A DELAY OF GERMINATION 1 (DOG1)-like protein regulates spore germination in the moss Physcomitrium patens

DELAY OF GERMINATION 1 is a key regulator of dormancy in flowering plants before seed germination. Bryophytes develop haploid spores with an analogous function to seeds. Here, we investigate whether DOG1 function during germination is conserved between bryophytes and flowering plants and analyse the underlying mechanism of DOG1 action in the moss Physcomitrium patens. Phylogenetic and in silico expression analyses were performed to identify and characterise DOG1 domain-containing genes in P. patens. Germination assays were performed to characterise a Ppdog1-like1 mutant, and replacement with AtDOG1 was carried out. Yeast two-hybrid assays were used to test the interaction of the PpDOG1-like protein with DELLA proteins from P. patens and A. thaliana. P. patens possesses nine DOG1 domain-containing genes. The DOG1-like protein PpDOG1-L1 (Pp3c3_9650) interacts with PpDELLAa and PpDELLAb and the A. thaliana DELLA protein AtRGA in yeast. Protein truncations revealed the DOG1 domain as necessary and sufficient for interaction with PpDELLA proteins. Spores of Ppdog1-l1 mutant germinate faster than wild type, but replacement with AtDOG1 reverses this effect. Our data demonstrate a role for the PpDOG1-LIKE1 protein in moss spore germination, possibly alongside PpDELLAs. This suggests a conserved DOG1 domain function in germination, albeit with differential adaptation of regulatory networks in seed and spore germination.

Nt-acetylation-independent turnover of SQUALENE EPOXIDASE 1 by Arabidopsis DOA10-like E3 ligases

The acetylation-dependent (Ac/)N-degron pathway degrades proteins through recognition of their acetylated N-termini (Nt) by E3 ligases called Ac/N-recognins. To date, specific Ac/N-recognins have not been defined in plants. Here we used molecular, genetic, and multiomics approaches to characterize potential roles for Arabidopsis (Arabidopsis thaliana) DEGRADATION OF ALPHA2 10 (DOA10)-like E3 ligases in the Nt-acetylation-(NTA)-dependent turnover of proteins at global- and protein-specific scales. Arabidopsis has two endoplasmic reticulum (ER)-localized DOA10-like proteins. AtDOA10A, but not the Brassicaceae-specific AtDOA10B, can compensate for loss of yeast (Saccharomyces cerevisiae) ScDOA10 function. Transcriptome and Nt-acetylome profiling of an Atdoa10a/b RNAi mutant revealed no obvious differences in the global NTA profile compared to wild type, suggesting that AtDOA10s do not regulate the bulk turnover of NTA substrates. Using protein steady-state and cycloheximide-chase degradation assays in yeast and Arabidopsis, we showed that turnover of ER-localized SQUALENE EPOXIDASE 1 (AtSQE1), a critical sterol biosynthesis enzyme, is mediated by AtDOA10s. Degradation of AtSQE1 in planta did not depend on NTA, but Nt-acetyltransferases indirectly impacted its turnover in yeast, indicating kingdom-specific differences in NTA and cellular proteostasis. Our work suggests that, in contrast to yeast and mammals, targeting of Nt-acetylated proteins is not a major function of DOA10-like E3 ligases in Arabidopsis and provides further insight into plant ERAD and the conservation of regulatory mechanisms controlling sterol biosynthesis in eukaryotes.

DELLA functions evolved by rewiring of associated transcriptional networks

DELLA proteins are land-plant specific transcriptional regulators that transduce environmental information to multiple processes throughout a plant's life^1-3. The molecular basis for this critical function in angiosperms has been linked to the regulation of DELLA stability by gibberellins and to the capacity of DELLA proteins to interact with hundreds of transcription factors^4,5. Although bryophyte orthologues can partially fulfil functions attributed to angiosperm DELLA^6,7, it is not clear whether the capacity to establish interaction networks is an ancestral property of DELLA proteins or is associated with their role in gibberellin signalling^8-10. Here we show that representative DELLAs from the main plant lineages display a conserved ability to interact with multiple transcription factors. We propose that promiscuity was encoded in the ancestral DELLA protein, and that this property has been largely maintained, whereas the lineage-dependent diversification of DELLA-dependent functions mostly reflects the functional evolution of their interacting partners.

DELLA proteins regulate spore germination and reproductive development in Physcomitrium patens

Proteins of the DELLA family integrate environmental signals to regulate growth and development throughout the plant kingdom. Plants expressing non-degradable DELLA proteins underpinned the development of high-yielding 'Green Revolution' dwarf crop varieties in the 1960s. In vascular plants, DELLAs are regulated by gibberellins, diterpenoid plant hormones. How DELLA protein function has changed during land plant evolution is not fully understood. We have examined the function and interactions of DELLA proteins in the moss Physcomitrium (Physcomitrella) patens, in the sister group of vascular plants (Bryophytes). PpDELLAs do not undergo the same regulation as flowering plant DELLAs. PpDELLAs are not degraded by diterpenes, do not interact with GID1 gibberellin receptor proteins and do not participate in responses to abiotic stress. PpDELLAs do share a function with vascular plant DELLAs during reproductive development. PpDELLAs also regulate spore germination. PpDELLAs interact with moss-specific photoreceptors although a function for PpDELLAs in light responses was not detected. PpDELLAs likely act as 'hubs' for transcriptional regulation similarly to their homologues across the plant kingdom. Taken together, these data demonstrate that PpDELLA proteins share some biological functions with DELLAs in flowering plants, but other DELLA functions and regulation evolved independently in both plant lineages.

Evolution of DELLA function and signaling in land plants

DELLA proteins are master growth regulators that repress responses to a group of plant growth hormones called gibberellins (GAs). Manipulation of DELLA function and signaling was instrumental in the development of high‐yielding crop varieties that saved millions from starvation during the “Green Revolution.” Despite decades of extensive research, it is still unclear how DELLA function and signaling mechanisms evolved within the land plant lineage. Here, we review current knowledge on DELLA protein function with reference to structure, posttranslational modifications, downstream transcriptional targets, and protein–protein interactions. Furthermore, we discuss older and recent findings regarding the evolution of DELLA signaling within the land plant lineage, with an emphasis on bryophytes, and identify future avenues of research that would enable us to shed more light on the evolution of DELLA signaling. Unraveling how DELLA function and signaling mechanisms have evolved could enable us to engineer better crops in an attempt to contribute to mitigating the effects of global warming and achieving global food security.

DELLA genes first appeared in the common ancestor of land plants and underwent two major duplications during land plant evolution.

DELLAs repress gibberellin responses in vascular plants but their function in nonvascular plants remains elusive.

PEATmoss (Physcomitrella Expression Atlas Tool): a unified gene expression atlas for the model plant Physcomitrella patens

Physcomitrella patens is a bryophyte model plant that is often used to study plant evolution and development. Its resources are of great importance for comparative genomics and evo-devo approaches. However, expression data from Physcomitrella patens were so far generated using different gene annotation versions and three different platforms: CombiMatrix and NimbleGen expression microarrays and RNA sequencing. The currently available P. patens expression data are distributed across three tools with different visualization methods to access the data. Here, we introduce an interactive expression atlas, Physcomitrella Expression Atlas Tool (PEATmoss), that unifies publicly available expression data for P. patens and provides multiple visualization methods to query the data in a single web-based tool. Moreover, PEATmoss includes 35 expression experiments not previously available in any other expression atlas. To facilitate gene expression queries across different gene annotation versions, and to access P. patens annotations and related resources, a lookup database and web tool linked to PEATmoss was implemented. PEATmoss can be accessed at https://peatmoss.online.uni-marburg.de.

Cross-kingdom signalling regulates spore germination in the moss Physcomitrella patens

Plants live in close association with microorganisms that can have beneficial or detrimental effects. The activity of bacteria in association with flowering plants has been extensively analysed. Bacteria use quorum-sensing as a way of monitoring their population density and interacting with their environment. A key group of quorum sensing molecules in Gram-negative bacteria are the N-acylhomoserine lactones (AHLs), which are known to affect the growth and development of both flowering plants, including crops, and marine algae. Thus, AHLs have potentially important roles in agriculture and aquaculture. Nothing is known about the effects of AHLs on the earliest-diverging land plants, thus the evolution of AHL-mediated bacterial-plant/algal interactions is unknown. In this paper, we show that AHLs can affect spore germination in a representative of the earliest plants on land, the Bryophyte moss Physcomitrella patens. Furthermore, we demonstrate that sporophytes of some wild isolates of Physcomitrella patens are associated with AHL-producing bacteria.

The Physcomitrella patens gene atlas project: large-scale RNA-seq based expression data

High-throughput RNA sequencing (RNA-seq) has recently become the method of choice to define and analyze transcriptomes. For the model moss Physcomitrella patens, although this method has been used to help analyze specific perturbations, no overall reference dataset has yet been established. In the framework of the Gene Atlas project, the Joint Genome Institute selected P. patens as a flagship genome, opening the way to generate the first comprehensive transcriptome dataset for this moss. The first round of sequencing described here is composed of 99 independent libraries spanning 34 different developmental stages and conditions. Upon dataset quality control and processing through read mapping, 28 509 of the 34 361 v3.3 gene models (83%) were detected to be expressed across the samples. Differentially expressed genes (DEGs) were calculated across the dataset to permit perturbation comparisons between conditions. The analysis of the three most distinct and abundant P. patens growth stages - protonema, gametophore and sporophyte - allowed us to define both general transcriptional patterns and stage-specific transcripts. As an example of variation of physico-chemical growth conditions, we detail here the impact of ammonium supplementation under standard growth conditions on the protonemal transcriptome. Finally, the cooperative nature of this project allowed us to analyze inter-laboratory variation, as 13 different laboratories around the world provided samples. We compare differences in the replication of experiments in a single laboratory and between different laboratories.

Furthering knowledge of seaweed growth and development to facilitate sustainable aquaculture

Macroalgae (seaweeds) are the subject of increasing interest for their potential as a source of valuable, sustainable biomass in the food, feed, chemical and pharmaceutical industries. Compared with microalgae, the pace of knowledge acquisition in seaweeds is slower despite the availability of whole-genome sequences and model organisms for the major seaweed groups. This is partly a consequence of specific hurdles related to the large size of these organisms and their slow growth. As a result, this basic scientific field is falling behind, despite the societal and economic importance of these organisms. Here, we argue that sustainable management of seaweed aquaculture requires fundamental understanding of the underlying biological mechanisms controlling macroalgal life cycles - from the production of germ cells to the growth and fertility of the adult organisms - using diverse approaches requiring a broad range of technological tools. This Viewpoint highlights several examples of basic research on macroalgal developmental biology that could enable the step-changes which are required to adequately meet the demands of the aquaculture sector.

Bacteria-induced morphogenesis of Ulva intestinalis and Ulva mutabilis (Chlorophyta): a contribution to the lottery theory

The green marine macroalgae of the class Ulvophyceae (Ulvophytes) are common algae distributed worldwide particularly in intertidal areas, which play a key role in aquatic ecosystems. They are potentially valuable resources for food, animal feed and fuel but can also cause massive nuisance blooms. Members of Ulvaceae, like many other seaweeds, harbour a rich diversity of epiphytic bacteria with functions related to host growth and morphological development. In the absence of appropriate bacterially derived signals, germ cells of the genus Ulva develop into ‘atypical’ colonies consisting of undifferentiated cells with abnormal cell walls. This paper examines the specificity of bacteria-induced morphogenesis in Ulva, by cross-testing bacteria isolated from several Ulva species on two Ulva species, the emerging model system Ulva mutabilis and the prominent biofouler species Ulva intestinalis. We show that pairs of bacterial strains isolated from species other than U. mutabilis and U. intestinalis can fully rescue axenic plantlets generated either from U. mutabilis or U. intestinalis gametes. This laboratory-based study demonstrates that different compositions of microbial communities with similar functional characteristics can enable complete algal morphogenesis and thus supports the ‘competitive lottery’ theory for how symbiotic bacteria drive algal development.

Surface sensing and stress-signalling in Ulva and fouling diatoms - potential targets for antifouling: a review

Understanding the underlying signalling pathways that enable fouling algae to sense and respond to surfaces is essential in the design of environmentally friendly coatings. Both the green alga Ulva and diverse diatoms are important ecologically and economically as they are persistent biofoulers. Ulva spores exhibit rapid secretion, allowing them to adhere quickly and permanently to a ship, whilst diatoms secrete an abundance of extracellular polymeric substances (EPS), which are highly adaptable to different environmental conditions. There is evidence, now supported by molecular data, for complex calcium and nitric oxide (NO) signalling pathways in both Ulva and diatoms being involved in surface sensing and/or adhesion. Moreover, adaptation to stress has profound effects on the biofouling capability of both types of organism. Targets for future antifouling coatings based on surface sensing are discussed, with an emphasis on pursuing NO-releasing coatings as a potentially universal antifouling strategy.

Surfing amongst Oil-Tankers: Connecting Emerging Research Fields to the Current International Landscape

The COST Action Phycomorph (FA1406) was initiated in 2015 from a handful of academic researchers, and now joins together 19 European countries and nine international partners. Phycomorph's goal is to coordinate and develop research on developmental biology in macroalgae. This is an ambitious project, as the related scientific community is small, the concepts are complex, and there is currently limited knowledge of these organisms and there are few technologies to study them. Here we report the first step in achieving this enterprise, the creation of the Phycomorph network. We share the associated strengths, pitfalls, and prospects for setting up the network in the hope that this might guide similar efforts in other fields.

The decision to germinate is regulated by divergent molecular networks in spores and seeds

Dispersal is a key step in land plant life cycles, usually via formation of spores or seeds. Regulation of spore- or seed-germination allows control over the timing of transition from one generation to the next, enabling plant dispersal. A combination of environmental and genetic factors determines when seed germination occurs. Endogenous hormones mediate this decision in response to the environment. Less is known about how spore germination is controlled in earlier-evolving nonseed plants. Here, we present an in-depth analysis of the environmental and hormonal regulation of spore germination in the model bryophyte Physcomitrella patens (Aphanoregma patens). Our data suggest that the environmental signals regulating germination are conserved, but also that downstream hormone integration pathways mediating these responses in seeds were acquired after the evolution of the bryophyte lineage. Moreover, the role of abscisic acid and diterpenes (gibberellins) in germination assumed much greater importance as land plant evolution progressed. We conclude that the endogenous hormone signalling networks mediating germination in response to the environment may have evolved independently in spores and seeds. This paves the way for future research about how the mechanisms of plant dispersal on land evolved.

An ancient and conserved function for Armadillo-related proteins in the control of spore and seed germination by abscisic acid

Armadillo-related proteins regulate development throughout eukaryotic kingdoms. In the flowering plant Arabidopsis thaliana, Armadillo-related ARABIDILLO proteins promote multicellular root branching. ARABIDILLO homologues exist throughout land plants, including early-diverging species lacking true roots, suggesting that early-evolving ARABIDILLOs had additional biological roles. Here we investigated, using molecular genetics, the conservation and diversification of ARABIDILLO protein function in plants separated by c. 450 million years of evolution. We demonstrate that ARABIDILLO homologues in the moss Physcomitrella patens regulate a previously undiscovered inhibitory effect of abscisic acid (ABA) on spore germination. Furthermore, we show that A. thaliana ARABIDILLOs function similarly during seed germination. Early-diverging ARABIDILLO homologues from both P. patens and the lycophyte Selaginella moellendorffii can substitute for ARABIDILLO function during A. thaliana root development and seed germination. We conclude that (1) ABA was co-opted early in plant evolution to regulate functionally analogous processes in spore- and seed-producing plants and (2) plant ARABIDILLO germination functions were co-opted early into both gametophyte and sporophyte, with a specific rooting function evolving later in the land plant lineage.

The green seaweed Ulva: a model system to study morphogenesis

Green macroalgae, mostly represented by the Ulvophyceae, the main multicellular branch of the Chlorophyceae, constitute important primary producers of marine and brackish coastal ecosystems. Ulva or sea lettuce species are some of the most abundant representatives, being ubiquitous in coastal benthic communities around the world. Nonetheless the genus also remains largely understudied. This review highlights Ulva as an exciting novel model organism for studies of algal growth, development and morphogenesis as well as mutualistic interactions. The key reasons that Ulva is potentially such a good model system are: (i) patterns of Ulva development can drive ecologically important events, such as the increasing number of green tides observed worldwide as a result of eutrophication of coastal waters, (ii) Ulva growth is symbiotic, with proper development requiring close association with bacterial epiphytes, (iii) Ulva is extremely developmentally plastic, which can shed light on the transition from simple to complex multicellularity and (iv) Ulva will provide additional information about the evolution of the green lineage.

Regulation of gametogenesis and zoosporogenesis in Ulva linza (Chlorophyta): comparison with Ulva mutabilis and potential for laboratory culture

Green Ulvophyte macroalgae represent attractive model systems for understanding growth, development, and evolution. They are untapped resources for food, fuel, and high-value compounds, but can also form nuisance blooms. To fully analyze green seaweed morphogenesis, controlled laboratory-based culture of these organisms is required. To date, only a single Ulvophyte species, Ulva mutabilis Føyn, has been manipulated to complete its whole life cycle in laboratory culture and to grow continuously under axenic conditions. Such cultures are essential to address multiple key questions in Ulva development and in algal-bacterial interactions. Here we show that another Ulva species, U. linza, with a broad geographical distribution, has the potential to be grown in axenic culture similarly to U. mutabilis. U. linza can be reliably induced to sporulate (form gametes and zoospores) in the laboratory, by cutting the relevant thallus tissue into small pieces and removing extracellular inhibitors (sporulation and swarming inhibitors). The germ cells work as an ideal feed stock for standardized algae cultures. The requirement of U. linza for bacterial signals to induce its normal morphology (particularly of the rhizoids) appears to have a species-specific component. The axenic cultures of these two species pave the way for future comparative studies of algal-microbial interactions.

AtMYB93 is a novel negative regulator of lateral root development in Arabidopsis

Plant root system plasticity is critical for survival in changing environmental conditions. One important aspect of root architecture is lateral root development, a complex process regulated by hormone, environmental and protein signalling pathways. Here we show, using molecular genetic approaches, that the MYB transcription factor AtMYB93 is a novel negative regulator of lateral root development in Arabidopsis. We identify AtMYB93 as an interaction partner of the lateral-root-promoting ARABIDILLO proteins. Atmyb93 mutants have faster lateral root developmental progression and enhanced lateral root densities, while AtMYB93-overexpressing lines display the opposite phenotype. AtMYB93 is expressed strongly, specifically and transiently in the endodermal cells overlying early lateral root primordia and is additionally induced by auxin in the basal meristem of the primary root. Furthermore, Atmyb93 mutant lateral root development is insensitive to auxin, indicating that AtMYB93 is required for normal auxin responses during lateral root development. We propose that AtMYB93 is part of a novel auxin-induced negative feedback loop stimulated in a select few endodermal cells early during lateral root development, ensuring that lateral roots only develop when absolutely required. Putative AtMYB93 homologues are detected throughout flowering plants and represent promising targets for manipulating root systems in diverse crop species.

AtMYB93 is an endodermis-specific transcriptional regulator of lateral root development in arabidopsis

Plant root systems are critical for survival, acting as the primary interface for nutrient and water acquisition, as well as anchoring the plant to the ground. As plants grow, their root systems become more elaborate, which is largely mediated by the formation of root branches, or lateral roots. Lateral roots initiate deep within the root in the pericycle cell layer, and their development is controlled by a wide range of internal signaling factors and environmental cues, as well as mechanical feedback from the surrounding cells. The endodermal cell layer, which overlies the pericycle, has emerged as an important tissue regulating LR initiation and formation. We recently identified the AtMYB93 transcription factor as a negative regulator of lateral root development in Arabidopsis. Interestingly, AtMYB93 expression is highly restricted to the few endodermal cells overlying developing lateral root primordia, suggesting that this transcriptional regulator might play a key role in mediating the effect of the endodermis on lateral root development. Here we discuss our recent findings in the wider context of root system development - with a particular focus on the role of the endodermis - and propose several potential models to explain AtMYB93 function during lateral root organogenesis.

ARABIDILLO gene homologues in basal land plants: species-specific gene duplication and likely functional redundancy

ARABIDILLO proteins regulate multicellular root development in Arabidopsis thaliana. Conserved ARABIDILLO homologues are present throughout land plants, even in early-evolving plants that do not possess complex root architecture, suggesting that ARABIDILLO genes have additional functions. Here, we have cloned and characterised ARABIDILLO gene homologues from two early-evolving land plants, the bryophyte Physcomitrella patens and the lycophyte Selaginella moellendorffii. We show that two of the PHYSCODILLO genes (PHYSCODILLO1A and -1B) exist as a tail-to-tail tandem array of two almost identical 12 kb sequences, while a third related gene (PHYSCODILLO2) is located elsewhere in the Physcomitrella genome. Physcomitrella possesses a very low percentage of tandemly arrayed genes compared with the later-evolving plants whose genomes have been sequenced to date. Thus, PHYSCODILLO1A and -1B genes represent a relatively unusual gene arrangement. PHYSCODILLO promoters are active largely in the haploid gametophyte, with additional activity at the foot of the sporophyte. The pattern of promoter activity is uniform in filamentous and leafy tissues, suggesting pleiotropic gene functions and likely functional redundancy: the latter possibility is confirmed by the lack of discernible phenotype in a physcodillo2 deletion mutant. Interestingly, the pattern of PHYSCODILLO promoter activity in female reproductive organs is strikingly similar to that of an Arabidopsis homologue, suggesting co-option of some PHYSCODILLO functions or regulation into both the sporophyte and gametophyte. In conclusion, our work identifies and characterises some of the earliest-evolving land plant ARABIDILLO homologues. We confirm that all land plant ARABIDILLO genes arose from a single common ancestor and suggest that PHYSCODILLO proteins have novel and pleiotropic functions, some of which may be conserved in later-evolving plants.

Function and evolution of 'green' GSK3/Shaggy-like kinases

Glycogen synthase kinase 3 (GSK3) proteins, also known as SHAGGY-like kinases, have many important cell signalling roles in animals, fungi and amoebae. In particular, GSK3s participate in key developmental signalling pathways and also regulate the cytoskeleton. GSK3-encoding genes are also present in all land plants and in algae and protists, raising questions about possible ancestral functions in eukaryotes. Recent studies have revealed that plant GSK3 proteins are actively implicated in hormonal signalling networks during development as well as in biotic and abiotic stress responses. In this review, we outline the mechanisms of Arabidopsis GSK3 action, summarize GSK3 functions in dicot and monocot flowering plants, and speculate on the possible functions of GSK3s in the earliest-evolving land plants.

ARABIDILLO proteins have a novel and conserved domain structure important for the regulation of their stability

ARABIDILLO proteins are F-box-Armadillo (ARM) proteins that regulate root branching in Arabidopsis. Many F-box proteins in plants, yeast and mammals are unstable. In plants, the mechanism for this instability has not been fully investigated. Here, we show that a conserved family of plant ARABIDILLO-related proteins has a unique domain structure consisting of an F-box and leucine-rich repeats (LRRs) followed by ARM-repeats. The LRRs are similar to those found in other plant and animal F-box proteins, including cell cycle proteins and hormone receptors. We demonstrate that the LRRs are required for ARABIDILLO1 function in vivo. ARABIDILLO1 protein is unstable: we show that ARABIDILLO1 protein is associated with ubiquitin and is turned over by the proteasome. Both the F-box and LRR regions of ARABIDILLO1 appear to enable this turnover to occur. Application of known lateral root-regulating signals has no effect on ARABIDILLO1 stability. In addition, plants that lack or overexpress ARABIDILLO proteins respond normally to known lateral root-regulating signals. Thus, we suggest that the signal(s) regulating ARABIDILLO stability in vivo may be either highly specific or novel. The structural conservation between ARABIDILLOs and other plant and animal F-box proteins suggests that the stability of other F-box proteins may be controlled by similar mechanisms.

The Armadillo repeat protein PF16 is essential for flagellar structure and function in Plasmodium male gametes

Malaria, caused by the apicomplexan parasite Plasmodium, threatens 40% of the world's population. Transmission between vertebrate and insect hosts depends on the sexual stages of the life-cycle. The male gamete of Plasmodium parasite is the only developmental stage that possesses a flagellum. Very little is known about the identity or function of proteins in the parasite's flagellar biology. Here, we characterise a Plasmodium PF16 homologue using reverse genetics in the mouse malaria parasite Plasmodium berghei. PF16 is a conserved Armadillo-repeat protein that regulates flagellar structure and motility in organisms as diverse as green algae and mice. We show that P. berghei PF16 is expressed in the male gamete flagellum, where it plays a crucial role maintaining the correct microtubule structure in the central apparatus of the axoneme as studied by electron microscopy. Disruption of the PF16 gene results in abnormal flagellar movement and reduced fertility, but does not lead to complete sterility, unlike pf16 mutations in other organisms. Using homology modelling, bioinformatics analysis and complementation studies in Chlamydomonas, we show that some regions of the PF16 protein are highly conserved across all eukaryotes, whereas other regions may have species-specific functions. PF16 is the first ARM-repeat protein characterised in the malaria parasite genus Plasmodium and this study opens up a novel model for analysis of Plasmodium flagellar biology that may provide unique insights into an ancient organelle and suggest novel intervention strategies to control the malaria parasite.

Shoot Na+ exclusion and increased salinity tolerance engineered by cell type-specific alteration of Na+ transport in Arabidopsis

Soil salinity affects large areas of cultivated land, causing significant reductions in crop yield globally. The Na+ toxicity of many crop plants is correlated with overaccumulation of Na+ in the shoot. We have previously suggested that the engineering of Na+ exclusion from the shoot could be achieved through an alteration of plasma membrane Na+ transport processes in the root, if these alterations were cell type specific. Here, it is shown that expression of the Na+ transporter HKT1;1 in the mature root stele of Arabidopsis thaliana decreases Na+ accumulation in the shoot by 37 to 64%. The expression of HKT1;1 specifically in the mature root stele is achieved using an enhancer trap expression system for specific and strong overexpression. The effect in the shoot is caused by the increased influx, mediated by HKT1;1, of Na+ into stelar root cells, which is demonstrated in planta and leads to a reduction of root-to-shoot transfer of Na+. Plants with reduced shoot Na+ also have increased salinity tolerance. By contrast, plants constitutively expressing HKT1;1 driven by the cauliflower mosaic virus 35S promoter accumulated high shoot Na+ and grew poorly. Our results demonstrate that the modification of a specific Na+ transport process in specific cell types can reduce shoot Na+ accumulation, an important component of salinity tolerance of many higher plants.

Branching out in new directions: the control of root architecture by lateral root formation

Plant roots are required for the acquisition of water and nutrients, for responses to abiotic and biotic signals in the soil, and to anchor the plant in the ground. Controlling plant root architecture is a fundamental part of plant development and evolution, enabling a plant to respond to changing environmental conditions and allowing plants to survive in different ecological niches. Variations in the size, shape and surface area of plant root systems are brought about largely by variations in root branching. Much is known about how root branching is controlled both by intracellular signalling pathays and by environmental signals. Here, we will review this knowledge, with particular emphasis on recent advances in the field that open new and exciting areas of research.

Armadillo-related proteins promote lateral root development in Arabidopsis

Armadillo/beta-catenin and related proteins have important functions during animal and Dictyostelium development, regulating cell differentiation, proliferation, and adhesion. Armadillo-repeat-containing proteins also exist in plants, but the majority have unknown roles. The Arabidopsis genes that show greatest sequence homology to Armadillo/beta-catenin are called ARABIDILLO-1 and -2. Here, we demonstrate that ARABIDILLO-1 and -2 promote lateral root development. arabidillo-1/-2 mutants form fewer lateral roots, and ARABIDILLO-1-overexpressing lines produce more lateral roots than wild-type seedlings. ARABIDILLO-yellow fluorescent protein fusions are nuclear. ARABIDILLO proteins contain an F-box motif, and thus may target other proteins for proteasomal degradation. Overexpression of ARABIDILLO-1 protein fragments, including F-box fragments, in wild-type seedlings reduces lateral root formation to the level of the arabidillo-1/-2 mutant. We have shown that plant beta-catenin-related proteins regulate root development. We suggest that ARABIDILLO proteins may target an inhibitor of lateral root development for degradation and propose that Arabidopsis beta-catenin-related proteins define a previously uncharacterized pathway that promotes root branching.

A prehistory of cell adhesion

Cell adhesion is a basic property of animal cells, but is also present in many other eukaryotes. Did cell adhesion systems arise independently in different eukaryotic groups, or do they share common origins? Recent results show that cell adhesion proteins related to cadherin, IgG-like CAM and C-type lectin are present both in sponges, the most distant animal branch, and in eukaryote groups outside the metazoan lineage, indicating that these forms of adhesion arose prior to animal evolution. Furthermore, proteins containing features of animal adhesion systems, such as Fas-1 and thrombospondin domains, are distributed throughout the eukaryotes and function in cell adhesion.

Armadillo repeat proteins: beyond the animal kingdom

Armadillo (Arm) repeat proteins contain tandem copies of a degenerate protein sequence motif that forms a conserved three-dimensional structure. Animal Arm repeat proteins function in various processes, including intracellular signalling and cytoskeletal regulation. A subset of these proteins are conserved across eukaryotic kingdoms, and non-metazoa such as Dictyostelium and Chlamydomonas possess homologues of members of the animal Arm repeat family. Higher plants also possess Arm repeat proteins, which, like their animal counterparts, function in intracellular signalling. Notably, these plant Arm proteins have novel functions. In addition, genome sequencing has identified a plethora of Arm-related proteins in Arabidopsis.

Antagonistic pathways in neurons exposed to body fluid regulate social feeding in Caenorhabditis elegans

Wild isolates of Caenorhabditis elegans can feed either alone or in groups. This natural variation in behaviour is associated with a single residue difference in NPR-1, a predicted G-protein-coupled neuropeptide receptor related to Neuropeptide Y receptors. Here we show that the NPR-1 isoform associated with solitary feeding acts in neurons exposed to the body fluid to inhibit social feeding. Furthermore, suppressing the activity of these neurons, called AQR, PQR and URX, using an activated K(+) channel, inhibits social feeding. NPR-1 activity in AQR, PQR and URX neurons seems to suppress social feeding by antagonizing signalling through a cyclic GMP-gated ion channel encoded by tax-2 and tax-4. We show that mutations in tax-2 or tax-4 disrupt social feeding, and that tax-4 is required in several neurons for social feeding, including one or more of AQR, PQR and URX. The AQR, PQR and URX neurons are unusual in C. elegans because they are directly exposed to the pseudocoelomic body fluid. Our data suggest a model in which these neurons integrate antagonistic signals to control the choice between social and solitary feeding behaviour.

Loss of the beta-catenin homologue aardvark causes ectopic stalk formation in Dictyostelium

Aardvark (Aar) is a Dictyostelium beta-catenin homologue with both cytoskeletal and signal transduction roles during development. Here, we show that loss of aar causes a novel phenotype where multiple stalks appear during late development. Ectopic stalks are preceded by misexpression of the stalk marker ST-lacZ in the surrounding tissue. This process does not involve the kinase GSK-3. Mixing experiments show that ectopic ST-lacZ expression and stalk formation are cell non-autonomous. The protein-cellulose matrix surrounding the stalk of aar mutant fruiting bodies is defective, and damage to the stalk of wild-type fruiting bodies leads to ectopic ST-lacZ expression. We postulate that poor synthesis of the stalk tube matrix allows diffusion of a stalk cell-inducing factor into the surrounding tissue.

Cell-cell adhesion and signal transduction duringDictyosteliumdevelopment

The development of the non-metazoan eukaryote Dictyostelium discoideum displays many of the features of animal embryogenesis, including regulated cell-cell adhesion. During early development, two proteins, DdCAD-1 and csA, mediate cell-cell adhesion between amoebae as they form a loosely packed multicellular mass. The mechanism governing this process is similar to epithelial sheet sealing in animals. Although cell differentiation can occur in the absence of cell contact, regulated cell-cell adhesion is an important component of Dictyostelium morphogenesis, and a third adhesion molecule, gp150, is required for multicellular development past the aggregation stage.

Cell-cell junctions that appear to be adherens junctions form during the late stages of Dictyostelium development. Although they are not essential to establish the basic multicellular body plan, these junctions are required to maintain the structural integrity of the fruiting body. The Dictyostelium β-catenin homologue Aardvark (Aar) is present in adherens junctions, which are lost in its absence. As in the case of its metazoan counterparts, Aar also has a function in cell signalling and regulates expression of the pre-spore gene psA.

It is becoming clear that cell-cell adhesion is an integral part of Dictyostelium development. As in animals, cell adhesion molecules have a mechanical function and may also interact with the signal-transduction processes governing morphogenesis.

Adherens junctions and beta-catenin-mediated cell signalling in a non-metazoan organism

Mechanical forces between cells have a principal role in the organization of animal tissues. Adherens junctions are an important component of these tissues, connecting cells through their actin cytoskeleton and allowing the assembly of tensile structures. At least one adherens junction protein, beta-catenin, also acts as a signalling molecule, directly regulating gene expression. To date, adherens junctions have only been detected in metazoa, and therefore we looked for them outside the animal kingdom to examine their evolutionary origins. The non-metazoan Dictyostelium discoideum forms a multicellular, differentiated structure. Here we describe the discovery of actin-associated intercellular junctions in Dictyostelium. We have isolated a gene encoding a beta-catenin homologue, aardvark, which is a component of the junctional complex, and, independently, is required for cell signalling. Our discovery of adherens junctions outside the animal kingdom shows that the dual role of beta-catenin in cell-cell adhesion and cell signalling evolved before the origins of metazoa.