Eukaryotic cells typically deviate from spherical shapes due to complex interactions between elements of their cytoskeleton and the extracellular matrix (ECM). Communication between the cytoskeleton and ECM is one of the most characteristic features of cellular mechanics and allows cells to respond

Cytoskeleton-Plasma Membrane-Cell Wall Continuum in Plants. Emerging Links Revisited

The roles of the phytotoxin coronatine (COR) and salicylic acid (SA)-mediated defenses in the interaction of Pseudomonas syringae pv. tomato DC3000 and tomato (Solanum lycopersicum) were investigated. Unlike findings reported for Arabidopsis thaliana, DC3000 mutants impaired for production of COR or one of its components, coronafacic acid (CFA) or coronamic acid (CMA), induced distinctly different disease lesion phenotypes in tomato. Tomato plants inoculated with the CFA- CMA- mutant DB29 showed elevated transcript levels of SlICS, which encodes isochorismate synthase, an enzyme involved in SA biosynthesis in S. lycopersicum. Furthermore, expression of genes encoding SA-mediated defense proteins were elevated in DB29-inoculated plants compared with plants inoculated with DC3000, suggesting that COR suppresses SlICS-mediated SA responses. Sequence analysis of SlICS revealed that it encodes a protein that is 55 and 59.6% identical to the A. thaliana ICS-encoded proteins AtICS1 and AtICS2, respectively. Tomato plants silenced for SlICS were hypersusceptible to DC3000 and accumulated lower levels of SA after infection with DC3000 compared with inoculated wild-type tomato plants. Unlike what has been shown for A. thaliana, the COR- mutant DB29 was impaired for persistence in SlICS-silenced tomato plants; thus, COR has additional roles in virulence that are SA independent and important in the latter stages of disease development. In summary, the infection assays, metabolic profiling, and gene expression results described in this study indicate that the intact COR molecule is required for both suppression of SA-mediated defense responses and full disease symptom development in tomato.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-20-8-0955

The phytotoxin coronatine contributes to pathogen fitness and is required for suppression of salicylic acid accumulation in tomato inoculated with Pseudomonas syringae pv. tomato DC3000

In plants, an active defense against biotrophic pathogens is dependent on a functional continuum between the cell wall (CW) and the plasma membrane (PM). It is thus anticipated that proteins maintaining this continuum also function in defense. The legume-like lectin receptor kinase LecRK-I.9 is a putative mediator of CW-PM adhesions in Arabidopsis and is known to bind in vitro to the Phytophthora infestans RXLR-dEER effector IPI-O via a RGD cell attachment motif present in IPI-O. Here we show that LecRK-I.9 is associated with the plasma membrane, and that two T-DNA insertions lines deficient in LecRK-I.9 (lecrk-I.9) have a ‘gain-of-susceptibility’ phenotype specifically towards the oomycete Phytophthora brassicae. Accordingly, overexpression of LecRK-I.9 leads to enhanced resistance to P. brassicae. A similar ‘gain-of-susceptibility’ phenotype was observed in transgenic Arabidopsis lines expressing ipiO (35S-ipiO1). This phenocopy behavior was also observed with respect to other defense-related functions; lecrk-I.9 and 35S-ipiO1 were both disturbed in pathogen- and MAMP-triggered callose deposition. By site-directed mutagenesis, we demonstrated that the RGD cell attachment motif in IPI-O is not only essential for disrupting the CW-PM adhesions, but also for disease suppression. These results suggest that destabilizing the CW-PM continuum is one of the tactics used by Phytophthora to promote infection. As countermeasure the host may want to strengthen CW-PM adhesions and the novel Phytophthora resistance component LecRK-I.9 seems to function in this process.

/pdf/the-lectin-receptor-kinase-lecrk-i-9-is-a-novel-phytophthora-etfapgpili.pdf

The Lectin Receptor Kinase LecRK-I.9 Is a Novel Phytophthora Resistance Component and a Potential Host Target for a RXLR Effector

Mycotoxins are secondary fungal metabolites, toxic to humans, animals and plants. Among the hundreds of known mycotoxins, aflatoxins, citrinin, patulin, penicillic acid, tenuazonic acid, ochratoxin A, cytochalasins, deoxynivalenol, fumonisins, fusarin C, fusaric acid, and zearalenone are considered the types that most contaminate cereal grain. The majority of the mycotoxins in these groups are produced by three fungal genera: Aspergillus, Penicillium and Fusarium. These metabolites primarily affect the seed quality, germination, viability, seedling vigour, growth of root and cleoptile. Additionally, since the fungi responsible for the production of these mycotoxins are often endophytes that infect and colonize living plant tissues, accumulation of mycotoxins in the plant tissues may at times be associated with development of plant disease symptoms. The presence of mycotoxins, even in the absence of disease symptoms, may still have subtle biological effects on the physiology of plants. Several studies highlight the toxic effects of mycotoxins on animals and cell lines but little is known about the mode of action of most of these metabolites on plant cells. The most important mycotoxins with phytotoxic effects and their producers in addition to their discovery are briefly outlined below and will be addressed in this article.

/pdf/mycotoxins-producing-fungi-and-mechanisms-of-phytotoxicity-1ftbicdzut.pdf

Mycotoxins: Producing Fungi and Mechanisms of Phytotoxicity

The plant pathogenic fungus Pyrenophora tritici-repentis secretes host-selective toxins (HSTs) that function as pathogenicity factors. Unlike most HSTs that are products of enzymatic pathways, at least two toxins produced by P. tritici-repentis are proteins and, thus, products of single genes. Sensitivity to these toxins in the host is conferred by a single gene for each toxin. To study the site of action of Ptr ToxA (ToxA), toxin-sensitive and -insensitive wheat (Triticum aestivum) cultivars were treated with ToxA followed by proteinase K. ToxA was resistant to protease, but only in sensitive leaves, suggesting that ToxA is either protected from the protease by association with a receptor or internalized. Immunolocalization and green fluorescent protein tagged ToxA localization demonstrate that ToxA is internalized in sensitive wheat cultivars only. Once internalized, ToxA localizes to cytoplasmic compartments and to chloroplasts. Intracellular expression of ToxA by biolistic bombardment into both toxin-sensitive and -insensitive cells results in cell death, suggesting that the ToxA internal site of action is present in both cell types. However, because ToxA is internalized only in sensitive cultivars, toxin sensitivity, and therefore the ToxA sensitivity gene, are most likely related to protein import. The results of this study show that the ToxA protein is capable of crossing the plant plasma membrane from the apoplastic space to the interior of the plant cell in the absence of a pathogen.

Localization of Ptr ToxA Produced by Pyrenophora tritici-repentis Reveals Protein Import into Wheat Mesophyll Cells

/pdf/interaction-between-cell-wall-and-plasma-membrane-via-rgd-ti24iqdqco.pdf

Interaction between Cell Wall and Plasma Membrane via RGD Motif is Implicated in Plant Defense Responses

When inorganic phosphate, a common and essential element for organisms, was applied endogenously, a rejection reaction and superoxide generation were induced in pea tissues but phytoalexin production was not. Phosphate-induced superoxide generation was sensitive to cycloheximide (CHX) and salicylhydroxamic acid (SHAM), indicating that part of the generation was dependent upon the expression of peroxidase gene(s). Peroxidases (POXs) are well known not only to scavenge hydrogen peroxide with phenolics but also to generate superoxide via NADH oxidation in the presence of p-coumaric acid and manganese ion. We cloned five pea POX cDNAs that are predicted to be located outside of the cells. The accumulation of five POX mRNAs, NTPase mRNA, and phenylalanine ammonia-lyase mRNA was measured by semiquantitative reverse transcription-polymerase chain reaction. The expression of the five POX genes was induced by a fungal elicitor. On the other hand, inorganic phosphate induced the accumulation of POX11, POX14, and POX21 mRNAs but not of POX13, POX29, and PsPAL1 mRNAs within 1–3 h after treatment of pea seedlings. In view of these findings, we discuss inorganic phosphate as a signal transmitter inducing part of the plant defense responses.

Induction of defense responses in pea tissues by inorganic phosphate

Cytochalasin A (CA) blocked the accumulation of phytoalexin and phenylalanine ammonia-lyase (PAL)-protein in pea tissues treated with a fungal elicitor but scarcely affected the PAL-mRNA content. Further analysis showed that CA decreased the PAL-mRNA bound to ribosomes. These results indicate that actin filaments are tightly associated with the translational process of the PAL gene.

/pdf/cytochalasin-a-inhibits-the-binding-of-phenylalanine-ammonia-2xiiacnybu.pdf

Megumi Sugimoto

Papers

Interaction between Cell Wall and Plasma Membrane via RGD Motif is Implicated in Plant Defense Responses

Induction of defense responses in pea tissues by inorganic phosphate

Cytochalasin A inhibits the binding of phenylalanine ammonia-lyase mRNA to ribosomes during induction of phytoalexin in pea seedlings.