Bimodal regulation of ICR1 levels generates self-organizing auxin distribution

Crosstalk between Rho of Plants GTPase signalling and plant hormones

Rho of Plants (ROPs) constitute a plant-specific subset of small guanine nucleotide-binding proteins within the Cdc42/Rho/Rac family. These versatile proteins regulate diverse cellular processes, including cell growth, cell division, cell morphogenesis, organ development, and stress responses. In recent years, the dynamic cellular and subcellular behaviours orchestrated by ROPs have unveiled a notable connection to hormone-mediated organ development and physiological responses, thereby expanding our knowledge of the functions and regulatory mechanisms of this signalling pathway. This review delineates advancements in understanding the interplay between plant hormones and the ROP signalling cascade, focusing primarily on the connections with auxin and abscisic acid pathways, alongside preliminary discoveries in cytokinin, brassinosteroid, and salicylic acid responses. It endeavours to shed light on the intricate, coordinated mechanisms bridging cell- and tissue-level signals that underlie plant cell behaviour, organ development, and physiological processes, and highlights future research prospects and challenges in this rapidly developing field.

Microtubule-associated ROP interactors affect microtubule dynamics and modulate cell wall patterning and root hair growth

Rho of plant (ROP) proteins and the interactor of constitutively active ROP (ICR) family member ICR5/MIDD1 have been implicated to function as signaling modules that regulate metaxylem secondary cell wall patterning. Yet, loss-of-function mutants of ICR5 and its closest homologs have not been studied and, hence, the functions of these ICR family members are not fully established. Here, we studied the functions of ICR2 and its homolog ICR5. We show that ICR2 is a microtubule-associated protein that affects microtubule dynamics. Secondary cell wall pits in the metaxylem of Arabidopsis icr2 and icr5 single mutants and icr2 icr5 double mutants are smaller than those in wild-type Col-0 seedlings; however, they are remarkably denser, implying a complex function of ICRs in secondary cell wall patterning. ICR5 has a unique function in protoxylem secondary cell wall patterning, whereas icr2, but not icr5, mutants develop split root hairs, demonstrating functional diversification. Taken together, our results show that ICR2 and ICR5 have unique and cooperative functions as microtubule-associated proteins and as ROP effectors.

ROP Interactive Partners are Involved in the Control of Cell Division Patterns in Arabidopsis Leaves

Animal Rho GTP-binding proteins and their plant counterparts, Rho of plants (ROPs), regulate cell polarity, but they do so through different effector proteins. A class of ROP effectors, interactor of constitutive active ROPs (ICRs)/ROP interactive partners (RIPs), has been implicated in diverse biological processes; however, there are limited analyses of RIP loss-of-function mutants. Here, we report an analysis of the functions of the Arabidopsis thaliana RIPs in the leaf epidermis. Green Fluorescent Protein (GFP) fusion proteins of all the RIPs colocalized to cortical microtubules. RIP1, RIP3 and RIP4, but not RIP2 and RIP5, colocalized with the preprophase band (PPB), spindles and phragmoplasts. RIP2 and RIP5 did not colocalize with the PPB, spindles or phragmoplasts even when they were expressed under a promoter active in proliferative cells, indicating that there are differences among RIP protein properties. The overexpression of RIP1 or RIP4 resulted in the fragmentation of cortical microtubules, and the rip1 2 3 4 5 quintuple mutant showed increased growth rate of microtubules at their plus ends compared with the wild type. The rip1 2 3 4 5 mutant leaves and petals were narrow, which was explained by the decreased cell number along the transverse axis compared with that of the wild type. The rip1 2 3 4 5 mutant leaf epidermis possessed fewer PPBs oriented close to the long axis of the leaf compared with wild type, indicating the involvement of RIPs in cell division plane regulation and leaf shape determination.

eIF4E1 Regulates Arabidopsis Embryo Development and Root Growth by Interacting With RopGEF7

Eukaryotic translation initiation factor 4E1 (eIF4E1) is required for the initiation of protein synthesis. The biological function of eIF4E1 in plant-potyvirus interactions has been extensively studied. However, the role of eIF4E1 in Arabidopsis development remains unclear. In this study, we show that eIF4E1 is highly expressed in the embryo and root apical meristem. In addition, eIF4E1 expression is induced by auxin. eIF4E1 mutants show embryonic cell division defects and short primary roots, a result of reduced cell divisions. Furthermore, our results show that mutation in eIF4E1 severely reduces the accumulation of PIN-FORMED (PIN) proteins and decreases auxin-responsive gene expression at the root tip. Yeast two-hybrid assays identified that eIF4E1 interacts with an RAC/ROP GTPase activator, RopGEF7, which has been previously reported to be involved in the maintenance of the root apical meristem. The interaction between eIF4E1 and RopGEF7 is confirmed by protein pull-down and bimolecular fluorescent complementation assays in plant cells. Taken together, our results demonstrated that eIF4E1 is important for auxin-regulated embryo development and root growth. The eIF4E1-RopGEF7 interaction suggests that eIF4E1 may act through ROP signaling to regulate auxin transport, thus regulating auxin-dependent patterning.

Solving the Puzzle of Shape Regulation in Plant Epidermal Pavement Cells

The plant epidermis serves many essential functions, including interactions with the environment, protection, mechanical strength, and regulation of tissue and organ growth. To achieve these functions, specialized epidermal cells develop into particular shapes. These include the intriguing interdigitated jigsaw puzzle shape of cotyledon and leaf pavement cells seen in many species, the precise functions of which remain rather obscure. Although pavement cell shape regulation is complex and still a long way from being fully understood, the roles of the cell wall, mechanical stresses, cytoskeleton, cytoskeletal regulatory proteins, and phytohormones are becoming clearer. Here, we provide a review of this current knowledge of pavement cell morphogenesis, generated from a wealth of experimental evidence and assisted by computational modeling approaches. We also discuss the evolution and potential functions of pavement cell interdigitation. Throughout the review, we highlight some of the thought-provoking controversies and creative theories surrounding the formation of the curious puzzle shape of these cells.

ROP INTERACTIVE PARTNER b Interacts with RACB and Supports Fungal Penetration into Barley Epidermal Cells

Rho of Plants (ROP) G-proteins are key components of cell polarization processes in plant development. The barley (Hordeum vulgare) ROP protein RACB is a susceptibility factor in the interaction of barley with the barley powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh). RACB also drives polar cell development, and this function might be coopted during the formation of fungal haustoria in barley epidermal cells. To understand RACB signaling during the interaction of barley with Bgh, we searched for potential downstream interactors of RACB. Here, we show that ROP INTERACTIVE PARTNER b (RIPb; synonym: INTERACTOR OF CONSTITUTIVE ACTIVE ROP b) directly interacts with RACB in yeast and in planta. Overexpression of RIPb supports the susceptibility of barley to Bgh RIPb further interacts with itself at microtubules. However, the interaction with activated RACB largely takes place at the plasma membrane. Both RIPb and RACB are recruited to the site of fungal attack around the neck of developing haustoria, suggesting locally enhanced ROP activity. We further assigned different functions to different domains of the RIPb protein. The N-terminal coiled-coil CC1 domain is required for microtubule localization, while the C-terminal coiled-coil CC2 domain is sufficient to interact with RACB and to fulfill a function in susceptibility at the plasma membrane. Hence, RIPb appears to be localized at microtubules and is then recruited by activated RACB for a function at the plasma membrane during formation of the haustorial complex.

A novel Ca2+-binding protein that can rapidly transduce auxin responses during root growth

Signaling cross talks between auxin, a regulator of plant development, and Ca2+, a universal second messenger, have been proposed to modulate developmental plasticity in plants. However, the underlying molecular mechanisms are largely unknown. Here, we report that in Arabidopsis roots, auxin elicits specific Ca2+ signaling patterns that spatially coincide with the expression pattern of auxin-regulated genes. We have identified the single EF-hand Ca2+-binding protein Ca2+-dependent modulator of ICR1 (CMI1) as an interactor of the Rho of plants (ROP) effector interactor of constitutively active ROP (ICR1). CMI1 expression is directly up-regulated by auxin, whereas the loss of function of CMI1 associates with the repression of auxin-induced Ca2+ increases in the lateral root cap and vasculature, indicating that CMI1 represses early auxin responses. In agreement, cmi1 mutants display an increased auxin response including shorter primary roots, longer root hairs, longer hypocotyls, and altered lateral root formation. Binding to ICR1 affects subcellular localization of CMI1 and its function. The interaction between CMI1 and ICR1 is Ca2+-dependent and involves a conserved hydrophobic pocket in CMI1 and calmodulin binding-like domain in ICR1. Remarkably, CMI1 is monomeric in solution and in vitro changes its secondary structure at cellular resting Ca2+ concentrations ranging between 10-9 and 10-8 M. Hence, CMI1 is a Ca2+-dependent transducer of auxin-regulated gene expression, which can function in a cell-specific fashion at steady-state as well as at elevated cellular Ca2+ levels to regulate auxin responses.

A Comprehensive Toolkit for Inducible, Cell Type-Specific Gene Expression in Arabidopsis

Understanding the context-specific role of gene function is a key objective of modern biology. To this end, we generated a resource for inducible cell type-specific transactivation in Arabidopsis (Arabidopsis thaliana) based on the well-established combination of the chimeric GR-LhG4 transcription factor and the synthetic pOp promoter. Harnessing the flexibility of the GreenGate cloning system, we produced a comprehensive set of transgenic lines termed GR-LhG4 driver lines targeting most tissues in the Arabidopsis shoot and root with a strong focus on the indeterminate meristems. When we combined these transgenic lines with effectors under the control of the pOp promoter, we observed tight temporal and spatial control of gene expression. In particular, inducible expression in F1 plants obtained from crosses of driver and effector lines allows for rapid assessment of the cell type-specific impact of an effector with high temporal resolution. Thus, our comprehensive and flexible method is suitable for overcoming the limitations of ubiquitous genetic approaches, the outputs of which often are difficult to interpret due to the widespread existence of compensatory mechanisms and the integration of diverging effects in different cell types.

A Conceptual Framework for Cell Identity Transitions in Plants

Multicellular organisms develop from a single cell that proliferates to form different cell types with specialized functions. Sixty years ago, Waddington suggested the 'epigenetic landscape' as a useful metaphor for the process. According to this view, cells move through a rugged identity space along genetically encoded trajectories, until arriving at one of the possible final fates. In plants in particular, these trajectories have strong spatial correlates, as cell identity is intimately linked to its relative position within the plant. During regeneration, however, positional signals are severely disrupted and differentiated cells are able to undergo rapid non-canonical identity changes. Moreover, while pluripotent properties have long been ascribed to plant cells, the introduction of induced pluripotent stem cells in animal studies suggests such plasticity may not be unique to plants. As a result, current concepts of differentiation as a gradual and hierarchical process are being reformulated across biological fields. Traditional studies of plant regeneration have placed strong emphasis on the emergence of patterns and tissue organization, and information regarding the events occurring at the level of individual cells is only now beginning to emerge. Here, I review the historical and current concepts of cell identity and identity transitions, and discuss how new views and tools may instruct the future understanding of differentiation and plant regeneration.

ROP GTPases Structure-Function and Signaling Pathways

Interactions between receptor like kinases and guanyl nucleotide exchange factors together with identification of effector proteins reveal putative ROP GTPases signaling cascades.

Auxin 2016: a burst of auxin in the warm south of China

The luxurious vegetation at Sanya, the most southern location in China on the island of Hainan, provided a perfect environment for the 'Auxin 2016' meeting in October. As we review here, participants from all around the world discussed the latest advances in auxin transport, metabolism and signaling pathways, highlighting how auxin acts during plant development and in response to the environment in combination with other hormones. The meeting also provided a rich perspective on the evolution of the role of auxin, from algae to higher plants.

Computational Modeling of Auxin: A Foundation for Plant Engineering

Since the development of agriculture, humans have relied on the cultivation of plants to satisfy our increasing demand for food, natural products, and other raw materials. As we understand more about plant development, we can better manipulate plants to fulfill our particular needs. Auxins are a class of simple metabolites that coordinate many developmental activities like growth and the appearance of functional structures in plants. Computational modeling of auxin has proven to be an excellent tool in elucidating many mechanisms that underlie these developmental events. Due to the complexity of these mechanisms, current modeling efforts are concerned only with single phenomena focused on narrow spatial and developmental contexts; but a general model of plant development could be assembled by integrating the insights from all of them. In this perspective, we summarize the current collection of auxin-driven computational models, focusing on how they could come together into a single model for plant development. A model of this nature would allow researchers to test hypotheses in silico and yield accurate predictions about the behavior of a plant under a given set of physical and biochemical constraints. It would also provide a solid foundation toward the establishment of plant engineering, a proposed discipline intended to enable the design and production of plants that exhibit an arbitrarily defined set of features.

A barley SKP1-like protein controls abundance of the susceptibility factor RACB and influences the interaction of barley with the barley powdery mildew fungus

In an increasing number of plant-microbe interactions, it has become evident that the abundance of immunity-related proteins is controlled by the ubiquitin-26S proteasome system. In the interaction of barley with the biotrophic barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh), the RAC/ROP [RAT SARCOMA-related C3 botulinum toxin substrate/RAT SARCOMA HOMOLOGUE (RHO) of plants] guanosine triphosphatase (GTPase) HvRACB supports the fungus in a compatible interaction. By contrast, barley HvRBK1, a ROP-binding receptor-like cytoplasmic kinase that interacts with and can be activated by constitutively activated HvRACB, limits fungal infection success. We have identified a barley type II S-phase kinase 1-associated (SKP1)-like protein (HvSKP1-like) as a molecular interactor of HvRBK1. SKP1 proteins are subunits of the SKP1-cullin 1-F-box (SCF)-E3 ubiquitin ligase complex that acts in the specific recognition and ubiquitination of protein substrates for subsequent proteasomal degradation. Transient induced gene silencing of either HvSKP1-like or HvRBK1 increased protein abundance of constitutively activated HvRACB in barley epidermal cells, whereas abundance of dominant negative RACB only weakly increased. In addition, silencing of HvSKP1-like enhanced the susceptibility of barley to haustorium establishment by Bgh. In summary, our results suggest that HvSKP1-like, together with HvRBK1, controls the abundance of HvRACB and, at the same time, modulates the outcome of the barley-Bgh interaction. A possible feedback mechanism from RAC/ROP-activated HvRBK1 on the susceptibility factor HvRACB is discussed.

Clathrin-Mediated Auxin Efflux and Maxima Regulate Hypocotyl Hook Formation and Light-Stimulated Hook Opening in Arabidopsis

The establishment of auxin maxima by PIN-FORMED 3 (PIN3)- and AUXIN RESISTANT 1/LIKE AUX1 (LAX) 3 (AUX1/LAX3)-mediated auxin transport is essential for hook formation in Arabidopsis hypocotyls. Until now, however, the underlying regulatory mechanism has remained poorly understood. Here, we show that loss of function of clathrin light chain CLC2 and CLC3 genes enhanced auxin maxima and thereby hook curvature, alleviated the inhibitory effect of auxin overproduction on auxin maxima and hook curvature, and delayed blue light-stimulated auxin maxima reduction and hook opening. Moreover, pharmacological experiments revealed that auxin maxima formation and hook curvature in clc2 clc3 were sensitive to auxin efflux inhibitors 1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid but not to the auxin influx inhibitor 1-naphthoxyacetic acid. Live-cell imaging analysis further uncovered that loss of CLC2 and CLC3 function impaired PIN3 endocytosis and promoted its lateralization in the cortical cells but did not affect AUX1 localization. Taken together, these results suggest that clathrin regulates auxin maxima and thereby hook formation through modulating PIN3 localization and auxin efflux, providing a novel mechanism that integrates developmental signals and environmental cues to regulate plant skotomorphogenesis and photomorphogenesis.