Plant biology

Polarized by the Light

Science Signaling  12 Apr 2011:
Vol. 4, Issue 168, pp. ec103
DOI: 10.1126/scisignal.4168ec103

Growth of plants toward light, a response known as phototropism, is primarily mediated in Arabidopsis thaliana by perception of blue light by phototropin family receptors such as PHOT1. In plants undergoing a phototropic response, the distribution and transport of the hormone auxin is asymmetric, such that auxin preferentially accumulates on the shaded side to promote growth toward the light (phototrophic bending). The auxin efflux carrier PIN3 also shows a polarized distribution during phototrophic bending, leading Ding et al. (see also the commentary by Grebe) to determine the mechanisms that mediate the polarized distributions of auxin and PIN3. In Arabidopsis expressing a reporter with activity that correlates with auxin concentrations, increased reporter activity was detected on the shaded side of hypocotyls in unilateral light, which was not observed in phot1 or pin3 mutants. In the dark, PIN3 tagged with green fluorescent protein (PIN3-GFP) had a nonpolarized distribution in endodermal cells of hypocotyls. When these plants were moved into the light, PIN3-GFP was no longer present at the outer lateral membrane (which is closest to the outside of the plant) but remained at the inner membrane that faced the vasculature. In unilateral light, PIN3-GFP redistribution occurred only at the illuminated side, such that PIN3-GFP was found predominantly opposite the light source, a distribution that would enable the directed transport of auxin to the shaded side. The sensitivity of PIN3-GFP redistribution and phototrophic bending to brefeldin A (BFA) suggested a role for endocytic vesicle trafficking. Plants expressing a BFA-insensitive form of the ARF guanine exchange factor GNOM, which regulates vesicle budding, showed phototropic bending and PIN3-GFP redistribution when treated with BFA. In vitro assays indicated that the serine-threonine kinase PINOID (PID) phosphorylated PIN3. Light repressed the transcription of PID, a process that showed a partial dependence on the phototropins PHOT1 and PHOT2, implying that PID is primarily active in cells in the shaded side. Thus, light-dependent phosphorylation of PIN3 by PID determines whether PIN3 undergoes nonpolar or polar trafficking in the hypocotyl epidermis.

Z. Ding, C. S. Galván-Ampudia, E. Demarsy, Ł. Łangowski, J. Kleine-Vehn, Y. Fan, M. T. Morita, M. Tasaka, C. Fankhauser, R. Offringa, J. Friml, Light-mediated polarization of the PIN3 auxin transporter for the phototropic response in Arabidopsis. Nat. Cell Biol. 13, 447–452 (2011). [PubMed]

M. Grebe, Out of the shade and into the light. Nat. Cell Biol. 13, 347–349 (2011). [PubMed]