Article by JENNIFER PAYNE: Recipient of the ComBio 2015 ASPS Student Poster Prize
Plants can’t run and and hide from their enemies. They are rooted to the spot and consequently have developed sophisticated defence mechanisms to shield them from potential invaders. Unlike animals they don’t have an adaptive immune system and rely instead on physical barriers such as waxy cuticles, secondary metabolites and innate immunity proteins for protection against potential microbial pathogens. Plant defensins, are a crucial part of this innate immune system. They are small, stable, cysteine-rich proteins that are produced by all plant taxa and most plant tissues. NaD1 a plant defensin from the ornamental tobacco Nicotiana alata, has potent antifungal activity against a range of serious plant pathogens that can devastate crop yields and we have been exploring its potential application in crop protection. NaD1’s antifungal mechanism is complex involving multiple steps, beginning with specific interaction with the fungal cell wall, followed by disruption of the plasma membrane and entry into the cytoplasm where it initiates cell death. Unlike most antifungal molecules, NaD1 requires an intact cell wall for its antifungal activity. The fungal cell wall is unique to fungi, creating an ideal selective target for new generation fungicides. It is composed of three layers; an outer glycoprotein layer, a β-1,3-glucan layer and a chitin layer immediately adjacent to the plasma membrane. Direct binding assays and chemical and genetic alteration of the thickness of the cell wall layers revealed that NaD1 binds with a higher affinity to chitin than β-1,3-glucan. This led to the hypothesis that NaD1 might be directed through the wall and onto the plasma membrane by an affinity gradient, a novel mechanism for passage through the cell wall. Once through the wall, NaD1 must pass through the plasma membrane to enter the cytoplasm and kill the fungal cell. The interaction between NaD1 and the membrane was studied using liposomes and bilayers of different lipid composition together with dual polarisation interferometry. This revealed that; NaD1 only interacts with membranes containing phosphatidylinositol 4, 5-bisphosphate, the membrane becomes disordered upon NaD1 binding, and NaD1 does not dissociate from the membrane after binding (Payne et al 2016). These data supported the work of Poon and colleagues (2014) who reported that NaD1 and phosphatidylinositol 4, 5-bisphosphate form an elegant arc shaped oligomer and highlighted a new mechanism for membrane disruption by an antimicrobial peptide.
Payne, J. A., Bleackley, M. R., Lee, T. H., Shafee, T. M., Poon, I. K., Hulett, M. D., . . . Anderson, M. A. (2016). The plant defensin NaD1 introduces membrane disorder through a specific interaction with the lipid, phosphatidylinositol 4,5 bisphosphate. Biochim Biophys Acta, 1858(6), 1099-1109. doi:10.1016/j.bbamem.2016.02.016
Poon, I. K., Baxter, A. A., Lay, F. T., Mills, G. D., Adda, C. G., Payne, J. A., … Hulett, M. D. (2014). Phosphoinositide-mediated oligomerization of a defensin induces cell lysis. eLife, 3, e01808. http://doi.org/10.7554/eLife.01808
For more information email: firstname.lastname@example.org (La Trobe University, Melbourne Victoria Australia)