Deetiolation Enhances Phototropism by Modulating NON-PHOTOTROPIC HYPOCOTYL3 Phosphorylation Status

Abstract

Phototropin (phot) receptor kinases play important roles in promoting plant growth by controlling light-capturing processes, such as phototropism. Phototropism is mediated through the action of NON-PHOTOTROPIC HYPOCOTYL3 (NPH3), which is dephosphorylated following phot activation. However, the functional significance of this early signaling event remains unclear. Here, we show that the onset of phototropism in dark-grown (etiolated) seedlings of Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) is enhanced by greening (deetiolation). Red and blue light were equally effective in promoting phototropism in Arabidopsis, consistent with our observations that deetiolation by phytochrome or cryptochrome was sufficient to enhance phototropism. Increased responsiveness did not result from an enhanced sensitivity to the phytohormone auxin, nor does it involve the phot-interacting protein, ROOT PHOTOTROPISM2. Instead, deetiolated seedlings showed attenuated levels of NPH3 dephosphorylation and diminished relocalization of NPH3 from the plasma membrane during phototropism. Likewise, etiolated seedlings that lack the PHYTOCHROME-INTERACTING FACTORS (PIFs) PIF1, PIF3, PIF4, and PIF5 displayed reduced NPH3 dephosphorylation and enhanced phototropism, consistent with their constitutive photomorphogenic phenotype in darkness. Phototropic enhancement could also be achieved in etiolated seedlings by lowering the light intensity to diminish NPH3 dephosphorylation. Thus, phototropism is enhanced following deetiolation through the modulation of a phosphorylation rheostat, which in turn sustains the activity of NPH3. We propose that this dynamic mode of regulation enables young seedlings to maximize their establishment under changing light conditions, depending on their photoautotrophic capacity.

In Arabidopsis, low blue light enhances phototropism by releasing cryptochrome 1-mediated inhibition ofPIF4expression

“…Moreover, NPH3, which is essential for phototropism in etiolated and green seedlings (Motchoulski and Liscum, 1999; Goyal et al, 2016) was also required for the response in our conditions (Figure 3B). One of the first steps in phot1 signaling is NPH3 de-phosphorylation, which was recently implicated in modulating the phototropic response as reduced NPH3 de-phosphorylation correlates with accelerated phototropism in seedlings treated for a few hours with light prior to phototropic stimulation (Sullivan et al, 2019). We therefore tested whether the LBL treatment that accelerates phototropism led to changes in NPH3 phosphorylation.…”

“…In the conditions we tested phot1 is the primary photoreceptor controlling hypocotyl reorientation and the enhanced response of cry1 mutants depends on phot1 (Figure 3). In seedlings treated with a few hours of light to initiate de-etiolation NPH3 phosphorylation has an effect on phototropism (Sullivan et al, 2019). In our conditions, we found that NPH3 is essential for phototropism but we did not detect an effect of LBL on NPH3 phosphorylation (inferred from mobility shifts on SDS-PAGE gels) (Figure 3).…”

Light Triggers the Search for Light

“…Even though phototropism is a phenotypic adaptation seen in multiple organs and stages of plant development, much of our understanding is based on studies using dark-grown (etiolated) seedlings. In this issue of Plant Physiology, Sullivan et al (2019) show that phototropism is much stronger in deetiolated seedlings. They expose dark-grown seedlings to light for 8 h, enough to promote deetiolation: opening, expansion, and greening of cotyledons (Gommers and Monte, 2018).…”

“…The work of Sullivan et al (2019) unravels an interesting interaction between deetiolation and phototropism. They point out that the difference between light and dark is made at the level of NPH3 phosphorylation.…”

“…Light-exposed seedlings will stop elongating the hypocotyl, initiate root development, and, importantly, expand the cotyledons and develop mature chloroplasts (Gommers and Monte, 2018). Sullivan et al (2019) find that both phytochrome-and cryptochrome-mediated deetiolation delay NPH3 dephosphorylation and promote phototropism, but it remains speculative exactly which players downstream of the photoreceptors are responsible. The pif quadruple mutant, which lacks PIF1, PIF3, PIF4, and PIF5, is constitutively photomorphogenic, with a short hypocotyl and open, expanded cotyledons in darkness.…”