Spatial IMA1 regulation restricts root iron acquisition on MAMP perception.

Iron is critical during host-microorganism interactions1-4. Restriction of available iron by the host during infection is an important defence strategy, described as nutritional immunity5. However, this poses a conundrum for externally facing, absorptive tissues such as the gut epithelium or the plant root epidermis that generate environments that favour iron bioavailability. For example, plant roots acquire iron mostly from the soil and, when iron deficient, increase iron availability through mechanisms that include rhizosphere acidification and secretion of iron chelators6-9. Yet, the elevated iron bioavailability would also be beneficial for the growth of bacteria that threaten plant health. Here we report that microorganism-associated molecular patterns such as flagellin lead to suppression of root iron acquisition through a localized degradation of the systemic iron-deficiency signalling peptide Iron Man 1 (IMA1) in Arabidopsis thaliana. This response is also elicited when bacteria enter root tissues, but not when they dwell on the outer root surface. IMA1 itself has a role in modulating immunity in root and shoot, affecting the levels of root colonization and the resistance to a bacterial foliar pathogen. Our findings reveal an adaptive molecular mechanism of nutritional immunity that affects iron bioavailability and uptake, as well as immune responses.

Sequence Exchange between Homologous NB-LRR Genes Converts Virus Resistance into Nematode Resistance, and Vice Versa.

Plants have evolved a limited repertoire of NB-LRR disease resistance (R) genes to protect themselves against myriad pathogens. This limitation is thought to be counterbalanced by the rapid evolution of NB-LRR proteins, as only a few sequence changes have been shown to be sufficient to alter resistance specificities toward novel strains of a pathogen. However, little is known about the flexibility of NB-LRR R genes to switch resistance specificities between phylogenetically unrelated pathogens. To investigate this, we created domain swaps between the close homologs Gpa2 and Rx1, which confer resistance in potato (Solanum tuberosum) to the cyst nematode Globodera pallida and Potato virus X, respectively. The genetic fusion of the CC-NB-ARC of Gpa2 with the LRR of Rx1 (Gpa2CN/Rx1L) results in autoactivity, but lowering the protein levels restored its specific activation response, including extreme resistance to Potato virus X in potato shoots. The reciprocal chimera (Rx1CN/Gpa2L) shows a loss-of-function phenotype, but exchange of the first three LRRs of Gpa2 by the corresponding region of Rx1 was sufficient to regain a wild-type resistance response to G. pallida in the roots. These data demonstrate that exchanging the recognition moiety in the LRR is sufficient to convert extreme virus resistance in the leaves into mild nematode resistance in the roots, and vice versa. In addition, we show that the CC-NB-ARC can operate independently of the recognition specificities defined by the LRR domain, either aboveground or belowground. These data show the versatility of NB-LRR genes to generate resistance to unrelated pathogens with completely different lifestyles and routes of invasion.

Systemic above- and belowground cross talk: hormone-based responses triggered by Heterodera schachtii and shoot herbivores in Arabidopsis thaliana.

Above- and belowground plant parts are simultaneously attacked by different pests and pathogens. The host mediates these interactions and physiologically reacts, e.g. with local and systemic alterations of endogenous hormone levels coupled with coordinated transcriptional changes. This in turn affects attractiveness and susceptibility of the plant to subsequent attackers. Here, the model plant Arabidopsis thaliana is used to study stress hormone-based systemic responses triggered by simultaneous root parasitism by the cyst nematode Heterodera schachtii and shoot herbivory by the thrips Frankliniella occidentalis and the spider mite Tetranychus urticae. First, HPLC/MS and quantitative reverse transcriptase PCR are used to show that nematode parasitism strongly affects stress hormone levels and expression of hormone marker genes in shoots. Previous nematode infection is then demonstrated to affect the behavioural and life history performance of both arthropods. While thrips explicitly avoid nematode-infected plants, spider mites prefer them. In addition, the life history performance of T. urticae is significantly enhanced by nematode infection. Finally, systemic changes triggered by shoot-feeding F. occidentalis but not T. urticae are shown to make the roots more attractive for H. schachtii. This work emphasises the importance of above- and belowground signalling and contributes to a better understanding of plant systemic defence mechanisms against plant-parasitic nematodes.

Gas exchange, growth, and defense responses of invasive Alliaria petiolata (Brassicaceae) and native Geum vernum (Rosaceae) to elevated atmospheric CO2 and warm spring temperatures.

PREMISE OF STUDY: Global increases in atmospheric CO2 and temperature may interact in complex ways to influence plant physiology and growth, particularly for species that grow in cool, early spring conditions in temperate forests. Plant species may also vary in their responses to environmental changes; fast-growing invasives may be more responsive to rising CO2 than natives and may increase production of allelopathic compounds under these conditions, altering species' competitive interactions. METHODS: We examined growth and physiological responses of Alliaria petiolata, an allelopathic, invasive herb, and Geum vernum, a co-occurring native herb, to ambient and elevated spring temperatures and atmospheric CO2 conditions in a factorial growth chamber experiment. KEY RESULTS: At 5 wk, leaves were larger at high temperature, and shoot biomass increased under elevated CO2 only at high temperature in both species. As temperatures gradually warmed to simulate seasonal progression, G. vernum became responsive to CO2 at both temperatures, whereas A. petiolata continued to respond to elevated CO2 only at high temperature. Elevated CO2 increased thickness and decreased nitrogen concentrations in leaves of both species. Alliaria petiolata showed photosynthetic downregulation at elevated CO2, whereas G. vernum photosynthesis increased at elevated temperature. Flavonoid and cyanide concentrations decreased significantly in A. petiolata leaves in the elevated CO2 and temperature treatment. Total glucosinolate concentrations and trypsin inhibitor activities did not vary among treatments. CONCLUSIONS: Future elevated spring temperatures and CO2 will interact to stimulate growth for A. petiolata and G. vernum, but there may be reduced allelochemical effects in A. petiolata.

Effects of an innovative strategy to contain grapevine Bois noir: field treatment with resistance inducers.

Grapevine Bois noir (BN) is a phytoplasma disease that is widespread in most viticultural regions of the world, and it can result in heavy reductions to yields and grape juice quality. At present, there is no effective strategy to reduce the incidence of BN-infected grapevines. However, phytoplasma-infected plants can recover through spontaneous or induced symptom remission. Five elicitors (chitosan, two glutathione-plus-oligosaccharine formulations, benzothiadiazole, and phosetyl-Al) were applied weekly to the canopy of BN-infected 'Chardonnay' grapevines from early May to late July. The best and most constant recovery inductions were obtained with benzothiadiazole and the two glutathione-plus-oligosaccharine formulations. The plants that recovered naturally or following the elicitors showed qualitative and quantitative parameters of production no different from healthy plants. In another vineyard, diseased plants showed reduced shoot length and production compared with healthy plants, and there were no negative effects on these parameters for grapevines sprayed with a glutathione-plus-oligosaccharine formulation. The application of resistance inducers promoted the recovery of BN-infected grapevines with no adverse effects on the plants. Therefore, grapevine can be used as a model species to test this innovative strategy to contain phytoplasma diseases.

Differences in ascorbate and glutathione levels as indicators of resistance and susceptibility in Eucalyptus trees infected with Phytophthora cinnamomi.

In this study we investigated the role that ascorbate (AA) and glutathione (GSH) play in the plant pathogen interaction of susceptible Eucalyptus sieberi L. A. Johnson and resistant Eucalyptus sideroxylon Woolls with Phytophthora cinnamomi Rands root infection. In a glasshouse study, seedlings were grown in soil-free plant boxes to facilitate the inoculation of the root systems by a P. cinnamomi zoospore solution. Ascorbate and GSH concentrations were measured in infected roots and leaves, along with leaf gas exchange, chlorophyll fluorescence and carbohydrate concentrations over a time course up to 312 h (13 days) post-inoculation (pi). At the early stages of infection (from 24 h pi), significant decreases in AA and GSH concentrations were observed in the infected roots and leaves of the susceptible E. sieberi seedlings. At the later stage of infection (312 h pi), the earlier AA decreases in the leaves of infected plants had become significant increases. In contrast, late, significant AA increases in the absence of any GSH changes were observed in the infected roots of the resistant E. sideroxylon seedlings. In E. sideroxylon leaves, a significant GSH increase occurred at 24 h pi; however, by 312 h pi the earlier increase had become a significant decrease, while no changes occurred in AA. In E. sieberi, photosynthesis (A), stomatal conductance (g(s)) and PSII quantum efficiency (Φ(PSII)) were reduced by ~60, 80 and 30%, respectively, in infected plants and remained significantly lower than uninfected controls for the duration of the experiment. Significant reductions in these parameters did not occur until later (120 h pi for g(s) and 312 h pi for A and Φ(PSII)), and to a lesser extent in the resistant species. Non-structural carbohydrate analysis of roots and leaves indicate that carbohydrate metabolism and resource flow between shoots and roots may have been altered at later infection stages. This study suggests that reduced antioxidant capacity, leaf physiological function and carbohydrate metabolism are associated with susceptibility in E. sieberi to P. cinnamomi infection, while AA increases and new root formation were associated with resistance in E. sideroxylon.

Immunolocalization of integrin-like proteins in Arabidopsis and Chara.

Integrins are a large family of integral plasma membrane proteins that link the extracellular matrix to the cytoskeleton in animal cells. As a first step in determining if integrin-like proteins are involved in gravitropic signal transduction pathways, we have used a polyclonal antibody against the chicken beta1 integrin subunit in western blot analyses and immunofluorescence microscopy to gain information on the size and location of these proteins in plants. Several different polypeptides are recognized by the anti-integrin antibody in roots and shoots of Arabidopsis and in the internodal cells and rhizoids of Chara. These cross-reactive polypeptides are associated with cellular membranes, a feature which is consistent with the known location of integrins in animal systems. In immunofluorescence studies of Arabidopsis roots, a strong signal was obtained from labeling integrin-like proteins in root cap cells, and there was little or no immunolabel in other regions of the root tip. While the antibody stained throughout Chara rhizoids, the highest density of immunolabel was at the tip. Thus, in both Arabidopsis roots and Chara rhizoids, the sites of gravity perception/transduction appear to be enriched in integrin-like molecules.