Edinburgh, Scotland, United Kingdom - TAIR

Regulation of shade avoidance by a network
of bHLH class transcription factors
Light is a source of energy for plants, but also an important source of information
about the surrounding environment. Since plants are sessile organisms it is of
major importance that they adapt growth to changing light conditions. One wellstudied
phenomenon is the shade avoidance response. In high vegetational
density the red:far-red (R:FR) ratio decreases, because photoactive pigments of
neighboring plants absorb R light, whereas FR light is mainly transmitted and
reflected. In Arabidopsis thaliana this change of light quality is detected by R/FR
photoreceptors known as phytochromes (phyA-phyE) and leads to the shade
avoidance response. In order to reach direct sunlight several morphological and
molecular changes take place. At the phenotypical level, shade avoidance is
characterized by elongation growth of stems and petioles at the expense of leaf
development. In addition plants have elevated leaf angles (hyponasty) and an
increased apical dominance leading to reduced lateral branching. At the
molecular level shade rapidly upregulates the expression of a number of
transcription factors including HFR1, ATHB2 and PIL1.
The Phytochrome-Interacting Factors PIF4 and PIF5 interact with the light
activated photoreceptor and promote growth responses under vegetational
shade. In direct sunlight they interact with phytochrome resulting in rapid
degradation of those bHLH class transcription factors. In shaded conditions the
phytochrome photoequilibrium shifts towards the inactive form of the
photoreceptor, that does not interact with PIF4 and PIF5. PIF4 and PIF5 thus
accumulate in low R:FR and lead to elongation growth responses. The related
bHLH class transcription factor HFR1 (long Hypocotyl in FR 1) limits excessive
shade-induced responses. We have studied the relationship between PIF4, PIF5
and HFR1 during shade avoidance at the molecular and physiological levels.
46
L21
Friday 10:00 - 10:30
Signalling in Development
Christian Fankhauser
Patricia Hornitschek
Séverine Lorrain
University of Lausanne
Switzerland