Auxin protects Arabidopsis thaliana cell suspension cultures from programmed cell death induced by the cellulose biosynthesis inhibitors thaxtomin A and isoxaben.

Thaxtomin A (TA) is a natural cellulose biosynthesis inhibitor (CBI) synthesized by the potato common scab-causing pathogen Streptomyces scabies. Inhibition of cellulose synthesis by TA compromises cell wall organization and integrity, leading to the induction of an atypical program of cell death (PCD). These processes may facilitate S. scabies entry into plant tissues. To study the mechanisms that regulate the induction of cell death in response to inhibition of cellulose synthesis, we used Arabidopsis thaliana cell suspension cultures treated with two structurally different CBIs, TA and the herbicide isoxaben (IXB).The induction of cell death by TA and IXB was abrogated following pretreatment with the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) and the natural auxin indole-3-acetic acid (IAA). The addition of auxin efflux inhibitors also inhibited the CBI-mediated induction of PCD. This effect may be due to intracellular accumulation of auxin. Auxin has a wide range of effects in plant cells, including a role in the control of cell wall composition and rigidity to facilitate cell elongation. Using Atomic Force Microscopy (AFM)-based force spectroscopy, we found that inhibition of cellulose synthesis by TA and IXB in suspension-cultured cells decreased cell wall stiffness to a level slightly different than that caused by auxin. However, the cell wall stiffness in cells pretreated with auxin prior to CBI treatment was equivalent to that of cells treated with auxin only.Addition of auxin to Arabidopsis cell suspension cultures prevented the TA- and IXB-mediated induction of cell death. Cell survival was also stimulated by inhibition of polar auxin transport during CBI-treatment. Inhibition of cellulose synthesis perturbed cell wall mechanical properties of Arabidopsis cells. Auxin treatment alone or with CBI also decreased cell wall stiffness, showing that the mechanical properties of the cell wall perturbed by CBIs were not restored by auxin. However, since auxin's effects on the cell wall stiffness apparently overrode those induced by CBIs, we suggest that auxin may limit the impact of CBIs by restoring its own transport and/or by stabilizing the plasma membrane - cell wall - cytoskeleton continuum.