Seminars

Because plants depend on light for energy their development is highly modified in response to the light environment. One example is de-etiolation: plants use light as a signal for soil emergence; seedling perception of light inhibits hypocotyl elongation. Our time-lapse imaging of seedling growth revealed that in diurnal conditions cell-elongation peaks at dawn. Subsequent analysis revealed that this pattern results from the coordinated action of both the internal circadian oscillator and the light signaling pathways. Specifically the clock regulates transcription of PIF4 and PIF5, growth-promoting transcription factors, whereas light regulates their protein stability. This work provides a detailed mechanism for clock/environment interactions.

A second example of light-regulated growth occurs during shade avoidance, a suite of traits that includes increased stem and petiole elongation, early flowering, and changes in resource allocation. Others have established that auxin is involved in shade-induced increases in cell elongations. To examine shade/auxin interactions in real-time, we used the synthetic auxin responsive promoter DR5 to drive expression of luciferase (LUC). We observed a strong increase in DR5:LUC bioluminescence after inducing the shade-avoidance syndrome plants with end-of-day far red (EOD-FR). The pattern of induction suggested an increase in auxin biosynthesis and we found that three putative auxin biosynthesis genes, YUCCA5, 8, and 9, are all induced by shade treatment. Mutant analysis shows that these genes are required for shade-induced increases in auxin signaling and full-elongation in response to EOD-FR, indicating a new connection between the light and auxin signaling pathways.