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. 2022 May;14(5):a039859. doi: 10.1101/cshperspect.a039859

Figure 2.

Figure 2.

PIN-dependent auxin transport network generating local auxin maxima and gradients. (1) Maternally produced auxin is transported to the embryo proper, where it accumulates and regulates development (Robert et al. 2018). At early globular stage, auxin production starts at the embryo apex, auxin transport routes reverse, and a new auxin maximum is generated for the root pole specification (Robert et al. 2013; Wabnik et al. 2013). Auxin production is indicated by green stars. (2) Converging auxin fluxes in the epidermis of the shoot apical meristem generate auxin maxima for organ initiation. From the developing primordia, auxin is canalized toward preexisting vasculature (Benková et al. 2003). (3) Light diverges auxin fluxes toward the shaded side of the shoot (Ding et al. 2011), where auxin accumulates (Friml et al. 2002b) and promotes bending. It remains unclear whether this is the primary, causal mechanism. (4) Gravity stimulation diverges auxin fluxes toward the lower side of the shoot (Rakusová et al. 2011), where auxin accumulates and promotes bending. At a later stage, auxin accumulation leads to PIN relocation to the inner cell sides, thus equalizing the auxin distribution and terminating bending (Rakusová et al. 2016). (5) Gravity stimulation diverges auxin fluxes in root columella toward the lower root side, where auxin accumulates (Luschnig et al. 1998; Friml et al. 2002b). Auxin promotes its own flow along the lower root side (Baster et al. 2013) delivering auxin to the elongation zone where it inhibits growth (Fendrych et al. 2018), leading to downward bending of the root. (6) Multiple, redundant PIN fluxes converge to generate an auxin gradient in the central root meristem, which maintains cell divisions and fate specification (Friml et al. 2002a). (7) Auxin flow through the lateral root primordium generates an auxin maximum at the tip, which is partly dissipated by the flow through the epidermis (Benková et al. 2003). Gray arrows indicate direction of growth after gravity or light stimulation. Green shading shows auxin accumulation. Darker green indicates higher auxin levels than lighter green. g denotes direction of gravity.