Abstract
Many signal perception mechanisms are connected to Ca(2+)-based second messenger signaling to modulate specific cellular responses. The well-characterized plant hormone auxin elicits a very rapid Ca(2+) signal. However, the cellular targets of auxin-induced Ca(2+) are largely unknown. Here, we screened a biologically annotated chemical library for inhibitors of auxin-induced Ca(2+) entry in plant cell suspensions to better understand the molecular mechanism of auxin-induced Ca(2+) and to explore the physiological relevance of Ca(2+) in auxin signal transduction. Using this approach, we defined a set of diverse, small molecules that interfere with auxin-induced Ca(2+) entry. Based on annotated biological activities of the hit molecules, we found that auxin-induced Ca(2+) signaling is, among others, highly sensitive to disruption of membrane proton gradients and the mammalian Ca(2+) channel inhibitor bepridil. Whereas protonophores nonselectively inhibited auxin-induced and osmotic stress-induced Ca(2+) signals, bepridil specifically inhibited auxin-induced Ca(2+) We found evidence that bepridil severely alters vacuolar morphology and antagonized auxin-induced vacuolar remodeling. Further exploration of this plant-tailored collection of inhibitors will lead to a better understanding of auxin-induced Ca(2+) entry and its relevance for auxin responses.