ABSTRACT A large part of a plants’ developmental plasticity relies on the activities of the phytohormone auxin and the regulation of its own distribution. This process involves a cohort of transcriptional and non-transcriptional effects of auxin on polar auxin transport, regulating the abundancy, biochemical activity and polar localization of the molecular components, predominantly PIN auxin exporters. While the transcriptional auxin signaling cascade has been well characterized, the mechanism and role of non-transcriptional auxin signaling remains largely elusive. Here, we addressed the potential involvement of auxin-induced Ca 2+ signaling in auxin’s inhibitory effect on PIN endocytic trafficking. On the one hand, exogenous manipulations of Ca 2+ availability and signaling effectively antagonized auxin effects suggesting that auxin-induced Ca 2+ signaling is required for inhibition of internalization. On the other hand, we addressed the auxin-mediated inhibition of PIN internalization in the auxin signaling ( tir1afb2,3 ) or Ca 2+ channel ( cngc14 ) mutants. These mutants were strongly defective in auxin-triggered Ca 2+ signaling, but not in auxin-inhibited internalization. These data imply that, while Ca 2+ signaling may be required for normal PIN trafficking, auxin-mediated increase in Ca 2+ signaling is not a direct part of a downstream mechanism that mediates auxin effects on Brefeldin A-visualized PIN intercellular aggregation. These contrasting results obtained by comparing the mutant analysis versus the exogenous manipulations of Ca 2+ availability and signaling illustrate the critical importance of genetics to unravel the role of Ca 2+ in a process of interest.

Pharmacological and genetic manipulations of Ca2+ signaling have contrasting effects on auxin-regulated trafficking