Plants grown at high densities perceive a decrease in the red to far-red (R:FR) ratio of incoming light, resulting from absorption of red light by canopy leaves and reflection of far-red light from neighboring plants. These changes in light quality trigger a series of responses known collectively as the shade avoidance syndrome. During shade avoidance, stems elongate at the expense of leaf and storage organ expansion, branching is inhibited, and flowering is accelerated. We identified several loci in Arabidopsis, mutations in which lead to plants defective in multiple shade avoidance responses. Here we describe TAA1, an aminotransferase, and show that TAA1 catalyzes the formation of indole-3-pyruvic acid (IPA) from L-tryptophan (L-Trp), the first step in a previously proposed, but uncharacterized, auxin biosynthetic pathway. This pathway is rapidly deployed to synthesize auxin at the high levels required to initiate the multiple changes in body plan associated with shade avoidance.

Phytochromes are red and far-red photoreceptors that regulate plant growth and development in response to environmental light cues. Phytochromes exist in two photo-interconvertible conformational states: an inactive Pr form and an active Pfr form. The alteration of phytochromes' subcellular location functions as a major regulatory mechanism of their biological activities. Whereas phytochromes in the Pr form localize in the cytoplasm, phytochromes in the Pfr form accumulate in the nucleus, where they interact with transcription factors to regulate gene expression. The molecular details of the regulation of phytochrome translocation by light are poorly understood. Using Arabidopsis phyB as a model, we demonstrate that the C-terminal PAS-related domain (PRD) is both necessary and sufficient for phyB nuclear import and that the entire C terminus is required for nuclear-body (NB) localization. We also show that phyB's N-terminal bilin lyase domain (BLD) and PHY domain interact directly with the PRD in a light-dependent manner. In vivo localization studies indicate that BLD-PHY is sufficient to regulate phyB's nuclear accumulation. For phyB nuclear localization, our results suggest a molecular mechanism in which the nuclear-localization signal in the PRD is masked by interactions with phyB's chromophore-attachment domains and unmasked by light-dependent conformational changes.

Key points• Neurons can be damaged when tissues are stretched or swollen; while astrocytes can contribute to this process, the mechanosensitive response from neurons is unclear.• We show here that isolated retinal ganglion cell neurons respond to mechanical strain with a rapid, sustained release of the neurotransmitter ATP.• The conduit for ATP release was through pannexin hemichannels, with probenicid, carbenoxelone and 10 panx inhibiting release.• Once released, this ATP acts back on the neurons to autostimulate lethal P2X 7 receptors, as A438079, AZ 10606120 and zinc reduced currents in whole cell patch clamp recordings.• Blocking release of ATP through pannexin channels, or activation of P2X 7 receptors, might be neuroprotective for stretched or swollen neurons.• Stretch-dependent release of ATP through neuronal pannexins, combined with the autostimulation of the P2X 7 receptors, provides a new pathway by which neuronal activity and health can be altered by mechanical strain independently of glial activity.Abstract Mechanical deformation produces complex effects on neuronal systems, some of which can lead to dysfunction and neuronal death. While astrocytes are known to respond to mechanical forces, it is not clear whether neurons can also respond directly. We examined mechanosensitive ATP release and the physiological response to this release in isolated retinal ganglion cells. Purified ganglion cells released ATP upon swelling. Release was blocked by carbenoxolone, probenecid or peptide 10 panx, implicating pannexin channels as conduits. Mechanical stretch of retinal ganglion cells also triggered a pannexin-dependent ATP release. Whole cell patch clamp recording demonstrated that mild swelling induced the activation of an Ohmic cation current with linear kinetics. The current was inhibited by removal of extracellular ATP with apyrase, by inhibition of the P2X 7 receptor with A438079, zinc, or AZ 10606120, and by pannexin blockers carbenoxolone and probenecid. Probenecid also inhibited the regulatory volume decrease observed after swelling isolated neurons. Together, these observations indicate mechanical strain triggers ATP release directly from retinal ganglion cells and that this released ATP autostimulates P2X 7 receptors. Since extracellular ATP levels in the retina increase with elevated intraocular pressure, and stimulation of P2X 7 receptors on retinal ganglion cells can be lethal, this autocrine response may impact ganglion cells in glaucoma. It remains to be determined whether the autocrine stimulation of J. Xia and J. C. Lim contributed equally to this work. purinergic receptors is a general response to a mechanical deformation in neurons, or whether preventing ATP release through pannexin channels and blocking activation of the P2X 7 receptor, is neuroprotective for stretched neurons.