Auxin signalling and the commitment to shade avoidance

CASAL, JORGE JOSÉ

Haitang Bay, Sanya, China

Congreso; Auxin 2016 meeting; 2016

Shade reduces the activity of phytochrome B (phyB), which is one of the most influential photo-sensory receptors. phyB reduces the abundance and/or DNA-binding capacity of PHYTOCHROME INTERACTING FACTORs (PIFs), which are transcription factors that act mainly as positive regulators of transcription of their target genes. Shade releases PIFs from the inhibition imposed by phyB, and PIFs enhance the expression of many target genes, including YUCCA genes that encode a rate-limiting enzyme in the synthesis of auxin. Therefore, under shade the synthesis of auxin is enhanced and the rate of stem growth is promoted. Low phyB activity under shade also enhances the nuclear accumulation of CONSTITUTIVE PHOTOMORPHOGENESIS 1(COP1), which is an E3 ligase that targets negative regulators of PIFs to degradation. The COP1 regulatory loop is also necessary for the enhanced auxin synthesis observed in early response to shade. We will show that, although under persistent shade the promotion of stem growth is reinforced, the levels of auxin return to those observed before the rapid stimulation caused by shade. We will describe the temporal dynamics of hormone signalling components that result in sustained stem growth under persistent shade.