The Signal Transducer NPH3 Integrates the Phototropin1 Photosensor with PIN2-Based Polar Auxin Transport in Arabidopsis Root Phototropism

Abstract

Under blue light (BL) illumination, Arabidopsis thaliana roots grow away from the light source, showing a negative phototropic response. However, the mechanism of root phototropism is still unclear. Using a noninvasive microelectrode system, we showed that the BL sensor phototropin1 (phot1), the signal transducer NONPHOTOTROPIC HYPOCOTYL3 (NPH3), and the auxin efflux transporter PIN2 were essential for BL-induced auxin flux in the root apex transition zone. We also found that PIN2-green fluorescent protein (GFP) localized to vacuole-like compartments (VLCs) in dark-grown root epidermal and cortical cells, and phot1/NPH3 mediated a BL-initiated pathway that caused PIN2 redistribution to the plasma membrane. When dark-grown roots were exposed to brefeldin A (BFA), PIN2-GFP remained in VLCs in darkness, and BL caused PIN2-GFP disappearance from VLCs and induced PIN2-GFP-FM4-64 colocalization within enlarged compartments. In the nph3 mutant, both dark and BL BFA treatments caused the disappearance of PIN2-GFP from VLCs. However, in the phot1 mutant, PIN2-GFP remained within VLCs under both dark and BL BFA treatments, suggesting that phot1 and NPH3 play different roles in PIN2 localization. In conclusion, BL-induced root phototropism is based on the phot1/NPH3 signaling pathway, which stimulates the shootward auxin flux by modifying the subcellular targeting of PIN2 in the root apex transition zone.

Light Signaling, Root Development, and Plasticity

“…The movement of the root away from light sources, or root negative phototropism, is dependent upon blue light perception by PHOTs (Wan et al, 2012). Contrary to the negative response to blue light, a positive growth response of the root to R light has also been observed , but this response is weak and can only be observed in the absence of gravity sensing Kiss et al, 2003).…”

“…There are two PHOTs identified in Arabidopsis, phot1 and phot2, with similar function and structure. Besides their function in the shoot, phot1 is also expressed in the roots, regulating root bending (Wan et al, 2012;Zhang et al, 2013). In contrast to other photoreceptors, which are present in the nucleus or cytoplasm, PHOTs are located in the plasma membrane (Sakamoto and Briggs, 2002).…”

“…NPH3, a target of phot1 and phot2 involved in hypocotyl phototropism, also is an important player in root negative phototropism (Wan et al, 2012). Downstream of directional light perception, polar auxin transport directs auxin away from the illuminated side.…”

“…Root negative phototropism is induced by blue and white light, and the photoreceptors involved are mainly phot1 and phot2, with minor roles for phyA and cry1/ cry2 (Boccalandro et al, 2008;Wan et al, 2012;SilvaNavas et al, 2016). NPH3, a target of phot1 and phot2 involved in hypocotyl phototropism, also is an important player in root negative phototropism (Wan et al, 2012).…”

Cellular Auxin Homeostasis under High Temperature Is Regulated through a SORTING NEXIN1–Dependent Endosomal Trafficking Pathway

“…It has been demonstrated that the polar targeting of a subset of PIN proteins (e.g., PIN1 and PIN2) is regulated by various protein trafficking pathways, including ADP ribosylation factor GTP-exchanging factor-dependent clathrinmediated internalization, Rab GTPases, and retromer complexmediated endosomal pathways (Geldner et al, 2001;Jaillais et al, 2006Jaillais et al, , 2007Michniewicz et al, 2007;Kleine-Vehn et al, 2008). The dynamic trafficking process that contributes toward facilitating the polar targeting of the newly synthesized nonpolar PINs to the plasma membrane (Dhonukshe et al, 2008) can be modified following physiological or environmental changes, resulting in the formation of altered intracellular auxin gradient as well as redirection of auxin flow (Paciorek et al, 2005;Laxmi et al, 2008;Shibasaki et al, 2009;Wan et al, 2012). These observations raise an interesting possibility that auxin homeostasis under high temperature may be modulated by intracellular protein trafficking pathways.…”