Figure 2.

Analysis of the effect of microtubule and actin drugs on PCD. (a) SI response. SI in Papaver pollen triggers F-actin depolymerisation, then stabilisation, microtubule depolymerisation, and PCD. There is evidence using cytoskeletal drugs that both actin and microtubule cytoskeleton dynamics have a role in mediating PCD. Both the SI ligand PrsS and artificial depolymerisation of actin using Latrunculin B stimulates PCD in pollen. PCD can be alleviated by pre-treatment with jasplakinolide. Artificial depolymerisation of actin using Latrunculin B shows that this can trigger microtubule depolymerisation. Although artificial depolymerisation of microtubules using oryzalin does not stimulate PCD, this depolymerisation is important for the execution of PCD because treatment of pollen with the microtubule-stabilising drug taxol alleviates PCD. These data suggest that actin depolymerisation triggers microtubule depolymerisation and the two events together have a role in SI-mediated PCD, although actin depolymerisation (if great enough) alone is sufficient to trigger PCD. Treatment of pollen with Jasplakinolide also triggers PCD and this effect can be alleviated by the treatment by Latrunculin B, suggesting that actin stabilisation (observed during SI) also has a role in triggering PCD. However whether SI-mediated actin stabilisation itself triggers PCD has yet to be established. (b) HR. HR signals trigger bundling of F-actin and microtubule depolymerisation around the site of pathogen entry. Treatment with the F-actin depolymerising drug cytochalasin D and the microtubule depolymerising drug oryzalin can prevent PCD in some systems. F-actin depolymerises during later stages of PCD, and treatment with cytochalasin D can promote PCD. There appears to be no evidence that microtubule bundling has a role in mediating PCD in HR as taxol has no effect on levels of HR-PCD. The possible cross-talk between microtubules and F-actin has not yet been characterised. Treatment with the elicitor, cryptogein, induces depolymerisation of microtubules and bundling of F-actin. F-actin bundling may regulate the pace of PCD, as treatment with the actin-depolymerising drug, bistheonellide, speeds up PCD