Inositol trisphosphate-induced ca²⁺ signaling modulates auxin transport and pin polarity

The phytohormone auxin is an important determinant of plant development. Directional auxin flow within tissues depends on polar localization of PIN auxin transporters. To explore regulation of PIN-mediated auxin transport, we screened for suppressors of PIN1 overexpression (supo) and identified an inositol polyphosphate 1-phosphatase mutant (supo1), with elevated inositol trisphosphate (InsP₃) and cytosolic Ca²⁺ levels. Pharmacological and genetic increases in InsP₃ or Ca²⁺ levels also suppressed the PIN1 gain-of-function phenotypes and caused defects in basal PIN localization, auxin transport and auxin-mediated development. In contrast, the reductions in InsP₃ levels and Ca²⁺ signaling antagonized the effects of the supo1 mutation and disrupted preferentially apical PIN localization. InsP₃ and Ca²⁺ are evolutionarily conserved second messengers involved in various cellular functions, particularly stress responses. Our findings implicate them as modifiers of cell polarity and polar auxin transport, and highlight a potential integration point through which Ca²⁺ signaling-related stimuli could influence auxin-mediated development.