Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation

High concentrations of heavy metals (HM) in the soil have detrimental effects on ecosystems and are a risk to human health as they can enter the food chain via agricultural products or contaminated drinking water. Phytoremediation, a sustainable and inexpensive technology based on the removal of pollutants from the environment by plants, is becoming an increasingly important objective in plant research. However, as phytoremediation is a slow process, improvement of efficiency and thus increased stabilization or removal of HMs from soils is an important goal. Arbuscular mycorrhizal (AM) fungi provide an attractive system to advance plant-based environmental clean-up. During symbiotic interaction the hyphal network functionally extends the root system of their hosts. Thus, plants in symbiosis with AM fungi have the potential to take up HM from an enlarged soil volume. In this review, we summarize current knowledge about the contribution of the AM symbiosis to phytoremediation of heavy metal

Mediated Plastid RNA Editing in Plant Immunity

[EN] Plant regulatory circuits coordinating nuclear and plastid gene expression have evolved in response to external stimuli. RNA editing is one of such control mechanisms. We determined the Arabidopsis nuclear-encoded homeodomain-containing protein OCP3 is incorporated into the chloroplast, and contributes to control over the extent of ndhB transcript editing. ndhB encodes the B subunit of the chloroplast NADH dehydrogenase-like complex (NDH) involved in cyclic electron flow (CEF) around photosystem I. In ocp3 mutant strains, ndhB editing efficiency decays, CEF is impaired and disease resistance to fungal pathogens substantially enhanced, a process recapitulated in plants defective in editing plastid RNAs encoding NDH complex subunits due to mutations in previously described nuclear-encoded pentatricopeptide-related proteins (i.e. CRR21, CRR2). Furthermore, we observed that following a pathogenic challenge, wild type plants respond with editing inhibition of ndhB transcript. In parallel, rapid destabilization of the plastidial NDH complex is also observed in the plant following perception of a pathogenic cue. Therefore, NDH complex activity and plant immunity appear as interlinked processes.This work was supported by the Spanish MICINN (CONSOLIDER and BFU2012 to PV), and Generalitat Valenciana (Prometeo2010/020 to PV). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.García-Andrade Serrano, J.; Ramirez Garcia, V.; López Sánchez, A.; Vera Vera, P. (2013). Mediated Plastid RNA Editing in Plant Immunity. PLoS Pathogens. 9(10):1003713-1003713. https://doi.org/10.1371/ journal.ppat.1003713S1003713100371391

How Do Smut Fungi Use Plant Signals to Spatiotemporally Orientate on and In Planta?

Smut fungi represent a large group of biotrophic plant pathogens that cause extensive yield loss and are also model organisms for studying plant–pathogen interactions. In recent years, they have become biotechnological tools. After initial penetration of the plant epidermis, smut fungi grow intra—and intercellularly without disrupting the plant-plasma membrane. Following the colonialization step, teliospores are formed and later released. While some smuts only invade the tissues around the initial penetration site, others colonize in multiple plant organs resulting in spore formation distal from the original infection site. The intimate contact zone between fungal hyphae and the host is termed the biotrophic interaction zone and enables exchange of signals and nutrient uptake. Obviously, all steps of on and in planta growth require fine sensing of host conditions as well as reprogramming of the host by the smut fungus. In this review, we highlight selected examples of smut fungal colonization styles, directional growth in planta, induction of spore formation, and the signals required, pointing to excellent reviews for details, to draw attention to some of the open questions in this important research field

Genetic Manipulation of the Brassicaceae Smut Fungus Thecaphora thlaspeos

Investigation of plant–microbe interactions greatly benefit from genetically tractable partners to address, molecularly, the virulence and defense mechanisms. The smut fungus Ustilago maydis is a model pathogen in that sense: efficient homologous recombination and a small genome allow targeted modification. On the host side, maize is limiting with regard to rapid genetic alterations. By contrast, the model plant Arabidopsis thaliana is an excellent model with a vast amount of information and techniques as well as genetic resources. Here, we present a transformation protocol for the Brassicaceae smut fungus Thecaphora thlaspeos. Using the well-established methodology of protoplast transformation, we generated the first reporter strains expressing fluorescent proteins to follow mating. As a proof-of-principle for homologous recombination, we deleted the pheromone receptor pra1. As expected, this mutant cannot mate. Further analysis will contribute to our understanding of the role of mating for infection biology in this novel model fungus. From now on, the genetic manipulation of T. thlaspeos, which is able to colonize the model plant A. thaliana, provides us with a pathosystem in which both partners are genetically amenable to study smut infection biology

The role of fungal RNA biology during plant infection.

<p>Model of a filamentous pathogen (left) interacting with its plant host (right). RNA-dependent processes crucial for infection are numbered as follows: <b>1</b> translational regulation of <i>clp1</i> mRNA (ribosomal subunits in orange, mRNA in green); <b>2</b> 3′ end processing by the RNA-binding protein RBP35 in the nucleus (gray oval); <b>3</b> modulation of mRNA stability in processing bodies (PB, blue: accessory factors); <b>4</b> natural antisense transcripts (NAT) regulate mRNA stability; <b>5</b> endosomal mRNA transport along microtubule (black line, brown: endosome with motor proteins attached); <b>6</b> alternative splicing of <i>gapdh</i> mRNA to generate an enzyme with a peroxisomal targeting sequence; <b>7</b> translational read-through adding a peroxisomal targeting sequence; <b>8</b> antimicrobial peptide (AMP) Wheatwin1(Wwin1) with RNase activity; <b>9</b> ribosome-inactivating protein (RIP) alters rRNA; <b>10</b> pathogen effectors target host splice factors (SF) as shown for the bacterial effector HopU1 inactivating AtGRP7; <b>11</b> pathogen effectors interfere with the generation of small RNAs in the host as shown for oomycetes effectors. Further details are given in the text.</p

Loss of Albino3 Leads to the Specific Depletion of the Light-Harvesting System

The chloroplast Albino3 (Alb3) protein is a chloroplast homolog of the mitochondrial Oxa1p and YidC proteins of Escherichia coli, which are essential components for integrating membrane proteins. In vitro studies in vascular plants have revealed that Alb3 is required for the integration of the light-harvesting complex protein into the thylakoid membrane. Here, we show that the gene affected in the ac29 mutant of Chlamydomonas reinhardtii is Alb3.1. The availability of the ac29 mutant has allowed us to examine the function of Alb3.1 in vivo. The loss of Alb3.1 has two major effects. First, the amount of light-harvesting complex from photosystem II (LHCII) and photosystem I (LHCI) is reduced >10-fold, and total chlorophyll represents only 30% of wild-type levels. Second, the amount of photosystem II is diminished 2-fold in light-grown cells and nearly 10-fold in dark-grown cells. The accumulation of photosystem I, the cytochrome b(6)f complex, and ATP synthase is not affected in the ac29 mutant. Mild solubilization of thylakoid membranes reveals that Alb3 forms two distinct complexes, a lower molecular mass complex of a size similar to LHC and a high molecular mass complex. A homolog of Alb3.1, Alb3.2, is present in Chlamydomonas, with 37% sequence identity and 57% sequence similarity. Based on the phenotype of ac29, these two genes appear to have mostly nonredundant functions

One of Two Alb3 Proteins Is Essential for the Assembly of the Photosystems and for Cell Survival in Chlamydomonas

Proteins of the YidC/Oxa1p/ALB3 family play an important role in inserting proteins into membranes of mitochondria, bacteria, and chloroplasts. In Chlamydomonas reinhardtii, one member of this family, Albino3.1 (Alb3.1), was previously shown to be mainly involved in the assembly of the light-harvesting complex. Here, we show that a second member, Alb3.2, is located in the thylakoid membrane, where it is associated with large molecular weight complexes. Coimmunoprecipitation experiments indicate that Alb3.2 interacts with Alb3.1 and the reaction center polypeptides of photosystem I and II as well as with VIPP1, which is involved in thylakoid formation. Moreover, depletion of Alb3.2 by RNA interference to 25 to 40% of wild-type levels leads to a reduction in photosystems I and II, indicating that the level of Alb3.2 is limiting for the assembly and/or maintenance of these complexes in the thylakoid membrane. Although the levels of several photosynthetic proteins are reduced under these conditions, other proteins are overproduced, such as VIPP1 and the chloroplast chaperone pair Hsp70/Cdj2. These changes are accompanied by a large increase in vacuolar size and, after a prolonged period, by cell death. We conclude that Alb3.2 is required directly or indirectly, through its impact on thylakoid protein biogenesis, for cell survival