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Twitter: @ronaldpierik
www.plantphotobiology.com

Shade avoidance responses help plants escape from shade cast by neighbors and are primarily initiated upon detection of far-red-enriched light reflected by nearby neighbors. Responses are varied and include accelerated elongation of stems or petioles, upward leaf movement (hyponasty) and apical dominance. Shade avoidance responses to planting density, however, need to be balanced against a variety of other environmental challenges that co-occur. Here I will present our recent insights into crosstalk between shade avoidance and other environmental signaling pathways.
Plant responses to shade cues act primarily through perception of far-red light via phytochrome B, leading to accumulation of PIF transcription factors. PIFs typically act in a larger transcription factor network and activate auxin synthesis, transport and response to drive cell elongation both in local tissues, as well as in remote tissues.
We found that mild soil salinity dampens the activity of the PIF network through ABA and brassinosteroids, thereby reducing shade avoidance responses in saline soil. Interestingly, we found that biotic stress, such as infection with the pathogenic fungus Botrytis cinerea, only affects shade avoidance to some extent, but is itself strongly controlled by far-red light signaling in dense stands. This involves interaction with components of the immunity signaling network as well as with primary metabolism.
Although light signaling and response are obvious properties of shoot tissues, also root development is responsive to these light cues. Shade cues typically suppress lateral root formation even if the roots are not exposed to any light cues themselves. When simultaneously challenged by specific nutrient deficiencies, these responses are, however, modulated, indicating tight interaction of signaling between light and nutrient cues.
I will discuss the key insights obtained through these studies and how they provide new entries into understanding how plants deal with multiple challenges simultaneously.