize morphogenesis and immune responses is limited. Here, we review evidence for auxin-mediated processes in maize and describe promising areas for future research in the auxin field. Several recent transcriptomic and genetic studies have demonstrated that auxin is a key influencer of both vegetative and reproductive development in maize (namely roots, leaves, and kernels). Auxin signaling has been implicated in both maize shoot architecture and immune responses through genetic and molecular analyses of the conserved co-repressor RAMOSA ENHANCER LOCUS2. Polar auxin transport is linked to maize drought responses, root growth, shoot formation, and leaf morphogenesis. Notably, maize has been a key system for delineating auxin biosynthetic pathways and offers many opportunities for future investigations on auxin metabolism. In addition, crosstalk between auxin and other phytohormones has been uncovered through gene expression studies and is important for leaf and root development in maize. Collectively these studies point to auxin as a cornerstone for maize biology that could be leveraged for improved crop resilience and yield.

Auxin and abiotic stress responses

Plants are exposed to a variety of abiotic stresses; these stresses have profound effects on plant growth, survival, and productivity. Tolerance and adaptation to stress require sophisticated stress sensing, signaling, and various regulatory mechanisms. The plant hormone auxin is a key regulator of plant growth and development, playing pivotal roles in the integration of abiotic stress signals and control of downstream stress responses. In this review, we summarize and discuss recent advances in understanding the intersection of auxin and abiotic stress in plants, with a focus on temperature, salt, and drought stresses. We also explore the roles of auxin in stress tolerance and opportunities arising for agricultural applications.

The AFB1 auxin receptor controls the cytoplasmic auxin response pathway in Arabidopsis thaliana

The phytohormone auxin triggers root growth inhibition within seconds via a non-transcriptional pathway. Among members of the TIR1/AFB auxin receptor family, AFB1 has a primary role in this rapid response. However, the unique features that confer this specific function have not been identified. Here we show that the N-terminal region of AFB1, including the F-box domain and residues that contribute to auxin binding, is essential and sufficient for its specific role in the rapid response. Substitution of the N-terminal region of AFB1 with that of TIR1 disrupts its distinct cytoplasm-enriched localization and activity in rapid root growth inhibition by auxin. Importantly, the N-terminal region of AFB1 is indispensable for auxin-triggered calcium influx, which is a prerequisite for rapid root growth inhibition. Furthermore, AFB1 negatively regulates lateral root formation and transcription of auxin-induced genes, suggesting that it plays an inhibitory role in canonical auxin signaling. These results suggest that AFB1 may buffer the transcriptional auxin response, whereas it regulates rapid changes in cell growth that contribute to root gravitropism.