PIN auxin efflux carriers are necessary for pulse-induced but not continuous light-induced phototropism in Arabidopsis.

Ken Haga,Tatsuya Sakai

doi:10.1104/pp.112.202432

Abstract

Auxin efflux carrier PIN-FORMED (PIN) proteins are thought to have central roles in regulating asymmetrical auxin translocation during tropic responses, including gravitropism and phototropism, in plants. Although PIN3 is known to be involved in phototropism in Arabidopsis (Arabidopsis thaliana), no severe defects of phototropism in any of the pin mutants have been reported. We show here that the pulse-induced, first positive phototropism is impaired partially in pin1, pin3, and pin7 single mutants, and severely in triple mutants. In contrast, such impairment was not observed in continuous-light-induced second positive phototropism. Analysis with an auxin-reporter gene demonstrated that PIN3-mediated auxin gradients participate in pulse-induced phototropism but not in continuous-light-induced phototropism. Similar functional separation was also applicable to PINOID, a regulator of PIN localization. Our results strongly suggest the existence of functionally distinct mechanisms i.e. a PIN-dependent mechanism in which transient stimulation is sufficient to induce phototropism, and a PIN-independent mechanism that requires continuous stimulation and does not operate in the former phototropism process. Although a previous study has proposed that blue-light photoreceptors, the phototropins, control PIN localization through the transcriptional down-regulation of PINOID, we could not detect this blue-light-dependent down-regulation event, suggesting that other as yet unknown mechanisms are involved in phototropin-mediated phototropic responses.

Highlights

Auxin efflux carrier PIN-FORMED (PIN) proteins are thought to have central roles in regulating asymmetrical auxin translocation during tropic responses, including gravitropism and phototropism, in plants
We have evaluated the phototropic responses in mutants of ATP-BINDING CASSETTE subfamily B19 (ABCB19), and a PINregulating protein kinase, PINOID (PID), and discuss the function and role of red-light preirradiation in hypocotyl phototropism
It is known that the relationship between the hook position and direction of phototropic stimulation influences hypocotyl phototropism in Arabidopsis (Khurana et al, 1989)

Summary

Introduction

Auxin efflux carrier PIN-FORMED (PIN) proteins are thought to have central roles in regulating asymmetrical auxin translocation during tropic responses, including gravitropism and phototropism, in plants. HYPOCOTYL3 ortholog in rice (Oryza sativa), COLEOPTILE PHOTOTROPISM1, has been shown to be an essential mediator of auxin redistribution in coleoptiles during the phototropic response (Haga et al, 2005) It remains largely unknown, how auxin gradients are achieved in response to the activation of phototropin signaling (Iino and Haga, 2005; Holland et al, 2009). PIN3 is thought to play central roles in phototropism and gravitropism by regulating the lateral translocation of auxin (Friml et al, 2002; Ding et al, 2011; Rakusová et al, 2011). The regulation of PIN3 subcellular localization appears to be an underlying mechanism of the asymmetrical translocation of auxin during phototropic responses

Methods

Results

Conclusion