Green light augments far-red-light-induced shade response

Plants grown in shade exhibit changes in architecture and gene expression to best accommodate growth in photosynthetically challenging conditions. Adaptive changes in morphology include stem and petiole elongation and leaf hyponasty. These changes can be induced by low red to far-red ratio (R/Fr ratio) or by enrichment of green light relative to red and blue. In this report we demonstrate the relationship between far-red and green light in combination. Wild-type Arabidopsis thaliana plants were treated with a high and low R/Fr ratio background with or without supplemental green light. The addition of green light augmented the far-red response. Genetic analysis showed that the green effect operates independently of cry1, cry2, phot1, and phot2 receptors. Additive effects are not observed in phyA and phyB double mutants, but are observed in the phy signal transduction mutants pif4, pif5, pif7. The transcript levels of shade-associated genes (PIL1, ATHB2, and HFR1), are induced by low R/Fr ratio conditions and are reduced in the presence of green light, but not in phyAphyB mutants. The reduction in shade-related gene expression caused by supplementation of green light is inconsistent with the elongated petiole phenotype observed. These results suggest that phyA or phyB is required for the green light shade response, but they are not the main receptors because green light would increase the R/Fr ratio, leading to a non-shade phenotype.