Low light intensity delays vegetative phase change.

Plants that develop under low light (LL) intensity often display a phenotype known as the "shade tolerance syndrome (STS)". This syndrome is similar to the phenotype of plants in the juvenile phase of shoot development, but the basis for this similarity is unknown. We tested the hypothesis that the STS is regulated by the same mechanism that regulates the juvenile vegetative phase by examining the effect of LL on rosette development in Arabidopsis (Arabidopsis thaliana). We found that LL prolonged the juvenile vegetative phase and that this was associated with an increase in the expression of the master regulators of vegetative phase change, miR156 and miR157, and a decrease in the expression of their SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) targets. Exogenous sucrose partially corrected the effect of LL on seedling development and miR156 expression. Our results suggest that the response of Arabidopsis to LL is mediated by an increase in miR156/miR157 expression and by factors that repress SPL gene expression independently of miR156/miR157, and is caused in part by a decrease in carbohydrate production. The effect of LL on vegetative phase change does not require the photoreceptors and transcription factors responsible for the shade avoidance syndrome, implying that light intensity and light quality regulate rosette development through different pathways.