Re-organization of the cytoskeleton and endoplasmic reticulum in the Arabidopsis pen1-1 mutant inoculated with the non-adapted powdery mildew pathogen, Blumeria graminis f. sp. hordei

Abstract

Plant cells attacked by microorganisms rapidly translocate cytoplasm to the site of pathogen penetration, a response that usually involves rearrangement of actin microfilaments. In this study, we monitored re-organization of green fluorescent protein (GFP)-labelled actin microfilaments, microtubules and endoplasmic reticulum (ER) during infection by the powdery mildew pathogen Blumeria graminis f. sp. hordei in non-host Arabidopsis and in the rabidopsis penetration 1-1 (pen1-1) mutant, which shows increased penetration susceptibility to non-adapted pathogens. Comparison of pen1-1 with wild-type Arabidopsis showed that the actin, microtubule and ER networks all responded in the pen1-1 mutant as they do in wild-type plants. Actin microfilaments became focused on the penetration site and ER accumulated at the penetration site while the overall arrangement of micro-tubule arrays was largely unaffected. These results indicate that the block in vesicle secretion conferred by the pen1-1 mutation does not interfere with cytoplasmic aggregation or recruitment of actin or ER to the infection site. In the pen1-1 mutant, the higher rate of successful penetration by the non-adapted pathogen results in an increased incidence of hypersensitive cell death. In dying cells, the structure of the ER was rapidly destroyed, in contrast to actin microfilaments and microtubules which remained for a longer time after the initiation of cell death. In Arabidopsis with GFP-tagged tubulin, fluorescent vesicle-like structures appeared near the cell surface during the initiation of cell death. In both wild-type and pen1-1 mutant plants, in cells surrounding the dying cell, bundles of actin microfilaments focused on the anticlinal walls adjacent to the dead cell and ER accumulated in the cortical cytoplasm near the dead cell. These observations suggest that the neighbouring, non-infected cells use actin-based transport to secrete material at the surface adjacent to the dead cell. They also indicate that disruption of secretion (in the pen1-1 mutant) does not inhibit transmission of signals from the dying cells to their neighbours. Show more