2.A.69.1.1
Auxin efflux carrier, PIN-FORMED1 (PIN1) (Reinhardt et al., 2003; Carraro et al. 2012). Catalyzes auxin efflux without the participation of any other protein (Petrasek et al., 2006).  PIN1 determines the direction of intercellular auxin flow (Wang et al. 2014). It consists of two TMS bundles, each of 5 TMSs at the N-terminus and the C-terminus of the protein (Nodzyński et al. 2016), confirming previous bioinformatic predictions (Mansour et al. 2007). Arabidopsis VASCULATURE COMPLEXITY AND CONNECTIVITY (VCC) (TC# 8.A.175) is a plant-specific transmembrane protein that controls the development of veins in cotyledons. Yanagisawa et al. 2021 showed that the expression and localization of PIN1 is altered in vcc developing cotyledons, and that overexpression of PIN1-GFP partially rescues vascular defects of vcc mutants. Genetic analyses suggested that VCC and PINOID (PID), a kinase that regulates PIN1 polarity, are both required for PIN1-mediated control of vasculature development. VCC expression is upregulated by auxin, likely as part of a positive feedback loop for the progression of vascular development. VCC and PIN1 localized to the plasma membrane in pre-procambial cells but were actively redirected to vacuoles in procambial cells for degradation. In the vcc mutant, PIN1 failed to properly polarize in pre-procambial cells during the formation of basal strands, and instead, it was prematurely degraded in vacuoles. VCC plays a role in the localization and stability of PIN1, which is crucial for the transition of pre-procambial cells into procambial cells that are involved in the formation of basal lateral strands in embryonic cotyledons (Yanagisawa et al. 2021). Three inward-facing conformational structures of Arabidopsis thaliana PIN1: the apo state, bound to the natural auxin indole-3-acetic acid (IAA), and in complex with the polar auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) have been solved (Yang et al. 2022). The transmembrane domain of PIN1 shares a conserved NhaA fold. In the substrate-bound structure, IAA is coordinated by both hydrophobic stacking and hydrogen bonding. NPA competes with IAA for the same site in an intracellular pocket, but with a much higher affinity (Yang et al. 2022). PIN-FORMED is required for shoot phototropism/gravitropism and facilitates meristem formation in Marchantia polymorpha (Fisher et al. 2023). Phytopathogens causes worldwide crop losses. Parmagnani et al. 2023 showed that microbial volatile organic compound (mVOC; 1-nonanol and 1-dodecanol) profile of Erwinia amylovora, enhances A. thaliana shoot and root growth. E. amylovora mVOCs triggered early signaling events including plasma transmembrane potential V_m depolarization, cytosolic Ca²⁺ fluctuations, K⁺-gated channel activity, and reactive oxygen species (ROS) and nitric oxide (NO) bursts from a few minutes to 16 h upon exposure. These early events were followed by the modulation of the expression of genes involved in plant growth and defense responses as well as responses to phytohormones, including abscisic acid, gibberellin, and auxin (including via the efflux carriers PIN1 and PIN3). A unique mutation in PIN-FORMED1 has been identified, and a genetic pathway for reduced sensitivity of Arabidopsis roots to N-1-naphthylphthalamic acid has been proposed (Huang et al. 2023).  Transport of phenoxyacetic acid herbicides by PIN-FORMED auxin transporters has been reported (Schulz et al. 2025).