A pentangular plant inflammasome

Like animals, plants require, and have evolved, a robust innate immune system. Plants can detect and respond to pathogen-derived molecules (effectors) through cell surface receptors and intracellular receptors, typically encoded by disease resistance (R) genes. Analysis of plant genome sequences reveals hundreds of such nucleotide-binding, leucine-rich repeat (NLR) proteins encoded by putative R genes. How such NLR proteins function has long been a matter of speculation. On page 43 and 44 of this issue, Wang et al. and Wang et al., respectively, end much of the speculation by defining the mechanism of activation for at least one NLR, the Arabidopsis thaliana HOPZ-ACTIVATED RESISTANCE 1 (ZAR1) protein, which activates defense in response to several pathogenic bacterial effectors

Intracellular innate immune surveillance devices in plants and animals

Multicellular eukaryotes coevolve with microbial pathogens, which exert strong selective pressure on the immune systems of their hosts. Plants and animals use intracellular proteins of the nucleotide-binding domain, leucine-rich repeat (NLR) superfamily to detect many types of microbial pathogens.The NLR domain architecture likely evolved independently and convergently in each kingdom, and the molecular mechanisms of pathogen detection by plant and animal NLRs have long been considered to be distinct. However, microbial recognition mechanisms overlap, and it is now possible to discern important key trans-kingdom principles of NLR-dependent immune function. Here, we attempt to articulate these principles.We propose that the NLR architecture has evolved for pathogen-sensing in diverse organisms because of its utility as a tightly folded "hair trigger" device into which a virtually limitless number of microbial detection platforms can be integrated. Recent findings suggest means to rationally design novel recognition capabilities to counter disease

Con-Ca2+-tenating plant immune responses via calcium-permeable cation channels

Calcium serves as a second messenger in a variety of developmental and physiological processes and has long been identified as important for plant immune responses. We discuss recent discoveries regarding plant immune-related calcium-permeable channels and how the two intertwined branches of the plant immune system are intricately linked to one another through calcium signalling. Cell surface immune receptors carefully tap the immense calcium gradient that exists between apoplast and cytoplasm in a short burst via tightly regulated plasma membrane (PM)-resident cation channels. Intracellular immune receptors form atypical calcium-permeable cation channels at the PM and mediate a prolonged calcium influx, overcoming the deleterious influence of pathogen effectors and enhancing plant immune responses

New Horizons for Plant Translational Research

The world’s human population continues to expand and is predicted to reach ~9 billion by 2040, up from its current level of just over 7 billion. Some estimate that with this rate of population growth, accommodating the increased demand for food will require the world’s agricultural production to increase 50% by 2030. The planet’s water resources are also under pressure. As Pamela Ronald highlights in her accompanying Essay, the amount of fresh water available per person has decreased 4-fold in the last 60 years and of the water that is available ~70% is already used for agriculture. Thus, agricultural production must be intensified to feed more people with less water on the same amount of land (given that little undeveloped arable land remains and what does is being lost to urbanization, desertification, and environmental damage). Furthermore, pathogens that cause devastating crop losses continue to spread in the face of increased global commerce and climate change. Given these challenges, there is a pressing need for plant research to produce solutions to ensure food security in a sustainable and safe way. The need is acute in both developed countries and in the less developed parts of the world, where many people endure chronic malnutrition and suffer the long term consequences on their health and well being. Plant scientists, therefore, urgently need to increase the productivity, pathogen resistance, and sustainability of existing crops, and are challenged to domesticate new crops

Root microbiome modulates plant growth promotion induced by low doses of glyphosate

Glyphosate is a commonly used herbicide with a broad action spectrum. However, at sublethal doses, glyphosate can induce plant growth, a phenomenon known as hormesis. Most glyphosate hormesis studies have been performed under microbe-free or reduced-microbial-diversity conditions; only a few were performed in open systems or agricultural fields, which include a higher diversity of soil microorganisms. Here, we investigated how microbes affect the8 hormesis induced by low doses of glyphosate. To this end, we used Arabidopsis thaliana and a well-characterized synthetic bacterial community of 185 strains (SynCom) that mimics the root-associated microbiome of Arabidopsis. We found that a dose of 3.6 x 10-6 g acid equivalent/liter (low dose of glyphosate, or LDG) produced an ~14% increase in the shoot dry weight (i.e., hormesis) of uninoculated plants. Unexpectedly, in plants inoculated with the SynCom, LDG reduced shoot dry weight by 17%. We found that LDG enriched two Firmicutes and two Burkholderia strains in the roots. These specific strains are known to act as root growth inhibitors (RGI) in monoassociation assays. We tested the link between RGI and shoot dry weight reduction in LDG by assembling a new synthetic community lacking RGI strains. Dropping RGI strains out of the community restored growth induction by LDG. Finally, we showed that individual RGI strains from a few specific phyla were sufficient to switch the response to LDG from growth promotion to growth inhibition. Our results indicate that glyphosate hormesis was completely dependent on the root microbiome composition, specifically on the presence of root growth inhibitor strains

Help wanted: helper NLRs and plant immune responses

Plant nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins function as intracellular receptors in response to pathogens and activate effector-triggered immune responses (ETI). The activation of some sensor NLRs (sNLR) by their corresponding pathogen effector is well studied. However, the mechanisms by which the recently defined helper NLRs (hNLR) function to transduce sNLR activation into ETI-associated cell death and disease resistance remains poorly understood. We briefly summarize recent examples of sNLR activation and we then focus on hNLR requirements in sNLR-initiated immune responses. We further discuss how shared sequence homology with fungal self-incompatibility proteins and the mammalian mixed lineage kinase domain like pseudokinase (MLKL) proteins informs a plausible model for the structure and function of an ancient clade of plant hNLRs, called RNLs

T cell ontogeny: Organ location of maturing populations as defined by surface antigen markers is similar in neonates and adults

Earlier studies have suggested that splenic T cell populations in nursling mice (<18 d of age) have Lyt cell surface antigens that identify them as less mature than their adult counterparts. Studies presented here, however, demonstrate that the expression of the Thy-1, Lyt-1, Lyt-2, and Lyt-3 T cell antigens is virtually identical in 14-d-old and adult T cell populations even though at 14 d, T cells constitute <10% of the total spleen cell population. Because the expression of these antigens on the immature (cortical) thymocyte population differs substantially from their expression on peripheral T cells, the majority of splenic T cells as judged by these criteria is similar in nurslings and adults. Very few cells in the neonatal thymus 4 h after birth correspond, in terms of antigen expression, to the more mature (medullary) thymocyte population of adults, but such cells develop rapidly during the first few days of life. They are present, therefore, sufficiently early to serve as the immediate source of peripheral T cells, as they apparently do in the adult. This then suggests that the locations for the major T cell maturational events are established within the first 2 wk of life of the mouse and maintained as such thereafter. The use of monoclonal antibodies and quantitative immunofluorescence analysis in our studies probably explains the differences between our findings and those reported previously, which relied on cytotoxic depletion by alloantisera and complement to estimate the frequencies of cells carrying the Lyt differentiation antigens in nurslings