ABSTRACT Phototropic bending of plants towards a light source allows them to position their photosynthetic tissues to optimize light capture. In light‐grown (de‐etiolated) Arabidopsis seedlings, phototropic bending of the hypocotyl is inhibited by light with a high red:far‐red ratio (HRFR) and high levels of blue light (HBL). This occurs via activation of the phytochrome B (phyB) and cryptochrome 1 (cry1) photoreceptor signaling pathways. Both phyB and cry1 act upstream of PHYTOCHROME INTERACTING FACTOR (PIF) transcription factors, which are required for hypocotyl bending in light‐grown seedlings. Presently, it is not known whether other pathways are involved in the inhibition of PIF‐mediated phototropism in light‐grown seedlings. To address this, we conducted a screen to identify mutants with increased phototropic bending relative to wild type in HRFR + HBL conditions. Through this screen, we identified EARLY FLOWERING 3 (ELF3), a member of the Evening Complex (EC), as a key inhibitor of phototropic bending in green seedlings. We show that both ELF3 and LUX, another component of the EC, inhibit phototropic bending upstream of PIF4/PIF5. Furthermore, we show that phototropic bending in Arabidopsis seedlings is subject to circadian regulation in an ELF3‐dependent manner. Finally, we provide evidence that ELF3 in the grass Brachypodium distachyon also affects phototropism but in an opposite way than in Arabidopsis .
Cobb et al. (Mon,) studied this question.