Guillaume Tena

Immunity: NLR population control.

The immune response in plants is costly because every cell defends itself, so it is only induced when pathogen presence is sensed through membrane or cytosolic receptors. NLRs are cellular sensors that perceive the presence or activity of pathogen effectors being delivered into the plant cell, initiating a strong immune response. If this control is lost, defence can be constitutively switched on and plants then suffer from autoimmunity, leading to dwarfed growth. To find new immune regulators that may be difficult to discover in a wild-type background, Shuai Huang, Xin Li and colleagues from the University of British Columbia, Canada, searched for mutations that enhanced the autoimmune phenotype of the mutant snc1, in which the NLR protein SNC1 is stabilized. They identified two redundant TRAF-like proteins named MUSE. TRAFs are major players in animal immunity, and despite the large family in Arabidopsis, these proteins have been seldom studied in plants. A double muse mutant shows very severe autoimmune symptoms, confirming the role of MUSE proteins as negative regulators of immunity. After an elegant series of genetic and molecular experiments, the authors concluded that, just like in animals, the two MUSE proteins mediate the formation of signalling complexes called TRAFasomes, which bring together NLRs and proteasome components. These complexes control NLR homeostasis, so that plants can choose between growth and defence. GT

Auxin signalling: Grass on acid.

It has been known for hundreds of years that floral scent is related to pollination, but Alexander Haverkamp and colleagues from the Max Planck Institute for Chemical Ecology in Jena have now shown that it is not a simple case of attraction. The researchers studied wild tobacco, Nicotiana attenuate, which is pollinated by the hawkmoth Manduca sexta and whose flowers emit a fairly simple scent dominated by benzyl acetone (BA). Using RNAi, the researchers silenced the production of BA to compare emitting and non-emitting plants. Somewhat surprisingly, odourless flowers were not ‘invisible’ to the moths, which showed no preference for visiting either type of flower both in tent and wind-tunnel tests. However, the odoured plants were pollinated more efficiently than those emitting no BA. Closer investigation revealed that the hawkmoths remained on scented flowers for longer and were more persistent at attempting to insert their proboscis into the bloom. The researchers identified cells on the very tips of the proboscis as responsible for sensing BA and showed that the behaviour of the moth was affected only once the proboscis had entered the flower. Rather than being a long-range signal (which could also attract nectar thieves, herbivores and female moths looking for sites to lay their eggs), BA is a local signal indicating the presence of ample nectar and so encouraging the moth to remain longer on the flower, thereby increasing the likelihood of pollination. CS

Cell wall formation: Xylem differentiation.

Densely packed clusters of short, hairy roots enhance nutrient uptake in many plants. A series of controlled-environment experiments suggest that soil bacteria that stimulate plant growth promote production of these root clusters. Although particularly important in nutrient-scarce environments, the factors that regulate root-cluster formation have remained unclear. Byron B. Lamont, of Curtin University, Australia, and colleagues tested the effects of two species of plantgrowth-promoting soil bacteria on cluster formation in representatives of the major root-cluster-bearing families — Fabaceae and Proteaceae — under four nutrient regimes. The bacteria stimulated cluster formation in six of the sixteen treatments. The stimulation, which was underpinned by increases in root length and the number of clusters per unit root length, was more pronounced in the presence of nitrogen. The researchers attribute the stimulation of cluster formation to bacterial secretion of the plant hormone auxin, known to promote the growth of these root systems. They suggest that the variable response observed stems from bacterial-dependent differences in auxin secretion, and plant-dependent differences in auxin sensitivity. AA

A window into the past

The Arabidopsis era is slowly fading away. Now is the time to embrace the full complexity of plants important for food security by doing translational research directly on model crops in the field. However, an opposite trend of plant biology has recently emerged through the use of living relatives of ancient plants with simpler genomes that can reveal mechanisms hidden by genetic redundancy. Molecular genetics in these humble non-vascular models, such as liverwort Marchantia polymorpha and moss Physcomitrella patens, also opens an unprecedented window into key evolutionary innovations. Liverworts diverged from every other land plant more than 450 million years ago, and represent one of the most ancient land plant lineages. Two recent examples highlight the use of Marchantia to understand the evolution of interactions between plants and their environment. The first study, led by Sebastian Schornack in Cambridge, UK, describes how a filamentous oomycete of the Phytophthora genus can colonize Marchantia photosynthetic layers and cause disease. Despite some atypical aspects, the pathogen develops intracellular infection structures, expresses typical effectors and switches from biotrophic to necrotrophic lifestyles; while the liverwort deals with this microbial invasion by activating a set of common responses, much like a modern plant does. The second study, headed by Roberto Solano in Madrid, demonstrates that the jasmonate receptor COI1 is functionally conserved in Marchantia for wounding and defence responses, a surprising result given that the stress hormone is not synthesized in liverwort. The reason is that COI1 in Marchantia binds another signalling molecule, a jasmonate precursor. So, jasmonate perception in modern plants comes from a balancing co-evolution game between a receptor and biosynthesis of its ligand. If complex crop models allow us to hopefully envisage ourselves in a future with less hunger, simpler models take us back in time to contemplate the ancient origin and fascinating evolutionary history of land plants.

Recruiting microbial bodyguards

Soil is more than decaying organic matter and some rocks. A teaspoon of a healthy topsoil layer probably contains billions of microorganisms encompassing thousands of species, many of them not yet described. The current unabating progress in sequencing power — enabling increasingly sensitive metagenomic studies — is helping to reveal the functional importance of the soil microbiome and its effect on plant development, nutrition and responses to environmental changes. Writing in Nature Biotechnology, Jihyun Kim and colleagues demonstrate that in tomato, the microbiome composition is critical for resistance to Ralstonia solanacearum, a soil pathogen that colonizes the xylem and causes bacterial wilt. The team applied a metagenomics approach to compare the rhizosphere microorganisms of resistant and susceptible tomato varieties. Transplant experiments showed that the soil by itself confers some level of resistance to the susceptible genotype. Finally, the authors isolate and characterize a novel flavobacterium that can partially suppress bacterial wilt symptoms when added to soil. This study reveals that plants recruit beneficial microorganisms to help them fight against pathogens or more generally to tolerate non-optimal conditions. It is easy to conceive potential strategies to protect crops from diseases in the future, for example by directly inoculating soils or seeds, or by modifying genomes to exudate attractive chemicals, in order to recruit the right kind of beneficial bacteria. Crop diseases are responsible for huge economic losses and threaten food security worldwide, and every solution is welcome.

Dr Rhodo and Mr Coccus

Microbes have a bad reputation. However, one of the most profound discoveries in biology over the past decades is the realization that microorganisms have a tremendous influence, often positive, on the animals and plants they closely associate with. Microbes can be commensal, beneficial or pathogenic for the host. These categories were thought to be quite different in terms of genomic sequences, and relatively easy to classify. However, a team of researchers led by Jeff Chang now demonstrates that a beneficial microorganism can easily switch to pathogenic with the acquisition of a single plasmid. Rhodococcus is a genus of bacteria that associates with many plants, and is the causative agent for the leafy gall disease that affects plant growth and development. The researchers started with a genomic epidemiology approach by collecting many bacterial isolates from nurseries, for which they obtained genome and plasmid sequences. They realized that bacteria lacking one plasmid that contains three virulence genes are beneficial for their host, as they enhance root architecture. On the other hand, the acquisition of this plasmid is the only change necessary for beneficial bacteria to become pathogenic and cause disease, regardless of its genomic background. This type of horizontal gene transfer may happen frequently in agricultural settings. The consequences of this research go beyond the discoveries about sudden evolutionary transitions. The authors also developed a more accurate method to diagnose the presence of Rhodococcus in commercial pistachio orchards, and to discriminate between beneficial and pathogenic bacteria. In a strongly worded discussion that criticizes previously published results, the authors mention that this could help to avoid potential misdiagnoses that may have already caused the unfortunate felling of entire orchards.

Stratospheric order mutants

Many Arabidopsis genes are still not associated with a biological function. Reverse genetics, applied mostly through insertional mutagenesis and recently with genome editing, is a powerful approach to assign a function to one gene. Unfortunately, the redundancy of plant genomes often makes it necessary to mutate several genes before obtaining a phenotype. The amount of time-consuming background work makes it difficult to produce high-order mutants. However, two recent studies, on abscisic acid (ABA) and immune signalling, used mutants of a level likely unseen before. Jian-Kang Zhu and collaborators produced plants mutated on 12 (duodecuple) and all 14 (quattuordecuple) members of the PYR/PYL family of Arabidopsis ABA receptors, by combining insertion mutants and multiplex CRISPR–Cas9. The plants were extremely insensitive to ABA for various processes, such as stomatal closure, germination and senescence, but unexpectedly some ABAindependent responses to osmotic stress were enhanced. Another project, headed by Jian-Min Zhou, focused on receptor-like cytoplasmic kinases (RLCK) subfamily VII. Members of this subfamily seem to connect membrane perception of pathogen signals to downstream events, such as MAPK activation. There are 46 RLCK VII in Arabidopsis, distributed in 9 subgroups. Once again combining insertion and CRISPR–Cas9 mutants, the authors produced high-order mutants for each subgroup. They characterized their response to various patterns, and found, for example, that one subgroup is necessary for chitin signalling. These studies show that old and new techniques can be merged to obtain genetic tools that were unreachable only a few years before. It is now possible to knock out entire gene families and, hopefully, this will help to reduce the percentage of Arabidopsis genes with unknown function.

Calcium channel helpers

Glutamate is an important neurotransmitter in vertebrates. Its perception across synapses is mediated by transmembrane receptors. Plants also contain glutamate receptor-like (GLR) proteins, which function as nonspecific channels with calcium permeability and have been involved in many biological processes, from immunity to metabolism. José Feijó and colleagues have identified molecular partners that are needed for proper function and subcellular localization of GLRs during pollen tube growth. Arabidopsis contains 20 GLRs. A close inspection of individual and higherorder mutants for pollen-expressed GLRs suggested that their role is more complex than simple plasma membrane calcium channels. A few of them could regulate calcium homeostasis in pollen by redirecting the ion away from the cytosol and into intracellular reservoirs, which implies specific subcellular localizations. Using analogy with animal systems, the authors focused on the CORNICHON (CNIH) family of proteins needed for trafficking of GLRs in vertebrates. Indeed, in a double cnih mutant, GLRs — but not other similarly targeted proteins, indicating some specificity — are not correctly sorted and end up trapped in the endomembrane network. Furthermore, the presence of CNIH proteins increases the GLR ion channel activity, even without its ligand, in electrophysiology assays. This study shows that understanding biological functions is a fascinating neverending game of molecular Russian nesting dolls, with many layers of complexity adding on top of each other: calcium is a necessary signal for pollination, GLRs modulate calcium fluxes, CNIH controls GLR localization and activity. Now may be the time to address how CNIH proteins are regulated.

Alone in the dark

Caves are fascinating and almost completely isolated worlds within a world. The saturated humidity, buffered temperature and of course absolute darkness deep within caves mean that plants cannot grow, but speleologists know that there is often a unique ecosystem that develops in the very limited zone in and around the entrance. Conditions there are peculiar due to the transition between both worlds, particularly in porous limestone regions known as karsts. For example, during a dry Mediterranean summer, it is not unusual to find rather large tropical-looking ferns that enjoy both the humidity and low light. Now, botanists from the Royal Botanic Gardens at Kew and from Guangxi in South China joined forces to survey the flora from cave entrances around karstic regions in South China. The microclimate is not the only distinguishing characteristic of cave entrance ecosystems. Small population sizes, fragmented geography and human influence through tourism or agriculture all have an impact on the biodiversity of cave plants. The team spent several years documenting dozens of caves, identifying hundreds of species of vascular plants, mostly angiosperms and some ferns, and classifying them into three growing zones with decreasing light levels: entrance, twilight and dark. Most of the species come from the surrounding karst forest, even when this forest has been eliminated, suggesting a role for caves as microrefuges and passive guardians of ancient forest biodiversity. The authors conclude that caves must be protected from degradation because of their high value for conservation and restoration purposes in the study region, which has seen massive deforestation during the twentieth century. This work also demonstrates once again the extraordinary adaptability of plants to peculiar and sometimes harsh environments on the planet.

Immunity: No sugar for you

With their immense reserves of carbohydrates, plants are very tempting targets for many microorganisms. Bacteria, fungi and oomycetes have all evolved various strategies to proliferate by tapping into the nutritional reserves of plants. Previous research showed that bacterial pathogens increase the flow of sucrose towards the apoplast, where they grow, by manipulating plant plasma membrane sugar transporters. However, according to new research by Yoshitaka Takano and colleagues from Japan, the plants fight back by reabsorbing the sugars inside the cells, in effect starving the pathogens. The authors discover that an Arabidopsis line mutated in two hexose transporters of the sugar transport protein (STP) family is more susceptible to the bacterial pathogen Pseudomonas syringae. They reconstruct the molecular pathway step by step to explain this result. After perception of the microbial signature peptide flg22 (a conserved domain of the flagellin protein) by its receptor FLS2 (FLAGELLIN-SENSING 2), two events occur: co-receptor BAK1 (BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED RECEPTOR KINASE 1) phosphorylates STP13, increasing the activity of the sugar transporter; and STP13 levels become higher in the leaf. So plants react to incoming bacteria both post-translationally and transcriptionally, increasing the reabsorption of hexoses by the cell. This results in less sugar in the apoplast, which restricts proliferation and virulence of the invading bacteria. This study highlights once again how sugar distribution is an important aspect of plant– pathogen warfare. The high prevalence and conservation of sugar transporters in other plants points towards possible novel resistance strategies in crops.