Search

Zinc is an essential nutrient at low concentrations, but toxic at slightly higher ones. It has been proposed that hyperaccumulator plants may use the excess zinc to fend off pathogens and herbivores. However, there is little evidence of a similar response in other plants. Here we show that Arabidopsis thaliana leaves inoculated with the necrotrophic fungus Plectosphaerella cucumerina BMM (PcBMM) accumulate zinc and manganese at the infection site. Zinc accumulation did not occur in a double mutant in the zinc transporters HEAVY METAL ATPASE2 and HEAVY METAL ATPASE4 (HMA2 and HMA4), which has reduced zinc translocation from roots to shoots. Consistent with a role in plant immunity, expression of HMA2 and HMA4 was up-regulated upon PcBMM inoculation, and hma2hma4 mutants were more susceptible to PcBMM infection. This phenotype was rescued upon zinc supplementation. The increased susceptibility to PcBMM infection was not due to the diminished expression of genes involved in the salicylic acid, ethylene, or jasmonate pathways since they were constitutively up-regulated in hma2hma4 plants. Our data indicate a role of zinc in resistance to PcBMM in plants containing ordinary levels of zinc. This layer of immunity runs in parallel to the already characterized defence pathways, and its removal has a direct effect on resistance to pathogens.

Arabidopsis heterotrimeric G-protein complex modulates pathogen-associated molecular pattern-triggered immunity (PTI) and disease resistance responses to different types of pathogens. It also plays a role in plant cell wall integrity as mutants impaired in the G- (agb1-2) or G-subunits have an altered wall composition compared with wild-type plants. Here we performed a mutant screen to identify suppressors of agb1-2 (sgb) that restore susceptibility to pathogens to wild-type levels. Out of the four sgb mutants (sgb10-sgb13) identified, sgb11 is a new mutant allele of ESKIMO1 (ESK1), which encodes a plant-specific polysaccharide O-acetyltransferase involved in xylan acetylation. Null alleles (sgb11/esk1-7) of ESK1 restore to wild-type levels the enhanced susceptibility of agb1-2 to the necrotrophic fungus Plectosphaerella cucumerina BMM (PcBMM), but not to the bacterium Pseudomonas syringae pv. tomato DC3000 or to the oomycete Hyaloperonospora arabidopsidis. The enhanced resistance to PcBMM of the agb1-2 esk1-7 double mutant was not the result of the re-activation of deficient PTI responses in agb1-2. Alteration of cell wall xylan acetylation caused by ESK1 impairment was accompanied by an enhanced accumulation of abscisic acid, the constitutive expression of genes encoding antibiotic peptides and enzymes involved in the biosynthesis of tryptophan-derived metabolites, and the accumulation of disease resistance-related secondary metabolites and different osmolites. These esk1-mediated responses counterbalance the defective PTI and PcBMM susceptibility of agb1-2 plants, and explain the enhanced drought resistance of esk1 plants. These results suggest that a deficient PTI-mediated resistance is partially compensated by the activation of specific cell-wall-triggered immune responses. Significance Statement The plant heterotrimeric G protein complex is an essential component of Pathogen Associated Molecular Pattern-triggered immunity (PTI) and of plant disease resistance, QC 20171124

Glutathione peroxidases (GPXs; EC 1.11.1.9) are key enzymes of the antioxidant network in plants and animals. In order to investigate the role of antioxidant systems in plant chloroplasts, we generated Arabidopsis (Arabidopsis thaliana) transgenic lines that are depleted specifically in chloroplastic (cp) forms of GPX1 and GPX7. We show that reduced cpGPX expression, either in transgenic lines with lower total cpGPX expression (GPX1 and GPX7) or in a gpx7 insertion mutant, leads to compromised photooxidative stress tolerance but increased basal resistance to virulent bacteria. Depletion of both GPX1 and GPX7 expression also caused alterations in leaf cell and chloroplast morphology. Leaf tissues were characterized by shorter and more rounded palisade cells, irregular spongy mesophyll cells, and larger intercellular air spaces compared with the wild type. Chloroplasts had larger and more abundant starch grains than in wild-type and gpx7 mutant plants. Constitutively reduced cpGPX expression also led to higher foliar ascorbic acid, glutathione, and salicylic acid levels in plants exposed to higher light intensities. Our results suggest partially overlapping functions of GPX1 and GPX7. The data further point to specific changes in the chloroplast ascorbate-glutathione cycle due to reduced cpGPX expression, initiating reactive oxygen species and salicylic acid pathways that affect leaf development, light acclimation, basal defense, and cell death programs. Thus, cpGPXs regulate cellular photooxidative tolerance and immune responses., authorCount :10

Part of urn:nbn:se:su:diva-418