Pandemonium Breaks Out:Disruption of Salicylic Acid-Mediated Defense by Plant Pathogens

Salicylic acid (SA) or 2-hydroxybenoic acid is a phenolic plant hormone that plays an essential role in plant defense against biotrophic and semi-biotrophic pathogens. In Arabidopsis, SA is synthesized from chorismate in the chloroplast through the ICS1 (isochorismate synthase I) pathway during pathogen infectioru The transcription co-activator NPR1 (Non-Expresser of Pathogenesis-Related Gene 1), as the master regulator of SA signaling, interacts with transcription factors to induce the expression of anti-microbial PR (Pathogenesis-Related) genes. To establish successful in fections, plant bacterial, oomycete, fun gal, and viral pathogens have evolved at least three major strategies to disrupt SA-mediated defense? The first strategy is to reduce SA accumulation directly by converting SA into its inactive derivatives. The second strategy is to interrupt SA biosynthesis by targeting the ICS1 pathway. In the third major strategy, plant pathogens deploy different mechanisms to interfere with SA downstream signaling. The wide array of strategies deployed by plant pathogens highlights the crucial role of disruption of SA-mediated plant defense in plant pathogenesis. A deeper understanding of this topic will greatly expand our knowledge of how plant pathogens cause diseases and consequently pave the way for the development of more effective ways to corn trol these diseases.

Extracellular superoxide dismutase VdSOD5 is required for virulence in Verticillium dahliae

Plants produce reactive oxygen species(ROS) to defend pathogens. To counteract this attack, certain pathogens express superoxide dismutases(SODs) to scavenge host-derived ROS. However, the roles of SODs in Verticillium dahliae, an important vascular pathogen, are not clear. Our previous study has shown that a putative extracellular SOD(VdSOD5) of V. dahliae is significantly induced by culturing in cotton tissues, suggesting that VdSOD5 may play an important role in host–pathogen interactions and virulence. Here, we showed that VdSOD5 encoded a superoxide dismutase with a cofactor copper-binding site and a functional signal peptide that can conduct protein secretion in an invertase-mutated yeast strain. The mutations in VdSOD5(ΔVdSOD5) did not change the normal vegetative growth and conidial production but reduced the virulence of V. dahliae on susceptible host cotton. Further studies showed that the transcription of Vd SOD5 was significantly up-regulated during the early stage of infection, and the loss-of-function of VdSOD5 decreased culture filtrate and fungal tissue SOD activities of V. dahliae by 74 and 28%, respectively. Compared to the wild-type strain Vd991, the ΔVdSOD5 showed the same sensitivity to the intracellular ROS generator menadione. Furthermore, nitroblue tetrazolium(NBT) staining demonstrated that VdSOD5 functioned in the detoxification of superoxides generated by host roots during infection. These results suggest that VdSOD5 of V. dahliae is an important virulence factor, secreted out of cells to combat host-derived ROS.

A comparative proteomics approach to detect unintended effects in transgenic Arabidopsis

Recently, as part of biosafety assessments, unintended effects have been given much attention. In this study, we applied a proteomics approach to elucidate the unintended effects of random T-DNA insertion in transgenic plants. Separated proteins extracted from 12 transgenic Arabidopsis thaliana with different T-DNA insertion sites and from wild-type （ecotype Col-o） were analyzed. In the transgenic plants, 102 significantly altered protein spots were detected, in which 59 were up-regulated and 43 down-regulated. MALDI-TOF MS analysis showed that most of these expression level-altered proteins were involved in energy transfer, oxidative respiration and photosynthesis. However, none of these proteins was a toxic protein or allergen. Using plants with or without cold treatment, a natural environmental stress, as controls, we found that the number of the altered proteins was even less in those transgenic plants than those triggered by the cold treatment, suggesting that the transgenic events had a weaker impact on the plants than the environmental stresses. Interestingly, the phosphinothricin acetyl transferase （PAT）, the BAR-encoded protein, was detected in nine out of twelve different T-DNA insertion lines at five different insertion sites. These data suggest that the most significant impact of transgenic events on the host plants is from the transgene itself, i.e., from the predictable intended effects, rather than unintended effects. This study also suggests that the proteomics approach has the potential to detect the unintended effects in transgenic plants.