Perception and transduction of shade signals: A network with multiple inputs, outputs and connections.

CASAL, JORGE J

Edinburgh

Simposio; The 2013 International Symposium on Plant Photobiology; 2013

The low red / far-red ratios of shade light shift the steady state of phytochrome B to its inactive form, which sequentially enhances the activity of the PIF bHLH transcription factors, the expression of auxin-synthesis genes, the levels of auxin and the stem-growth rates, compared to the plants grown in open places. In addition to this elegant, short pathway, other events are important in plant responses to shade. First, shade reduces not only the red / far-red ratios; it also reduces irradiance levels and we will show that phyB is involved in the perception of these changes in irradiance. Second, PIFs not only promote auxin synthesis and stem growth; some of the direct targets of PIFs are growth repressors and their action is important to adjust stem growth under dynamic light environments. Third, shade not only enhances PIF activity: Reduced phytochrome and /or cryptochrome activity under shade allows the re-accumulation of COP1 in the nucleus, a response that is surprisingly rapid, given the slow kinetics of nuclear exclusion during de-etiolation. At least some direct targets of PIF transcriptional control also require COP1 for normal responses to shade. Forth, shade not only initiates shade-avoidance responses such as the promotion of stem or petiole growth; shade light signals also induce acclimation responses, which reduce the energetic cost of enhanced stem growth by lowering the stem content of flavonoids and the stem respiration rate.