The Enhancement of Phototropin-Induced Phototropic Curvature in Arabidopsis Occurs via a Photoreversible Phytochrome A-Dependent Modulation of Auxin Responsiveness

Abstract

The induction of phototropism in etiolated (dark-grown) seedlings exposed to an unidirectional pulse or extended irradiation with low fluence rate blue light (BL) requires the action of the phototropin (nph1) BL receptor. Although cryptochromes and phytochromes are not required for phototropic induction, these photoreceptors do modulate the magnitude of curvature resulting from phototropin activation. Modulatory increases in the magnitude of phototropic curvature have been termed "enhancement." Here, we show that phototropic enhancement is primarily a phytochrome A (phyA)-dependent red/far-red-reversible low fluence response. This phyA-dependent response is genetically separable from the basal phototropin-dependent response, as demonstrated by its retention under extended irradiation conditions in the nph4 mutant background, which normally lacks the basal BL-induced response. It is interesting that the nph4 mutants fail to exhibit the basal phototropin-dependent and phyA-dependent enhancement responses under limiting light conditions. Given that NPH4 encodes a transcriptional activator, auxin response factor 7 (ARF7), we hypothesize that the ultimate target(s) of phyA action during the phototropic enhancement response is a rate-limiting ARF-containing transcriptional complex in which the constituent ARFs can vary in identity or activity depending upon the irradiation condition.

PHYTOCHROME KINASE SUBSTRATE 1 is a phototropin 1 binding protein required for phototropism

“…Phytochrome-mediated induction of PKS1 and PKS2 expression (22) may thus partly explain the reduced phototropism in phyA mutants in response to blue-light pulses (Fig. 1C) (27). Indeed, PKS1 protein and mRNA are light-induced by a phyAdependent VLFR (22).…”

“…1C). Under these conditions phot1 functions as the essential blue-light receptor mediating perception of directional light (26,27). phyA mutants show a reduced phototropic response possibly because phyA is required to inhibit gravitropism and͞or because the phytochromes are required to modulate the level or activity of phototropism signaling components (3,6).…”

“…For long-term irradiation experiments seedlings were grown on vertically oriented half-strength Murashige and Skoog plates treated and measured as described (6). Phototropism in response to pulses of blue light was performed as described (27).…”

Phytochrome A Mediates Rapid Red Light–Induced Phosphorylation ofArabidopsisFAR-RED ELONGATED HYPOCOTYL1 in a Low Fluence Response

“…PhyB is well known to mediate both HIR to red light and the R/FR reversible low-fluence response (LFR) (Shinomura et al, 1996). Recently, there have been several reports that imply phyA might also be involved in LFR (Long and Iino, 2001;Stowe-Evans et al, 2001;Takano et al, 2005). However, direct evidence for a phyA-mediated LFR biochemical process has been lacking.…”

Auxin and Light Control of Adventitious Rooting in Arabidopsis Require ARGONAUTE1

“…Moreover, the ARF10 mRNA showed a twofold increase, and no significant difference was detected for ARF6, ARF8, or ARF16. Additionally, no difference in expression was detected for either ARF7/NPH4 or ATHB2, analyzed as transcription factors putatively not targeted by miRNAs and because they were shown previously to act in the cross talk between light and auxin signaling pathways (Stowe-Evans et al, 2001;Morelli and Ruberti, 2002).…”