Abstract
Plants have developed sophisticated systems to monitor and rapidly acclimate to environmental fluctuations. Light is an essential source of environmental information throughout the plant’s life cycle. The model plant Arabidopsis thaliana possesses five phytochromes (phyA-phyE) with important roles in germination, seedling establishment, shade avoidance, and flowering. However, our understanding of the phytochrome signaling network is incomplete, and little is known about the individual roles of phytochromes and how they function cooperatively to mediate light responses. Here, we used a bottom-up approach to study the phytochrome network. We added each of the five phytochromes to a phytochrome-less background to study their individual roles and then added the phytochromes by pairs to study their interactions. By analyzing the 16 resulting genotypes, we revealed unique roles for each phytochrome and identified novel phytochrome interactions that regulate germination and the onset of flowering. Furthermore, we found that ambient temperature has both phytochrome-dependent and -independent effects, suggesting that multiple pathways integrate temperature and light signaling. Surprisingly, none of the phytochromes alone conferred a photoperiodic response. Although phyE and phyB were the strongest repressors of flowering, both phyB and phyC were needed to confer a flowering response to photoperiod. Thus, a specific combination of phytochromes is required to detect changes in photoperiod, whereas single phytochromes are sufficient to respond to light quality, indicating how phytochromes signal different light cues.
Highlights
Plant photoreceptor signaling networks are sensitive to a large dynamic range of light inputs
We studied the interactions among members of the phytochrome family of photoreceptors, which detect the changes in light quality that occur upon shading by other plants, as well as the duration of daylength, which indicates seasonal changes
We conducted these studies in Arabidopsis, which bears five phytochromes
Summary
Plant photoreceptor signaling networks are sensitive to a large dynamic range of light inputs. Plant light signaling systems are sensitive enough to induce germination in response to extremely short exposures of light, as encountered during soil tillage, and very low light intensities, as experienced under soil litter, and yet able to detect subtle variations in light quality under full sunlight or changes in photoperiod. These abilities depend partly on the existence of multiple photoreceptor families with differential spectral properties and on the sub-functionalization of photoreceptor family members, which resulted in the emergence of photoreceptors with distinct properties and the capacity to interact to modulate light sensitivity [1, 2]. Phytochromes promote germination by stimulating gibberellin (GA) synthesis and sensitivity [6, 7], promote deetiolation during early seedling development, inhibit hypocotyl and stem elongation by altering auxin levels [8], entrain the circadian clock, and regulate flowering time [2]
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